Journal of The Electrochemical Society, 2023 170 021505
1945-7111/2023/170(24021505/18/540.00 © 2023 The Electrochemical Society (
of ECS by I0P Publishing Limited
). Published on behalf
Al rights, including for text and data mining, Al training, and similar technologies, are reserved.
CrossMark
Review—Electrochemical Migration in Electronic Materials:
Factors Affecting the Mechanism and Recent Strategies for
Inhibition
Ee Lynn Lee," 7O Yi Sing Goh,">® A. S. M. A. Haseeb
" Yew Hoong Wong,"**® Mohd
Faizul Mohd Sabri,"*® and Boon Yew Low’
! Deparmment of Mechanical Engineering, Faculty of Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
2Centre of Advanced Materials, Faculty of Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
? Department of Nanomaterials and Ceramic Engineering, Faculty of Chemical and Materials Engineering. Bangladesh
University of Engineering and Technology, 1000 Dhaka, Bangladesh
$ Centre for Energy Sciences, Faculty of Engineering, Universi Malaya, 50603 Kuala Lumpur, Malaysia
3 Process Innovation, NXP Malaysia Sdn. Bhd., 47300 Petaling Jay . Selangor, Malaysia
Electrochemical migration (ECM) is one of the serious failure modes encountered in electronic devices due to the electrochemical
reactions triggered by the presence of moisture and bias voltage, leading to the growth of dendrites and short circuits. The classical
ECM mechanism consists of four consecutive stages: (i) electrolyte formation, (ii) anodic dissolution, (iii) ion transport, and (iv)
dendrite growth. ECM is a delicate process that involves a combination of a good number of factors, such as the clectrode
properties. climatic conditions, contaminants, electric field, additives. etc. We intend to provide a comprehensive review of the
complex effects that these factors have on each stage of ECM and provide insights into the recent developments in ECM research.
Previous finding , current debates and recent discoveries are covered in this article. This review paper also provides a review of
recent strategies for ameliorating ECM failures in electronics.
© 2023 The Electrochemical Society (“ECS”). Published on behalf of ECS by IOP Publishing Limited. [DOI: 10.1149/1945-7111/
acbólal All rights, including for text and data mining, Al training, and similar technologies, are reserved.
Manuseript submitted October 12, 2022; revised manuscript received January 21, 2023. Published February 15, 2023. This paperis
part of the JES Focus Issue on Critical Factors in Localized Corrosion in Honor of Gerald Frankel.
List of Symbols
Material dependent parameter
Concentration of ions
Diffusivity or diffusion coefficient
Arrhenius pre-cxponential factor
Spacing between two oppositely charged electrodes
Electric field
Standard electrode potential
Activation energy
Ew
Ecorr
Epiu
F
fcorr
Adsorption energy
Corrosion potential
Pitting potential
Faraday’s constant
Corrosion current density
Flux in the transport of ions
Electric current de
Acid ionization cons nt
Solubility product
Total amount of product
Valency of metal
Number of electrons transferred per mole of product
Number of particles per unit volume
Negative base—10 logarithm of K,
Total charge
Carrier charge
Molar gas constant
Absolute temperature
Applied bias voltage
Drift velocity of a migrating ion
Velocity of the ion in a flow at x
Concentration gradient at distance x
Potential gradient
Tonic mobility
Electric conductivity
Tonic conductivity
“E-mail: eelynn@um.edu.my; haseeb@nce buet.ac bd; yhwong@um edu.my
Electrochemical migration (ECM) is one of the main reliability
issues encountered in the electronic packages and batteries.'™ ECM
is a phenomenon involving electrochemical reactions in the presence
of bias voltage and electrolyte. During ECM, a metal anode
corrodes, dissolves, and redeposits at the cathode in dendrite form.
ECM ultimately leads to short circuit as the dendrite forms a bridge
that comnects two adjacent conductive materials.
"" resulting in
catastrophic failure. The increasing need to operate in harsh
environment and the increased use of no-clean fluxes along with
uncoated surfaces due to cost factors put the electronic devices at the
risk of exposure to contaminants. The miniaturization trend in the
electronics results in a smaller spacing between the conduction lines
and makes the cleaning process more difficult. The smaller spacing
also leads 0 a lager elecuic field and a higher cutent density. The
stringent conditions faced by the different electronic devices nowadays lead to a higher risk of ECM failure, as shown in Fig. 1.
Many metals and alloys used in electronic devices, such as the
surface finishes (i.e., Au, Cu, Sn), solder interconnects (Sn-based
alloys and Ag), Cu wirebonds and thin films (Au and Pd) arc
susceptible to ECM."" These materials make up a large portion of
the exposed arca in an electronic device. Other than the electronic
packages, zinc (Zn)-air, lithium (Li) and sodium (Na) metal batteries
in the consumer electronics such as electric vehicles, sensors and
portable electronics also suffer from the safety issues caused by the
uncontrollable dendrite growth and internal short circuit. "
However, it is important to note that the electrolyte in the batteries
is different from that involved in electronic packages. Therefore, the
mechanism of ECM in materials constituting clectronic packages
differ somewhat from that of ECM in batteries. The electrolytes
the batteries are generally ester-based, ether-based, and carbonatebased, while the electrolytes in electronic packages mainly contain
moisture layers resulting from the condensation process. The
electrolyte volume also differs in the two cases, the moisture volume
being very small in electronics. The use of electrolyte in a battery is
essential but the presence of clectrolytes in the electronic materials is
undesirable.
Based on the literature,” 4.6.7 most of the electronic materials share
similar ECM mechanism. Among the few theoretical models of
ECM mechanisms, the classical model is the most common
mechanism of ECM encountered by electronic materials. The
Journal of The Electrochemical Society, 2023 170 021505
Harsh Environment
- Higher humidity
Highest
- Higher temperature
risk of ECM
failure
- More exposure to
contaminants
(i.e., dust, chemical pollutants,
microbial pollutants)
Figure 1. Driving force of ECM.
classical ECM model involves four consecutive stages (Fig. 2). It
starts with the formation of a continuous layer of moisture on the
surface of an electronic material via adsorption or condensation. In
the presence of bias voltage, the continuous moisture layer acts as
the conductive electrolyte that connects two adjacent conductive
materials, thereby forming an electrochemical cell. One of the
conductive materials acts as an anode while another acts as a
cathode. The presence of the electrolyte layer causes corrosion of
metal anode and results in the dissolution of metal that forms metal
ions. These positively charged metal ions then transport or migrate
through the electrolyfe layer towards the cathode under the influence
of electrical field, diffusion or convection. The metal ions are
redeposited at the cathode in the form of dendrite. The dendrite
then continues to grow and propagate towards the anode. Once the
tip of the dendrite reaches the anode, a conductive bridge is formed
Stage 1:
Electrolyte
formation
between two oppositely charged electrodes and this ultimately leads
to short circuit of the system.”*
In general, the ECM mechanism of metals that are highly
susceptible such as Ag, Cu, Sn and Pb follows the typical four
consecutive steps described in the classical model.""? Some
materials experience ECM involving a slightly different mechanism.
For example, when contaminants such as halogens were present, the
formation of complex ions consisting of metal ions and halides was
involved in the ECM of Pd and Pt, which were reported to be ECM-
free in the past.” Not only pure metals, the ceramic-based electronic
components such as Bi,O, and CuO can produce metal ions that
migrate and deposit at the cathode.” The metal ions are formed by
the reduction of metal oxides by H'. Some studies’"' reported that
Cu and Ni can experience the growth of conductive anodic filaments
(CAF) during ECM, where the conductive filament emanates from
Stage 3:
Stage 2:
-—
Anodic
dissolution
-—
lon
transport
Stage 4:
-—
Dendrite
growth
Growth direction
Figure 2. Schematic of the classical model for ECM mechanism.Ó"
Journal of The Electrochemical Society, 2023 170 021505
the anode when there are formations of complex secondary anions
with negative charges. Despite the differences between the mechanisms involved in different materials, the basic steps are similar to
those of the classic four-step ECM mechanism.
is applied and the electrolyte layer is present, these materials become
electrodes, and an electrochemical cell that can lead to ECM forms.
During ECM, one of the metal electrodes acts as an anode and
oxidizes to form metal cations as shown in Eq. |
Therefore, this
article focuses on the classical ECM model.
A critical review of the ECM phenomenon is crucial to providing
in-depth knowledge for adopting appropriate mitigation measures
and future studies. A few review papers on ECM have appeared in
the literature in the past. Previous review articles covered ECM
Me — Me"* + ne™
where Me and n represent the metal the valency of the metal,
respectively.
It wes reported that the standard electrode potential (£°) of a
metal involved affects the anodic dissolution of ECM. E" is defined
as the individual potential of an electrode with respect to the
standard hydrogen clectrode under standard conditions (1 mol
mechanisms,”™ ECM test methods,* advances in suppressing ECM
of Sn-based solders,' and factors influencing the ECM behaviour of
metals.”
% The findings in the
literature demonstrated that ECM is a
complex phenomenon that depends on a good number of factors and
the synergistic interplay among them."? However, a critical review
that links the effects of the factors to different stages of ECM is not
dm7?
major factors that affect the sequential steps in the mechanism of
ECM and suggest the advanced mitigation measures that help
ameliorate the ECM of electronic materials. More recent research
papers that were not covered in the past review papers are also
discussed in this article. The factors are discussed comprehensively
under the following categories: material properties, environmental
factors. electrolyte propertics. clectric field and surface conditions as
displayed in Fig. 3. These factors can be further divided into a few
subsidiary factors. It is demonstrated that each factor influences
multiple stages of ECM. Moreover, recent strategies for inhibition of
ECM at cach stage is also suggested.
difficult it is for a metal anode to dissolve. Noh et al.'® reported that
regardless of spacing and bias voltage, electroless Ni and immersion
gold (ENIG) have the highest time to failure (TTF) compared to Sn
and Ag due to the higher
£7 of gold (+1.498 V). Medgyes'” reported
that Ni with a lower E" (—0.25 V) has a higher tendency to undergo
electrochemical oxidation as compared with Cu (+0.34 V), therefore
Ni has a higher ability to produce metal ions and leads to a shorter
Theoretically, ECM is less feasible in metals with higher E°.
However, Ag with a relatively large value of £" (+0.799 V) shows a
Material Properties
Electrode material—Metals used in electronics (Sn, Ag, Cu, Ni,
Au, Sb, etc.) and automotive industry (Sn, Ag, Cu, Pb, Au, Pt, etc.)
high susceptibility towards ECM."?"?"? Some of the findings'”
recognized that the rates of ECM correlate better with the solubility
product (K.,) of the metal hydroxides rather than with the £" of the
have been found to be susceptible to ECM.5"" When the bias voltage
Electrode material
Environmental Factors
Temperatui
Electrolyte Properties
i
Contaminants
Form of electrolyte
eHur::i;:yre
Alloying elements
Elstatv_:;eltt
ectrolyte
formation
—————
Electric Field
Bias voltage
Spacing
ion concentration, room temperature and 1 atm pressure).'>
The standard data available represents the tendency of an electrode
to undergo reduction and gain electrons. The more positive the £°,
the higher the tendency for reduction to occur and the more difficult
it is for oxidation to occur.
Previous work""""" related anodic dissolution to the E" of the
metals by stating that the more positive the £" value, the more
available. This work aims at providing in-depth insights into the
Material Properties
SA(agZ
nodic
dissolution
[
lonic conductivity
pH of electrolyte
ot
Stage 3:
Stage 4:
lon fransport
formation
Dendrites
L
Surface Conditions
Surface Roughness
Oxidation state
Substrate
Self-assembled Monolayers
Surface coatings
Figure 3. Factors affecting the mechanism of ECM (in box) and strategies in mitigating ECM (in oval-shaped block)
Journal of The Electrochemical Society, 2023 170 021505
metals. K., represents the ability for a solute (metal hydroxides) to
dissolve, in a solution and form ions. Ky, can be represented by
Eq. 217829.
found present in the dendrite. They verified their findings using
scanning transmission electron microscopy (STEM) and confirmed
that Sb can participate in the ECM mechanism even with a minor
addition (£1.03 at%).
Ksp = [Me"*][OH]"
21
The more soluble a substance is, the higher the K, valueit has. The
metal cations (Me"") react with OH ™ ions during ion transport to
form Me(OH), precipitate. Higher K., of the precipitate results in
the dissolution of more metal ions, which are then transported to the
cathode for subsequent ECM steps. Precipitate with a lower K., on
the other hand, remains in the electrolyte and can physically obstruct
ion transport.
Harsdnyi and Inzelt"? demonstrated that in a pure metallization
system, the average TTF or the mean time to failure (MTTF) of a
conductor in THB tests decreased linearly with the K., of metal
hydroxides. Although £° of Ag is the highest among the four metals
(Ag, Cu, Pb and Sn) compared, the MTTF of Ag was the lowest
owing to its highest K.,. Ag* formed from anodic dissolution can
‘migrate directly to the cathode for electrodeposition as it has a lower
tendency to form hydroxide, causing Ag to be very prone to ECM
failure. Steppan et al.” reported that the K., of precipitate for Ag, Cu,
Pb. Zn and Sn is in decreasing order. and the ECM susceptibilityis
found to be in a similar order. Literature'>'>'® suggested that the
speed ofion transport and dendritic deposition are affected by the
concentration of metal ions inside the electrolyte which is controlled
by the hydroxide precipitate formation through K., However, this
rule is only applicable for the metals that follow the classical ECM
model in the absence of ionic contaminants, and when anodic
dissolution is not_hindered by intermetallic compounds (IMC) or
passivation layer,'>'* where passivation layer refers to the oxide or
hydroxide film that protects it against further corrosion.7" In general,
metals that can form a stable and defect-free passive film are usually
very resistant to ECM, for example Au, Al, Pd and Pt.”
In summary, according to the work reported, the type of material
determines the £° and K,j, of the electrodes. In most of the cases, K.,
plays a more significant role than £°. The material properties of an
electrode can affect three stages involved in classical ECM model:
anodic dissolution, ion transport and dendrite formation stages. The
influence of the type of materials on the electrolyte formation stage
was not reported before. However, it can be noted that the type of
‘materials exhibits different surface characteristics such as the surface
wettability and surface energy.7 This may in turn affect the
condensation of water on the material surface, and hence affect
the electrolyte formation.
Alloying elements.—Alloying clements are often added to the
metals to enhance their mechanical, thermal, and electrical properties. The demands for optimum performance and lifetime have
prompted the continuous development of innovative solder alloys.”
