IMC STUDY OF MID-PHOSPHOROUS
AND HIGH-PHOSPHOROUS ENIG FINISHES
Sandra Nelle, Thiago Pugliesi-Garcia, Britta Schafsteller, Gustavo Ramos
Atotech Deutschland GmbH
Berlin, Germany
sandra.nelle@atotech.com
I)
ABSTRACT
ENIG with mid-phosphorous (MP) nickel is used for
soldering and Al wire bonding applications. ENIG with highphosphorous (HP) nickel serves the niche of environmental
corrosive environments.
22M$
27M$
ENIG
42M$
i-Sn
178M$
However, an unspoken fear in the industry concerning highphosphorous nickel is a poor soldering performance and
creation of an extended P-rich layer. This paper aims a
comparison in regards of the soldering performance to shed
light towards the real soldering performance of MP vs. HP
nickel.
OSP
I-Ag
91M$
Others
88M$
E-Pd
Soldering performance will be evaluated by wetting balance,
wave soldering and HSS testing. The IMC as a major
indicator of solder joint reliability will be evaluated
comparing both phosphorus systems. Different reflow
conditions will contribute to the formation of different solder
alloys and simulate different ageing and soldering conditions.
Apart from solderability, environmental corrosion will be
evaluated by gas chamber testing. Further, nickel corrosion
performance will be checked as further indication of the
quality of the solder joint.
It is also the intention of this paper to evaluate whether there
are any further unintended benefits of an HP ENIG.
To evaluate the impact of the different electroless nickel
baths in combination with reduction assisted immersion gold,
data generated by Design of Experiment (DOE) will be used.
The results are expected to be production applicable and it is
hoped that they may clarify myths or misunderstandings
within the PCB manufacturing environment.
Key words: IMC, Highest reliability, Corrosion resistance,
Solderability, Black pad, High-phosphorous nickel, Midphosphorous nickel, 0,2 1 2
II)
INTRODUCTION
ENIG plays a major role in the worldwide share of final
finishes (Figure 1).
Figure 1: (top) PCB Fabrication Materials Value in 2018
(M$); share of final finishes [1]. (Bottom)This trend to be
unchanged in the future: PCB Market Growth Value by
Technology, 2018 vs. 2023F (M$) [2]
The share of Multilayer PCBs is expected to increase again.
As well as the overall value for HDI PCB.
ML-PCBs occupy a sweet spot in terms of capability and
price, as they provide a reasonable routing density at a
competitive price. From 2020 new functionalities such as 5G
and 3 D sensors is expected to boost the smartphone industry
again.
As ENIG plays an important role for both, HDI and
Multilayer, it is expected to be still the dominant finish in the
future. It is considered as all-round performance surface
finish for mass production as it is solderable and capable for
aluminum wire bonding.
Apart from its being established for long and therefore being
well technically understood, a new technical challenge
aroused.
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
881
The introduction of the latest version of IPC 4552 [3] sheds
Nickel corrosion into a new light.
While mid-phosphorous Nickel (MP-Ni) exhibits a
Phosphorus content (P-content) between 7% and 10%, highphosphorous Nickel (HP-Ni) shows Phosphorus contents
between 10% and 12%. That difference in P-content
contributes to different layer properties such as its crystal
structure, its appearance and its physical properties such as
corrosion resistance.
This paper aims to address similarities and differences of MP
and HP Nickel to draw potential conclusions about which
Nickel system should be preferred for which application.
III)
EXPERIMENTAL AND TEST METHODS
Atotech Testboards (SFTB1 Rev7) were cleaned via a wet chemical
approach before ENIG. The process flow is displayed in Figure 2
and Table 1
Cleaner
Process Step
T [°C]
t [min]
V (L)
Acidic Cleaner
40
5
145
Hot Rinse
50
1
135
DI Rinse
RT
2,5
135
1
135
DI Rinse
MicroEtch
30
2
145
DI Rinse
RT
0,5
135
2
135
Acid Rinse
Materials and process flow
Etch
Cleaner
Table 1: ENIG detailed process flow including plating
conditions
Activation
Figure 2: ENIG process flow
Electroless
Nickel
DI Rinse
1
135
Pre-dip
RT
3,5
135
Pd Catalyst
23
1,5
145
Pd Deactivation
RT
3
145
DI Rinse
RT
1
135
DI Rinse
1
135
DI Rinse
1
135
Electroless
Gold
E'less Nickel - MP
85
25
270
E'less Nickel - HP
82
37
270
DI Rinse
RT
0,5
135
DI Rinse
0,5
135
DI Rinse
0,5
135
10 (MP)
15 (HP)
0,5
140
DI Rinse
4
135
DI Rinse
3
135
E'less
Dragout Rinse
Gold
83
RT
135
Hot Rinse
50
2,5
135
Dryer
60
10
-
The thicknesses applied were 5 µm Nickel and 0,05 µm Gold.
