Mold Compound Adhesion to Bare Copper Lead
Frames – Effect of Laser Texturing
Joseph Fauty, James Knapp, Jay Yoder
5005 East McDowell Road
Phoenix, Arizona 85008
Phone: 602-244-5022
Fax: 602-244-5714
e-mail: Joseph.Fauty@onsemi.com
Abstract
This paper investigates the effect substrate preparation has on epoxy mold compound (EMC)
adhesion to bare copper leads. There have been four basic strategies employed to characterize
and subsequently improve adhesion between the EMC and copper:
1. Choosing a particular copper alloy, i.e. varying alloy effects in copper to improve oxidation
control and thus adhesion.
2. Modifying the mold compound chemistry.
3. Modifying the surface chemistry/ topography of the copper by metal plating, organic inhibitor
priming, vacuum deposition, ion implantation, UV cleaning, chemical oxidation, and various forms
of mechanical roughening including sand or bead blasting.
4. Controlling the rate of oxidation and CuO/Cu2O ratio during product assembly.
The effects of mold compound chemistry, copper alloying, control of oxidation level and surface
topography modification through various means such as laser texturing and Electrical Discharge
Machining (EDM) are investigated. It will be shown that laser surface texturing offers one possible
low cost solution to achieving significant improvement in adhesion by changing the surface
topography of the copper substrate.
The International Journal of Microcircuits and Electronic Packaging, Volume 25, Number 1, First Quarter, 2002 (ISSN 1063-1674)
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Key words
2. Adhesion
Epoxy Mold Compound, Copper Lead
Frames, Laser Surface Texturing, Electrical
Discharge Machining
Ensuring good package reliability with copper
lead frames in adverse conditions necessitates
the prevention of EMC/copper delamination
and subsequent package cracking [1]. There
have been four basic strategies employed to
characterize and subsequently improve
adhesion between the EMC and copper:
1. Introduction
Within ON Semiconductor alloy 42 (Ni/Fe) lead
frames have traditionally been used with AuSn eutectic die attach for high power discrete
devices. There is currently underway an effort
to migrate to larger die in smaller packages.
To achieve the necessary thermal and
electrical responses copper is being explored
as an alternative lead frame material. There
are however problems inherent to copper
which mitigate its universal use in plastic
encapsulated packages. One issue is copper’s
large thermal expansion coefficient which
precludes the use of eutectic die attach with
larger die. Another issue is the adhesion
strength between the epoxy molding
compound (EMC) and copper. Still another
issue facing plastic encapsulated metal lead
frames in general is resistance to moistureinduced damage especially from catastrophic
mechanical failure during solder reflow (i.e.
popcorn phenomena).
This paper will investigate the effect substrate
preparation has on mold compound adhesion
to bare copper substrates. The effects of mold
compound chemistry, copper alloying, control
of oxidation level and surface topography
modification through various means such as
laser texturing and electrical discharge
machining are investigated. It will be shown
that a significant improvement in adhesion can
be achieved by changing the surface
topography of the copper substrate by
mechanical means. It will also be shown laser
ablation can be a clean low cost solution for
this process.
1. Choosing a particular copper alloy, i.e.
varying alloy effects in copper to improve
oxidation control and thus adhesion.
2. Modifying the mold compound chemistry.
3. Modifying the surface chemistry/
topography of the copper by metal plating,
organic inhibitor priming, vacuum deposition,
ion implantation, UV cleaning, chemical
oxidation, and various forms of mechanical
roughening including sand or bead blasting.
4. Controlling the rate of oxidation and CuO/
Cu2O ratio during product assembly.
2.1. Alloy Effects
Choi et al [1] performed tests to select the best
copper alloy and determine an empirical factor
they termed “adhesion index parameter” as a
gauge measurement tool. Their tests showed
adhesion strength was affected by alloy
composition, oxide layer thickness, and CuO/
Cu2O oxide ratio. The oxide ratio proved to be
the most important factor in their study. Copper
has a strong affinity for oxygen and will readily
form an oxide even at room temperature.
Within the temperatures of interest (25-3000C)
both cupric (CuO) and cuprous (Cu2O) oxide
will form. Three copper alloys were tested
using Cr-Zr-Zn, Ni-Si-Mg, and Ni-Si-Zn as
hardening agents. After 40 minutes of heating
in air at 2200C the alloy containing Cr-Zr-Zn
exhibited the best adhesion strength. The
authors matched the alloy type with oxide
thickness and CuO/Cu2O ratios as a function
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of time at temperature and came to the
conclusion that the Cr-Zr-Zn alloy controlled
oxide growth to a much better degree than the
other two alloys. Plotting adhesion strength as
a function of the CuO/Cu2O ratio showed that
regardless of alloy content when the ratio was
between 0.2 and 0.3 the maximum adhesion
strength was achieved. The Cr-Zr-Zn alloy by
some mechanism maintained a ratio of 0.20.3 for at least 40 minutes at temperature while
the other two alloys peaked after roughly 10
minutes. Ohsuga et al [2] applied a mold
compound to various copper alloys and
measured adhesion (expanded on in Section
III). Their results indicated adhesion strength
was achieved in the following order from
highest to lowest: pure copper, Cu-NiSi, CuFe, Cu-Cr, and Cu-Sn. They determined that
pure copper provided the best adhesion so
alloying by some mechanism actually lowered
adhesion strength.
In a recent paper Y. Tomioka and J. Miyake [3]
investigated copper alloy dependence on oxide
film adhesion. Tests indicated that maximum
adhesion of the oxide film to the metal existed
at a certain oxide thickness and the optimum
film thickness varied with the type of alloy
chemistry employed and the heating
temperature. SEM analysis of the peeled
surfaces indicated that those with high
adhesion were rough and contained small pits
while those with low adhesion were fairly
smooth. X-Ray diffraction of the high adhesion
samples found either no CuO or very low CuO/
Cu2O ratios. High ratios were found for those
alloys that exhibited low adhesion strengths.
The authors concluded that if CuO forms on
top of Cu2O that would cause internal stresses
to develop in the film due to the difference in
the lattice structure of the two oxides. The
authors also stated that the alloys with the
lowest adhesion strength often contained Sn
as an alloying element. The speculation was
that SnO might also be forming providing a
low adhesion film.
2.2. Modifying Mold Compound
Chemistry
Berriche et al [4] compared ortho-cresol
novolac (OCN) to dicyclopentadiene (DCP)
chemistries for their adhesion strength to a FeZn-P based copper alloy. Oxidation
temperatures were 175 and 2000C, with bake
times varying from 5 minutes to 118 hours.
Results showed that DCP – Cu adhesion was
virtually unaffected by oxidation at both 175
and 200 0C for exposure times up to 50
minutes. OCN adhesion started out lower and
exhibited a sharp decrease in adhesion as both
time and temperature increased. The authors
attributed the better performance of DCP to
its lower viscosity (less than half that of OCN),
which allowed better wetting, a lower thermal
expansion coefficient and a possible role of
adhesion promoters in the chemistry. While
their work was concerned with alloy 42 Asai et
al [5] studied the effect of adding modifiers to
the mold compound and using various phenol
resins as curing agents. The authors noted that
conventional epoxy molding compounds
containing cresol novolac epoxy resin with a
phenol novolac or cresol novolac curing agent
while providing good electrical and mechanical
properties suffered from poor adhesion. Their
premise was that EMC compounds from
difunctional epoxy resins with phenol resins
as curing agents, low modulus at soldering
temperatures, low water content at equilibrium
and high adhesion strength developed few
cracks at solder temperatures. Compounds
made from multifunctional epoxy resins with
high cross-link densities and high water
absorption gave poor results. Their suggestion
was to use a biphenyl epoxy resin because of
its low modulus and low moisture absorption.
