Electromigration Performance of WLCSP Solder Joints

ELECTROMIGRATION PERFORMANCE OF WLCSP SOLDER JOINTS
Robert Darveaux
Jimmy-Dinh V Hoang
Bhuvaneshwaran Vijayakumar
Skyworks Solutions, Inc.
5221 California Ave.
M/S 21-1
Irvine, CA 92617
robert.darveaux@skyworksinc.com
ABSTRACT
Wafer Level Chip Scale Package (WLCSP) assemblies were
tested under high temperature and high current conditions.
Electromigration damage was observed along with
accelerated diffusion and intermetallic compound growth at
the solder / Under Bump Metallization (UBM) interface.
Final electrical failure typically occurred due to a void
created in the redistribution line (RDL) near the UBM. The
failure rate increased with higher temperature, higher
current density, and reduced RDL trace width. Both Ni
UBM pads and Cu pillar structures had superior
performance over Cu UBM pads. A failure model based on
Black’s equation was developed from the experimental data
and other published data. The model was then used to
develop recommended guidelines for accelerated testing and
qualification testing based on representative field use
conditions.
Key words: WLCSP, electromigration.
INTRODUCTION
WLCSP has become a popular package for devices such as
RF buck converters, camera flash drivers, backlight drivers,
and analog switches used in portable product applications
due to its small form factor. These devices require current
of up to 2A or more to be delivered through BGA solder
joints. One potential limiter to the maximum current rating
for a given device is field failure due to electromigration.
Electromigration failure in flip chip and WLCSP solder
joints occurs due to high current density driven diffusion
and intermetallic compound reactions that are accelerated at
high temperatures [1-34]. These effects can create voids
that open up and grow over time.
As the void size
increases, there is an increase in electrical resistance through
the joint, and eventually an open circuit occurs.
In most electromigration studies, design or material
variables are compared using a test matrix of current
densities and temperatures. The tests are often run until at
least half of the units in a given leg have failed, so that the
data can be fit to a log-normal or Weibull distribution. A
typical goal is to determine constants for a failure prediction
model, such as Black’s equation [27].
In the present study, electromigration behavior of lead free
WLCSP solder joints was studied with UBM diameters up
to 300um, test currents up to 3.5A, and solder temperatures
up to 194C. The sample size was 3 to 15 units per leg, and
the test duration never exceeded 168hrs. Given this method
and sample size, it was not always possible to get good
Weibull statistics on every leg of the experimental matrix.
However, several head-to-head evaluations were conducted,
so the impact of multiple material and design variables
could be quantified.
EXPERIMENTAL PROCEDURE
Three separate daisy chained test vehicles were used to
study electromigration performance of lead free WLCSP
solder joints. The bump lay out and daisy chain pattern for
Test Vehicle (TV) #1 is shown in Figure 1a. For most of
the experimental legs, eight bumps were used in the daisy
chain. Only two bumps were used in one of the legs. TV#1
had three different UBM pad metal stack ups as indicated in
Figures 1b, 1c, and 1d. The sputtered Ti/Cu layer between
the UBM pad and RDL is not shown.
The bump pattern for TV#2 is shown in Figure 2a. Only a
two bump chain was used in the testing. TV#2 had two
different UBM pad stack ups. A 4-mask process stack up
as shown in Figure 2b, and a 3-mask process stack up as
shown in Figure 2c. The sputtered Ti/Cu layer between the
UBM pad and RDL is not shown in Figure 2b.
The bump pattern for TV#3 is shown in Figure 3a. Only a
two bump chain was used in the testing. The UBM pad
stack up for TV#3 is shown in Figure 3b. This TV had both
a single metal layer RDL configuration (as in Figure 3b),
and a double metal layer RDL configuration.
In all cases, the motherboard had copper defined pads that
were finished with organic surface protectant (OSP). The
WLCSPs were surface mounted using a conventional reflow
profile and a flux dip process.
Electromigration testing was conducted under constant
current and constant oven ambient temperature conditions.
In all cases, the test duration was 168hrs.
a. Bump layout.
2.5 Ni
7.5 Cu
2.5
5
5
b. Stack up 1. Dimensions in (um).
a. Bump layout.
2
7.5
Ni
4
7.5
10 Cu
b. Stack up 1. Dimensions in (um).