The Nickel bath was aged from 0 MTO (Metal Turn Over)
until 3 MTO. Nickel bathes applied were mid-phosphorous
(MP) Nickel and high-phosphorous (HP) Nickel. The exact
distinction is at a range of 7 – 10 % Phosphor (P) for MP
Nickel and > 9.5 % P for HP Nickel. Plating of MP Nickel
and HP Nickel was done simultaneously at the same day and
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
882
the same pre- and post-treatment conditions. Different ageing
conditions are tested as well as a lower gold thickness for the
wetting tests (Table 2)
Table 2: Variations of Nickel and Gold.
Nickel
Nickel
Gold
System
MTO
Thickness
(µm)
0
0,03
0,05
3
Figure 3: EDX detector
0,03
HP
0,05
0
The measurement of Nickel-, Phosphor- and Tin-distribution
was captured at 5 kV, 3096.5s, 9116 cps count rate, 1045 x
926 Pixel and 10000 x magnification.
0,03
0,05
MP
3
0,03
0,05
Measurement tools and methods
Thickness Distribution
The thickness uniformity of the Nickel and Gold layer was
investigated by X-ray fluorescence (XRF) measurement.
HSS Testing
The test and ball soldering conditions for ball shear test at
higher speed (HSS) are the following. Solder balls used are
of Ball type Shenmao SM Ball PF684-S with SAC 305 alloy.
The ball diameter is 450 μm. Flux used is of type Kester
Tacky Flux TSF 6502. The reflow profile is sketched in
Figure 4.
Peeling Evaluation
For ENIG with 5 µm Nickel and 60 nm Gold thickness,
peeling was checked via tape test (Scott 3M)
As representative areas, two through-holes (TH) of diameter
sizes 1.2 mm and 0.8 mm were taken as well as two different
pads. To simulate SIT production, the boards were partially
covered with OSP (Organic Surface Protection) before ENIG
plating and etched three-times in the OSP-etch after ENIG
plating.
Wetting Tests
The different wetting tests Selective Wave Soldering and
Wetting Balance Test were performed according to IPC JSTD 003C. Coupons defined for that test are used as
described in IPC J-STD 003A.
Appearance and Nickel corrosion
according to IPC 4552 A and B
rating
The cross section samples were prepared by epoxy casting, grinding
and polishing. The samples were investigated by optical microscope
and scanning electron microscopy (SEM). The counting’s according
to IPC 4552 A and B were done by light microscope, as described
in this specification.
IMC Testing and investigation of the P-rich layer
IMC appearance was investigated with SEM microscopy.
The analysis of the P-rich layer was performed via EDX with
the detector X-Max Extreme at Oxford Instruments
(resolution≤127eV at MnKα
Figure 4: Linear lead-free reflow profile used for cold ball
pull and ball shear test; peak-T: 250°C
The reflow oven used for all soldering tests was Rehm
Compact Nitro B 2100 – 400. Reflow was under nitrogen
atmosphere.
For the tests TV3 boards manufactured by KSG
Leiterplattentechnik were used at SMD BGAs with SRO 380.
30 solder balls were applied per condition.
The equipment is DAGE4000. It was used a cartridge BS5kg
at a shear speed of 0,6 m/s.
Beneath as received conditions, aging conditions were tested
to simulate real usage of the boards in the field before and
after soldering (Figure 5).
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
883
IV)
RESULTS
Thickness Distribution
Figure 5: Aging conditions for solder tests
The Pre-Aging Condition is baking for 2h at 175°C, 5 min DI
Water rinsing at 60°C and cold air dry.
The Post Aging Condition is baking for 250 h, 500 h and
1000h at 150°C and 60% RH and two reflows (RF) (ball
attach and 1 RF).
In ENIG layer systems the Nickel layer acts as a barrier layer
to protect the underlying Copper and the Gold layer protects
the nickel layer from oxidation and ensures good wettability.