Table I shows some examples of the alloying elements added to the
electronic materials and their roles in the ECM mechanism. The
ECM steps affected by addition of alloying elements include (1)
anodic dissolution, (2) ion transport and (3) dendrite formation. The
examples given in Table I are mainly on Sn-based alloys and Agbased alloys. The effect of alloying elements on the ECM of Cu have
not been investigated in the past. Most of the publications were
based on pure Cu,! 11315220
Some of the alloying elements added to the base material
participate in the dissolution, migration, and deposition of the
ECM reactions. During ECM, free metal anions are produced from
the dissolution of the anode. These ions are crucial for the
subsequent ECM stages. The availability of metals in their free
existing form plays an important role in determining the ions
dissolved. For example, when Pb is added into Sp-based alloy, it
remains as a Pb-rich phase that can dissolve into Pb?" readily."" TI
articipation of Pbin the ECM of Sn-Pb was reported.'® Medgy
al."" reported an unexpected observation in the ECM of Sn-0.3Ag0.7 (SAC0307) and Sn-0.8Ag-0.7 (SAC0807): antimony (Sb) was
The ions participating in ECM are affected by K, of the metal
hydroxides. The solubility products of Sn(OH),, Zn(OH)2, PH(OH);,
Cu(OH) and Cu(OH), are 1.0 x 107,12 x 10717, 1.2 x 1071,
1.0 x 10~
and 5.0 x 107)? respectively. 13 As discussedin the
previous section. metal hydroxides with a higher K., arc morc
susceptible to ECM. For example, Ma et al.” reported that in Sn9Zn, Zn dissolves more easily compared to Sn owing to the higher
K. of Zn(OH), compared to that of Sn(OH),. Therefore, dendrites
deposited at the cathode are composed of more Zn (20.99 wt96) than
Sn (1.17 wt%), even though the concentration of Sn is much higher
than Zn in Sn-9Zn. Takemoto et al."" discovered that the dendrite
produced from the ECM of Sn-40Pb contains 81.4 wt% Pb. The
higher Pb content in the dendrite can be attributed to the larger K, of
Pb(OH); than Sn(OH),.
However, it is important to note that the susceptibility of alloys
towards ECM cannot be determined simply by the K., of their metal
hydroxides.'” This is attributed to two main reasons: (1) the
formation of IMC in the alloy and (2) the passivation on the surface
of the anode.
Addition of Ag or Cu into Sn form IMC such as CueSns and
AgzSn 57728 It was widely reported that the formation of these IMCs
prevents Ag and Cu from dissolving into ions for subsequent ECM
steps, even though pure Ag and pure Cu are highly susceptible to
ECM. A recent findingő shows that Sn-3.8Ag-0.7Cu-3.0Bi-1.4Sb0.15Ni (InnoLot) exhibits the highest resistance to ECM among 5
types of commonly used Sn-based alloys. This is attributed to the Ni
and Bi additions to the InnoLot that make the distribution of the
IMCs in the bulk alloys more homogeneous, which leads to a lower
susceptibility of ECM.>* However, it is important to note that the
ability of an IMC can be affected in the presence of higher voltages
, Ag? can escape from AgzSn), which will be discussed below.
Alloying elements can also affect the passivation behavior of a
material which in turn affect the anodic dissolution step, either by an
increase in the passive range or by the formation of new passive
oxide film with the solders involved.7975
This passive oxide layer
prevents a metal anode from being oxidized to form metal ions.
Previous investigations showed that alloying elements such as Cu,
Ag, Bi and Pb affect the passivation behavior of Sn-based materials.
Yoo et al.? reported that the additions of Cu and Ag into the Su
matrix improved the passivity of the 96.55n-3Ag-0.5Cu (SAC305)
solder. The authors linked the passivation behavior of the solders to
the corrosion properties such as corrosion potential (Eeey) and
pitting potential (Epi) and showed that SAC305 has higher values of
Ezorr and Epiu compared to pure Sn and Sn-3.5Ag. Therefore, it was
concluded that the addition of Ag and Cu improves the passivity of
Sn-based alloys.
Harsinyi and Inzelt'” investigated the passivation behaviors of
silver-palladium-platinum (Ag-Pd-Pt) using cyclic voltammetry
method. The authors concluded that alloying Ag with Pd and Pt
accelerates the formation of passive oxide films, which prevents the
anodic dissolution, thus eliminating ECM failure. Naguib and
MacLaurin® studied the effect of adding Pd to Ag and found that
the ECM failure time increased with Pd content. It was suggested
that the formation of PdO also acted as a barrier to Ag dissolution. A
recent finding by Yang et al.** also reported thatincorporating 19 at
% of indium (In) into pure Ag creates Ag-In alloy that is completely
resistant to ECM due to the formation of a passive indium oxide
layer (suggested to be In:O3).
Alloying clement addition also affects the third step in the
classical ECM model. the ionic transport step. Different ions have a
different speed of transport in the electrolyte. The solutes or the ions
can transport through bulk motion of fluid by advection. Metal ions
with a higher transport speed migrate faster across the electrolyte to
reach the cathode.®”
Ma et al."" proved that Zn has a higher transport
rate than Sn by plotting theion concentration profiles of Zn and Sn
Journal of The Electrochemical Society, 2023 170 021505
Table I. Alloying elements in electronic materials and their roles in the ECM mechanism.
Base material
sn
Alloying element
Ag
Cu
Roles in ECM mechanism
Forms stable IMC of AgsSn
Forms passive film on anode
Improves passivation behavior
Enhances pitting corrosion
Affects dendrite morphology
Forms stable IMC of CueSns
Forms passive film on anode
ECM steps affected
(1)Anodic dissolution
(3)Dendrite formation
References
8. 27,28
(hAnodic dissolution
8. 27. 28
(1)Anodic dissolution
27,29
(3)Dendrite formation
Improves passivation behavior
Enhances pitting corrosion resistance
Affects dendrite morphology
Bi
Improves corrosion resistance
Zn
Participates in dissolution, migration and deposition
Pb
Participates in dissolution, migration and deposition
Improves passivation behavior in NaxSO, environment
Affects dendrite morphology
Sn-Ag-Cu
In
Zn
Accelerates corrosion
Changes dendrite morphology
sb
Accelerates corrosion
Changes dendrite morphology
Participates in dissolution, migration and deposition
Ni -- Bi + Sb
Create homogeneous IMC distribution
(hAnodic dissolution
20
(2)lon transport
(3)Dendrite formation
(1)Anodic dissolution
15, 30
(1)Anodic dissolution
31
(DAnodic dissolution
(3)Dendrite formation
(hAnodic dissolution
31
(2)lon transport
(2)lon transport
(3)Dendrite formation
(3)Dendrite formation
32
(2)lon transport
(3)Dendrite formation
Sn-35.0Bi-1.0Ag
Ag
Ge
Pd
Improve corrosion resistance
Changes dendrite morphology
Forms passive film on anode
In
Forms passive film on anode
P
Forms passive film on anode
over time obtained from the ion chromatography results . In addition
to the transport speed, it was mentioned above that Ky, of Zn is
larger than that of Sn. Therefore, the dendrite composition of Sn-9Zn
alloy in deionized water at 5 V was mainly Zn due to the synergistic
effects of higher transport speed and larger Ky, even though the
composition of Sn is much higher than Zn in Sn-9Zn.
Apart from the anodic dissolution and ion transport steps,
alloying elements are found to affect the dendrite formation step.
The composition and morphology of the dendrites formed can be
affected by the alloying elements. Hua et al.** reported that the
morphology of the deposits of Sn-35.0Bi-1.0Ag solder changed from
floccus-like (rounder shape) to tree-like with the addition of 0.5 wt%
(1)Anodic dissolution
23
(3)Dendrite formation
(1)Anodic dissolution
33
12,34
(1)Anodic dissolution
35
(1)Anodic dissolution
2
Ge. It was also found that adding 0.8 wt% of In to SAC305 can
change the common tree-like dendrite into petal-like deposits, while
adding 5.0 Wi% of Zn to SAC305 can tum the tree-like dendrite into
rounded-shape deposiíts.7" The authors suggested that the change in
morphology of deposits was due to the presence of different IMCs
on the alloy
surface such as InsSn, InSn,, CusZny, CuZn,, and ZnO.
The deposits with a petal or round shape inhibit ECM effectively
because the length of the deposits is shorter than that of dendrites,
which are in a tree-like shape. Medgyes et al.*® reported the change
in dendrite morphology with increasing Ag content. The dendrites of
SACO807 are curved needle-like, while the dendrites of SAC0307
are straight and perpendicularly branched. Yu et al."7 showed that
Table II. Morphologies of the deposits of Sn-based alloys.
Morphology
Metal/ Alloy
References
“Standard” tree-shaped dendrite
Sn
39
Longer stick-shaped dendrite
Round-shaped deposit
$n-3.0Ag-0.5Cu
Sn-3.0Ag
Sn-58Bi
31,40
41
29,41
Sn-35.0Bi-1.0Ag-0.5Ge
33
Sn-37 0Pb
Sn-36.0Pb-2.0Ag
15
Sn-350Bi-1.0Ag
33
95 (SAC305)—5.0Zn
31
SAC0807
38
95 (SAC3OS)—0.8In
Transformed/ distorted tree-shaped dendrite
15
Sn-8.0Zn-3.0Bi
$n-0.7Cu
Sn-5.05b
31
15, 40
41
41
Journal of he Electrochemical Society, 2023 170 021505
Pb-bearing solders such as Sn-37Pb and Sn-36Pb-2Ag solders form
dendrites that are in longer stick shape compared to the tree-like
shape.
The morphologies of the deposits and the dendrites have
significant impacts on the ECM behavior. In general, thin films,
nodules and dendrites with plenty of branches are more favorable in
reducing ECM susceptibility because the propagation time of the
deposit is longer. The shape of the deposits and the dendrites is also
affected significantly by the physical factors (i.c., the temperature,
geometry of electrolyte, and bias voltage). The ECM resistance can
be improved by altering the deposit/dendrite morphology using
inhibitors and sclf-assembled monolayers (SAMs). These factors and
inhibition methods are discussed in detail in the following sections.
Table II summarizes the different morphologies of deposits
reported with the addition of various alloying elements to Sn-based
materials. They can be categorized into four major types, which
include tree shape, longer stick shape, rounder shape and other
transformed shapes. Among the four major types listed, dendrites
that are in tree-like shape are the “standard” deposits that form as a
result of ECM failure. A majority of the reported deposits that
caused short circuit are dendrites in tree-like shape.
In summary, the properties of a material, such as the E" and Ky,
of a substance are determined dircctly by the type of electrode
material. The K.y, of metal hydroxide, corrosion propertics (Ecor; and
E,i), passivation behaviour of the anode and transport speed of ions
are affected by the addition of alloying elements. The addition of
alloying elements can affect three stages involved in the classical
ECM model: anodic dissolution, ion transport, and dendrite formation stages (refer to Table 1).
Environmental Factors
Temperature and humidity.—An electronic device is subjected
to a wide range of temperatures and humidity levels during service.
The devices are used in a variety of geographical locations with
climates of different temperatures and humidity levels."? Besides.
the interior condition of an electronic device can be altered by the
flow of current and the subsequent increase in the resistance, which
generate excessive heat. The difference between the device interior
conditions and external climates creates temperature fluctuations to
the electronic device.
The effects of temperature and relative humidity (RH) on the
electrolyte formation stage in ECM have been comprehensively
reviewed by Zhong et al." In general, decreasing the temperature to
below the dew point leads to the formation of moisture layer on the
printed circuit board (PCB) surface. The layer of moisture evaporates when the temperature rises, resulting in a thinner moisture
layer.® On the other hand, increasing the RH accelerates the first
stage of ECM. The RH is inversely proportional to temperature.
Therefore, increasing the temperature impedes the formation of
role of temperature on passivation breakdown. The breakdown
potential of passive film or Eyy decreased as the temperature of
the solution increased. A stable passive film is very important in
resisting ECM by retarding the dissolution of anode. Therefore, an
increase in temperature promotes the anodic dissolution step of the
classical ECM model.
It is important to note that the presence of weak organic acids
(WOAs) at an increased temperature creates synergistic cffects on
ECM. Temperature has a significant impact on the corrosivity of
WOAs. Piotrowska et al.**reported that the corrosivity of WOAS (e.
.. palmitic acid, suberic acid, glutaric acid, etc.) was associated with
their solubility that increased as a function of temperature. The
increase in temperature raised the solubility of a WOA, and this
facilitated the anodic dissolution step of Sn-based PCB."" Further
discussion related to WOAS can be found below.
The third stage of ECM, which is the ion transport step, involves
the migration of the dissolved metal jons, Me"" from the anode to
the cathode under the influence of clectrical field, diffusion in the
presence of concentration gradient. and convection due to the flow in
the fluid." The transport of fons, which is a combination of
diffusion,
migration and convection, is nenemu(expressed by the flux (J) that
is governed by Nernst-Planck Eq.
Z—í—y:i—,Pi—Lv(x)
where D is the diffusivity or the diffusion coefficient; c/x is the
concentration gradient at distance x; ju is the ionic mobility; c is the
concentration of ions; d¢/dx is the potential gradient, and v(x) is the
velocity of the ion in a flow at x. A detailed discussion on the
numerical modelling of ECM based on the migration and diffusion
can be found in the literature.”>**~>" It should be noted that there are
differences between electrochemical deposition in electrochemistry
and ECM in electronic materials. He et al.”® put forward three such
differences: (1) the thickness of the transport media (lectrolyte) is
much smaller in ECM, (2) the ionic strength of the transport media is
lower, and (3) the electroactive ions or the metal ions are not
available prior to the application of potential difference. Therefore,
the application of these theories to ECM may be limited.””
The mass transport of ions to the electrode surface is promoted
by the increase in the temperature. ! The drift velocity of a migrating
ion is represented by’
v = puE
the RH is maintained high. Zhou et al.** reported that the surface
insulation resistance (SIR) of the PCB samples decreased by several
orders of magnitude once they were inserted into a high temperature
and humidity chamber (65°C-85°C/ 75%-90%RH), indicating the
immediate condensation of water and formation of water film on the
PCB surface in a harsh condition with high humidity and temperature.
The second stage of ECM involves the dissolution or corrosion of
metals. It is known that the corrosion rate ofa material is influenced
significantly by the temperature. The corrosion rate predicted by the
Arrhenius equation anticipates
that the corrosion rate doubles with
every increase of 10 °C.*" 4. Mohran et al.** showed that the corrosion
current density (icor) of Sn increased with temperature in polarization tests. Varying the temperature also affects the passivation layer
of the anode ‘material. In general, the stability of a passive film
decreases with temperature.”’ Parangusan et al.77 summarized the
4]
where E is the applied clectric field. 1 increases with temperature.
Accordingly, an increase in the temperature results in an increase in
ion transport velocity. The diffusivity of a species is often related to
the Arrhenius equation as follows:
moisture layer.
Although a higher temperature may lower the chances of
formation of moisture layer, its impact on ECM can be drastic if
131
51
where D is the Arrhenius pre-exponential factor, E is the
activation energy. R is the molar gas constant and 7 is the absolute
temperature. According to Eq. 5. the diffusivity increases with
temperature. Since the ion drift velocity and the mass transport of the
ions increase with temperature, increasing the temperature accelerates the ion transport step.
The last stage, dendrite formation stage is affected by the
temperature_as well. Aoki et al."7 found that the Cu dendrites
formed at 50 °C were thicker than those formed at 20 °C. 1
morphology of the dendrites changed from porous-sponge-like to
bamboo-leaf-like when the temperature was increased. It was
suggested that the higher ion mobility in the water and the higher
surface diffusion of the adatoms during nucleation and propagation
of dendrites increased the thickness of the dendrite branches. The
morphology of a dendrite significantly affects the ECM behavior
such as the propagation of a dendrite. Further investigation into this
Journal of he Electrochemical Society, 2023 170 021505
aspect is necessary for a better understanding on the effect of
temperature on the dendrite morphology.