2.5
5
5
c. Stack up 2. Dimensions in (um).
7.5
9 Cu
7.5
c. Stack up 2. Dimensions in (um).
Figure 2. Test vehicle #2. Two bump chain used.
50 Cu
2.5
5
5
d. Stack up 3. Dimensions in (um).
Figure 1. Test vehicle #1. Eight bump or two bump chains
used.
Shown in Figure 4 is the original printed circuit board
(PCB) configuration used in Legs 1 to 18. Nine units were
surface mounted to the PCB, and thermocouples were
soldered near each component to monitor the local
temperature. It was found that joule heating in the PCB
caused non-uniform test temperatures for the WLCSPs.
Hence the test data was divided into three groups as
indicated in Figure 5. Therefore, the effective sample size
was three units for legs 1a through 11c.
To improve the temperature uniformity, individual PCBs
were applied for each WLCSP as shown in Figure 6. The
sample size was increased to fifteen units per leg for data
sets 13 to 83. Thermocouples were soldered to each test
board to monitor the local temperature. A summary of the
experimental variables is shown in Table 1.
a. Bump layout.
0.70 Cu
0.65 Ni
8
6
Figure 6. Fifteen individual evaluation boards in oven for
data sets 13 through 83.
9.5
b. Stack up.
Figure 3. Test vehicle #3. Two bump chain used.
Figure 4. Nine-unit evaluation board inside oven for data
sets 1 through 11.
c1
c2
c3
c4
c5
c6
c7
c8
c9
group1
group2
group3
Figure 5. Three groups of units based on temperature
distribution from joule heating in evaluation board.
UBM Pad Stack Up (um)
0.65Ni / 0.70Cu, 2Ni,
7.5Cu/2.5Ni, 10Cu, 50Cu
UBM Diameter (um)
160 to 300
PCB Pad Diameter (um)
220 to 250
Solder Ball Diameter (um) 167 to 300
RDL Width (um)
90 to 340
Solder Alloy
SAC305, SAC405
Joints in Daisy Chain
2 to 8
Current (A)
1.8 to 3.5
Oven Temperature [C]
85 to 154
PCB Temperature [C]
103 to 164
Solder Temperature [C]
129 to 194
Test Duration (hrs)
168
Table 1. Range of Experimental Variables
ESTIMATION OF SOLDER JOINT TEMPERATURE
Finite element analysis was used to estimate the solder
temperature during electromigration testing for each leg in
the experiment. The power dissipation from joule heating in
the daisy chain traces was calculated from the measured
resistance and the applied current. The power was allocated
over the different portions of the daisy chain depending on
the PCB and RDL trace cross section. The oven ambient
temperature was a boundary condition in the model, and the
temperature distribution was solved using ANSYSTM. The
convective heat transfer coefficients were adjusted until the
calculated PCB temperature matched the measured value.
Example calculated temperature distributions are shown in
Figures 7, 8, and 9 for TV#1, TV#2, and TV#3,
respectively. It is seen that the 8-bump daisy chain case in
Figure 7 results in much more uniform die temperature
compared to the 2-bump daisy chain cases in Figures 8 and
9. Also, the temperature rise due to joule heating is much
greater in Figures 7 and 9 compared to Figure 8 due to the
higher DC resistance of the daisy chains.
DC resistance at 125C = 65mOhms
Power dissipation at 2.5A = 406.3mWatts
Oven 125C
Board temp = 137C (Solder ball temp = 158C)
Figure 7. Example calculated temperature distribution for
TV #1, Leg 2a, 8-bump daisy chain.
DC resistance at 154C = 19.5mOhms
Power dissipation at 2.5A = 122mWatts
Oven 154C
Board temp = 156C (Solder ball temp = 160C)
Figure 8. Example calculated temperature distribution for
TV#2, Leg 73a, 2-bump daisy chain.
DC resistance at 125C = 72mOhms
Power dissipation at 2.2A = 348mWatts
EXPERIMENTAL RESULTS
A summary of all test results is shown in Table 2. There
were a total of thirty three data sets with a sample size of
three units, and sixteen data sets with a sample size of
fifteen units. The key design attributes are shown in Table
2, as well as the test conditions and results after 168hrs of
testing.