To fulfil those tasks a good distribution of the layers all over
the board is of high importance. Especially the gold can be
critical in distribution to its nature of immersion reaction and
low overall thicknesses required. Therefore the gold
thickness distribution was measured and can be seen in
Figure 6.
Al-wire Bonding
The equipment for aluminum wire bond testing used are the
Automatic Wedge/Wedge-Bonder F&K Delvotec 5630 with
bond wires from Heraeus AlSi 1%, diameter of 25 μm and
break load of approx. 15 g. The pull tester was DAGE 4000;
with the Cartridge WP100 and a pull speed of 500 μm/s. No.
of pulls per sample was 30. The parameter settings used in
the bond of the two wedges can be read from Table 3.
Table 3: Parameter settings used for wire bond test
Parameter
US-Power [a.u.]
US-Time [ms]
Bond Force [cN]
First
Bond
90
25
20
Second
Bond
90
25
25
Environmental Tests
There were two environmental tests performed – the neutral
salt spray test (NSS) and the SO2 gas test, also known as
Kesternich test.
The NSS was performed according to ISO 9227 :2017. The
test specimen are elevated the corrosive media through spray
nozzles in one chamber for different, specified amount of
time. For NSS 5% NaCl solution is dissolved in water. The
chamber is operated at 35°C and the salty fog fills the
chamber completely and homogeneously.
The SO2 gas test simulates industrial atmospheres by
exposing the specimen to a high humidity environment
containing sulfur dioxide. It was performed according to ISO
6988. That means that the samples are exposed in a 300l
chamber for 6 consecutive cycles for each 24h at a SO2
content of 10 ppm, 100% r.h. and a temperature of 42°C.
Figure 6: Thickness distribution of different Nickel systems
at different bath ages
The overall ENIG Std. deviation of the gold is 3,2 nm on
average, which is acceptable.
Divided into the different Nickel systems the thickness range
of the gold on MP-Nickel is max. 15,9 nm while it is max.
7,8 nm on HP Nickel. The standard deviation of gold is on
MP-Nickel on average 3,2 nm while it is on average 2,2 nm
on HP Nickel.
It can be concluded that the thickness distribution of gold on
HP is slightly more even than on MP-Nickel. One
explanation for that can be seen in the difference in P-content.
To verify the P-content it was measured via EDX. For HP
Nickel it is on average 11,8%, ranging from 11,5% to 12,4 %.
For MP Nickel it is on average 8,1%.
The HP Nickel therefore generates a nickel layer with a
uniform, higher phosphorus content. Due to that
incorporation it exhibits a constant deposition speed during
lifetime and a supposing more even layer. Due to that
characteristic and the fact that the immersion exchange
reaction of the gold takes place more controlled at higher Pcontent, the gold distribution is assumed to be more even.
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
884
Peeling Evaluation
For final component reliability, no such defects such as layer
separation are of high importance for reliability. To
exaggerate the effect, very harsh conditions are applied with
a very aggressive etch 3x applied. The results of peeling
evaluation can be seen in Table 4
Table 4: Peeling Evaluation of selected areas of different
Nickel systems at different bath ages
Test
Peeling Evaluation
Conditions
MP
HP
NiSys.
MTO
3x OSP etch
Peeling
failure
description
mode
0
No peeling
Not Applicable
1
No peeling
Not Applicable
2
No peeling
Not Applicable
3
No peeling
Not Applicable
0
Peeling
1
Peeling
2
Peeling
3
Peeling
Hole area peeling (TH 0.8
and 1.2)
Slight area peeling at one
TH
Slight area peeling at one
TH
Hole area and pad area
peeling
Figure 7: Evaluation of THs for selective wave soldering
test.
Figure 7 a-c show samples, which are passing the test. Figure
6 d-f show samples, which are failing the test.
Following international standard IPC J-STD 003C the ageing
is “as received” (ASR) and 8h/72°C/85% r.h.
For simulating representative conditions, TH diameters of 0,8
and 1,2 mm are tested at different gold thicknesses, MTO and
Nickel systems. The results can be seen in the following
figures.
30 nm gold condition was evaluated, to check whether good
wetting could be still achieved at thinner gold layers.
It can be seen, that HP-Nickel outperforms MP Nickel in
terms of peeling under SIT production environment. That is
assumed due to the physical layer probabilities of the HPNickel like the amorphous structure.
Of course, the etching condition is exaggerated and not
applied to that extend in the field. But the information can be
extracted, that HP-Nickel is more able to withstand even
harshest conditions.