Ohsuga et al [2] in addition to looking at alloy
types also investigated the properties required
in a mold compound to give good adhesion to
copper. They developed a high filler type
chemistry using bi-phenyl epoxy resins and
elastic hardeners for alloy 42 and tried it on
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copper. They determined that the properties
of mold compounds required for good
adhesion consisted of low water absorption,
low thermal expansion, low modulus and high
flexural strength at high temperatures. Tada
and Fujioka [6] looked at modifying the glass
transition temperature (Tg) of molding
compounds to improve adhesion. They noted
that a common practice to lower Tg by
decreasing the cross-linking density of the
compound lead to increased resistance to
package cracking but at the same time
decreased resistance to moisture loading.
Tada and Fujioka determined that a low crosslinking density was necessary for resistance
to cracking but a reasonably high Tg was
necessary to pass high temperature/ high
humidity testing. The authors introduced a
method to lower the crosslinking density
without affecting Tg by introducing rigid
structural elements by mixing a naphthalene
structure into the matrix resin. The results were
impressive. Sauber et al. [7] and later Saitoh
et al. [8] used linear fracture mechanics to
investigate mold compound properties and
package geometry effects on delamination and
cracking of EMC on both alloy 42 and copper
lead frames. It was found for copper lead
frames that an EMC with a small Young’s
modulus and a specific coefficient of thermal
expansion (for the lead frame geometries
studied – 12 ppm) was recommended for
preventing delamination between the bottom
surface of the die pad flag and the EMC. They
also discovered that within the range of die
flag thickness studied a thinner package was
less susceptible to delamination at the bottom
surface of the die flag independent of the size
of the chip. In contrast once delamination starts
on the top surface of the die flag, lower values
of CTE for the mold compound, thinner
packages, and larger chips will enhance rather
than alleviate delamination along the bottom
surface of the die flag.
2.3 Modifying Surface Chemistry/
Topography
Chemical adhesion between two dissimilar
materials is possible because of weak
intermolecular forces known as Van der Waals
forces. The two primary attractive forces
operating are London dispersion and
Hydrogen bonding forces. Both forces of
attraction are possible because of dipole induced dipole or dipole - dipole interactions.
Though hydrogen bonding is the strongest of
the intermolecular forces dispersion bonding
tends to dominate and it is much weaker than
an interatomic force such as ionic or covalent.
Therefore to improve adhesion to the EMC
either the substrate surface chemistry must be
modified or a mechanical component added.
Asai et al [5] looked at surface treatment for
its effect on adhesion strength. They exposed
alloy 42 lead frames to a vacuum deposition
and rf sputtering at 13.52 Mhz to modify the
surface. Vacuum deposition was used to
change the surface to Al or SiO and rf
sputtering to change the SiO into SiO2 or Si3N4.
Layer thicknesses were measured in the
100nm range. All surface treatments resulted
in near zero delamination in the as-cured state.
Lap shear tests at 2150C after moisture loading
showed significant improvement over control
samples. In two back-to-back papers Evans
and Packham [9 and 10] investigated the
cause of enhanced adhesion between a
polyethylene polymer and a purposelyoxidized copper substrate. Previous studies
showed that adhesion of polyethylene to metal
substrates fell into two categories; one
dependent upon polymer oxidation and the
other independent of polymer oxidation but
dependent on surface topography. Studies of
adhesion of polyethylene to other metals such
as iron/steel found good adhesion associated
with the metal’s ability to oxidize the polymer.
In a series of experiments the authors showed
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that in the case of polymer adhesion to copper
oxide good adhesion was a consequence of
the surface roughness of the oxide. Love and
Packman [11] later investigated changes in
surface chemistry on the peel strength of
copper/polymer interfaces. After chemically
cleaning Oxygen Free High Conductivity
(OFHC) copper foils they deliberately created
two different surface morphologies by chemical
etching and mechanical sanding. All foils were
then chemically oxidized. The chemically
etched samples were further broken down into
three groups, one “as is” after oxidation, one
exposed to Cr+ ion implantation and the other
immersed in a known copper complexing
solution of benzotriazole/ ethylene glycol. Peel
strength tests as a function of time at
temperature (150 0 C) showed surface
morphology making no difference at time zero
but diverging dramatically with time at
temperature. The authors attributed the
difference to a mechanical locking effect of the
etched units. In addition samples of the etched
morphology exposed to ion implantation
resulted in peel strengths less than that for the
“as is” samples while an improvement was
noted for the samples immersed in the primer
solution. Although not mentioned as a cause
the authors did note that ion implantation
changed the oxide from CuO to Cu 2 O.
Improved adhesion with the primer was
attributed to a chemical interaction between
CuO and the azole-type primer. S. Kim [12]
looked at controlled chemical oxidation of
copper. He noted that growing oxides
chemically produced structures more uniform
and “dendritic” then the globular morphology
of normally heat generated oxides. While
adhesion strength was higher than for normal
heat-treated samples it was still too low to
resist delamination from the copper.
J. Kim et al [13] performed a study on the
effects of chemically dimpling the surface of
copper lead frames with various metal plating
materials. Surface metal plating materials
studied included bare C-194 copper,
microetched copper for surface roughening,
Ag, Au, Ni, Pd and CuO. The authors showed
that dimples etched into the lead frame surface
increased interface adhesion between the lead
frame and epoxy molding compound. For
some of the plating schemes in particular bare
copper and microetched copper the adhesion
strength increased linearly with the number of
dimples. All dimples (dimple size being about
8.0 mils in diameter by 3.0 mils deep) were
etched into the copper. The authors attributed
the increase in adhesion to mechanically
interlocking effect. They stressed that the
shape of the dimple was critical for the
interlocking. Square or round form factors were
effective while pyramid-shaped was not. It was
also noted in the paper that others have had
positive experimental results with adding holes
to the lead frames in various locations.
2.4 Controlling Rate of Oxidation
Cho et al [14] investigated oxidation effects
on a Ni-Si-Mg-P copper alloy. The mold
compound they used was a cresol novolac
epoxy resin with 84 wt% SiO2 filler. After
cleaning lead frames were exposed to 150,
200 and 3000C in an air-circulating oven.
Evidence of CuO was apparent even as low
as 10 minutes at 1500C. When plotted against
oxide thickness or time at temperature the
average adhesion strength exhibited a
phenomenon observed by many other
investigators; namely an increase in adhesion
to a maximum point then a decrease as oxide
thickness increased. The authors showed that
the oxidation time to reach maximum adhesion
became shorter as the temperature increased
but plotting adhesion strength vs. oxide
thickness showed pull strength was always a
function of the thickness and not the heat
treatment temperature. The kinds of oxides
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formed and their layer structure were almost
identical in the temperature range 150-3000C.
Maximum adhesion strength was always
obtained at an oxide thickness of 21-25 nm.
The adhesion failure between the EMC and
lead frame with an oxide film was caused by a
weakness in the CuO morphology or the
instability of CuO due to its inherent brittleness
and density difference from Cu2O. Cu2O is
known to grow with a specific epitaxial
relationship with the copper lead frame surface
at initial stages of oxidation while random
growth is favored for CuO. Therefore as
oxidation continues the weak link becomes the
CuO/Cu2O interface. The authors postulated
microvoid formation at the CuO/Cu2O interface
as oxidation proceeded as the cause of the
decrease in adhesion strength with time at
temperature. Two explanations were given for
the initial increase in adhesion. One was that
oxidation of the copper gave rise to changes
in the surface chemistry and topography which
may be beneficial to adhesion. A rougher
looking surface was noted as oxidation time
increased in the early stages. The second
explanation was surface wettability – contact
angle measurements showed a decrease in
contact angle during the early stages of
oxidation. Takano et al [15] investigated oxide
film properties and the effect of the film on
reliability performance of a packaged part. The
authors noted that growth of the oxide film was
suppressed when the oxygen concentration
was less than 5%. They also noted in
agreement with Cho that adhesion strength to
oxidized copper drastically decreased when
the film thickness exceeded 20 nm (~200Å).