Failure Mechanism
The failure mechanism was similar for all test vehicles and
experimental conditions evaluated in the present study.
Electromigration damage was observed due to accelerated
diffusion and intermetallic compound growth at the solder /
UBM interface. Final electrical failure typically occurred
due to a void created in the RDL near the UBM. A
representative cross section of a failed unit is shown in
Figure 10. A similar failure mode in WLCSP solder joints
was observed in Refs [22,28,34]
A representative resistance log for a nine sample population
is shown in Figure 11. There is a steady increase in
resistance for all units on test due to conversion of the
copper and solder into intermetallic compounds. However,
a rapid increase in resistance is detected when a void starts
to develop in the UBM and RDL regions as indicated by
samples C4, C5, and C6 in Figure 11.
Effect of Solder Temperature
Electromigration performance degraded with an increase in
solder temperature. This effect was demonstrated by
reducing the RDL width or increasing the oven temperature.
The temperature effect is seen by comparing legs 3a-c with
legs 1a-c. The RDL width was 90um in legs 3a-c and
270um wide in legs 1a-c. This resulted in more joule
heating, a 37C to 50C higher solder temperature, and a
much higher failure rate for legs 3a-c.
The temperature effect is also seen in comparing the
superior performance of legs 4a-c and 5a-c over legs 3a-c.
Reduced oven temperatures resulted in 28C to 51C lower
solder
temperatures,
and
better
electromigration
performance.
Effect of Current Density
Electromigration performance degraded with an increase in
current density. This effect was demonstrated by increasing
the test current, or by decreasing the UBM diameter.
Oven 125C
Board temp = 136C (Solder ball temp = 155C)
Figure 9. Example calculated temperature distribution for
TV #3, Leg 81, 2-bump daisy chain.
The electromigration performance degradation due to
increasing test current is seen in the following
combinations: leg 2b versus 1b, leg 17 versus 15, and legs
81,82 versus 80,83.
The electromigration performance degradation with a
decrease in UBM diameter is seen in the following
combinations: leg 3b versus 3c, and legs 6a-c versus 7a-c.
The effect of decreasing UBM diameter was also observed
in Refs [30,31,33].
Effect of RDL Width
Electromigration performance is degraded with a decrease
in RDL width for three reasons: 1) the current crowding
effect increases the local current density, 2) the joule
heating increases, and 3) a smaller void is required to cause
a resistance increase in eventual open circuit. The negative
impact of decreased RDL width is seen by comparing legs
3a-c versus 1a-c. Similar observations of increased current
crowding, increased joule heating and reduced lifetime due
to decreased RDL width were made in Refs [25,30,31].
A (pre-factor), n (current density exponent), and E a
(activation energy) are constants applicable to a given
metallurgical system. In a typical evaluation, a matrix of
test temperatures and current densities is defined and the
constants A, n, and E a can be determined.
The current study is not conducive to easily determining the
Black’s equation constants because 1) many of the tests
were not extended until 50% of the population had failed, 2)
legs 1 through 11 had only three unit sample sizes, and 3)
multiple test parameters (current, temperature, RDL width)
were varied at once. Nevertheless, it is possible to compare
the present results with those in literature by making some
reasonable assumptions.
Effect of UBM Pad Stack
Several UBM pad stacks were evaluated using across the
three test vehicles. For samples with a 240um to 250um
UBM diameter, a tall copper pillar (50um height) and a 2um
Ni UBM performed the best.
The copper pillar structure has at least three advantages: 1)
reduced current crowding to minimize localized high current
density, 2) reduced joule heating, and 3) provides a distance
barrier so solder cannot directly react with the copper RDL.
The improved performance of copper pillar samples can be
seen in the following combinations: legs 10a-c versus 3a-c,
and leg 13 versus 17.
Excellent electromigration
performance of copper pillar structures was also observed in
Refs [14,15,23].
UBM
RDL trace open
IMC
Solder
The 2um Ni UBM improves electromigration performance
by providing a metallurgical barrier that prevents reaction of
the solder and copper RDL. This effect can be seen by
comparing legs 73a-c versus legs 17 and 18. A similar trend
was reported in Ref [34].
Figure 10. Cross section of failed solder joint. Leg 6a,
TV#1, 7.5um Cu / 2.5um Ni UBM pad, 250um UBM
diameter, 90um RDL trace width, 1.8A, 177C, 69hrs.