Selective Wave Soldering
All wetting tests refer to the ability of the solder, to spread
evenly all over the surface of pads or PTHs. Good wetting
means a good connection to the later assembled component.
Figure 8: Selective wave soldering, As Received, TH 0,8
The main focus for selective wave soldering lays on the
through holes (THs), but also pads are measured. For THs the
solder has to be completely raised through the holes and wet
the surface around the top-side of the hole. Examples of
rating can be seen in Figure 7.
Figure 9: Selective wave soldering, Aged: 8h/72°C/85% r.h.,
TH 0,8
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
885
For the TH 0,8 both, ASR and the aged and humid condition
all samples are passed. The same result applies for TH 1,2
ASR.
Figure 10: Selective wave soldering, Aged: 8h/72°C/85%
r.h., TH 1,2
For TH 1,2 there is a slight amount of failed samples for MPNickel in the aged condition. This amount is not significant
yet.
For pads, at least 95% of the surface have to be wetted.
Figure 11: Selective wave soldering, As Received and Aged:
8h/72°C/85% r.h. for Pads
Figure 12: Steps of wetting balance test
The wetting force is defined as the difference between
maximum force and buoyancy time (red line).
After pre-defined times the test forces are measured (Table
5).
Table 5: Defined test times for wetting forces
Results of wetting balance test at described times at a gold
thickness of 50 nm and ageing of 0 and 3 MTO can be seen
in the following figures.
All conditions pass the Selective wave soldering test of pads
as seen in Figure 11.
Wetting Balance
In the wetting tests shown so far the adhesion was evaluated
in a qualitative matter. Wetting balance testing aims to
quantify the adhesion by strength (wetting force) and speed
(wetting time).
The test principle relies on measuring mentioned adhesion
forced during dipping a pad into solder and releasing it again.
This single test steps and corresponding wetting forces are
visualized in Figure 12.
Figure 13: Wetting time Ta
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
886
Ta is expected to be within 1,5 s, which is fairly achieved for
both layer systems. F2 is expected to be at minimum 25% of
the maximum theoretical wetting force. The maximum
theoretical wetting force can be read from the value F10 and
amounts approx. 12 mN for both layer systems. Therefore the
minimum value for F2 should be no less than 3 mN. That
could be fulfilled for both layer systems. F5 has the
acceptance criterion to be no less than F2 and this is fulfilled
for both Nickel systems. The last measured force should be
no less than 4,8 mN which is fairly achieved by both samples.
Therefore both layer systems pass the Wetting balance test in
a comparable manner.
12
11
10
9
8
7
6
5
4
3
2
1
0
HP
MP
For all wetting tests it can be concluded that HP and MP
Nickel are comparable, whereas HP Nickel is more suitable
for harsher and aged conditions.
Appearance and Corrosion Results
Figure 14: Wetting force at T2
Via SEM differences in appearance shall be captured.
Therefore, the surface and cross-section is compared at
different conditions, as described in the following figures.
Figure 17: HP-Nickel at 0 MTO, 50 nm Au, Right : higher
magnification
Figure 15: Wetting force at T5
Figure 18: MP-Nickel at 0 MTO, 50 nm Au, Right : higher
magnification
Figure 19: HP-Nickel at 3 MTO, 50 nm Au, Right : higher
magnification
Figure 16: Wetting force at T10
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
887
Therefore several issues and discussion were faced in the
industry, which was the reason for the reviewed and launched
IPC 4552A on August, 2017. That version tries to quantify
the Nickel corrosion events, targeting to derive acceptance
criteria.
Figure 20: MP-Nickel at 3 MTO, 50 nm Au, Right : higher
magnification
From surface view, it can be interpreted that HP- and MPNickel exhibit different crystal structure. This is due to the
amorphous character of HP-Nickel layers on the contrary to
crystalline MP-layers. That difference causes different
properties of HP- compared to MP-Nickel Systems. Lacking
grain boundaries is expected to mild Nickel-corrosion
attacks, as electrons needed by immersive gold are prone to
move from grain boundaries in particular.
To evaluate potential differences in Nickel corrosion,
following cross-sections are prepared.
However, this revision has caused insecurity within the
electronics industry and its distinct parties such as OEMs,
PCB manufacturers and chemical suppliers.