Overall, the susceptibility towards ECM increases with temperature or humidity if one of them is maintained constant. Hong et al.>'
showed that when the temperature was maintained at 85 °C, the
number of failed samples increased with the RH from 75% to 9596.
Also, when the RH was maintained constant at 85%, the number of
failed samples increased with temperature from 65 °C to 85 °C.
In short, the temperature and RH of the environment affect all
four stages of ECM including the moisture or electrolyte layer
formation, anodic dissolution, ion transport and dendrite formation
stages. An adequate RH levelis the key factor that determines the
moisture layer formation stage. A higher temperature increases the
rate of electrochemical reactions but impedes the formation of
moisture layer.
Contaminants.—Contaminants originate from the chemicals
used in the wafer fabrication processes and the surroundings of the
storage, transportation, and usage of clectronics. Due to the rapid
evolution of technologies in the past decades, the use of electronic
gadgets has expanded to wider fields such as the oil and gas sector,
militaries, and monitoring of volcanic activities. Contaminants are
largely found in chemically aggressive harsh environment such as
volcanic fields, industrial zones, military bases, combustion sites,
coastal areas, swamps, and sewers.
The roles of contaminants in the ECM of electronic materials are
complicated as they can have opposing effects on the ECM
mechanisms. A contaminant can either accelerate or hinder the
ECM process, depending on the properties of the contaminant. The
study on the effects of contaminants on the ECM of electronic
materials is cxpanded as the exposure of electronics to diverse
environment is inevitable. Dust is one of the most commonly found
pollutants from the service and industrial environments, 779 while
in harsh environments, more contaminants such as Hy$. SO."
voleanic ash™*
and mold™ are found. The pollutants or the
contaminants contain soluble ionic salts and acids which can have
severe impacts on the ECM of electronic materials.
Ionic contaminants.—lonic contaminants refer to the impurities
or substances with positive or negative charge (cations or anions)
that arise from the dissociation of pollutants in the presence of
moisture. Their presence causes the solution to become conductive,
which subsequently leads t9 cunent leakage and failures in an
clectronic system. lonic contaminants are emanated from the
pollutants such as dust particles, volcanic gases, rainwater, chemicals, etc. They are found in human handling, manufacturing process,
and service environments. 7895" Chloride (C1), sulfate (SO,%7)
and nitrate (NO;) are typical ionic contaminants found in the
environments. The results of ion chromatography showed that CI" ,
SO and NO;~ were highly present in the dust, rainwater and
mic ash. 333861-03 Minor amount of fluoride (F7), phosphate
#7) and nitrite (NO, ") were also detected in these pollutants.
Apart from the environment, ionic contaminants such as F , CI" and
bromide (Br") can arise from the halogen-containing chemicals such
as cleaning solvents and fluxes used in the PCB manufacturing
process.
lonic contaminants such as CI" are known to accelerate ECM
process due to their hygroscopic properties.597 They lower the
critical RH value of moisture adsorption, increase the total moisture
sorption by absorbing or adsorbing the water molecules from the
atmosphere, and allow the formation of moisture layer even when
the RH of the environment is low.*** The ionic contaminants
facilitate the electrolyte formation step in ECM by acting as moisture
condensation sites on the surface of electronics."7 Therefore, the
time required for the electrolyte formation step is shortened in the
presence of ionic contaminants.
An electrochemical cell is created upon the formation of moisture
layer, followed by the corrosion of the anode, which is referred to
the anodic dissolution step in ECM. Many studies revealed that the
presence of ionic contaminants has tremendous impacts on the anodic
dissolution step. Literature has reported that the presence of Cr, Br ,
SO7, NO; , SO, H,S, dust and volcanic ash increased the con-
ducuvny of the electrolyte as a function of concentration. 579961 9768
The conductivity of the electrolyteincreases with the number of ionic
species dissociated in the solution, which in turn increases the “aggressiveness™ of the electrolytes and results in a more rapid anodic
dissolution.
The presence of ionic contaminants affects the pH environment
of the electrolyte. Literature'*®” reported that the pH of region close
to the anode becomes acidic due to the generation of H' from the
anodic reactions, while that of the cathode becomes alkaline due to
the generation of OH" from cathodic reactions. The acidity/
alkalinity at the anode/cathode increased with the concentration of
ionic contaminants. An increase in the concentration of ionic
contaminants creates a more corrosive environment suitable for the
anodic dissolution. Fernandez et al.®” found that the pitting corrosion
of brass occurred in the presence of high H* or highly acidic
environment. In contrast, pitting corrosion of brass was inhibited in
the presence of high OH™ or highly alkaline environment. These
indicate that a metal is generally more prone to anodic dissolution in
an environment with a lower pH.
However. it is important to note that sometimes a lower pH does
not necessarily favor ECM. The pH determines the species formed
from the cathodic and anodic reactions. Researchers utilized the
Pourbaix potential-pH (E-pH) equilibrium diagram"" to estimate the
species formed from the ECM reactions.'**’ When there is the
formation of a certain species (e.g., complex, precipitate) due to the
presence of organic compounds, ECM can be inhibited. Further
discussion can be found below.
The exposure of metal surfaces to aggressive anions such as F
CI", and Br" can cause the dissolution and breakdown of a
thermodynamically stable passive film. The point defect model
(PDM) suggested by Lin et al."" depicted that an aggres:
can penetrate through and break the protective passive film.
Harsdnyi® reported that CI"
can accelerate the anodic dissolution
step by forming chlorine oxide and/or chlorous acid that destroy the
existing oxide film on the anode surface or inhibit the formation of
the protective oxide film. Some metals that can form a stable and
defect-fiee passive film such as Au, Pd, and Pt are usually very
resistant to ECM. However, it must be pointed out that the
can
suffer from ECM in the presence of aggressive anions.’ The
stability ofa passive film depends on the concentration öl aggressive
ions and the pH of the environment.7"9972 It was reported that the
Ecorr and Epi decrease monotonously with the increase in concen-
tration of Br , indicating that the passive film is less stable in the
presence of a higher concentration of ionic contaminants.”’ A
localized acidification leads to the breakdown of a passive film
while an alkaline condition with plenty of OH hinders the pitting
process.5199
However, it is also important to note that different materials
behave differently when subjected to the same contaminants. It was
demonstrated that the SnO; film formed in sodium chloride (NaCl)
was thicker and more stable than in NazSO, therefore the resistance
to ECM of pure Sn and SAC305 in Na,SO, was lower than in
NaCL”*"> On the other hand, opposite behavior was observed in
cutectic Sn-38Pb. The PbO passive film is more stable in NazSO,
than in NaC1."®
Different types of ionic contaminants have different levels of
“aggressiveness” which can be represented by the charge density of
an ion
ionic charge
Ton charge density = T8
— (61
volume of the ion
An ion with larger ionic charge and/or smaller radius has a larger
charge density,therefore has a higher efficiency in breaking down a
passive film."""" Previous work™ agreed well to this statement,
where it was shown that the CI" with smaller a ionic size has a
Journal of The Electrochemical Society, 2023 170 021505
©Sn eSn-3Pb mCu AlmAg ®AgNP
00 —o>
dendritic growth, which impacts the ECM of metals more significantly.
It'is
important to note that further increasing the concentration of
contaminants can lead to the redissolution of precipitates. The
excessive OH™ produced at the cathode creates an cxmmcly
1400
alkaline condition and releases complexions such as [Sn(OH)g]> "
No short circuit
Sn(OH); + 20H7 — [Sn(OH)e)*~
The complex ions are then reduced at the
dendrites as followső!
1000
171
cathode to form Sn
[Sn(OH)G]~ + 467 — Sn + 60OH-
®
8
s
18]
This mechanism is widely accepted in the alkaline plating process.7"
Despite the rapid formation of precipitates, which acts against ECM
at high concentrations, the redissolution of Sn(OH), precipitates in
extremely alkaline condition enables the deposition of ions and
dendrite growth.
Previous studies reveal that the ECM of metals has a similar
L
Mean time to failure (s)
1200
600
400 !
trendin the presence ofionic contaminants such as CI" , Br" , SO
$7-, and NOs . In a low concentration, the ECM susceptibility
increases with the concentration; in an intermediate concentration,
e.
200
there is only the formation of precipitates without dendritic growth;
®
0
0.001
0.0001
001 — 0.4
1
-A
10
Concentration of Cl" (M)
Figure 4. Effect of CI" concentration on the mean time to failure of Sn,?
Sn-3Pb,"" Cu.” immersion Ag (ImAg),7 and Ag nanoparticles,'” measured
at applied bias of 3 V.
higher probability to ECM failure than Br" even though they have
the same ionic charge (—1).
Apart from anodic dissolution, ionic contaminants also affect the
ion transport step significantly. During ECM, metal hydroxides form
from the reactions between metal ions and hydroxides. Precipitates
such as AgCl, SnBr:, SnS,, etc. were also formed during ECM.
Precipitates are the products of ECM and usually co-exist with
metallic dendrites. They are neutral in electric charge and are not
affected by the electric field. However, excessive precipitates
impede the transport of ions and act against the growth of dendrites.
Many researchers reported that the amount of precipitates increased
with the conceniration of ionic contaminants including but not
limited to CI" , Br" , SO." , S" , NOz , ete. 847%™ Thisis
due to the increase in the conductivity and the “aggressiveness” of
electrolyte. When the concentration of ionic contaminants reached a
certain extent, dendrite could not be observed but only precipitates
were produced from the ECM process of metals. The precipitates act
as a wall-like barrier to block the transport of ions mechanically.
Zhong et al."" utilized a 3D micrograph to measure the height of
precipitates formed during the ECM of Sn in the presence of 17 mM
CI , at 5 V. They found that the height of precipitates (162 7m) was
higher than the thickness of the electrolyte layer (100 ym) and
proved that there was no dendritic growth.”” Liao et al."? even
utilized the additives such as NazS that facilitate formation of
precipitate as a method to inhibit the growth of dendrites. Further
discussion on ECM inhibitors can be found below.
While some claimed that the time-to-failure of ECM decreases
in a high concentration, ECM of metals is plausible due to the
redissolution ofprecipitates. Figure 4 displays the comparison of the
ECM behavior of different electronic materials (Ag. Cu, Sn and Sn3Pb) under chloride contaminations. It shows that these materials
behaved in a similar way when exposed to various concentrations of
chloride contaminants: (i) low concentration of CI~ short-circuited
the system, (ii) intermediate concentration resultedin a large amount
of precipitate and hindered the ECM process, while (i
concentration of CI" caused the redissolution of precipitats
allowed dendritic growth.
In summary, ionic contaminants influence the ECM behavior by
acting as condensation site and by affecting the conductivity of
electrolyte, “aggressiveness” of electrolyte, pH environment, stability of passive film and species formed as a product of ECM. An
ionic contaminant usually accelerates the electrolyte formation and
anodic dissolution. However, the effect of ion contaminants on the
ion transport and the growth of dendrites is complicated. Depending
on the pH ecnvironment and stability of species form. an ion
contaminant can either accelerate or decelerate the ion transport
and the growth of dendrites.
Weak organic acids (WOAs).—The usc of no-clean flux systems
in the soldering process of printed circuit board assembly (PCBA)
has become a major trend due to the cost factors.
chemicals in such flux systems are assumed to decompose into
volatile substances during the soldering process and this minimizes
reliability concerns.*> However, it has been reported that a significant ‘amount of WOA residues remained on the PCB after
subjecting it to the simulated temperature regime (25, 170, 250 "
C) of a soldering process.7797$5 These WOA residues contribute as
part of the major sources of contamination on the PCB. The common
WOAS present in the flux residues include adipic acid, glutaric acid,
malic acid, oxalic acid and palmitic acid.®
WOAs accelerate the electrolyte formation with their hygroscopic propci
he polar functional groups present in the WOAs
attract and hold the water molecules within their lattices. 6899 This
facilitates the collection of moisture from the environment to form
an electrolyte layer on the PCB surface for the subsequent ECM
high ionic concentrations. The contradictory effects on the ECM of
steps.® The critical RH for electrolyte formation to occur decreases
with the presence of WOAs. **>* A lower RH is sufficient for the
was attributed to the opposing effects on the anodic dissolution step
and the ionic transport step. Although the anodic dissolution rate was
believed to be the rate determining step of ECM, the rapid anodic
dissolution creates precipitates that act as physical obstacles to the
with a high solubility dissolves more casily in the thin electrolyte
with the ionic concentration, some found that ECM is inhibited at
metals were first reported by Harsányi in 1999.° The contradiction
moisture adsorption to oceur in the presence of WOAs compared to
the RH required on a clean PCB surface.
The anodic dissolution step is affected by the solubility and the
acid ionization constants (K,) of the WOAs present.7"9 A WOA
Journal of The Electrochemical Society, 2023 170 021505
layer formed by moisture collection®®; a WOA with a lower pK,
(pK, = negative base—10 logarithm of K,) value has a stronger
acidity and can be ionized more easily in water. The pK, values
measure the dissociation ability and the strength of the acids.™
Piotrowska et al."" studied the effect of WOAs (DL-malic, palmitic,
glutaric, suberic, adipic, succinic) on the Sn specimen using a
climatic chamber (25 °C/99% RH). It was found that the presence of
DL-malic acid and glutaric acid with high solubilities at 25°C (591.8
217; 540.1
g 17")*7and low pK, values (3.4;
4.34) resulted
in a high
leakage current, severe corrosion, and heavy ECM failure. Zhou et
al."" reported that under 60 °C/ 9396 RH, the corrosion of Cu was the
most severe in the presence of malic acid with the lowest pK, value
(3.4), and the least severe in the presence of abietic acid with the
highest pK, value (6.7). Dendrite growth due to ECM was observed
in the presence of high concentration of malic acid, whereas no ECM
failure occurred in the presence of abietic acid. It is important to
highlight that the solubility ofa WOA is affected by the temperature.
It was reported that a higher temperature raised the solubility of the
WOAs, resulting in higher leakage currents and more severe
corrosion.’®* Therefore, the presence of WOAs along with an
increase in temperature worsens the ECM phenomenon.
WOAS can also affect the anodic dissolution stage with their
ability to act as either corrosion inhibitor or activator. depending on
the nature of the acid.”> Ambat et al.” showed that at 5 V and 10 V,
the ECM of Sn was suppressed in the presence of flux residue
containing adipic acid. It was suggested that adipic acid had a
passivation effect on the Sn anode. Minzari et al.'* reported that at
12 V, ECM of Sn did not occur in the presence of 10 ppm, 1000
ppm, and saturated (approximately 24000 ppm) adipic acid. They
suggested that it was due to the promotion of passivation on Sn
surface by the organic compound.
Other than the electrolyte formation and anodic dissolution,
WOASs affect the ion transport step. It is known that the bulk pH
of the electrolyte is lowered with higher concentration of WOAs due
to the dissociation of acid. The pH of an electrolyte determines the
stability of a species present.'” Depending on the pH, precipitates or
complex species can be formed in the presence of WOAs. These
species affect the ion transport step. Verdingovas et al."? found that
at high concentration (above 92 1g em"?) of flux residues containing
adipic acid, ECM was suppressed due to the ex
e precipitation
of Sn(OH), that acted against the ion transport, which was caused by
pH change in the electrolyte. Liao et al.* suggested that the
[S(C204)217" complex anivns were formed fiom Wc reaction
Mold species such as Aspergillus niger can spread and grow well
on PCB materials.77 Metals suffer from corrosion-related damages
due to the presence of mold, including ECM.***** ECM of metals
that occurred in the presence of mold was reported recently in
2015.” When exposed to outdoor environment in Xishuangbanna,
China for 6 months and applied bias of 12 V, Sn and Cu dendrites
were found co-existing with the mold spores on the surface of leadfree hot air solder leveling printed circuit boards (PCB-HASL).””