The authors developed two equations for oxide
growth that mimic both die attach and wire
bond. Both equations involved heating in air
so the difference in the two was related to the
heat transfer mechanism; natural convection
for die cure in an oven versus thermal
conduction for a substrate sitting on a wire
bond pedestal. The authors also stressed that
precise control of oxide thickness was required
in order to avoid problems with adhesion to
copper. Chong et al [16] postulated the
presence of voids at the oxide/metal interface
as the cause of poor adhesion and subsequent
delamination. The degree of voiding increased
as the degree of oxidation increased during
normal assembly of product. They also found
that post mold cure had a negative impact on
interfacial integrity of an oxidized surface. Their
paper focused on oxidation as a result of wire
bonding conditions. Oxide thickness and rate
was measured after various temperature and
heating time combinations in open air.
Oxidation of copper at higher wire bond
temperatures (2800C) showed very rapid initial
rate then a reduction as time duration
increased. An oxide thickness of 250 nm was
possible in less than 200 seconds at 2800C.
As the temperature was lowered through
2000C the oxidation rate decreased until at
2000C it was low enough to remain constant
out to 5 minutes. CuO/Cu 2 O ratios were
measured as a function of temperature. It was
found that the ratio increased (i.e. CuO grows)
as temperature increased. The Cu2O always
appeared first with CuO growing in intensity
as temperature increased. All units tested had
no observable delamination since those with
delamination were excluded from the tests. So
even with good wetting adhesion strength
suffered as temperature and time increased.
In fact lead frames heated at 2000 C for 400
seconds produced better adhesion results than
clean copper. J. Kim et al [13] deliberately grew
a thick black copper oxide plating (most likely
CuO) which they reported as being at least
twice as thick as the ideal thickness reported
by others. Their results indicated excellent
adhesion of this thick oxide in apparent
contradiction of previous reported results.
There are issues with all the above-described
methods for improving adhesion. Seeking out
specific copper alloys limits one in the choice
of metals to use plus results in added cost for
special formulations. Modifying EMC chemistry
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has the same limitations, namely narrow range
of choice and added cost for the addition of
special adhesion promoters. The third method
for improving adhesion is arguably the best
choice – i.e. modifying the lead frame surface
either chemically or mechanically. However,
almost all methods described add extra
processing and almost certainly involve added
cleaning steps before the lead frame is suitable
for use. The last method, controlling the
oxidation of the copper surface also has good
results but suffers from extreme difficulty in
maintaining sufficient control in the various
assembly processes that use heat. All the
methods described add costly processing and/
or do not lend themselves to easy
implementation on the manufacturing floor.
What is needed is a simple cost effective
process that can easily be implemented into
the assembly process. One purpose of this
paper is to describe a very simple cost effective
method of providing enhanced EMC/copper
adhesion strengths using basically any copper
alloy and low cost mold compound chemistry.
This method enlists the use of laser ablation
to cleanly texture the lead frame surface prior
to molding. Implementation of a laser ablation
system onto the assembly floor is as easy as
setting up a traditional laser-marking machine.
As shown below the ablation process is fast
enough for production volumes, needs no post
process cleaning operations and as long as
the standard precautions are taken for any
temperature processing steps the ablation
process can be employed at any point in the
assembly flow prior to mold though the closer
to molding it is the higher the adhesion strength
achieved.
3. Objective of Research Work
As mentioned earlier Au-Sn eutectic die attach
is a standard process for Alloy 42 lead frames.
With the switch to copper Au-Sn becomes
untenable when die sizes increase not much
above 0.030 inches. For larger die sizes epoxy
die attach becomes necessary. Nishimura et
al [17] compared alloy 42 with copper lead
frames and determined differences in package
cracking mechanisms existed under conditions
of temperature cycling. In alloy 42 cracking
usually occurred at the interface between the
EMC and the bottom of the die bond flag
(metal) due to the large thermal mismatch
between the two (5 ppm vs. 20 ppm). In the
case of copper cracking was found to occur
on the top of the die flag along the sides. Since
copper has a thermal expansion coefficient
close to the EMC (17 ppm vs. 20 ppm) the
mold compound is not the issue; rather the use
of compliant adhesives to relieve thermal
stress on the die becomes the issue. The
hypothesis was that the adhesive allowed the
die flag to slide under the silicon die (17 ppm
vs. 3 ppm) during temperature excursions
allowing large tensile stresses to build up at
the edge of the die flag. Due to relatively poor
adhesion strength of the EMC to copper
delamination occurs which leads to cracks.
The authors stressed the fact that if the
adhesion of the EMC to copper were of
sufficient strength the stress developed would
be so small cracking would not likely occur.
Takano et al [15] determined that copper lead
frame package cracking during solder reflow
was due poor adhesion of the mold compound
to the copper. In addition to this the advent of
lead free soldering means exposure to higher
temperatures (2600C) during surface mount.
All this adds up to increased demands on the
quality of the adhesive strength between the
mold compound and lead frame.
As mentioned previously an objective of this
investigation is to describe the laser texturing
process and through experimental results
show it to be a simple and robust method for
improving EMC adhesion to copper. By way
of contrast laser ablation will be compared to
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Electrical Discharge Machining (EDM), another
method investigated for mechanical
roughening. While EDM significantly improved
adhesion it suffered from the following:
2. Alloying controls the oxidation process in
some manner so that at a specific time at
temperature some copper alloys act better
than others.
1. The need to expose lead frames to either
water or oil during the process.
They measured adhesion strength as a
function of oxide thickness and showed as
others have that adhesion increased at first,
went through a maximum then decreased with
further oxidation. However in contrast to Cho
[14] and others they found that maximum
adhesion was reached at different thickness
for each alloy type. Good adhesion was
evidenced by separation of the mold
compound from the oxide while poor adhesion
was evidenced by separation of the oxide from
the copper.
2. A tendency to leave carbon deposits on
the lead frame surface.
3. Difficulty in implementing a large bulky and
expensive machine in a production
manufacturing flow.
4. A post machining cleaning step is required.
To confirm viability of laser texturing the ability
of packages to withstand moisture loading
(850C/85%RH for 168 hours followed by solder
reflow) was used as the reliability gate to
measure the success of this process.
4. Sample Preparation / Test Setup
4.1. Materials
Copper Alloys
A previous study carried out by Ohsuga et al
[2] determined mold compound adhesion to
copper varied with the type of alloy used. For
alloys receiving no heat treatment prior to mold
in order starting with the highest adhesion was
pure copper, Cu-NiSi, Cu-Fe, Cu-Cr, and then
Cu-Sn. When subjected to heat treatment
(2000C for 40 minutes) the order changed to
Cu-Fe, Cu-NiSi, Cu-Cr, pure Cu and then CuSn. Their hypothesis was that alloying affected
adhesion in two ways:
1. Pure copper provided the best adhesion
so alloying by some unknown mechanism
lowers adhesion strength.
Three different copper alloys were chosen for
this study in order to determine whether
alloying had a significant impact on adhesion
strength of a laser prepared surface. The
chemical makeup of each alloy along with
industry advertised mechanical properties are
given in Table 1.
C110 copper is essentially pure with a
maximum impurity level of 0.05 wt% oxygen.
Alloys C151 and 194 use a dispersion
strengthening mechanism to reach higher
strength levels. In dispersion strengthening
atoms of the alloying species form small
particles in the pure copper matrix. It is these
particles that directly impact the strength of the
alloy. In C151 a fine dispersion of CuZr 3
particles form within the copper matrix while
in C194 small particles of iron and
phosphorous form within the matrix. Because
of their strength electrical and thermal
conductivity both alloys find wide use in
integrated circuit lead frame applications.
Samples of each copper alloy pull tab (see
Section D for pull tab details) were submitted
for tensile testing without any mold compound
to derive a baseline for subsequent mechanical
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Table 1. Copper Lead Frame Alloy
Hardness
Tensile
Strength
(kgf/mm2)
Yield
Strength
(kgf/mm2)
Elong
(%)
C110
Electrolytic Tough
Pitch (ETP)
Pure copper with
.05 wt% O2 max.
Full hard
(89 RF)
30-37
32
9
C151
99.9 Cu, 0.05-0.15
Zr, Al/Mn/Fe 0.005
max.