Misinterpretations of the nickel hyper-corrosion evaluation
may lead to falsely negative judgements and to the rejection
of perfectly functioning ENIG production. This can be
verified by the fact that production that has been proven
acceptable for decades could now be classified as rejectable
because of the misinterpretation of the new IPC 4552A
specification. Therefore a new revision is planned for 2019 IPC 4552 B.
Keeping those reservations in mind, the IPC 4552 A Nickel
corrosion criteria were used for this paper, to get a
quantitative comparison between HP-Nickel and MP-Nickel.
The method of IPC 4552A Nickel corrosion evaluation will
be described in brief. The basic principle is working with
Levels 1-3, from which acceptance, rejection or further
analysis (AABUS: As Agreed Between User and Supplier)
can be derived. That is displayed in Table 6.
While for HP-Nickel no incidents could be observed at 0 and
3 MTO, at MP-Nickel a corrosion event at the aged condition
is detected. That indicates support of the thesis above, that
HP-Nickel is more resistant to Nickel-corrosive attacks.
Table 6: Hyper corrosion Level based on Hyper-Corrosion
investigation according to IPC 4552A
Hyper
HyperCorrosion Corrosion
Disposition
Level
Investigation
Acceptable – This
≥60%
of
level
of
hyperinvestigated
1
corrosion
activity
locations show
will not degrade
Level 1
solder joint integrity.
AABUS – Will
require
extra
All
other
2
analysis and testing
observations
to
ensure
acceptability.
Rejectable – This
≥40%
of
level
of
hyperinvestigated
3
corrosion
will
locations show
degrade solder joint
Level 3
integrity.
The question arises to which extend Nickel corrosion would
be still acceptable for ENIG finishes. That question is
currently revised by the industrial specification IPC 4552 and
it will be therefore discussed in the following paragraph.
The mentioned locations define 5 specific positions at pads
and 7 positions at PTHs excluding the influence of Solder
Mask and Cu-etching quality. The investigation has to be
conducted via light microscope.
Nickel Corrosion rating according to IPC 4552
Pictures underneath show exemplary cases of Level 0 and
Level 3 corrosion.
Figure 21: Left : HP-Nickel at 0 MTO ; Right : HP-Nickel at
3 MTO, 50 nm Au
Figure 22: Left : MP-Nickel at 0 MTO ; Right : MP at 3
MTO, 50 nm Au
The first version of IPC 4552, released on October 2002, did
not contain any definition of corrosion and during almost 15
years this topic was not covered by the specification.
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
888
Table 7: Hyper Corrosion Level according to IPC 4552 A
Hyper
MP
HP
Corrosion Level
79%
100%
Level 1
Level 2
21%
0%
Level 3
0%
0%
Total
100%
100%
It can be seen, that overall corrosion rating of HP Nickel with
100% Level 1 rating outperforms a conventional MP Nickel
system with 79% Level 1 rating.
Figure 23: Example of Level 0 Corrosion and therefore
Acceptance according to IPC 4552A
As the IPC 4552 B is expected to become relevant soon, there
was further a corrosion evaluation according to IPC 4552 B
performed:
Level 0
Level 1
Level 2
Level 3
Figure 26: Product Rating / Corrosion Evaluation according
to IPC 4552 B : Left : MP-Nickel ; Right : HP-Nickel
Figure 24: Example of Level 3 Corrosion and therefore
Rejection according to IPC 4552A
Further details such as a full rating example can be found in
IPC specification itself.
It can now be understood, that the example of Figure 22
(right) displays an incident of Level 2 Corrosion event of MPNickel. To quantify and justify those events, in the following
a statistical IPC corrosion evaluation was derived.
The corrosion evaluation according to IPC 4552 A was used
to compare HP- and MP-Nickel:
Level 1
Table 8: Product Rating / Corrosion Evaluation according to
IPC 4552 B
Product Rating MP
HP
Level 0
Level 1
10%
90%
70%
10%
Level 2
20%
0%
Level 3
0%
0%
Total
100%
100%
From both tests it can be concluded that, quantitatively, HPNickel outperforms MP-Nickel in terms of Nickelhypercorrosion.
In the field, also MP-Nickel is proven to be a reliable finish
with acceptable corrosion. It may be concluded that for more
tight demands in terms of Nickel corrosion HP-Nickel could
serve as in that context new, reliable option.
Level 2
IMC Testing and investigation of the P-rich layer
Level 3
The appearance of the Inter-metallic Compound (IMC) was
proven in old studies to reflect the quality of the solder joint
and give indications about its adhesive ability [4].