The presence of mold spores facilitates the first step of ECM,
which is the adsorption of moisture or the formation of an
electrolytic path on the PCB surface. Mold spores can inoculate,
“sprout” to form mycelia and grow on the surface of the PCB
‘materials such as Ag and Cu. The metabolic process of mold absorbs
moisture from the air and secretes extracellular polymeric substances
(EPS). which contain enzymes, proteins, organic acids, etc.7777 The
viscous EPS facilitates the adsorption of spores and mycelia onto the
surface of a metal. This is then followed by the continuous
absorption of the moisture in the air on the metal surface.”™*”
The metabolic process of mold also accelerates the second step of
ECM, the dissolution of the anode. EPS contains ionic contaminants
(e.g., S0477, PO,>,etc.) and WOAs (e.g., acetic acid, malic acid,
oxalic acid, citric acid, etc).” The environment of the PCB surface is
therefore severely deteriorated. The presence of negatively charged
ions and WOAs can accelerate the corrosion of the metals. The
corrosion phenomenon induces the dissolution of the anode, which
produces metallic ions for migration via the electrolytic path. Apart
from the influence of EPS, the mycelia associated with the mold are
electrically conductive when moisture is adsorbed. This also helps in
accelerating the corrosion of the PCB materials. Yi et al."" reported
that in the presence of mold spores and an applied bias voltage of 5
V and above, Cu underwent serious corrosion and formed dendrites.
Whereas Ag dendrites started to form at 20 V and above. Xiao et
al."7 also reported that the presence of mold caused the growth of
dendrites on the PCB-Cw/ENIG surface at 25 V applied bias voltage.
On the other hand, ECM did not occur without the presence of mold
after 84 days of exposure to 25 V bias. Therefore, it can be
concluded that mold can lead to ECM.
In summary, the metabolism of mold absorbs moisture and
seeretes the metabolic product,
EPS that contains ionic contaminants
and WOAs. The mycelium of the mold is electrically conductive.
Therefore, when mold is present, the electrolyte formation step and
the anodic dissolution step of ECM are accelerated. Subsequently, in
the presence of both elecuic field and mold, metal ions can form at
between Sn and oxalic acid at pH 4.0 and they retarded the transport
of Sn”" ions for subsequent ECM steps. The presence of WOAs may
result in the formation of either precipitate or complex species.
which in turn affect the ion transport step.
In summary, three major steps in the ECM mechanism are
affected by the following properties of WOAs: hygroscopic properties, solubility, pK., nature to act as corrosion inhibitors/ activators,
and ability to alter pH. In general, the presence of WOA facilitates
the electrolyte layer formation due to its hygroscopicity. WOA with
a higher solubility and lower pK, increases the leakage current and
accelerates the anodic dissolution step. However, increasing. the
concentration of WOA can also suppress the ion transport step due
to the formation of excessive precipitate or complex species at lower
pH.
the anode, migrate, and deposit at the cathode to form dendrite.
Mold.—Mold is scientifically referred to hyphae that grow in the
form of multicellular_filaments or is more commonly known as
microscopic fungi.7795 Mold is a type of microbial poliutant that is
widely distributed in the environment. It spreads through mold
spores floating in the air. Mold grows quickly at outdoors where the
environment is temperate and humid or in indoors where the
ventilation is poor. The presence of mold not only causes damages
to buildings and health problems, but it also deteriorates the lifespan
of electronic materials. For example, the reliability of PCBs and
connectors in International Space Station encountered challenges
used to express the ionic conductivity, o, is as follow""":
due to the growth of mold. 599697
Electrolyte Properties
The presence of electrolyte is one of the prerequisites for the
ECM events. The electrolyte can be categorized into invisible
adsorbed thin moisture film and bulk solution of condensed
water.6064-100 The chemical and physical properties of an electrolyte
such as the pH environment, ionic conductivity and geometry of the
electrolyte affect the ECM mechanism significantly.
Ionic conductivity of electrolyte.—Ionic conductivity is the
clectric conductivity of a liquid electrolyte due to the motion of
fonic charge in response to an electric field."! A typical equation
o= gnv
191
where g is the carrier charge, n; is the number of particles per unit
volume and v is the average drift velocity of a carrier in an applied
electric field, E.
Previous workóf-58-64.68.93 reported that the ionic conductivity of
a solution in.
s with the concentration of the contaminants such
as CI" , Br , SO, NO, , SO,, H2S, dust, volcanic ash and flux
content in WOAS. This is in accordance with Eq. 9, where increasing
the number of charged particles increases the 0;. Subsequently. the
Journal of The Electrochemical Society, 2023 170 021505
some WOAs (e.g.. oxalic acid, glutaric acid), instead of being
reduced at the cathode, the metal ions are consumed to form
complex species.*! The presence of acidic organic compounds
®TEL +WDT
0-0-0-
C
L
@ 1200
=
s
o 800
£
may promote the passivation as well. On the other hand, in highly
alkaline region, the precipitates such as Sn(OH); can disproportionate 1o [Sn(OH)]*~ complex, which can be reduced to pure
metallic Sn at the cathode.'*
No short circuit
In short, the pH level of an clectrolyte affects the stability of
passive film and determines the ion specics in the electrolyte. These
properties affect the anodic dissolution, ion transport and dendrite
formation significantly.
v
£
=
5 400
[]
=
o
.
.
0.00001 0.0001
[]
0.001
.
.
°
001 — 0.
.
]
1
Concentration of Cr (M)
Figure 5. Mean time to failure of Sn vs concentration of CI" measured at 3
V using TEL” and WDT.'*
current passing through the electrolyte is increased. This accelerates
the overall rates of reactions such as the rate of anodic dissolution,
fon transport and the growth of dendrites. Likewise, the rates of
anodic, cathodic and other reactions such as the evolution of bubbles
and the formation of precipitate are accelerated with the ionic
conductivity.
An important remark must be made, where increasing the ionic
conductivity and the overall rate of reactions does not equal to
increasing the tendency of ECM. This is because some of the
reactions can retard ion transport and the growth of dendrites, such
as the formation of precipitate and the evolution of bubbles.
pH of electrolyte—The pH levels in a static micro-volume
droplet change with the contaminants present and the localized
reactions at the anode and cathode.'*""7>'°% In general, a pH
Form of electrolyte—ECM occurs in both adsorbed moisture
layer (thin film) and condensed water droplet (droplet).””'™ The
adsorbed moisture layer is as thin as a few nm and invisible, whereas
the condensed water droplet is a state where liquid is concentrated in
a region and is relatively thicker and visible. Different environmental conditions and test methods lead to the formation of
clectrolytes of different forms and geometries.
The anodic dissolution or the corrosion can be affected by the
geometry of the electrolyte such as its thickness.'”* According to
Zhong et al.,""? increasing the electrolyte layer thickness led to the
increase in the volume of the electrolytic solution. indicating that the
total quantity of charged particles increased with the thickness
Therefore, the rate of anodic dissolution as well as the amount of Sn
ions to be deposited at the cathode increased with the thickness.
However, the TTF did not decrease with electrolyte thickness. The
authors suggested that the TTF could not be simply correlated to the
rate of reactions and concentration of charged particles, but it should
be correlated to the local concentration of the tin ions, where the
concentration of tin ions was reported to first decrease, and then
increase with the thickness of the electrolyte layer.'™*
The geometry of electrolyte also impacts the fluid motion inside
the liquid medium. This affects the third step of ECM as hydro-
dynamic flow is one of the means for ion transport in liguid.
A
comparison was made between the results obtained from two
different findings using different test methods."?"95 Zhong et al.””
utilized 100 ym-thin film in the thin electrolyte layer test (TEL),
while Medgyes et al.""? utilized droplet of 25 4l in the water drop
test (WDT). Both tests were
ed out using the same parameters,
gradient will be developed in an electrolyte that is initially neutral
due to the generation of H" at the anode and OH™ at the cathode.
where the electrodes were composed of Sn, spacing was set at 0.5
mm, applicd bias was 3 V and electrolytes selected were NaCl
This makes the region close 0 the anode acidic and that of the
sulutions. However, both showed different uend (Fig. 5). In TEL
cathode alkaline."7"" A higher applied bias
accelerates the localized
reactions and creates a steeper pH gradient. However, the pH
gradient will eventually be smoothened out due to the rapid
disturbances caused by the natural convection of the droplet.
However, the presence of contaminants such as WOAS in the flux
residue and HSO;" shifts the bulk pH to an acidic environment.*>"
In general, a lower pH undermines the stability of the passive
film of an electrode and accelerates the corrosion or the anodic
dissolution step. Liao et al.’” reported that the MTTF of SAC305
decreased as the initial pH value of electrolyte decreased. A more
detailed discussion of the effect of pH on the anodic dissolution step
can be found above.
The stability of a metal species depends on pH environment of
clectrolyte. A Pourbaix E-pH diagram is often referred to in
ng the possible species in a certain pH condition. For
example, when a Sn ion migrates from anode to cathode, it
encounters various pH levels. According to the standard Pourbaix
E-pH diagram,”” the precipitate, Sn(OH); which acts against the ion
transport and dendrite growth is one of the stable species in a wide
range of pH. When the anodic reactions arc high and the pH
environment is conducive for precipitation, Sn(OH), is produced
more rapidly. Therefore, an increase in dissolution of anode does not
necessarily lead to increase in dendrite formation because formation
of precipitates consumes the metal ions before reaching the cathode
and acts as a barrier to block the transport of ions. In the presence of
test, ECM of Sn was inhibited at NaCl concentrations from 0.01 M
10 0.03 M due to the formation of excessive precipitates: in WDT,
ECM of Sn occurred at these concentrations. This can be attributed
to the different hydrodynamics flow in water droplet, which allowed
the transportation of ions in the presence of precipitates.
Different forms of clectrolyte also affect the dendrite mor-
phology. Warren et al.""" compared the ECM behaviours under
three different forms of electrolyte including 10000 ;m bulk
solution, 1 4m thin electrolyte layer and 0.02 m adsorbed moisture
layer. Similar dendrite morphology and dendrite growth rate were
observed in bulk solution and 1 zm thin electrolyte layer but were
different in 0.02 yim adsorbed moisture layer. Dendrites in adsorbed
moisture layers exhibit a more distinct two-dimensional structure
compared to_three-dimensional ones in bulk solution.'*® Tsou et
al.)?? compared the microstructure of Ag dendrite formed from
WDT and THB tests. The Ag dendrites formed from WDT were
denser compared to those formed from THB.
In short, the form of electrolyte layer is determined by the
environment and type of ECM test conducted. The rate of anodic
dissolution and the local concentration of ions are affected by the
thickness of electrolyte. The form of clectrolyte also plays significant roles in hydrodynamic flow and dendrite morphology which
affect the ion transport and dendrite formation respectively.
Therefore, the researchers shall select the ECM test conducted
based on their application.
Journal of he Electrochemical Society, 2023 170 021505
lower limits.*''* No ECM phenomenon can be observed below the
Electric Field
ECM involves electrochemical reactions due to the presence of
moisture and bias voltage. When bias voltage is applied, electric
current flows across the electrochemical cell from the positive
terminal to negative terminal. The electric current density (j) passing
through the clectrolyte in an clectrochemical cell can be represented
critical bias voltage, while other types of failurei.e. dielectric failure
occurs beyond the upper limit instead of ECM.*'"* Different types
of bias voltage such as AC voliage, DC yoltage and pulsed voitage
cause huge differences in ECM behavior.®! %1% This article focuses
on the effect of DC voltage on ECM.
One of the affected ECM steps is the anodic dissolution, whichis
the second stage of ECM.'*''®!'7 One of the recent discoveries
Jj=oE
[10]
where 0 represents the electric conductivity and E reprcsents the
electric field. The electric current density in an electrochemical cell
is affected by factors such as the electric field, the conductivity of
both the electrodes and the solution, etc.'”*'"” The rates of reactions
for the electrochemical reactions involved including the anodic and
cathodic reactions increase with the electric current. Although the
risks of ECM in general increase with electric current, it is important
to note that increasing the rate of reactions sometimes does not
promote ECM as some of the accelerated reactions such as formation
of precipitate and evolution of bubbles retard the ion transport and
dendritic growth.
The application of external electric field to an ion in electrolytic
solution imparts drift velacity to the ion 32199 If the ion is in the
vacuum, the acceleration continues until theion strikes its respective
electrode. However, instead of vacuum, the ECM phenomenon
occurs in an electrolytic solution with a large number of other
ions, where these ions are bound to collide, stop for some time and
accelerate again under the influence of external electric field.'™ It
can be seen from Eq. 10 where increasing increases the electric
current density passing through the clectrolytic solution. An electric
field £, between two oppositely charged electrodes which are
parallelly alignedis represented byő7:
a
11
[y
where V is the applied bias voltage and is the spacing between two
oppositely charged electrodes. As such, the electric field is directly
proportional to V and inversely proportional to d. However, it should
be noted that the cquation has not taken the factors such as the
geometry of electrolyte droplet and surface roughness of electrodes
into account. Nevertheless, the equation shows that the demand for
miniaturized electronic devices that causes smaller spacing between
electronic components has results in a larger E. The need for higher
performance has also led to a larger E during the operation of
devices, increasing the risk of ECM failures in the electronics
Apart from the influences mentioned above, it is worthwhile to
consider the role of E on the electrolyte layer formation stage of the
ECM mechanism, as water is a well-known polar molecule.
Researchers suggested a correlation between E and the water
condensation phenomena.''*"'? It was reported that the presence of
external £ can influence the polar water molecules and hence affect
the condensation rate.""9-? According to Wang et al.,""?
the major
effect of electric field on water condensation is the exertion of a
torsional force on the water molecules (in gaseous state), causing them
to rotate and rearrange themselves in the direction of the applied E.
The intensity of rearrangement increases with electric field intensity.
Vertical electric field can rotate the cluster of water molecules, reduce
their contact area with surface, which in turn reduce the condensation
rate. To-date, there is no published report on ECM-related study that
investigates the effects ofE on the electrolyte formation stage. Future
research is expected to reveal the effect of £ on the electrolyte
formation stage of ECM, which is one of its prerequisites.
Bias voltage.—Applied bias voltage, V is claimed as the driving
force for ECM.*!” It is directly proportional to the force applied to a
charged particle or an ion. An electric field, E is generated across the
clectrochemical cell when a bias voltage, V is applied. It has been
reported that ECM occurs in a range of bias voltage with upper and
proved that gold (Au) which was believed to be inert to ECM
without halogen ionic compounds, is susceptible to ECM in distilled
water when the applied bias voltage is increased beyond 5 V.""" The
authors suggested that the anodic dissolution of Au to form Au""
following the classical ECM mechanism is enabled only above a
threshold polarization voltage level. Incre:
voltage accelerates the rates of reactions. Lee et al.
the current measured between anode and cathode was proportional to
the applied bias voltagein a current-voltage plot, indicating that the
rates of reactions were faster at higher voltage. Liao et al.'®>
measured the anodic potentials and cathodic potentials of Sn in 1
mM CI" and 1 mM citrate ions at the range of 2 V to 10 V. They
found that with increasing voltage, the anodic potential became more
positive whereas the cathodic potential became more negative. The
anode immediately commences to dissolve as soon as its potential
exceeds the electrolytic dissolution potential.""? Therefore, increasing the anodic potential releases more metal ions for the
subsequent stages of the dissolution reaction.