Half hard
(37 RB)
30-36
29
16
C194
97.0 Cu min., 2.12.6 Fe, 0.05-0.20
Zn, 0.03 Pb,
0.015-0.150 P
Half hard
(59RB)
37-44
32
17
Designation
Chemical Make
Up (wt%)
Table 2. Measured Data
Designation
Stress at
Max
Load Cold
(kgf/mm
)
2
Max
Load at
Break Cold
(kgf)
Stress at
Max Load
- PMC
(kgf/mm )
Max
Load at
Break PMC
(kgf)
Stress at
Max Load
– 260C
Reflow
2
(kgf/mm )
Max Load
at Break
– 260C
Reflow
(kgf)
2
C110
26.7
+ 0.8
50.8
+ 0.2
26.3
+0.1
50.0
+0.2
22.4
+0.1
42.5
+ 0.3
C151
27.4
+ 0.1
52.0
+ 0.1
27.3
+0.2
51.8
+0.4
23.4
+0.4
44.4
+0.4
C194
40.2
+ 0.4
76.3
+ 0.7
40.2
+0.1
76.4
+0.1
38.1
+0.1
72.4
+0.1
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testing. Machine set up is discussed in another
section of this paper. Samples of each alloy
were tensile tested as is (cold) without any
processing, after a simulated post mold cure
of 175C for 3 hours and again after a simulated
solder reflow at 260C (maximum furnace
temperature set at 360C). The results are given
in Table 2. Comparing this data to that given
in Table 1 alloy C194 cold stress at maximum
load measured roughly in the middle of industry
advertised range while both C110 and C151
measured lower than the advertised range.
While post mold cure did not appear to affect
mechanical strength there was a noted drop
in value after exposure to solder reflow
temperatures.
4.2 Mold Compound Chemistry
A crucial part of this experiment was to prove
that no special mold compound chemistry is
needed to enhance adhesion when laser
texturing is used. With this in mind three
different mold chemistries were chosen to
represent standard inexpensive compounds in
widespread use in the industry versus a
relatively expensive compound specifically
formulated for high adhesion strength to
copper. A fourth compound was added later
on to access the true nature of the interface
strength between copper and mold compound.
Properties of the compounds chosen for this
study are given in Table 3.
Mold compound A represents a standard low
cost compound that is used on almost 80% of
the product manufactured in ON
Semiconductor. This compound has previously
been shown not to adhere too well to copper
after temperature excursions. The compound
designated as B represents ‘green’ chemistry
specifically formulated for adhesion to copper
and rated as MSL level 1; therefore fairly
expensive. Compound C represents a low cost
standard mold resin chemistry that has been
cited as having good adhesion to copper but
not rated as MSL 1. Compound D was chosen
because it develops a lower than usual bond
strength to copper and tends to degrade rapidly
once past the supplier’s recommended post
mold cure time.
5. Description of Laser System
A Lumonics Light Writer SPe Nd-YAG laser
with a 0.060 inch aperture was the principal
laser system used. It has an output power of
50 watts with a maximum 40 amp power
supply, a wavelength of 1064 nanometers and
a pulse frequency that is selectable from
continuous wave (CW) to 64 kHz.
6. Description of Electrical Discharge
Machining (EDM)
The EDM process depends upon an electrical
discharge between two electrodes to produce
a plasma field that basically erodes both
electrode surfaces via a strictly controlled
cavitation-erosion type mechanism. Erosion is
symmetrical and depends on many factors
including polarity, electrical discharge intensity
and duration, thermal conductivity, electrical
properties of the intervening dielectric material
etc. An EDM machine can use either a wire or
a solid metal piece called a “sinker” electrode.
Wire and sinker electrode material will vary
depending on the particular process, material
makeup of the work piece etc. Typical materials
used for both include brass, tungsten, gold,
beryllium, or copper to name a few. The wire
or sinker electrode forms one of the two
electrodes in the circuit. The work piece (the
part to be machined) forms the second
electrode. An electric field develops between
the machine electrode and work piece. This
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Table 3. Mold Compound Properties
Resin Type
A
Epoxide Cresol
Novolac
B
Biphenyl
C
D
DicycloPentadienyl /
Biphenyl
Epoxide Cresol
Novolac
Note 1: x10-6 (ppm)
Cure
Agent
Flexural
Modulus
[2]
Flexural
Strength
[3]
Water
Absorption
[4]
124
~0.50
Tg
(0C)
CTE
[1]
190
24
130
12
190
170
0.15
PhenolNovolac
150
8
270
170
0.12
PhenolNovolac
160
15.6
117
114
~0.30
Anhydride
Proprietary
155
Note 2: x 102 N/mm2 Note 3: N/mm2
field accelerates free positive ions and
electrons to high speeds forming an ionized
conductive channel that rapidly reaches very
high temperatures (8,000 to 12,0000C). The
high temperature causes localized melting of
material from both the work piece and
electrode and also causes gas bubbles to form.
When the current is turned off the sudden drop
in temperature causes the bubbles to implode
forcing previously molten material away from
the electrode surfaces. The molten material
resolidifies in the dielectric and is washed
away. Wire EDM usually uses water as the
dielectric while sinker electrode EDM uses oil.
The particular machine used for this analysis
was a Charmilles Technologies model with a
sinker electrode and oil dielectric. The sinker
material used was graphite.
Note 4: % in boiling water
7. Tensile Pull Sample / Instron
Machine Setup
Figure 1 shows a photo of a tensile strength
test sample once it is molded and separated
into individual units. The pull tab is specifically
designed so that only adhesion of the mold
compound to the tab is tested. No mechanical
interlocks are present to confound the results.
An Instron Model 5566 test system with a 10kN
load cell was used to measure the strength of
the copper-encapsulant interface. One end of
the package was held rigid while an external
force was applied the other end. All tests were
carried out at room temperature with a
crosshead speed of 4.0 mm/min. The machine
was set up to automatically record load as a
function of time. Maximum values of load at
failure were automatically recorded from the
load versus time curves.
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Figure 1. Photo showing a tensile strength test sample once it is molded and
separated into individual units. The encased end of the pull tab is
shown in Figure 5.
8. Level of Oxidation
A small experiment was conducted to baseline
the level of oxidation. No attempt was made
to directly measure the thickness of the oxide
film. Oxidation levels were scaled according
to the amount of time the test units were
exposed to temperature. Heating was carried
out on a hot plate in air to simulate wire
bonding. Test units were used ‘as is’ from the
supplier. No special effort was made to clean
the test strips prior to the experiment. As part
of the fabrication process all panels are
cleaned in a dilute muratic acid bath followed
by an alcohol rinse and forced air drying. All
panels are wrapped in corrosion resistant
paper and stored in a nitrogen dry box prior to
use. In this experiment all molding was
performed using mold compound ”C”. Bare
C194 type copper strips were heated on a
hotplate set at 200 and 2500C for times ranging
between 0 and 20 minutes then immediately
molded. As part of the mold process the strips
were exposed to 170-1750C for an additional
1-2 minutes prior to actual transfer of mold
compound. After mold the units were subjected
to post mold cure at 1750C for 3 hours in a
nitrogen atmosphere according to
manufacturers specifications. Even though
PMC will have an effect on oxidation it was
considered necessary since normal production
environment uses PMC. In addition it has been
proven necessary for novolac cured mold
compounds in order to raise cross-link density
and Tg to an acceptable value to withstand
further processing with the application of heat.
Figure 2 shows the results of tensile strength
as a function of oxidation at 2000C. Three
groups are shown in the graph. One group with
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Adhesion Strength (Kg)
80
Adhesion Strength as a Function of Oxidation
60
40
20
0
5
10
15
20
Oxidation Time (min)
No Laser Treat
Laser then Oxidize
Oxidize then Laser
Figure 2. Effects of oxidation level measured as exposure time at 2000C on
adhesion strength as a function of laser treatment using C194 copper and mold
compound C. As expected lead frames laser treated after oxidation showed the
better adhesion then those laser treated before oxidation. For comparison
purposes untreated copper controls are also shown. Data points represent the
mean value of at least five measurements.
no laser treatment was used as a control while
the other copper strips were exposed to laser
ablation under two sets of conditions. One
group was oxidized and then laser treated
while the other group was laser treated then
oxidized.