Figure 25: Hyper Corrosion Level according to IPC 4552 A :
Left : MP-Nickel ; Right : HP-Nickel
SEM microscopy will be utilized to detect the character of the
IMC to see if there are differences between HP-Nickel and
MP-Nickel. Bathes were plated at 0 and 3 MTO for that test
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
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and ageing conditions range from ASR over 250 h, 500 h and
1000 h at 1000°C. The results are displayed in the following
figures.
Figure 30: SEM cross-section of HP-Nickel at 3 MTO
Figure 27: SEM cross-section of MP-Nickel at 0 MTO
Both Nickel Systems display a continuous IMC without any
defects. There is no clear difference observed between HPNickel and MP-Nickel.
To quantify those results in a statistical manner, IMC area
and thickness measurements of both Nickel systems are
performed. An example of measurement method can be seen
in Figure 31.
Figure 31: IMC area measurement method
Figure 28: SEM cross-section of HP-Nickel at 0 MTO
The quality of the IMC is determined by its mechanical bond.
Therefore, a higher area is correlated with a better quality of
the solder joint reliability. The area will be quantified at three
different solder balls at three different positions to achieve
significance. Beneath the bath ageing 0 and 3 MTO, also the
ASR and 1000h aged condition are compared for both Nickel
systems.
The quantified results are displayed in Figure 32and Figure
33.
Figure 29: SEM cross-section of MP-Nickel at 3 MTO
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
890
Before getting to HSS testing, another aspect of the IMC will
be examined. It is common knowledge that in Nickel based
IMCs a phosphorus enriched phase occurs – the Ni3P layer.
It is also known as “P-rich band”. It occurs as the nickel is
dissolving into the solder during heat exposure whereas the
phosphor stays in the IMC layer. This P-rich band is assumed
to be more brittle than the rest of the IMC. While MP-ENIG
exhibits P-contents of approx. 15% in the P-rich band, this
value is expected to be higher for HP-ENIG. To check this
hypothesis, via EDX a quantitative line scan of the samples
was performed (Figure 34).
Figure 32: IMC area and thickness measurements, HPNickel
Figure 34: Electron Image with line scan measurement of
HP-Nickel at 0 MTO
For measurement only the elements Ni, Sn and P were
checked. The quantitative results normalized in atom-% can
be read from Table 9.
Table 9: Line scan measurement of HP-Nickel at 0 MTO
Figure 33: IMC area and thickness measurements, MPNickel
From all conditions, there is no significant difference for HPNickel and MP-Nickel. This leads to the statement that both
Nickel Systems display a continuous IMC of comparable area
without any defects. The physical abilities of the solder joint
of both systems therefore will be further checked via High
Speed Shear Testing.
The Ni3P layer of HP-Nickel exhibits a P-content of 15,5%
on average. Therefore the P-rich band of HP-ENIG is
expected to be no higher in P-content than the one of MPENIG.
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
891
However, one difference in the P-rich band is assumed from
previous studies: HP-ENIG is showing a higher P-rich band
thickness than MP-ENIG. The question arises, whether this
would increase its brittleness of the solder joint. If that would
be the case, a higher brittleness could be reflected in lower
Total Energy (TE) values of HSS testing. Therefore HSStesting was conducted.
The shear strength of both Nickel Systems show comparable
results. However, when only comparing the aged condition,
HP-Nickel shows a better performance (Figure 37).
High Speed Shear Testing (HSS)
HP-ENIG and MP-ENIG were compared by total energies:
Figure 37: Shear Strength (g) after ball attach and 500h at
150°C aging, 50 nm Au
This significance is statistically proven with a confidence
(0,95Prob < t ) of 0,0250.
Like from wetting tests above, this test might give an
indication about a superior solder joint reliability
performance of HP-Nickel at aged conditions.
Figure 35: Analysis of Total Energy (mJ) after ball attach
and all aging conditions, 50 nm Au
The fracture modes are good for both Nickel Systems at all
aging conditions:
The Total Energy and therefore ductility of the solder joint is
slightly higher for HP-Nickel. Therefore it can be concluded
that the brittleness of the solder joint will not increase and
therefore the solder joint reliability of ENIG finishes would
not decrease by usage of a HP-Nickel.
Further, the shear strength of both Nickel systems was
derived from HSS testing (Figure 36).