The amount of metal ions required to form the conductive
dendrite that short-circuit the system is constant. This can be
explained by the Faraday’s law which correlates the total charge,
0 passed through an electrochemical cell to the total amount of
product, N"
O = n,FN
121
where F is the Faraday"s constant (F = 96,485.3 C mol"") and n, is
the number of electrons transferred per mole of product. Lee et al.!"5
measured the total electric charge required to form dendrites by
integrating the current-time curve, assuming that there is no loss of
current during the electrochemical process. The curve showed that
varying the applied bias voltage did not change the total electric
charge required to form dendrite. This proved that the amount of
metal ions required to form the conductive dendrite that led to short
circuit is constant. A higher applicd bias voltage gencrates a larger
amount of metal ions in a specified time.'” Consequently, the time
required to generate sufficient metal ions from the anodic dissolution
is shorten and this leads to a shorter time to insulation failure. Apart
from increasing the rate of ion generation, varying applied bias
voltage also affects the second stage of ECM in terms of corrosion
behaviour. Lee et al.""? reported that in 0.001 wi% NaCl, Sn
underwent uniform corrosion at 0.5 V. whileincreasing the applied
bias voltage to 1.0 V changed this step to pitting corrosion. The
former did not experience ECM failure, while the latter shortcircuited at approximately 600 s.
The ion transport step or the third stage of ECM is also
significantly influenced by the applied bias voltage. The positively
charged metallic ions move and migrate from the anode to cathode
under the influence of electric field. According to Eq. 11, the electric
field intensity increases with the applied bias voltage. The drift
velocity, v of a charged ion increases with the electric field as shown
in Eq. 4. Increasing the applied bias voltage increases the magnitude
of electric field and the drift velocity ofions that are moving through
the electrolyte solution. ™ The transport or the flux of ions is related
to the diift velocity:
Flux of ior
ion drift velocity X concentration of ions
[13]
According to Eq. 13, it can be concluded that the ion transport step
in ECM accelerates with ion drift velocity when the bias voltage is
increased.
Journal of The Electrochemical Society, 2023 170 021505
— 8— SAC305 (6.5V) 8—
—
SAC305 (15V)
——4— Sn-37Pb (6.5V) — — Sn-37Pb (15V)
— 4 —ImAg (1.5V) — — 4 —ImAg (2V)
— 4 — IMAg (3V)
— 4 —ImAg (5V)
ssseb sss Cufinish (3V) ----#--- Cu finish (6.5V)
ss.h sss Cu finish (10V)
: - -- - Cu finish (15V)
% Cu finish (18V)
1200
higher than the critical potential for the dissolution of IMC. Zhong et
al." reported that Ag dissolved and escaped from Ag;Sn at 2.38 V.
vs saturated calomel electrode (SCE), resulting in the cathodic
deposition of Ag" in the ECM of SAC305 alloy.
Liao et al.'™ investigated the combination effects of varying
voltage and adding inhibitor (0.4 g 17" cetyltrimethylammonium
bromide, CTAB) on ECM of Sn in I mM NaCl. It was found that as
the bias voltage was increased, the morphology of the dendrite was
more likely to be particle-like, which was less likely to bridge the
electrodes due to its short and round shape. This has indicated that
the CTAB was more effective in inhibiting ECM in the presence of a
higher voltage. In contrast, the result is different for another inhibitor
(tri-sodium citrate dehydrate), where increasing the voltage to 5 V
and 8 V enabled the deposition of ions and growth of Sn metallic
dendrite, given the dendritic growth was retarded at 3 V with the
presence of only 1 mM tri-sodium citrate dehydrate.' This was
attributed to the formation of [Sn(OH)J*~ ions at high applied bias
voltage.
In summary, the rates of the anodic and cathodic reactions
involved in ECM inerease with the applied bias voltage owing to its
proportional relationship with the electric field and electric current.
This is reflected in the increased rates of anodic dissolution, ion
transport and dendrite growth. Consequently. increasing the applied
bias voltage usually increases the risks of short circuit due to ECM
However, it is also important to note that the rates of precipitates
formation and effervescence which hinder the dendrite growth also
increase with applied bias voltage. Other than the rate of reactions,
the composition of the dendrite formed from alloys is determined by
the applied bias voltage. Increasing the bias voltage to 5 V also
enables the ECM of Au which has a high resistance to ECM at low
voltage. Varying the voltage can alter the effects of ECM inhibitors,
i.e., increasing bias voltage increases the effectiveness of CTAB but
decreases the effectiveness of tri-sodium citrate dehydrate on
inhibiting the ECM of Sn.
g
2
o
Time to failure (s)
1000
400
200
15
Distance between electrodes
(mm)
Figure 6. Time to failure vs distance between clectrodes in various literature
studies 26127125
The applied bias voltage plays an important role in the dendrite
growth or the fourth stage of ECM. First, similar to the anodic
dissolution and ion migration steps, the dendritic growth rate also
increases with the applied bias voltage. The deposition of the
positively charged metal ions is a cathodic reaction which increases
with the applied electric field. Second, the structure of the dendrite
formed is affected by the magnitude of voltage. Hua et al.** found
that at a lower voltage (I V), the branches of dendrite formed from
ECM of 99.5(64Sn-35Bi-1Ag)—0.5Ge was less obvious. Increasing
the voltage to 3 V resulted in more branches produced at both sides
of main trunk due to the increased crystal growth rate of dendrite
with the voltage.™
Varying applied bias voltage also affects the composition of
dendrite formed from the ECM of alloys. The dendrites produced
from the ECM of Sn-9Zn were composed of different compositions
of Snand Zn at 3 V and 5 V.7" The composition of Zn formed at 3 V
and 5 V was 3.12 wi% and 20.99 wt% respectively. The increase in
applied bias voltage promoted the migration of Zn"" and resulted in
the deposition of Zn"" although the alloy consisted of only 9 wt% of
Zn. The addition of Ag to Sn-based solders was believed to result in
the formation of Ag,Sn, consequently retarding the participation of
Ag in ECM even though pure Ag is highly susceptible to ECM.
However, dissolution of IMC can occur if the applied potential is
Spacing.—Spacing refers to the distance between two oppositely
charged electrodes which are made up of conductive materials such
as the surface finish and the conduction lines in the electronic
products. The spacing between the electronic components has
become shorter to meet the demand for miniaturized packages.
Today, the typical pitch distance of the ball grid arrays (BGA) is
ranged at 0.4~1.0 mm, while that of flip chip or controlled collapse
chip connection (C4) is as short as 0.1-0.15 uu. 4125
Researchers have carried out tremendous amount of work to
understand the effects of spacing on the ECM behaviour as the
decreased spacing is one of the drivers for ECM. It was reported that
the time required for the initial step of ECM depends on the spacing
between two adjacent electrodes.'*! The in-situ investigation of the
Thermal Humidity Bias (THB) showed that the water droplet
initially formed on the surface of the electrodes (Ag). The water
droplets then accumulated and eventually connected to form a water
bridge that covered the insulation substrate, the glass fibre epoxy or
flame-retardant class 4 (FR4) between the conduction lines.'* The
water bridge acted as the path for the transport of ions and growth of
dendrites. Longer distance between the conduction lines resulted in
more time required for the longer path or electrolyte layer formation
step."?" In this case, the formation of clectrolyte layer is independent
of the electric field as a longer spacing exhibits longer incubation
period prior to the subsequent ECM steps, lengthening the overall
TTF even if the applied bias voltage is large.'”" This was shown by
Zhan et al.,'>! where no failure was observed at the samples with
largest spacing (0.64 mm) after 168 h of THB test even when a high
applied bias voltage (42 V) was applied.
Apart from the electrolyte layer formation step, spacing also
plays important roles in the transport of ions and the growth of
dendrites. The path for the migration of ions and the growth of
dendrites is decreased, leading to a shorter TTF. Besides, the
reduction in the distance between the electrodes increases the
electric field as indicated in Eq. 11. This means that a stronger
Journal of The Electrochemical Society, 2023 170 021505
force is applied to a charged particle or an ion. Thus, the speed of ion
transport and dendrite growth increases while the spacing decreases.
Figure 6 summarizes the results obtained from multiple
work.>'?"1? WDTs were performed using cither distilled water
127128 or deionized water "" on test vehicles with varying pitch of
in a wide range of surface roughness of the electrodes, for example
as low as a few nm in the electrodeposited Sn method'*” and as high
as a few hundred nm in the immersion Sn.'*'
Surface roughness of clectrodes affects the electrolyte formation
step.17192193 A rougher surface contains more localized regions for
water adsorption, for example, pores, pits, and scratches. 13134
comb patterns. The results showed that the TTF decreases with the
decrease in spacing for all types of materials involved, indicating
that the risk of ECM increases as the pitch distance decreases.
Decreasing the distance between the conductive materials enhances the risk of ECM. The components are more difficult to clean
and can easily leave the hygroscopic contaminants behind and
facilitate the adsorption of moisture. A shorter distance also leads
to a shorter electrolytic path, migration path and dendrite growth
path, therefore the time required for the short circuit to take place is
shorter. Besides, decreasing the distance leads to a higher electric
field, increasing the speed of ion transport and dendrite growth.
RH for clectrolyte formation is lowered with higher surface
roughness.'® Therefore, an increase in the surface roughn
expedites the electrolyte layer formation step.
Surface roughness also affects the anodic dissolution step in
ECM. It was reported that the corrosion rate of metals and metal
The relationship between electric field, voltage and spacing.—
The state-of-art of art approach in relating the spacing and voltage to
V (Eg. 11) estimation. 813
the electric field is the simple E — —
d
However, it was suggested that the approach of £ = £ could not
rougher cathode. Reiss et al.' investigated the effect of roughness of
estimate the electric field accurately. Reiss et al."? analyzed the
relationship between the electric field and the spacing between Cu
conduction lines by the means of modelling as well as validation b
tests. Based on the findings, they suggested the relationship to be'™:
aV
206
[14]
where a is the material dependent parameter such as the roughness
of the electrodes. Therefore, it is important to note that the electric
field and the TTF required for ECM failure do not simply depend on
the E = % (Ea. 11) relationship.
It was known that a stronger electric field accelerates the
migration of charged particles (ions). Zhou et al."? showed the
relationship between the time required for ECM failures of ImAg in
deionized water using WDT. The plot proved that the TTF decreases
with applied electric field. However, it was highlighted that the
influences of bias voltage and spacing on TTF can be different even
though the resultant elecuic field, £ was equal. For instance, both 1.5
V/0.318 mm combination and 3 V/ 0.635 mm combination resulted
in same E. but the TTF were 308.57 s and 172.38 s respectively. It
was noticed that the bias voltage had larger impact on TTF because
the bias voltage did not only affect the electric field, but also the
number of charged particles in electrolyte.*® The relationships
between the electric field, applied bias voltage and spacing are
rather complicated. More factors such as the geometry of the
electrochemical cell and the environment should be taken into
consideration while predicting the ECM failure from the electric
field.
Surface Conditions
ECM involves the growth of dendrites in the presence of
moisture layer, usually on the surfaces of conducting clectrodes
and substrate made of insulating material between the electrode:
Therefore, the physical properties (i.e., surface roughness), chemical
properties (i.e.. oxidation state) of the surfaces of electrodes and
substrate surface can affect the ECM behaviors.
Surface roughness of electrodes.—A surface finish is applied on
the conductive tracks of a PCB to protect the exposed Cu from
oxidation and provide a solderable surface. Some common examples
of surface finishes include hot air solder levelling (HASL), ImAg,
immersion Sn, electroless nickel immersion gold (ENIG) and
organic surface protectant (OSP).'>” Different surface finishes result
These localized regions facilitate the accumulation of water mole-
cules due to capillary forces.'® In addition, the minimum ambient
alloys accelerates with higher surface roughness. 135137 Takemoto et
al."7 found that ECM did not occur on carefully polished Sn
electrodes, but occurred on rough Sn surfaces. They also combined
electrodes with different surface roughness (i.e., rough anode/
smooth cathode and smooth anode/rough cathode). They found
that a rougher anode was more likely to accelerate ECM than a
Cu foil on the electric field and the TTF for ECM using modelling
approach. Compared to a smooth geometry, the rougher Cu foil/
insulating material interface increases the electric field."? Increasing
the electric field is known to accelerate the corrosion. Therefore, the
anodic dissolution accelerates with roughnes:
sing more metal
ions to be released for the subsequent ECM steps.
Surface roughness can affect the ion transport and the dendrite
formation steps. Increasing the surface roughness causes increase in
electric field and leads to the acceleration of ionic drift velocity.™*
The electrical field intensity was suggested to be steeper at the
peaked regions, leading to much faster ion transport at these areas.
As for the dendrite formation step, according to the literature, a
dendritic precursoris required to initiate the dendrite growth."" The
major function of this dendritic precursor is to disturb the current
flow toward the electrode, resulting in a higher deposition rate at the
peak of the precursor than that of a flat electrode surface.” The
dendritic precursors suggested by the researchers are a surface
imperfection, a screw dislocation pyramid and a surface roughness
clement formed by the codeposition.** It was suggested that the
nucleation of Sn dendrites often occurs at sharp_corners, kinks and
protrusions of the surface of a cathode surface.71)9" Tsou et al.)97
showed that Ag dendrites formed intensively at regions with
prottusions. They explained that the location wilh protusions has
shorter diffusion lengths for Ag* ions, resulting in a larger electric
field strength.
Based on previous studies, the following is deduced: the rougher
the surface, the higher the electric field and ECM susceptibility, and
there are more kinks and protrusions which can act as sites for
initiation of dendrites. In summary, the surface roughness of the
clectrodes affects the anodic dissolution, ion transport and dendrite
formation steps.
Oxidation states of electrodes.—The oxidation state of a metal
alters according to the fabrication process and the service environment. Cu is a typical metal that exhibits different states of oxidation.
It was reported that heat annealed Cu leads to oxidized surface and
chemically cleaned Cu gives oxide-frec surface. This results in
different dissolved ion species. Cu" formed from the anodic
dissolution of oxide-free Cu surface while Cu"" formed from the
anodic dissolution of oxidized surface."" In most of the cases, Cu"
and Cu?" coexist and the ratio depends on the level ofcleaning. The
solubility product constants, K. of Cu(OH) and Cu(OH), are 1.0 x
1071 " and 5.0 x 10
" ™ respectively. Different levels of
cleaning results in different resistance of Cu to ECM. Larger K.
leads to higher ability to dissolve, therefore, Cu™ results in a shorter
TTF compared to Cu"".