It should be noted that a 2.5um Ni layer on top of a 7.5um
Cu pad does not act as an effective metallurgical barrier.
The solder is able to wet to the copper pad edge and provide
a path where the copper RDL can react with Sn. The failure
shown in Figure 10 is such an example.
Also, it is seen that legs 6a-c and 7a-c with the Cu/Ni pad
had similar or worse performance than legs 8a-c and 9a-c
with a Cu pad.
However, the 0.65umNi/0.70umCu pad used in TV#3 does
provide a reasonable metallurgical barrier to protect the Cu
RDL trace. The performance of legs 80 to 83 with a 160um
UBM diameter was comparable to legs 15 to 18 with a
250um UBM diameter.
DISCUSSION
Electromigration failure data is commonly fit to some form
of Black’s equation [27]
t = AJ-n exp(E a /kT)
(1)
where t is the time to failure, J is the current density, T is
temperature, and k is Boltzman’s constant. The constants
Time (hrs)
Figure 11. Resistance increase during electromigration
failure for 3of 9 units. Leg 7, TV#1, 7.5um Cu / 2.5um Ni
UBM pad, 300um UBM diameter, 90um RDL trace width,
1.8A, 178C to 190C solder temperature.
Test
Leg Vehicle
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
5a
5b
5c
6a
6b
6c
7a
7b
7c
8a
8b
8c
9a
9b
9c
10a
10b
10c
11a
11b
11c
13
14
15
17
18
50
55
56
66
73a
73b
73c
80
81
82
83
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
3
3
3
3
Solder
# Bumps
Ball
UBM Pad
RDL Solder in Daisy
Oven
UBM
PCB Pad
Stack
Diameter Diameter Diameter Width Alloy
Chain Current Temp
[C]
(um)
(um)
(um)
(um)
(A)
(um)
8
1.8
125
270 SAC305
250
300
250
7.5Cu/2.5Ni
125
280
8
1.8
7.5Cu/2.5Ni
250
300
270 SAC305
125
8
1.8
300
270 SAC305
300
250
7.5Cu/2.5Ni
125
8
2.5
270 SAC305
250
300
7.5Cu/2.5Ni
250
125
8
2.5
270 SAC305
250
300
7.5Cu/2.5Ni
280
125
8
2.5
300
270 SAC305
250
7.5Cu/2.5Ni
300
1.8
125
8
90 SAC305
250
300
7.5Cu/2.5Ni
250
125
8
1.8
250
300
90 SAC305
280
7.5Cu/2.5Ni
125
8
1.8
90 SAC305
250
300
7.5Cu/2.5Ni
300
85
1.8
90 SAC305
8
250
300
250
7.5Cu/2.5Ni
8
1.8
85
90 SAC305
300
7.5Cu/2.5Ni
280
250
85
90 SAC305
8
1.8
250
300
7.5Cu/2.5Ni
300
8
1.8
100
90 SAC305
300
7.5Cu/2.5Ni
250
250
8
1.8
100
90 SAC305
300
7.5Cu/2.5Ni
280
250
8
1.8
100
90 SAC305
7.5Cu/2.5Ni
300
250
300
1.8
125
8
250
300
90 SAC305
250
7.5Cu/2.5Ni
125
8
1.8
250
250
300
90 SAC305
7.5Cu/2.5Ni
1.8
125
300
90 SAC305
8
250
250
7.5Cu/2.5Ni
1.8
125
300
90 SAC305
8
7.5Cu/2.5Ni
300
250
8
1.8
125
250
300
90 SAC305
7.5Cu/2.5Ni
300
8
1.8
125
250
300
90 SAC305
7.5Cu/2.5Ni
300
8
1.8
125
300
90 SAC305
10Cu
250
250
1.8
125
90 SAC305
8
10Cu
250
250
300
8
1.8
125
250
250
300
90 SAC305
10Cu
1.8
125
250
300
90 SAC305
8
10Cu
300
1.8
125
250
300
90 SAC305
8
10Cu
300
90 SAC305
8
1.8
125
300
250
300
10Cu
8
1.8
125
50Cu
250
167
90 SAC305
250
180
90 SAC305
8
1.8
125
50Cu
280
250
8
1.8
125
188
90 SAC305
50Cu
300
250
8
2.2
125
250
167
90 SAC305
50Cu
250
8
2.2
125
280
250
180
90 SAC305
50Cu
90 SAC305
8
2.2
125
50Cu
300
250
188
8
2.5
125
250
250
167
270 SAC305
50Cu
8
3
125
50Cu
250
250
167
270 SAC305
270 SAC305
8
1.8
125
10Cu
250
250
300
250
300
270 SAC305
8
2.5
125
10Cu