Various authors have established that mold
compound adhesion initially increases with
oxidation to a maximum and thereafter
decreases to a minimum value. There does
not seem to be a clear increase in strength as
a function of exposure time though there is a
small spike in the data with all three conditions
in the 1.5 to 2.0 minute range. Laser treating
after oxidation appears to be the best
alternative followed by laser treating before
oxidation then no laser treatment. No laser
treatment shows the most dramatic drop off in
adhesion strength. Laser treating after
exposure does show a drop off but then a fairly
flat response after 4-5 minutes exposure. The
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80
Pull Strength (Kg)
Adhesion Strength as a Function of Oxidation
(2000C vs. 2500C)
60
40
20
0
5
10
15
20
Oxidation Time (min)
250 °C
200 °C
Figure 3. Effects of oxidation level measured as exposure time at both 2000C and
2500C on adhesion strength as a function of oxidation temperature for untreated
C194 copper pull tabs molded with compound C. As expected there is a rapid
drop off in adhesion with exposure at 2500C. A maximum was detected at about 30
seconds exposure time. Data points represent the mean value of at least five
measurements.
drop off in adhesion strength may be due the
inherent weakness of the oxide in the nonlasered portions overtaking the added strength
given by the laser pits acting as micro mold
locks. The laser pits may also not be deep
enough but we were at the power limits of our
laser equipment. Figure 3 compares data for
untreated C194 at both 200 and 2500C. In the
case of exposure to 2500C an initial increase
in adhesion strength was noted at the 30second readout. The subsequent drop in
strength was much more dramatic than that
for 2000C.
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9. Experiment / Results
9.1. Surface Texturing
Initial Studies – EDM versus Laser
Texturing
Which method of surface preparation yields
the best results with respect to adhesion? In a
series of experiments various methods to
modify the topography of the copper surface
were explored. Among those tried were
chemical treatments, plasma treatments with
nitrogen, bead blasting, EDM, and laser
texturing. No significant results were recorded
for the chemical and plasma treatments so no
further data will be reported in this paper. Of
the remaining groups EDM and laser textured
units formed the experimental groups while a
set of untreated parts formed the control group.
In addition to the control units bead blasted
units were used as a measure of the best
process for mold adhesion. Mold compound
C (Dicyclo-Pentadienyl / Biphenyl) and C194
copper were used for all samples. Figure 4
shows the results of the first round of tensile
strength tests performed. Six different levels
of EDM process parameters were investigated
along with five levels of laser treatments. Also
included are the bead blasted and control
groups. A statistical analysis performed on the
data indicated all but EDM groups 4, 5, and 6
to have statistically higher tensile strengths
than the control group.
Figures 5, 6, and 7 show photos of the surfaces
of the EDM and laser treated groups. Not all
of the EDM test cells are shown. Figure 5A
shows the lightest while 5B shows the heaviest
machine parts. All other groups were in
progression from light to heavy texture.
An interesting observation was made for both
EDM and laser treated groups. The roughest
surfaces yielded the lowest tensile strengths
in each group. This would seem to indicate an
upper limit for surface modification. Failure
mode played an important part in the recorded
data. The bead blasted and laser treated
groups with the exception of laser group 2A
had very tight distributions compared to the
other test groups. This was because almost
all failures in the test groups were due to
copper breaks and not adhesion failures nor
plastic breaks. The standard deviations in
these cases represented the spread in copper
tensile strength. Data pooling was performed
on all test groups. EDM groups 1, 2, and 3
were not statistically different from each other
so were pooled into one group. The same was
done for laser treated groups 2B-2E. The best
process settings for each group were then
compared to each other along with the bead
blasted and control groups. The results are
shown in Figure 8. A Tukey-Kramer analysis
on the resultant groups indicated the bead
blasted and laser treated groups were
statistically the same while the EDM and
control groups formed their own sample
populations with statistically significant lower
tensile strengths. The means and standard
deviations for each test group are listed in
Table 4.
The experimental data seems to indicate a very
robust process using laser texturing. Based
on these analysis it was decided to concentrate
further effort in characterizing the laser
process.
9.2. Copper Alloy Effect
Pull tabs representing the three different
copper alloys were molded with the DicycloPentadienyl / Biphenyl material (compound C).
The pull tabs were organized into four groups.
Half were molded as received and half molded
after being exposed to 2000C for five minutes.
Those two groups were further divided into
units that were post mold cured at 175 0C for 3
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Adhesion Strength as a Function of Various Surface Treatments
80000
75000
70000
Tensile Strength
Table Strenght
65000
60000
55000
Control
group
average
value
50000
45000
40000
Bead Blasted
Control
EDM 1 EDM 2 EDM 3 EDM 4
EDM 5
EDM 6 Laser2A Laser2B Laser2C Laser2D Laser2E
Description
Description
Figure 4. Box plot of showing the results of three different methods for surface
texturing copper for improved mold compound adhesion. Compared against a
control group in which no texturing was performed are samples that have been
bead blasted, exposed to sinker etched Electrical Discharge Machining (EDM 1-6)
and those exposed to various levels of laser ablation (Laser 2A-2E). The bead
blasted units were used for comparison purposes since this process has been
shown to give superior results in the past. The tight grouping (small standard
deviations) for the bead blasted and laser treated groups was due to the fact that
the failure mode was copper breaks, therefore the only differences between the
groups is the scatter in copper tensile strength.
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Figure 5a
Figure 5b
Figure 5c
Figure 5. Photos showing surface texture effects for test samples exposed to
sinker etch EDM. Figure 5a is representative of group EDM 1, 5b of group EDM 6,
and 5c shows a typical unetched control unit. From Figure 4 it is seen that EDM
group 1 had better tensile strengths than EDM 6. Close ups of both etched parts
are shown in Figure 6.
Figure 6a
Figure 6b
Figure 6. High magnification pictures of surface texture for samples shown in
Figures 5a and b. Sample units with the surface topography of 6a had much
higher adhesion strengths than those with the surface texture of 6b.
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Laser Group 2A
Laser Group 2B
Laser Group 2D
Laser Group 2C
Laser Group 2E
Figure 7. Photos showing surface topography of test units from laser textured
groups Laser 2A through 2E respectively. As with the sinker-etched parts it
appears that the less textured units possessed the higher tensile strengths. Of
the five test cells only Laser group 2A showed lower tensile strengths. The other
four groups possessed high strengths with almost 100% copper breaks. All
photos are the same magnification. The basic difference in surface topography is
row and column spacing for the lasered pits.
Table 4. Comparison of Processes
Level
Number
Bead Blasted
16
24
EDM 123
GroupCTL
16
Laser2BCDE
20
Mean
73794.2
69010.3
59213.2
73420.7
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Std Dev
574.80
2828.04
7889.52
240.68
Results of Data Pooling of Adhesion Strength for Laser Treatment and EDM
80000
75000
Tensile Strength
Tensile Strength
70000
65000
60000
55000
50000
45000
40000
Bead Blasted
EDM 123
GroupCTL
Description
Laser2BCDE
All Pairs
Tukey-Kramer
0.05
Description
Figure 8. The best process settings for EDM and laser textured units were
compared to each other along with untreated control and bead blasted units. EDM
groups 1, 2, and 3 were not statistically different from each other so were pooled
into one group. The same was done for laser groups 2B-2E. A Tukey-Kramer
analysis on the resultant groups showed that the bead blasted and laser textured
groups were statistically the same while the EDM and Control groups formed
separate populations with statistically significant lower tensile strengths.
hours and those that received no PMC. Two
questions were to be answered in this
experiment – does the copper alloy itself affect
adhesion strength and do the alloys oxidize at
rates different enough to affect adhesion.
Figure 9 shows adhesion strength as a function
of copper alloy type in the as-molded state and
after post mold cure at 175 0C for 3 hours.