Another metal which possesses multiple oxidation states is Sn
which consists of Sn(II) species and Sn(1V) species. Upon exposure,
a thin protective film of stannic oxide (SnO2) is formed
Journal of The Electrochemical Society, 2023 170 021505
spontancously on Sn surface by the reaction with the atmospheric
oxygen. It was suggested that the oxide film consisted of a layered
structure of SnO,/SnO/Sn.”® The stability of Sn(II) species and Sn
V) species, however, is not determined by the surface condition,
but varics as a function of pH. The pH of the clectrolyte is
determined by the presence of contaminants or by the local pH
change at anode and cathode due to the electrochemical reactions.
The Pourbaix E-pH diagram is usually referred to check for the
stability of Sn ions in the solutions as discussed. Both Sn(I) species
and Sn(IV) species have been reported to take part in ECM. They
can migrate, react with OH" to form precipitates, form complex
species or undergo reduction at the cathode to form Sn dendrite
during ECM.*'* However, the effect of oxidation state of Sn on the
ECM susceptibility was not reported.
Substrate—Substrate is the body of the base material that
supports the circuit components and tracks of conduction lines of a
PCB."* The substrates are often categorized into flexible and rigid
base materials.'*” Some of the common flexible materials include
polyethylene terephthalate (PET), polyimide (PI) and polyethylene
naphthalate (PEN), while some examples of rigid materials include
FR4, bismaleimide triazine, liquid crystal polymers (LCP), etc.'*
The properties of substrate material such as surface roughness
and thermal diffusivity can affect the water condensation rate that
influences the electrolyte layer formation step.'*'** The surfaces
with scratches and pits have higher surface energy areas which
facilitate water condensation.'”™* Medgyes et al."
investigated
the water condensation behaviour of two different substrate mate-
rials (FR4 and PI) in in-situ THB tests. It was reported that the water
condensation intensity of FR4 was higher than that of PI, where
water droplets were observed forming on the FR4 and PI substrates
at 0 s and 10 s respectively. This was due to the surface of FR4
which was more porous and of higher roughness. Thercfore, FR4 has
a larger effective surface and higher surface energy than Pl. Apart
from that, FR4 also possessed hi%hcr thermal diffusivity (5.43 x 10
7) compared to PI (3.16 x 10
~7)."* Consequently, the condensa-
tion rate on FR4 was faster than on Pl, and the MTTF value of ImAg
4 was lower than that on Pl.
The roles of the substrate material on the subsequent ECM steps
including anodic dissolution, ion transport and dendrite growth were
reported to be less significant. Medgyes et al.'** compared the
MTTEF of FR4 and PI using WDT technique which excludes the
elecuolyte formation step. The results of WDT validated that there
the recent discoveries, particularly on the inhibitors, SAMs and
surface coatings.
Inhibitors for ECM mitigation.—The corrosion inhibitors and
additives such as surfactants, grain refiners and brighteners are often
added to the electronics to improve their lifespan and to produce
desirable deposits.'>'” These additives have emerged as potential
inhibitors in retarding ECM.'~!1-123-141
An ECM inhibitor can suppress ECM in a few ways, one of them
is by retarding the anodic dissolution which forms ions for the
subsequent steps. Liao et al.""? reported that adding 1 mM of tri-
sodium citrate dehydrate was sufficient to result in less anodic
dissolution of pure Sn in 1 mM NaCl. It was demonstrated that the
Epiu of Sn increased from —202 mV vs SCE (without citrate) to —26
mV vs SCE (with | mM citrate).""? Polyethylene glycol (PEG) and
Imidazole arc used as suppressors o suppress the reduction rate in
the Cu electrodeposition.'’ Song et al.""
investigated the effect of
adding PEG and Imidazole on the ECM of Cu in the presence of
CI . It was found that the addition of 100 ppm of Imidazole and
PEG helped in reducing the corrosion rate and therefore significantly
increased the TTF. They acted as corrosion inhibitors that suppressed the anodic dissolution step in the ECM of Cu. Hong and
Kim'* found that as an oxygen scavenger. carbohydrazide effec-
tively slowed down the ECM process of Ag and Cu in the presence
of C1" by removing the oxygen content in the electrolyte. Devos et
al.77 reported that the anodic dissolution of Cu in 107? M oxalic acid
was prevented by using deacrated solutions, thus inhibiting the dendritic
growth even when the ECM test was continued for a long time, i.e.,
5000 s. It was also demonstrated that the ECM of Cu was effectively
inhibited by adding anodic and cathodic corrosion inhibitors such as 3methyl-1,2,4-triazole-5-thiol and 1-methyltetrazole-5-thiol.>* The corro-
sion inhibitors also help in the formation of passive film on the anode
surface and limit the diffusion of the reducing species towards cathode.
Another corrosion inhibitor introduced to inhibit ECM of Sn recently
was disodium hydrogen phosphate (NasHPO,)."* It was reported that
upon adding 500-1000 mg 17" of Na,HPO to 1 mM NaCl, a compact
film mainly consisting of Sn, O and P formed on Sn anode and slowed
down the rate of anodic dissolution. It is important to note that the ECM
inhibitors are different from the corrosion inhibitors. A corrosion
inhibitor is added to the electronics to reduce the corrosion rate; an
ECM inhibitor does not only consist of corrosion inhibitor but also other
additives such as surfactants, grain refiners and brighteners.
Other than Wc anodic dissolution step, the tansport of metallic
was no significant difference between MTTE of both substrate
ions step can be suppressed by the addition of ECM inhibitor. The
materials after water layer was formed.'** However, another work
reported that FR4 possessed a lower TTF (~45 %) compared to PI in
the results of WDT.""" Apparently, there are still ambiguities in the
citrate ions (Cit ) in the tri-sodium citrate dehydrate can form tin
effects of substrate materials on the anodic dissolution, ion transport
and dendrite growth. From the electrochemical point of vicw, the
substrate material or the insulation material does not affect the rate
of metal corrosion, ion transport and redeposition of charged ions.
In summary, the type of substrate material plays important roles
in the clectrolyte layer formation step in ECM. The substrate
material with rougher surface and higher thermal diffusivity facilitates the water condensation and accelerates the ECM proc
Strategies for ECM Inhibition
The development of inhibition for reducing ECM failures is
extremely important to improve the ECM lifetime of the miniaturized electronic products in harsh environments. Previous review
papers by Zhong et al." and Liao et al." made significant contribu-
tions to the understanding in the methods of suppressing ECM of Sn
and Sn-based solder alloys. Zhong et al." summarized the roles of
alloying clements, pulsed or AC voltage and inhibitors in the
mitigation of
ECM. Liao et al." provided some
strategies such as
proper storage, appropriate material selection, addition of alloying
clements, heat treatment, surface coating and application of inhibitors. On top of the available review papers. this section focuses on
citrate complexes with tin ions such as SnCit™ and SnHCit. The
formation of these complexes consumed the Sn ions and prevented
the transport of tin ions from the anode to the cathode." It was
reported that WOAs such as adipic acid and succinic acid can form
Cu complexes or chelates with Cu.'** Therefore, WOA can act as
inhibitors for ECM. Liao et al.*” reported that adding 0.5 mM Na,S
and above into 0.1 mM NaCl led to formation of large amount of
precipitates made up of SnS, SnS,, Sn(OH); and Sn(OH),. These
precipitates acted as a wall-like barrier and blocked the migration of
ions.
The authors presented that the thickness
of accumulated
precipitates (570 im) was higher than that of the electrolyte layer
(200 £zm) in a 3D micrograph. In general, the transport of ions is
stopped by the inhibitors either through the formation of complex
ions which consumes the ions or by the formation of precipitates
which block the transport of ions physically.
The addition of an ECM inhibitor can affect the dendrite
deposition step and in turn inhibit ECM failures. The addition of
CTAB into electrolyte solution with | mM C17 resulted in a change
in the morphology of Sn deposits.'>* This was attributed to the
CTA™ adsorbed on the Sn cathode surface. This had strengthened
the deposition of Sn ions at the lower energy {101} plane to form
particle-like deposits. In the absence of CTAB, the original deposits
were tree-like dendrites, but with increasing CTAB concentration
from 0.1-0.8 g 17!, the tree-like dendrites became thicker while the
Journal of The Electrochemical Society, 2023 170 021505
particle-like deposits, which were short and round in shape became
denser. This effectively inhibited the propagation of dendrites
towards anode, in which no ECM failure was detected after 10 h
of ECM test."77 The same phenomenon was observed in another
work by Liao et al."? that added hexadecyltrimethylammonium
chloride (CTAC) to Ag nanopaste. With the addition of 0.1 mM of
CTAC, the TTF of Ag nanopaste in 1 mM NaCl increased from
293.4 s to 1781.7 s at 3 V. Addition of more than 10 mM CTAC led to
retardation of ECM failure even after 3000 s of WDT. The adsorption
of excessive CTA™ on the cathode reduced the growth of the high
energy crystalline and accelerated the dendritic growth along lower
energy plane.'” Tsai et al."" reported that adding complexing agents
which consist of citric acid, ethylenediaminetetraacetic acid (EDTA)
and PEG helped to suppress the dendrite formation of 30Sn-70Bi
during electroplating process. With the addition of these complexing
agents, the authors reported that instead of dendrite, resultant deposits
with relatively compact, polycrystalline, and smooth morphology were
found adhering on cathode.
Table 111 summarizes the types of ECM inhibitors investigated in
previous work. The ECM inhibitors slow down or eliminate ECM of
electronic materials in a few ways: (1) suppressing the anodic
dissolution by forming passivation or reducing the corrosion rate of
anode: (2) interrupting or blocking the ion transport by forming
complex compounds or wall-like barrier (precipitate); (3) strengthening the deposition of metal ions at the lower energy planes and
forming dendrites with different morphologies.
The investigation into the development of ECM inhibitors is
crucial to retarding the ECM of electronic materials and help in
prolonging the lifespan of the electronics. The additions of anodic
type, complexing type, precipitate type and cathodic (ype inhibitors
are equally effective based on the literature. 519123
The selection
of suitable ECM inhibitors should consider the effectiveness in
inhibition, cost-effectiveness as well as the side-effects of the
inhibitors such as the toxic effects.
Self-assembled monolayers (SAMs).—SAMs are surface films
of highly organized molecular structure that form spontancously on
the surface of a metal or a metal oxide.""*'"7
A typical SAM is
composed of a head group that binds to the surface of a substrate, an
end group which defines the surface properties and a spacer group or
a backbone which connects the head and end groups.'*”'* The
formation of SAMs film involves two steps: fast adsorption of the
head group 04 substiate suiface and slow
reartangement öl
molecules. The density and stability of the thin film increase with
the processing time and concentration of solution.'*” SAM technique
is used to modify the surface in order to achieve certain function-
alities and to improve the durability.'*® Recently, the application of
SAMS in electronic devices has gained considerable attention due to
their chemical stability, simple forming process and cost
effectiveness without sacrificing the conductivity, and appearance
of the devices.'*"'*%'>! An SAMs covered metal surfaces were
found to improve the ECM durability as well.'**
SAMs can act as ultrathin anticorrosive layer that prevents the
contact between the substrate (electrode) and moisture or corrosive
species such as CI" , O, and H,S. This is due to the presence of the
hydrophobic molecular chains of SAMs which makes the surface of
a noble metal such as Au, Ag and Cu more hydrophobic.'**'** It
was widely reported that the water contact angle increases with the
application of SAMs, 952153 which further proved that SAMs act
as a barrier against moisture effectively. Therefore, it is expected
that SAMs defer the clectrolyte layer formation step in ECM.
The resistance to anodic
dissolutionis also greatly improved by
SAMs. The utilization of SAMs is one of the well-established and
effective approaches in mitigating_corrosion of metals such as Fe,
Ag and Cu.'"15%155 Rao et al.”™ investigated the effects of 2-
(octadecylthio) benzothiazole (2-OTBT) SAM on the corrosion of
Cu. The polarization test showed that ico, of Cu surface covered by
2-OTBT was lower than bare Cu. Based on the AFM image and XPS
results, they found that after exposing the bare Cu to 0.02 M NaCl
environment for 24 h, CuzO formed on the surface due to oxidation
and pitting corrosion, whereas 2-OTBT SAM that formed on the
surface of Cu remained stable. This was attributed to the presence of
SAM which acted as barrier that protected the Cu from the corrosion
environment. Liu et al."? also reported that the oxidation of Ag
nanowires was effectively inhibited by the self-assembled organothiols such as azole derivatives, silane, and alkancthiolate. The
SAMs were achieved by formation of Ag—S covalent bond that acts
as barrier layer and isolates the surface of Ag manowires from
corrosive species such as O, and H2S.""
Sekine et al."7? investigated the effects of SAMs on ECM of Ag.
They introduced the SAMs by immersing the Ag substrate in a 5 mM
pentafluorobenzenethiol solution at ambient temperature (25 °C) for
5 min and proved that the growth of dendrites was suppressed. SAMtreated Ag electrodes exhibit substantial improvement in surface
morphology and ECM reliability. The TTF of parallelly aligned Ag
electrodes increased from 20 s to 130 s in pure water. It increased the
water contact angle of the Ag clectrode from 60.5 £ 2.3° to 92.2 =
3.8°. The thiol groups of SAMs bonded well with Ag, improving the
surface morphology. It was suggested that the thiol group prevents the
contact between the electrodes and the corrosive species.
In addition to corrosion inhibition, SAMs are also used as resists
for pattern formation in etching and deposition processes owing 10
their excellent anti-adherent properties.***'*’ Therefore, the dendrite
growth step of ECM is expected to be retarded by SAMs. However,
it is important to note that excess metal ions can penetrate through
the organic layer and deposit on the electrode surface via the
reduction process. The defects in the SAM layer can act as
nucleation sites for the deposition.'™’ It was also found that Ag
Table III. Types of ECM inhibif
Material
ECM Inhibitor
Function
ECM Steps affected
References
Sn
Tri-sodium citrate dehydrate
Complexing agent
102
Na:HPO,
Corrosicn inhibitor
(1) Anodic dissolution
(2) Ion transport
NazS
Additive
(2) Ton transport
80
Citric acid + EDTA -- PEG
Complexing agents
(3) Dendrite formation
145
PEG/ Imidazole
WOA
Reduction suppressor
Flux
(1) Anodic dissolution
11
3-methyl-1,2 4-triazole-5-thiol & 1-methyltetrazole-5-thiol
Corrosien inhibitor
(1) Anodic
24
Carbohydrazide
CTAC
Oxygen scavenger
Surfactant/ Corrosion inhibitor
CTAB
30Sn-70Bi
Cu
Ag
Carbohydrazide
Surfactants
Oxygen scavenger
(1) Anodic dissolution
(2) Ton transport
(3) Dendrite formation
(1) Anodic dissolution
(2) lon transport
dissolution
143
123
142
144
(3) Dendrite formation
(1) Anodic dissolution
142
(3) Dendrite formation — 19
Journal of he Electrochemical Society, 2023 170 021505
ions can be deposited on top of a SAM at high overpotentials.
97158
The controversy reveals that the deposition of metal ions of SAMs
covered electrode is a complex process. To-date, the information on
the effects of SAMSs on dendrite growth is still scarce. It is foreseen
that the deposition or retardation mechanisms of dendrite can be
understood in the future.
In summary, SAMs improve the ECM resistance of electronic
materials with their hydrophobic properties, anti-corrosive behaviour
and anti-adherent properties. With the application of SAMs, the
electrolyte layer formation, anodic dissolution and dendrite growth
steps in ECM mechanism are retarded.