Figure 38: Fracture Mode after ball attach and all aging
conditions, 50 nm Au
Figure 36: Shear Strength (g) after ball attach and all aging
conditions, 50 nm Au
To summarize the results of soldering and IMC appearance it
can be concluded that the thickness of the phosphorus
enriched layer plays smaller role for solder joint reliability
but the structure of the IMC plays a more significant role. As
both, HP- and MP-ENIG show comparable IMCs, their
solder joint reliability proven by HSS testing is comparable.
At aged conditions HP-ENIG seems to outperform MPENIG.
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
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Al-wire Bonding
Beneath soldering, also Al-wire bonding is an important
application for ENIG finishes. Although the gold is expected
to play the main role for bonding performance, also the
substrate influences bond adhesion. As HP-and MP-Nickel
have different crystal structures, a possible influence towards
bonding shall be examined.
weaknesses of the coating are detected through corrosion
spots. Possible reasons for defects lay i.e. in pores or an
unsuitable alloy composition.
Results of the Kesternich test can be seen in Figure 41 and of
NSS test in Figure 42.
Figure 39: Pull Strength (g) – all ageing conditions
As visible in Figure 39, the pull strength of mid-phosphorous
Nickel opposed to high-phosphorous Nickel is comparable.
But, when only checked the aged condition, HP Nickel
outperforms the MP Nickel (Figure 40).
Figure 41: Environmental Corrosion Test: Kesternich (50
nm Au)
Both Nickel layer systems exhibit similar results at the as
received and aged condition. From that observation could be
concluded, that the state of bath ageing does not influence the
environmental corrosion resistance. Further, both Nickel
layer systems exhibit comparable resistance in Kesternich
stress test. First, slight shadowing appears for HP Nickel at
48h stressing, while at 24h for MP-Nickel. However, the
significance of that observation would require further testing.
Figure 40: Pull Strength (g) after 1000h at 150°ageing
This results are of statistical significance with a confidence
(0,95Prob < t ) of < 0,0001.
Environmental Corrosion Tests
Part of the reliability of final finishes is the ability to
withstand high humidity containing sulfur dioxides or salt.
That applies even more for mobile applications as well as for
countries with a high humidity. Resistance agains such
influences is measured via Kesternich and NSS test. Both
tests exhibit good reproducibility. Via optical inspection
Figure 42: Environmental Corrosion Test: NSS (50 nm Au)
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
893
As seen in Kesternich test before, both Nickel layer systems
exhibit comparable resistance at their different bath ages 0 vs.
3 MTO.
Over all NSS test duration times, HP Nickel exhibits less
corrosion spots than MP-Nickel. That difference gets even
more significant at longer testing times. At 72 h exposition,
already more than 50% of the Pad surface of MP Nickel
plated pads is completely corroded. That stage cannot be
observed at HP-Nickel plated pads, even not at 144h of
environmental exposition. It is therefore concluded, that HPNickel provides better environmental protection than MPNickel.
This could be explained with its high phosphorous content
which results in an amorphous crystal structure for the nickel
diffusion barrier. The crystal structure for Aurotech HP
Nickel is amorphous. This eliminates the crystal boundaries
along where copper can diffuse into the gold.
V)
SUMMARY
ENIG will remain of major interest for the HDI and
multilayer market. This investigation aimed to compare the
two ENIG Nickel Systems with either high-phosphorous
(HP) or mid- phosphorous (MP) content.
Wetting performance, solder joint reliability and bond ability
are found to be equal for both Nickel Systems.
Major benefits of HP- compared to MP-Nickel detected are a
better overall corrosion resistance against both,
environmental corrosion and Nickel corrosion. This was
explained with differences in layer structure. The crystal
structure of HP-Nickel is amorphous. This eliminates the
crystal boundaries along where copper can diffuse into the
gold. These corrosion resistant properties are beneficial for
high humidity environments as well as corrosion resistance
during the immersion gold plating.
VI)
SOURCES
[1] The Printed Circuit Report, Prismark Partners Report,
2018.
[2] Prismark Printed Circuit Report Q1 2019
[3] IPC- 4552A specification -Performance Specification for
Electroless Nickel/Immersion Gold (ENIG) Plating for
Printed Boards.
[4] R. Nichols; S. Heinemann: The impact of deposition
thickness on HSS test results, SMTAi Rosemont, 2017
Proceedings of SMTA International, Sept. 22-26, 2019, Rosemont, IL, USA
894