The as-molded adhesion strength data
appeared to be alloy dependent; C194 copper
clearly had the highest adhesion strength
however metal yielding limited the adhesion
strength of C110 and C151. All C110 samples
failed via metal break before the compound/
metal adhesion could be tested. Also
considerable metal strain was noted for the
C151 alloy units before final failure at the mold
compound/metal interface and so may have
confounded the results. After post mold cure
all samples failed at the mold compound/metal
interface. The data seems to indicate that if
any difference in strength between the alloys
existed as-molded it disappeared once the
samples were exposed to post mold cure. The
drop in strength for C194 is real and significant
however no conclusions can be made with
regards to C110 or C151 because of the metal
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Post Mold Cured Adhesion Strength
75
75
76
76
70
70
65
65
60
60
55
55
50
50
45
45
40
40
C110
C151
C194
Alloy Type
Adhesion Strength (Kgs)
80
80
Adhesion Strength (Kgs)
80
80
Adhesion Strength (Kgs)
Adhesion Strength (Kgs)
As-Molded Adhesion Strength
72
72
68
68
64
64
60
60
56
56
52
52
48
48
44
44
40
40
C110
C151
C194
Alloy Type
Alloy Type
Alloy Type
Figure 9. Adhesion strength as a function of copper alloy type as-molded and
after post mold cure at 1750C for 3 hours. As-molded adhesion strength appears
to be alloy dependent however in reality all C110 samples failed via metal break
before the compound/metal adhesion could be tested. Also considerable metal
strain was noted for the C151 alloy units before final failure at the mold
compound/metal interface and so may have confounded the results. The C194
samples experienced true adhesion failure before and after PMC so the loss in
adhesion is relevant. It is interesting to note post mold cured data indicates no
real difference between any of the alloys.
strain. For post mold cured samples the
inherent weakness in adhesion strength
apparently makes the alloy type basically
irrelevant. Figure 10 shows what happens
when test units are exposed to 2000C for 5
minutes on a hotplate in air. As-molded
strength does not seem to differ from those
groups not exposed to oxidation.
A Tukey-Kramer analysis performed on each
alloy type in the as-molded condition with and
without oxidation indicated no differences
within each pair. This was expected with C110
and C151 due to metal and not compound
adhesion failure but the result was also true
for C194, which experienced actual adhesion
failures. Post mold cure did however bring out
some real differences. Both the C110 and
C194 alloys showed significant drops in
adhesion strength when compared to the asmolded condition. Only the C151 alloy units
appeared to remained unchanged, however
there is no way of knowing whether metal strain
affected the adhesion strength in the asmolded condition and so how high the actual
strength may have been. In this experiment it
appears that C151 (Zr alloy) reacted the best
followed by C110 (pure Cu) and then C194
(Fe-P alloy).
The post mold cure data was analyzed to
determine the effects of oxidation without metal
yielding confounding the results. Analysis of
Variance indicated both main variables alloy
type and oxidation along with their interaction
was relevant. If post mold cured data with no
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50
45
40
35
30
25
20
15
10
C110
C151
C194
Alloy Type
Alloy Type
Adhesion Strength (Kgs)
65
60
55
Post Mold Cured Adhesion Strength
Oxidized 5 Minutes at 2000C
Adhesion Strength (Kgs)
Adhesion Strength (Kgs)
Adhesion Strength (Kgs)
As-Molded Adhesion Strength
Oxidized 5 Minutes at 2000C
65
60
55
50
45
40
35
30
25
20
15
10
C110
C151
C194
Alloy Type
Alloy Type
Figure 10. Box plots of adhesion strength as a function of copper alloy type in
both the as molded and post mold cured states. When exposed to 2000C for 5
minutes on a hotplate in air the as-molded strength does not seem to differ from
that not oxidized. A Tukey-Kramer analysis performed on each alloy type in the
as-molded condition with and without oxidation indicated no differences within
each pair. This was expected with C110 and C151 due to metal and not compound
adhesion failure but the result was also true for C194, which experienced actual
adhesion failures. Post mold cure did however bring out some differences. Both
C110 and C194 alloys showed significant drops in adhesion strength when
compared to the as-molded condition. Only the C151 alloy units remained
unchanged, however there is no way of knowing whether metal strain affected the
adhesion strength in the as-molded condition and so how high the actual
strength may have been.
oxidation is compared to that with oxidation
(compare Figure 9 to Figure 10) it is apparent
that alloy type controls the rate of oxidation
and therefore adhesion strength. Both C110
and C194 experienced a loss of adhesion
strength in the post mold cured state while
C151 changed very little. Since all failures were
adhesion related these results are real. This
seems to indicate that C151 is more robust
with respect to resistance to the effects of
oxidation.
9.3. Effect of Laser Treating
Does laser ablation mitigate the effects of PMC
and copper alloy chemistry? To answer this
question another experiment was performed.
Pull tabs representing the three copper alloys
were once again molded with the DicycloPentadienyl / Biphenyl material (compound C).
The pull tabs were organized into four groups.
Half were laser treated then molded while the
other half were molded as is to act as a control
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group and confirmation run for the previous
experiment. The groups were subdivided into
units that were post mold cured at 175 0C for 3
hours and those that were not. The as molded
and post mold cured data for the bare copper
tabs tracked the data from the first experiment.
An Analysis of Variance (ANOVA) of all data
points indicated all three main variables
(copper alloy, PMC and laser treatment) were
significant. Two interaction terms were also
cited as significant but the inherent softness
of both the C110 and C151 alloys were
affecting the results making the interaction
effects suspect. An important observation was
noted. For all three alloys the failure mode for
laser treated units with no PMC was 100%
metal fracture including C194 samples. Metal
fracture with C194 was a new occurrence.
Concentrating on the laser treated data only
yielded different results. Adhesion strength
became independent of PMC. Only the copper
alloy mattered. Accepting the proposition
inherent softness of C110 and C151 was
confounding the results an analysis of the
C194 data by itself was performed. Post mold
cure became relevant again. The data (Figure
11) showed laser treatment improved the
overall adhesion to C194, PMC tended to
degrade it and the interaction between the two
had no affect.
With laser treatment adhesion remained high
regardless of PMC while significant loss was
noted for units without laser treatment. There
may very well be a larger rate of decrease in
adhesion for the laser treated units since it we
do not really know how high the adhesion
strength could be with those units receiving
no PMC. The data seems to indicate laser
treatment will increase adhesion quality of
mold compound to copper. Failure mode for
all copper alloys laser treated and not exposed
to PMC was 100% metal fracture. This means
the adhesion strength was never tested since
the copper failed first.
In an effort to make adhesion independent of
copper yield strength and mold compound
fracture strength and so access the true nature
of the adhesion interface a fourth mold
compound chemistry (compound D) was used.
This compound (an epoxidized cresol novolac
with a phenol novolac curing agent) was known
from past experiments to have lower asmolded adhesion strength and to degrade very
rapidly when exposed to post mold cure for
more than 30 minutes. This particular
compound is designed to reach maximum
cross-linking with a PMC of 15 minutes at
1750C and therefore not meant to go much
beyond 20 minutes at temperature. C194
copper tabs both laser treated and as-is were
molded then exposed to PMC at 1750 in a
nitrogen atmosphere for up to three hours. In
this experiment all units failed via adhesion to
the copper. Figure 12 shows the results from
adhesion testing of the parts. The graph
depicts a least squares linear fit to the mean
of the results at each test level. A statistical
analysis of the data indicated both laser
treatment and PMC were significant variables
however the interaction effect was not. With
laser treatment the adhesion strength is higher
at every PMC level but post mold cure will
always degrade strength; in effect laser
treatment improved the overall adhesion at all
levels of PMC time. The rate of decrease
appears to be the same for both laser treated
and bare copper. Therefore laser treatment
does improve adhesion quality. It also appears
laser treatment helps mitigate the effects of
PMC but will not make adhesion strength
independent of it.