Surface coatings.—The use of surface coatings on the PCBA is a
well-established method of insulating the components from external
influences such as moisture, dust and other contaminants.'> The
coatings are thin layers of materials which are applied to conform to
the contours of the PCBs. The five major types of conformal
coatings include polyamide, polyurethane, silicone, parylene and
acrylic coatings. These coatings are usually applied in thin layer
(30210 zzm), depending on the type of coating.
One of the major benefits of surface coatings is the suppression
of electrolyte layer formation. Zhan et al.'>! m\esuga(ed the effect
of applying conformal coatings on the ECM behaviours in the
presence of no-clean fluxes using IPC-B-25 test coupons in THB. It
was reported that the conformal coatings prolonged the formation of
electrolyte layer and increased the overall TTF of the system. PCBs
with conformal coatings were less susceptible to SIR degradation as
the coatings protected the PCBs from condensation of moisture and
provided a barrier for the subsequent steps such as ion transport and
dendrite growth."7" However, the voids within the conformal coat-
ings can promote the adsorption of moisture. Since cleaning step was
eliminated with the application of no-clean fluxes, the organic fibres
which were introduced and trapped beneath the conformal coatings
provided a medium for the accumulation of moisture. Overall,
silicone gave the best protection, followed by urethane and
acrylic."?" Rathinavelu et al."> also showed that the TTF of silicone
and acrylic coated PCBAS was significantly higher than the uncoated
PCBA, regardless of the type of flux residues present.
Apart from the electrolyte formation step, the application of
surface coatings can have significant effect on anodic dissolution. Yi
etal.'® studied the effects of ceramic coatings on the ECM resistance
of SAC305 solders in CI" thin electrolyte. 1 zm-thick ceramic coating
(Z103, TiO2 o1 ALO3) was deposited on the suiface of SAC305 by
magnetron sputtering technique. It was found that the presence of
ceramic coatings suppressed the anodic dissolution of SAC305 by
inhibiting the infiltration of corrosive CI".
The effectiveness of these
coatings depends on their stability in acidic environment which affects
their suppression ability against corrosion. AlOz and ZrO, can
dissolve in basic or acidic environment duc to their amphoteric nature,
causing the substrate to undergo serious anodic dissolution.'® ZrO,
on the other hand is stable in a wide pH value range (around 1.5-11.5)
compared to that of ALO; (around 4 to 8.5), thus possessing higher
corrosion resistance than the latter. The adsorption energy (Eqq) of CI
on each coating was found to be in the order of E (TiO2) < Eu
(210,) < E,(ALO3), which shows that CI" adsorption is more likely
to occur in Al:O; coating, leading to the local oxide thinning and
anodic dissolution.'® Therefore, TiO, coating with smallest E,q, is the
best in terms of suppressing anodic dissolution and protecting the
substrate from ECM.
In short, the application of surface coatings provides barrier to
moisture condensation and improves the corrosion resistance. Thus,
the electrolyte layer formation and anodic dissolution steps of ECM
are suppressed.
ons and Perspectives
This article critically reviewed the factors affecting the classical
four-step electrochemical migration (ECM) mechanism of electronic
materials and linked the effects of material properties, environment,
Conclu:
electrolyte properties, electric field and surface conditions to the
ECM mechanism. ECM is an exquisite process which occurs when
there is a balance between the factors involved. A minor change in
one factor can lead to a huge difference in the mechanism of ECM.
The effects of each factor on ECM and the latest inhibition strategies
are discussed based on the sequence of the steps in ECM
mechanism.
Step 1 of the ECM mechanism involves the formation of an
electrolyte layer that covers both the oppositely charged electrodes
This step is one of the prerequisites for ECM. It is accelerated by the
increase in RH, the presence of hygroscopic contaminants and the
decrease in spacing between electrodes. The surface properties such
as the surface roughness, surface energy and thermal diffusivity of
the substrate materials and electrodes play significant roles in step 1
of ECM. The application of surface coatings and self-assembled
monolayers can defer the electrolyte formation.
Step 2 involves the anodic dissolution of the positive terminal.
This step involves the formation of metal ions for subsequent ECM
steps from the corrosion of metals. The properties of clectrode
materials such as the standard electrode potential, solubility product
and electrochemical parameters can determine the rate of anodic
dissolution. The harsher environment and higher electric field
accelerate the corrosion of metals. Great efforts have been devoted
to inhibiting this step as the presence of metal ions is detrimental to
ECM failure. Corrosion inhibitors, additives, conformal coatings,
SAMs, etc. are employed to suppress the anodic dissolution of the
conductive materials.
Step 3 of the ECM mechanism or the ion transport step is mainly
governed by the transport kinetics and the presence of neutral
precipitates. The electric field is the main driving force for the
migration of the charged particles. The convection and mass
transport of ions are accelerated with an increase in temperature.
On the other hand, the formation of insoluble precipitates which act
as barrier that block the path for ion transport is the main obstacle to
ion transport. Precipitations are part of the ECM reactions and are
ineluctable. The formation of precipitates is often affected by the
type of materials involved and the electrolyte chemistry. The
inhibitors which facilitate precipitates such as NazS are found to
inhibit ECM
The final step of the classical ECM mechanism is the growth of
dendrites which involves the reduction of metal ions at the cathode.
Although the location of dendritic growth is random, it is believed
that the dendrites nucleate at points wilh sur facec imper fections, such
as screw dislocations, kinks and protrusions. The electrode material,
environmental factors, electrolyte properties, electric field and surface properties of electrodes affect the rate of dendritic growth.
Altering the temperature and bias voltage can affect the dendrite
morphology. The additions of alloying clements and inhibitors are
capable of transforming the tree-like dendrite morphology which is
facile in dendritic propagation to particle-like dendrites which arc
intricate in propagation.
Controversies exist. Some claimed that the rate of anodic
dissolution is the rate-determining step for ECM, but some of the
findings showed that the growth of dendrites decreased with anodic
reactions. This is attributed to the two opposing results due to the
increased rate of anodic reactions: (1) increased dissolution of ions
which facilitates dendrite growth and (2) increased formation of
precipitate which acts against dendrite growth.
Researchers have made tremendous efforts to understand the
effects of various factors on ECM. The mitigation of ECM in
clectronic materials is still the main challenge. The addition of
alloying elements as part of the development of electronic materials
and the application of inhibitors are found to be potential means to
retard ECM. It is foreseen that SAMs have a great potential for
mitigating ECM. These findings are crucial for the selection of
appropriate materials and the development of reliable electronic
products. In the future, attention needs to be drawn to the effects of
no-clean fluxes on the ECM of clectronic materials due to their
growing use.
Journal of The Electrochemical Society, 2023 170 021505
ORCID
Ee Lynn Lee ( https:/orcid.org/0000-0002-6932-9109
Yi Sing Goh 0 hutps:/orcid.org/0000-0002-8974-2662
Yew Hoong Wong 0 hitps:/orcid.org/0000-0003-3254-9420
Mohd Faizul Mohd Sabri 0 hitps://orcid.org/0000-0001-8096-2709
References
. B. Liao, H. Wang, W. Xiao, Y. Cai,
31, 13001 (2020).
and X. Guo,J. Mater. Sci., Mater. Electron.,
2. M. Sun. H-G. Liao. K. Niu, and H. Zheng. Sci. Rep.. 3, 3227 (2013).
3. K. Wang, ACS Omega, 5, 10225 (2020).
4. X. Zhong, S. Yu, L. Chen,J. Hu, and Z. Zhang, J. Mater. Sci., Mater. Eleciron.,
28,2279 (2017).
5. B. Medgyes, X. Zhong, and G. Harsányi, J. Mater. Sci., Mater. Eleciron., 26,2010
015).
6. J. Steppan,J. Roth, L. Hall, D. Jeannotte, andS. Carbone, /. Electrochem. Soc.,
134. 175 (1987).
7. G. Harsanyi, [EEE Trans. Compon., Packag.. Manf. Technol., Part A, 18. 602
(1995).
8. X. Zhong, L. Chen, B. Medgyes,Z. Zhang, S. Gao, and L. Jakab, RSC Adv., 7.
28186 (2017).
9. L. Ma, J. Cui, $. Yao, X. Liu, Y. Luo, X. Shen, and J.-K. Kim, Energy Sror.
Mater., 27. 522 (2020).
10. K. Miyazaki, Y. S. Lee, T. Fukutsuka, and T. Abe, Electrochemisiry, 80, 725
2012).
11. S.-I. Song, S.-R. Choi, and J.-G. Kim, J. Electroandl. Chem., 832,75 (2019).
12. G. Harsányi and G. Inzelt, Microelecrron. Reliab., 41, 229 (2001).
13. G. Reiss,B. Kosednar-Legenstein,J. Riedler, and W. EBI, Microelectron. Reliab.,
117, 114035 (2021).
14. D. Minzari, M. S. Jellesen, P. Moller, and R. Ambat, Corros. Sci., 53, 3366
2011.
15. D. O. Yu, W. Jillek, and E. Schmitt, J. Mater. Sci, Mater. Electron., 17, 219
(2006).
16. B. I. Noh, J. B. Lee, and $. B. Jung, Microelectron. Reliab.,
48, 652 (2008).
17. B. Medgyes,J. Mater. Sci., Mater. Electron., 28, 18578 (2017).
18. B. Medgyes. B. Illés, and G. Harsányi, J. Mater. Sci.. Mater. Electron., 23. 551
(2012).
19. B. Liao, H. Wang, L. Kang,S. Wan, X. Quan, X. Zhong, and X. Guo,/. Marer.
Sci., Mater. Electron., 32, 5680 (2021).
20. H. R. Ma, A. Kunwar,J. Chen, L. Qu. Y. P. Wang, X. G. Song, P. Raback, H.
T. Ma, and N. Zhao, Microcleciron. Reliab., 83. 198 (2018).
21. H. Parangusan, J. Bhadra, and N. Al Thani, Emergent Mater., 4, 1187 (2021).
22. L. Wang, K. Wang,N. Erkan,
Y. Yuan,J. Chen, B. Nie,F. Li, and K. Okamoto,
Appl. Surf. Sci., 511, 145555 (2020).
23. F. Li, V. Verdingovas, K. Dirscherl, G. Harsányi, B. Medgyes, and R. Ambat,
J. Mater. Sci., Mater. Electron., 31, 15308 (2020).
24. O. Devos, C. Gabrielli, L. Beitone, C. Mace, E. Ostermann, and H. Perrot,
J. Electroanal. Chem., 606, 85 (2007).
25. K. Xiao, P. Yi, C. Dong. S. Zou, and X. Li, Marer. Lett., 210, 283 (2019).
26. Y. Zhou, P. Yang, C. Yuan, and Y. Huo, Chem. Eng. Trans., 33, 559 (2013).
27. H. Tanaka, "Factors leading to ionic migration in lead-free solder." ESPEC
Technology Report (2002)p. 114.
28. Y. R. Yoo, H. S. Nam,J. Y. Jung, S. B. Lee,Y. B. Park,Y. C. Joo, and Y. S. Kim,
Corros. Sci. Technol., 6. 50 (2007).
29. Y. R. Yoo and Y. S. Kim, Met. Mater. Int., 16,739 (2010).
30. Y. R. Yoo and Y. S. Kim, Mer. Marer. /nr., 13, 129 (2007).
31. L. Hua and C. Yang, Microclectron. Reliab., 51, 2274 (2011).
32. B. Medgyes, B. Horváth, B. Il és, T. Shinohara, A. Tahara, G. Harsányi, and
O. Krammer, Corros. Sei., 92, 43 (2015).
L. Hua and H. N. Hou, Microelectron. Reliab., 75, 27 (2017).
H. Naguib and B. MacLaurin, [EEE Trans. Compon. Hybrids Manuf. Technol.,2,
196 (1979).
35. C. A. Yang,). Wu, C. . Lee, andC. R. Kao, J. Mater. Sci., Mater. Electron., 29,
13878 C2018).
36. A.G. Massey, N. R. Thompson,B. F. G. Johnson, and R. Davis, The Chemistry of
Copper, Silver and Gold: Pergamon Texts in Morganic Chemistry (Oxford)
(Pergamon)p. 16 (1975).
37. T. Takemoto, R. M. Latanision,T. W. Eagar, andA. Matsunawa, Corros. Sci., 39,
1415 (1997).
38. B. Medgyes, B. Illés, andG. Harsányi, Period. Polytech.,
57, 49 (2013).
39. X. Zhong, G. Zhang, Y. Qiu, Z. Chen, X. Guo, and C. Fu, Corros. Sci., 66, 14
2919.
40. D. O. Yu, W. Jillek, and E. Schmitt, J. Mater.
, Mater. Electron., 17, 229
(2006).
S. Jiang, B. Liao,Z. Chen, andX. Guo, /nr. J. Electrochem. Sci., 13, 9942 (2018).
K. Piotrowska and R. Ambat, [EEE Trans. Compon., Packag., Manuf. Technol.,
10, 1617 (2020).
. Y. Zhou, Y. Zhao, L. Yang, Y. Li, and W. Lu, /EEE Access, 8, 182580 (2020).
R. E. Melchers and R. J. Jeffrey, Reliab. Eng. Syst. Saf., 93, 423 (2008).
H. S. Mohran, A.-R. El-Sayed, and H. M. Abd El-Lateef, /. Solid Srare
Electrochem., 13, 1279 (2009).
K. Piotrowska, R. Ud Din, F. B. Grumsen, M. S. Jellesen, and R. Ambat,
J. Electron. Mater., 47. 4190 (2018).
47. A. J. Bard, L. R. Faulkner, and H. S. White, Electrochemical Methods:
Fundamentals and Applications (New York)(Wiley)p. 183 (2022).
48. B. Illés,B. Medgyes, K. Dusek, D. Busek, A. Skwarek, andA. Géczy, /nt.J. Heat
Mass Transfer, 184, 122268 (2022).
49. P. Rasouli,C. I. Steefel, K. U. Mayer, and M. Rolle, Comput. Geosci., 19, 523
(2015).
50. X. He, M. H. Azarian, and M. G. Pecht, Elecrrochim. Acta, 142, 1 (2014).
51. G. G. Wildgoose, D. Giovanelli, N. S. Lawrence, and R. G. Compton,
Electroanalysis, 16, 421 (2004).
52. 1. O. M. Bockris and A. K. N. Reddy, Volume/: Modern Electrochemistry: Tonics,
ed. J. O. M. Bockris (Berlin)(Springer)p. 361 (1998).
T. Aöki, Y. Li, and M. Saka, Marer. Lerr., 236, 420 (2019).
W. S. Hong and C. Oh, /. Electron. Mater., 49. 48 (2020).
. Y. Zhou, Y. Li, Y. Zhao, and W. Lu, /EEE Trans. Device Mater. Reliab., 21, 117
(202),
X. Zhong, J. Mater. ci., Mater. Electron., 31, 8996 (2020).
. B. Lao, Z. Li, Y. Cai, and X. Guo, J. Mater. Sci., Mater. Electron., 30, 5652
2
Acknowledgments
This work was supported by the Collaborative Rescarch in
FEngineering, Science and Technology (CREST) [Grant number:
P12C1-14] and the Partnership Grant of Universiti Malaya [Grant
number: RK003-2016].
(2019).