10. Mold Compound Chemistry
Due to the addition of adhesion promoters and
properties of specific resin chemistries some
mold compounds adhere to copper better than
others [4-6]. An experiment was designed to
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Adhesion Strength as a Function of Laser Treatment and Post Mold Cure
65
55
45
No
Yes
Laser Treat
Laser Treat
Post Mold Cure Data Pooled
75
65
55
45
No
Yes
Post
MoldCure
Cure
Post
Mold
Laser Treated Datapooled
LS Means Plot PMC- Laser
Adhesion StrengthLS Means
Adhesion
Strength LS Means
75
LS Means Plot PMC
Adhesion
LS
Means
AdhesionStrength
StrengthLS
Means
Adhesion
LS
Means
AdhesionStrength
StrengthLS
Means
LS Means Plot Laser Treatment
Yes
Laser
75
65
No
55
Laser
45
No
Yes
PostMold
Mold Cure
Post
Cure
Laser Treatment vs. PMC
Figure 11. Least Squares Means plots showing the effects of laser treatment and
PMC on the adhesion of mold compound C to C194 copper alloy tabs. Laser
treatment improves the overall adhesion of C194. PMC tends to degrade adhesion
of C194. There is no interaction between PMC and laser treat. With laser treatment
adhesion remains high regardless of PMC while significant loss was noted for
units with no laser treatment. There may still be a larger slope (rate of decrease)
for the test cell laser/no PMC since it is not known how high the adhesion may be
for the test group laser treated with no PMC (100% metal fracture).
determine if laser ablation could reduce the
dependency on mold compound specific
chemistries. Three compounds were chosen
to represent resin chemistries containing
epoxide cresol novolac, biphenyl, and dicyclopentadienyl biphenyl (compounds A-C
respectively). According to reference [5] the
biphenyl resin based compound should
achieve the best results. Copper C194 pull tabs
were organized into 12 groups. Sample tabs
both laser treated and untreated were molded
with each compound. These six groups were
further divided into those which received three
hours of post mold cure at 1750C in nitrogen
and those that did not. Figure 13 shows the
results for all three compounds. Adhesion
quality for the biphenyl resin (compound B)
was virtually independent of both laser
treatment and PMC. Compound B represents
new resin chemistry specially formulated for
adhesion to copper and shows excellent
results with or without PMC – a rarity for mold
compounds in general. Since the purpose of
the experiment was to show compounds with
less desirable adhesion quality but also less
expensive could work just as well, compound
B results were excluded and the data analyzed
once more. All three main effects (laser
treatment, compound chemistry and PMC)
became significant, however no interaction
effects were. As shown in Figure 13 the
dicyclo-pentadienyl biphenyl compound
tended to have higher adhesion strengths than
the epoxide cresol novolac compound at all
test levels while PMC tended to degrade
adhesion. At all test levels laser treated
samples has consistently higher adhesion
strengths than untreated units. The superior
quality of the laser treated units was borne out
by an analysis of the failure modes. Thirty
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Adhesion Strength (kg)
Adhesion Strength as a Function of Post Mold
Cure Time
60
40
20
0
1
min(X) - 0.5
2
3
X
max(X) - 0.5
PMC Time (hours)
Bare Metal Data
Least-squares fit
Laser Data
Least-squares fit
Figure 12. Adhesion strength to C194 copper of an epoxidized cresol novolac
compound with a phenol novolac curing agent (compound D) as a function of
post mold cure at 1750C. The graph shows a least squares linear fit to the mean of
the data at each test level. As can be laser treatment improved the overall
adhesion at all levels of PMC time. The rate of decrease appears to be the same
for both laser treated and bare copper. In this experiment all units failed via
adhesion to the copper.
percent of the tested samples failed by metal
fracture when laser treated compared to 7%
when left bare. Forty-one percent of the bare
units failed for adhesion to the copper
compared to 8% of the laser treated parts.
Laser treatment clearly enhances the quality
of adhesion to copper for compounds not
specifically design to do so.
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Adhesion Strength as a Function of Mold Compound Chemistry and
Laser Treatment
Compound B
73
66
59
52
45
-1
0
1
2
3
PMC (Hours)
80
73
66
59
52
45
-1
Compound C
Adhesion Strength (Kg)
80
Adhesion Strength (Kg)
Adhesion Strength (Kg)
Compound A
0
1
2
3
PMC (Hours)
80
73
66
59
52
45
-1
0
1
2
3
PMC (Hours)
Compound A (Laser)
Compound B (Laser)
Compound C (Laser)
Compound A (no Laser)
Compound B (no Laser)
Compound C (no Laser)
Figure 13. Adhesion strength of compounds A, B, and C versus PMC with and
without laser treatment. Compound B is the biphenyl resin compound that was
specifically formulated for adhesion to copper and has been shown in other work
to form a better quality bond than either a cresol novolac or a dicylco-pentadienyl
based resin system. An important point to note here is that with laser treatment
all three compounds have approximately the same as-molded adhesion strength
and with post mold cure the loss in the cresol novolac and dicylco-pentadienyl
resin based systems is less than without laser treatment.
11. Moisture Loading
In order to confirm the viability of laser texturing
a group of C194 copper pull tabs both laser
treated and as-is were molded with
compounds C and D. C194 copper was
chosen so that the copper alloy itself would
not confound the experiment after exposure
to temperature. Compound D was specifically
chosen because of its poor adhesion
performance compared to the other compound
formulations. The purpose of the experiment
was to determine if laser treatment could
improve the adhesion quality to the point that
any standard low cost mold compound would
become a viable candidate for adhesion to
copper. After molding and the appropriate post
mold cure the four groups were exposed to
moisture loading at 850C / 85% RH for 0, 48
and 168 hours. They were then submitted for
simulated solder reflow in a forming gas
purged furnace. The units were reflowed three
times at a peak temperature of 2300 C then
submitted for tensile testing. The resultant data
is shown in Figure 14. Mean values of
adhesion strength are plotted as a function of
moisture loading for each compound
formulation. Statistical analysis of the data
indicated that both main variables laser
treatment and moisture loading were
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Adhesion Strength as a Function of Moisture
Loading and Laser Treatment
Compound C
Compound D
75
Adhesion Strength (Kg)
Adhesion Strength (Kg)
75
62
49
36
23
10
-10
28
66
104
Moisture Time (hr)
142
180
62
49
36
23
10
-10
28
66
104
Moisture Time (hr)
142
180
Laser treated
Bare
Laser treated
Bare
Figure 14. Mean values of adhesion strength as a function of moisture loading for
mold compounds C and D. Samples of both mold compounds were exposed to
85% RH / 850C for 0, 48, and 168 hours then reflowed in an inert gas atmosphere
furnace at a peak temperature of 2300C three times. Laser treatment created a
significant improvement in both compounds to the extent that laser treated units
exposed to 168 hours of moisture then reflowed had higher adhesion levels than
those not exposed to moisture (but still reflowed).
significant while the interaction effect was
mixed. Compound C had no interaction effect
while compound D showed a strong effect. This
may indicate a reliance on mold compound
chemistry but more compound formulations
would have to be tested to prove this. Some
important points can be noted from the plots
in Figure 14. Laser treatment represents a
definite improvement in adhesion quality. Up
to the limit of the test at 168 hours laser treated
parts outperformed un-treated units even those
exposed to no moisture. For compound D laser
treatment almost made adhesion independent
of moisture loading and reflow within the tested
range.
12. Alloy Recommendation
Because of the inherent softness of C110 the
choice is really between C151 and C194. While
temperature induced softening of C151 limited
its use for some experiments in this
investigation an opinion can still be made. Alloy
C194 consistently shows the highest adhesion
strength in the as-molded state but usually
experiences a large drop after post mold cure.
In contrast the adhesion strength for C151
changed very little as a function of both PMC
and exposure to oxidation (2000 for 5 minutes).
The post mold cure adhesion strength for C151
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seems to be every bit as good as that for C194.
Taking the effects of oxidation into account
C151 appears to be more robust than C194 in
the post mold cured state. While adhesion
strengths for both C151 and C194 are roughly
the same absolute value the C194 losses a
large portion of its strength after post mold
cure. This loss of strength may be reflected in
thermal mismatches that could cause other
issues such as delamination and cracking. If
the data for C151 is accurate the delamination
and cracking issues may be reduced. The
limited softening of the alloy during post mold
cure may actually be a benefit for stress relief
between the metal and mold compound.
Level 1 are for delamination of the mold
compound from the die flag or leads. Laser
treatment can cure this and allow the use of
less expensive compound formulations but
performance after PMC and moisture testing
must be accessed.
While adhesion properties of C151 are
superior to those of C194, softening of the alloy
may cause issues for wire bonding after high
temperature reflow for thinner substrates (<
0.010 inches) or substrate designs with long
slender bond fingers. Lower limits for tensile
strength or hardness may have to be specified
so the alloy does not soften appreciably during
solder die attach or PMC.
Laser texturing improved adhesion quality of
all compound chemistries tested but some
chemistries such as the novolac based
compounds may be too weak to withstand
some quality requirements even with the aid
of laser ablation. Whenever possible biphenyl
or dicyclo-pentadiene resin chemistry should
be chosen over a novolac based one for better
adhesion to copper. With the aid of laser
texturing a specially formulated therefore high
cost compound may not be necessary for
some applications.
13. Does Laser Treatment Mitigate the
Use of Specially Formulated
Expensive Compounds?
Laser treatment definitely improves the quality
of adhesion to copper but does it do so to the
extent that any low cost formulation can be
used? The basic answer to this question would
be qualified yes but is clearly dependent on
the application and quality level one is trying
to achieve. There are a lot of factors that must
be considered when choosing an appropriate
compound. Copper alloy, lead frame design
(single sided QFN’s versus double sided
PDIPs) which governs the amount of copper
available for laser texturing, the quality level
one is trying to achieve and compound cost
are a few. Most quality test failures for MSL
14. Conclusions
Laser texturing has been proven to be a very
robust process yielding significantly higher
adhesion strengths for all epoxy mold
compounds tested.
References cited in this paper established
mold compound chemistry affected adhesion
to copper in the following order from highest
to lowest – biphenyl, dicyclo-pentadiene and
cresol-novolac. Our studies confirmed this.
Correlating as-molded and PMC adhesion
strength to mechanical properties of the
various mold compounds seems to suggest
the combination of low modulus, water
absorption and CTE gives the best results (i.e.
biphenyl compound). The two cresol-novolacs
faired the worst.
Laser treatment helps mitigate the effects of
PMC but will not make adhesion independent
of it.
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Laser texturing improved the performance of
all three copper alloys however the inherent
softness of C110 limits it use for electronic
applications. While both C151 and C194 will
work C151 seems to offer a better solution than
C194. C194 appears to interact adversely with
oxidation. Of the three alloys only C151
retained its adhesion quality after exposure to
2000 C for 5 minutes in air.
Lower limits for tensile strength or hardness
for C151 may have to be specified so the alloy
does not soften appreciably during solder die
attach or PMC.
Laser texturing can be performed at the start
of assembly or just before mold, however the
optimal place to insert is just prior to mold to
offset any uncontrolled oxidation effects.
References
[1] K-S Choi, T-G Kang, et al, “Copper Lead
Frame: An Ultimate Solution to the Reliability
of BLP Package”, IEEE Transactions on
Electronics Packaging Manufacturing, Vol. 23
No. 1, Jan. 2000, p. 32-38.
[2] H. Ohsuga, H. Suzuki, et al, “Development
of Molding Compounds Suited for Copper Lead
frames”” IEEE 44th Electronic Components and
Technology Conference, 1994, p.141-146.
[3] Y. Tomioka and J. Miyake, “Oxide Adhesion
Characteristic of Lead Frame Copper Alloys”,
49th Electronic Components and Technology
Conference, 1999, p.714-720.
[4] R. Berriche, S. Vahey, et al, “Effect of
Oxidation on Mold Compound – Copper Lead
frame Adhesion”, 1999 International
Symposium on Advanced Packaging
Materials, p.77-82.
[5] S. Asai, T. Ando, et al, “Adhesion Between
Ni/Fe Lead Frame and Epoxy Molding
Compounds in IC Packages”, Journal of
Adhesion Science, Technology, Vol. 10, No.
1, 1996, p.1-15.
[6] K. Tada, et al, “Properties of Molding
Compounds to Improve Package Reliability of
SMD’s”, IEEE Transactions on Components
and Packaging Technology, Vol. 22, No. 4, Dec
1999, p. 534-540.
[7] J. Sauber et al., “Fracture Properties of
Molding Compound Materials for IC Plastic
Packaging”, IEEE Transactions on
Components, Packaging and Manufacturing
Technology – Part A, Vol. 17, No. 4, December
1994, p. 533-541.
[8] T. Saitoh et al., “Linear Fracture Mechanics
Analysis on Growth of Interfacial Delamination
in LSI Plastic Packages under Temperature
Cyclic Loading – Part II: Material Properties
and Package Geometry Factors”, IEEE
Transactions on Advanced Packaging, Vol. 23,
No. 3, August 2000, P. 554-560.
[9] J. Evans and D. Packham, “Adhesion of
Polyethylene to Copper: Reactions between
Copper Oxides and the Polymer”, Journal of
Adhesion, Vol. 9, 1978, p. 267-277.
[10] J. Evans and D. Packham, “Adhesion of
Polyethylene to Copper: Importance of
Substrate Topography”, Journal of Adhesion,
Vol. 10, 1979, p. 39-47.
[11] B. Love and P. Packman, “Effects of
Surface Modifications on the Peel Strength of
Copper Based Polymer/Metal Interfaces with
Characteristic Morphologies”, Journal of
Adhesion, Vol. 40, 1993, p.139-150.
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[11] S. Kim, “The Role of Plastic Package
Adhesion in Performance”, IEEE Transactions
on Components, Hybrids, and Manufacturing
Technology, Vol. 14, No. 4, Dec. 1991, p. 809817.
[12] J-K Kim, et al., “Interface Adhesion
Between Copper Lead Frame and Epoxy
Molding Compound: Effects of Surface Finish,
Oxidation and Dimples”, 50 th Electronic
Components and Technology Conference,
2000, p.601-608.
[13] S-J Cho, K-W Paik, et al, “The Effect of
the Oxidation of Cu – Based Lead frame on
the interface Adhesion Between Cu Metal and
Epoxy Molding Compound”, IEEE
Transactions on Components, Packaging and
Manufacturing Technology – Part B. Vol. 21,
No. 2, May 1997, p. 167-175.
[14] E. Takano, et al, “The Oxidation Control
of Copper Lead frame Package for Prevention
of Popcorn Cracking”, IEEE 47 th Electronic
Components and Technology Conference,
1997, p.78-83.
[15] C.T. Chong, et al, “Investigation on the
Effect of Copper Lead frame Oxidation on
Package Delamination”, IEEE 45th Electronic
Components and Technology Conference,
1995, p.463-469.
[16] A. Nishimura, et al, “Effect of Lead Frame
Material on Plastic-Encapsulated IC Package
Cracking Under Temperature Cycling”, IEEE
Transactions on Components, Hybrids, and
Manufacturing Technology, Vol. 12, No. 4, Dec.
1989, p. 639-645.
Biographies
Joseph Fauty received his M.S. degree in
Materials Science from the State University of
New York at Stony Brook in 1975. He is a
Senior Principal Staff Engineer with ON
Semiconductor working in the Package
Technology Development Lab. Mr. Fauty has
25 years experience in hybrid and MCM
process technology. He is a member of IMAPS
and IEEE.
Jay Yoder is a Manufacturing Process
technician with ON Semiconductor’s Core
Technologies Packaging Lab. He has received
2600 hours of advanced Electronics training
while serving in the US Navy and is currently
pursuing a degree in Electro-Mechanical
Automation. Mr. Yoder has approximately 11
years of experience in the Semiconductor
Manufacturing Industry.
His current
responsibilities
include
process
characterization and optimization for new
product development, tooling design and
equipment maintenance for related back /frontend manufacturing.
James Knapp is the Package Technology
Development Lab manager. Mr. Knapp is an
industry recognized expert in plastic
encapsulation with over 20 patents in the field.
Mr. Knapp’s primary focus for the last year has
been power QFN packaging and package
within a package concepts.
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