K. Ito,N. Sakai, andM. Arai, /. Electron. Mater.,
47, 7179 (2018).
P. Y1, K. Xiao,C. Dong, 5. Zou, and X. Li, Bioelecrrochemisiry, 119, 203 (2018).
D. Minzari, M. Jellesen, P. Mollcr, P. Wahlberg, and R. Ambat, /JEEE Trans.
Device Mater. Reliab., 9, 392 (2009).
R. Huston, Y. Chan, H. Chapman,T. Gardner, andG. Shaw, Warer Res., 46. 1121
(2012).
B. Song, M. H. Azarian, and M. G. Pecht, /. Electrochem. Soc., 160, C97 (2013)
P. Salve, A. Maurya, S. Wate, and S. Devotta, Bull. Environ. Contam. Toxicol.,
80,
242 (2008).
F. L. Lee, A. Haseeb, W.J. Basirun, Y. H. Wong,M. F. M. Sabri, and B. Y. Low,
Sci. Rep., 11, 1 (2021).
G. Harsányi, Microelectron. Reliab., 39. 1407 (1999).
. A. Moncmanová, Environmental factors that influence the deterioration of
matedials." . Environmental Deterioration of Materials, ed. A. Moncmanová
(Southamptom)(WIT Press)p. 1 (2007).
. X. Zhong,
G. Zhang, Y. Qiu, Z. Chen, and X. Guo, Corro Sci., 74, 71 (2013).
B. Liao, Z. Chen, and X. Guo, Madridge J Anal Sci Instrum, 1, 4 (2016).
§. Fernández and M. Alvarez, Corros. Sci., 53, 82 (2011).
. M. Pourbaix, Azlas of electrochemical egullibria in agueous solution (Texas)
(National Association of Corrosion Engineers)p. 479 (1974).
. L. Lin, C. Chao, and D. Macdonald, /. Electrochem. S0c., 128. 1194 (1981).
. S. Cho, J.-H. An, $.-H. Lee, andJ. Kim, /nt. J. Electrochem. Sci., 15, 4406
(2020)
. W. E. O'Grady, D. F. Roeper, andP. M. Natishan, J. Phys. Chem. C, 115, 25298
2017.
-V Jung. S8 1.ee, Y.-C. oo H.-Y. Lee. and V.-B. Park, Microelectron. Fne..
85, 1597 (2008).
1.-Y. Jung, S-B. Lee, H-Y. Lee, Y.-C. Joo, and Y.-B. Park, /. Electron. Mater.,
37. 1111 (2008).
. J.-Y. Jung, $.-B. Lee, H.-Y. Lee, Y.-C. Joo, and Y.-B. Park, J. Electron. Mater.,
38. 691 (2009).
The Molecular Nature of Matter and Change (New
Yorkj(MeGrasw-Hillyp. 529 (2015).
8. A. Rostron, Corros. Sci., 19, 321 (1979).
79. X. Zhong, G. Zhang, Y. Qiu, Z. Chen, W. Zou, and X. Guo, Electrochen.
Commun., 27, 63 (2013).
80. B. Liao,L. Wei, Z. Chen, andX. Guo, RSC Adv.,7, 15060 (2017).
81 Y. Zhang, Modem electroplating, ed. M. Schlesinger and M. Paunovic (New
York)(Wiley) 5th ed.p. 139 (2011)
82. Y. Zhou, Y. Li, and Y. Chen,/. Electron. Mater., 49, 2066 (2020)
83. H. Conseil, V. Verdingovas, M. S. Jellesen, andR. Ambat,/. Mater. Sci., Mater.
Electron., 27. 23 (2016).
84. B. Liao, H. Cen, Z. Chen, and X. Guo, /nnovations in Corrosion and Materials
Science, 9, 74 (2019).
K. Piotrowska, M. S.Jellesen, andR. Ambat, Soldering5 1f. Mount Technol., 29,
133 2017)
44, 1116 (2015).
86. V. Verdingovas. M.$. Jellesen. andR. Ambat, /. Electron. Mater.,
87. 3. Sohn and U. Ray, Circuit World, 21, 22 (1995).
88. €. Peng, M. N. Chan, and C. K. Chan, Environ. Sci. Technol., 35, 4495 (2001).
89. M. 2. H. Rozaini, Atmospheric Aerosols: Regional Characteristics - Chemistry
and Fhysics, ed. H. Abdul-Razzak (Rijeka)InTech)p. 323 (2012).
90. S. H. Yalkowsky.Y. He. and P. Jain, Handbook of Aqueous Solubility Data (Boca
Raton, FLY(CRC Press)p. 99 (2016).
o Y.Q Zhou, L. Turbini,D. Ramjattan,B. Christian, and M. Pritzker, /. Eleciron
Mater., 42, 3609 (2013
92. R. Ambat, M. $. Jellesen, D. Minzari, U. Rathinavelu, M. A. K. Johnsen, P. J.
S. Westermann, and P. Moller, European Corrosion Congress 2009, Proceedings
of the Eurocorr 2009, Nice, France8141 (2009).
. M. S. Silberberg, Chemist;
Journal of The Electrochemical Society, 2023 170 021505
. V. Verdingovas, M. Jellesen, and R. Ambat, Corros. Eng. Sci. Technol., 48, 426
(2013).
E. Heseltine
andJ. Rosen, WHO Guidelines for Indoor
Air Quality: Dampness and
Mould (Rheinbach)(World Health Organization)p. 2 (2009).
. M. T. Madigan and J. M. Martinko, Brock Biology of Microorganisms (Pearson
Prentice Hall, Hoboken) (2006).
. R. Wasserbauer, /nt. Biodeterior. Biodegrad., 53. 171 (2004).
3.-D. Gu, Int. Biodeterior. Biodegrad., 59, 170 (2007).
. Y. Hu, K. Xiao, L. Yan, X. Hao, L. Huang, and Y. Lou, Surf: Topogr.: Metrol.
Prop., 10, 015022 (2022).
(2015).
100. S. J. Krumbein, IEEE Trans. Compon. Hybrids Manuf. Technol., 11, 5 (1988).
101. R. Job, Electrochemical Energy Storage: Physics and Chemisiry of Barteries (De
Gruyter, Berlin) (2020).
Z. Chen, Y. Qiu, G. Zhang, andX. Guo, Corros. Sci., 112, 393 (2016).
102. B. Liao,
n. N. Van den Steen. H. Terryn, andJ. Deconinck, Electrochin. Acta.
103. H.
209, 149 (2016).
104. X. Zhong, G. Zhang, and X. Guo. Corros. Sci., 96. 1 (2015).
105. B. Medgyes, D. Szivós, S. Ádám, L. Tar, P. Tamási, R. Berényi, and G. Harsányi,
2016 IEEE 22nd International Symposium for Design and Technology in
Electronic Packaging Proceedings of the IEEE, Oradea, Romania274 (2016).
106. G. W. Warren, P. Wynblatt, and M. Zamanzadeh, /. Electron. Mater., 18, 339
(1989).
107. €.-H. Tsou, K.-N. Liu, H.-T. Lin, and F.-Y. Ouyang, /. Electron. Mater., 45, 6123
(2016).
108. K. I. Popov. P. M. Zivkovié, and N. D. Nikolié,J. Serb. Chem. Soc., 76. 805 (2011).
109. M. Dalal, A Textbook of Physical Chemistry — Volume 1 (Dalal Institute, Rohtak)
394 (2018).
110. J. Aragones, L. MacDowell, J. Siepmann, and C. Vega, Phys. Rev. Lert., 107,
155702 (2011),
. R. Pillai, J. D. Berry, D. J. E. Harvie, d M. R. Davidson, Phys. Rev. , 92,
013007 (2015).
112. O. Wang, H. Xie, Z. Hu, and C. Liu, Nanomaterials, 9, 64 (2019).
113. J. A. Jachim, G. B. Frecman, and L. J. Turbini, IEEE Trans. Compon., Packag..
Manuf. Technol., Part B, 20, 443 (1997).
114. V. Verdingovas, M. S. Jellesen, and R. Ambat, J. Mater. Sci., Mater. Electron., 26.
7997 (2015).
115. X. Zhong,L. Chen,J. Hu, Y. Shi,Z. Zhang,D. Zeng, and T. Shi,/. Electrochem.
Soc.. 164, D342 (2017).
116. 5-B. Lee, M.-S. Jung, H.-Y. Lee, T. Kang, and Y.-C. Joo, EEE Trans. Device
Mater. Reliab., 9. 483 (2009).
17. B. I. Noh and S. B. Jung, J. Mater. Sci., Mater. Electron., 19, 952 (2008).
118. B. Medgyes, A. Gharaibeh, D. Rigler, and G. Harsányi, Maierials, 14, 5237
2021.
119. §. Glasstone, An Introduction 10 Electrochemistry (Redditch)(Read Books Ltd)p.
435 2011).
120. M. Ciobanu, J. P. Wilburn, M. L. Krim. and D. E. Cliffel, Handbook of
Electrochemistry, ed. C. G. Zoski (Amsterdam)(Elsevier) 15t ed.p. 11 (2007).
S. Zhan, M. H. Azarian, and M. G. Pecht, /EEE Trans. Electron. Packag. Manuf.,
29,217 (2006).
W. Lu, B. Liao, B. Medgyes, J. Hu, Y. Zheng, D. Zeng, andZ. Zhang,
122. X. Zhong,
Corros. Sci.. 156. 10 (20191.
123. B. . Liao, Z. Chen, Q. Qiu, and X. Guo, Corros. Sci., 118, 190 (2017).
124. S. Jangam and $. $. Iyer, /EEE Trans. Compon., Packag., Manuf. Technol., 11,
727 (2021).
125. E. Perfecto and K. Srivastava, Advanced Flip Chip Packaging, ed. H.-M. Tong,
Y.-S. Lai, and C. P. Wong (Berlin)(Springer)p. 23 (2013).
99. K. Ding, K. Xiao,C. Dong, S. Zou, P. Yi, and X. Li,J. Elecrron. Marer.,
44, 4405
. B. Medgyes, B. Illés, R. Berén and G. Harsányi, J. Mater. Sci, Mater.
Electron., 22, 694 (2011).
7. B. I. Noh and S. B. Jung, Circuit World, 34, 8 (2008).
. B. 1. Noh, J. W. Yoon, W. S. Hong, and S. B. Jung, J. Electron. Mater., 38. 902
(2009).
. M. Bacior, N. Sobczak, A. Sie rek, A. Kudyba, M. Homa, R. Nowak,
M. Dziula, and S. Maslon, /. Marer. Eng. Perform., 24, 754 (2015).
C. Xu, Y. Zhang,C. Fan, P. Chiu, andJ. Abys, Plar. Surf. Finish.,
87, 88 (2000).
. S. M. Ramkumar and K. Srihari, 2007 IEEE 571h Electronic Components and
Technology Conference, Proceedings of the IEEE, Sparks, Nevada, USAGS (2007).
S. Ztan, M. H. Azarian, andM. Pecht, IEEE Trans. Device Mater. Reliab.,8,426
(2008).
24, 2315
133. B. Medgyes, B. Illés, and G. Harsányi,J. Marer. Sci. Mater. Electron.,
2019.
134. K. Piotrowska, R. U. Din, M. S. Jellesen, and R. Ambat, /EEE Trans. Compor.,
Packag., Manuf. Technol., 8. 1756 (2018).
135. G. Chi, D. Yi. and H. Liu, J. Mater. Res. Technol., 9. 1162 (2020).
136. H. Ezuber, A. Alshater, S. O. Nisar, A. Gonsalvez, and $. Aslam, Surf. Eng. Appl.
Electrochem., 54, 73 (2018).
I.
137. H. A. Jaffery, M. F. M. Sabri, S. M. Said, S. W. Hasan, I. H. Sajid, N.
M. Nordin, M. M. I. M. Hasnan, D. A. Shnawah, and €. V. Moorthy,/. Alloys
Compd., 810, 151925 (2019).
138. W. A. Khan, K. Rahman, G. Hussain, and G. Abbas, Functional Reverse
Engineering of Strategic and Non-Strategic Machine Tools (Boca Raton, FL)
(CRC Press)p. 87 (2021).
139. P. Szlvo and M. Hemandez-Silveira, Wireless Medical Systems and Algoríthms:
Design and Applications (Boca Raton, FLXCRC Press) (2017).
Sci., 51, 690 (2009).
140. M. Mouanga, L. Ricg, J. Douglade, and P. Bergot, Corros.
141 C. O'Regan, X. Zhu, J. Zhong, U. Anand, J. Lu, H. Su, and U. Mirsaidov,
Langmir, 32, 3601 (2016).
/. Electron. Mater., 48, 5012 (2019).
142. M.-S. Hong and J.-G. Kim,
143. B. Liao, H. Wang, S. Wan, W. Xiao, and X. Guo, Merals, 10, 942 (2020).
144. B. A. Smith and L. J. Turbini, /. Electron. Mater., 28, 1299 (1999).
145. Y.-D. Tsai, C.-C. Hu, and C.-C. Lin, Elecrrochim. Acta, 53, 2040 (2007).
146. B. D. Ratner and A. S. Hoffman, Biomaterials Science: An Introduction 10
Materials in Medicine, ed. B. D. Ratner, A. S. Hoffman, F. J. Schoen, and J.
E. Lemons (Amsterdam)(Elsevier) 2nd ed.p. 201 (2004).
147. M. Singh, N. Kaur, and E. Comini, J. Mater. Chem. C. 8, 3938 (2020).
148. S. Kim and H. Yoo, Micromachines, 12, 565 (2021).
149. A. Ulman, Chem. Rev., 96. 1533 (1996).
150. B. A. Rao, M. Y. Igbal, and B. Sreedhar, Corros. Sci, 51, 1441 (2009).
151 H. Wei, H. Zhaohong, and Z. Qin, 2073 IEEE 15h Electronics Packaging
Technology Conference, Proceedings of the IEEE, Singapore555 (2013).
152. T. Sekine, J. Sato, Y. Takeda, D. Kumaki, and S. Tokito, ACS Appl. Mater.
Interfaces, 10, 16210 (2018).
153. G.-S. Liu, Y. Xu, Y. Kong, L. Wang, J. Wang, X. Xie, Y. Luo, and B.- . Yang,
ACS Appl. Mater. Interfaces, 10, 37699 (2018).
154. S. Ramachandran, B.-L. Tsai, M. Blanco. H. Chen, Y. Tang. and W. A. Goddard,
Langnuir, 12, 6419 (1996).
155. D.-O. Zhang, X.-M. He, O-R. Cai, LX. Gao, and G. S. Kim, I. Appl.
Electrochem., 39, 1193 (2009).
156. 0. Azzaroni, P. Schilardi, and R. Salvarezza, Electrochim. Acta, 48, 3107 (2003).
157. A. V. Walker. Encyclopedia_of [nterfacial Chemistry, ed. K. Wandelt
(Amsterdam)(Elsevienp. 752 (2018).
158. M. Esplandiu and H. Hagenström, Soid Srate Tonics, 150, 39 (2002).
159. U. Rathinavelu, M. S. Jellesen. and R. Ambat, Corros. Eng.. Sci. Technol. 48,436
2015.
160. P. Yi, C. Dong. M. Ao, and K. Xiao, Ceram. /nt., 46. 25568 (2020).