250
2.5
150
250
250
300
270 SAC305
2
10Cu
2
2.5
154
9Cu RDL
240
250
250
340 SAC405
2.0
125
2Ni
240
250
250
340 SAC405
2
2
125
2Ni
240
250
250
340 SAC405
2.5
2
3.3
152
2Ni
205
250
250
340 SAC405
250
250
340 SAC405
2
2.5
154
2Ni
240
340 SAC405
2
3.0
152
2Ni
240
250
250
250
250
340 SAC405
2
3.5
152
2Ni
240
0.65Ni/0.70Cu
160
220
170
100 SAC305
2
1.8
125
0.65Ni/0.70Cu
160
220
170
100 SAC305
2
2.2
125
220
170
100 SAC305
2
2.2
125
0.65Ni/0.70Cu
160
2
2.0
125
0.65Ni/0.70Cu
160
220
170
100 SAC305
Table 2. Summary of test results.
PCB
Temp
[C]
131
139
132
137
153
139
147
164
161
103
118
115
122
138
135
147
164
161
148
160
150
145
157
150
144
154
147
140
148
141
150
163
151
135
140
133
139
155
156
128
129
157
156
157
160
132
136
134
133
Units
Solder Failed /
Joint
Units
Temp Tested
[C]
140
0/3
0/3
148
141
0/3
158
0/3
174
1/3
160
0/3
177
3/3
194
3/3
0/3
191
128
0/3
0/3
143
0/3
140
0/3
149
165
0/3
162
0/3
177
2/3
3/3
194
191
2/3
178
0/3
190
3/3
0/3
180
175
1/3
187
1/3
180
1/3
174
2/3
184
1/3
177
0/3
160
0/3
167
0/3
0/3
161
181
0/3
194
0/3
181
0/3
152
0/15
163
0/15
142
0/15
2/15
158
159
2/15
160
0/15
130
0/15
132
0/15
162
0/15
160
0/15
162
0/15
0/15
166
145
0/15
155
2/15
153
1/15
148
0/15
Sixteen data sets in Table 2 had at least 1 measured failure.
Seven of the data sets had enough failures to determine
t50% directly. The average Weibull shape parameter
(slope) was 2.57 for these data sets. The remaining nine
data sets did not have enough failures to determine t50%
directly, so the average Weibull shape parameter of 2.57
was applied to estimate t50%.
Shown in Figure 12 is a plot of calculated versus measured
electromigration life using the Black’s equation constants
discussed above. It is seen that the data from Ref [20] were
well fit to the model. However, the model correlation for
the present study’s data depends on the UBM metal stack.
The 7.5um Cu / 2.5um Ni and 10um Cu data are above the
line, and hence they perform slightly worse than the data
from Ref [20]. The 0.65um Ni / 0.70um Cu data are below
the line, so this data set performs better than that from Ref
[20]. This is likely due to the fact that Gee et.al. UBM stack
had a thinner Ni layer (0.32um Ni / 0.8um Cu) [20].
It was assumed that only the pre-factor, A, varies with UBM
metal stack. Hence, a simple ratio of calculated to measured
life was used to estimate the pre-factor for each case. A
summary of the proposed Black’s equation constants for the
various UBM metal stacks is shown in Table 3.
field use reliability criteria for consumer product
applications is given in Table 4. If one assumes that the
field use environment has a PCB temperature of 60C and
there is 21C of joule heating, then the solder temperature is
81C. The specified acceptable cumulative failures are <
0.1% after 1yr (8760hrs) of service.
For the qualification test, the sample size is 77 units, so the
first failure would constitute a median rank cumulative
distribution of 0.9%. The test duration is 1000hrs at rated
current. If the joule heating is 25C, then the PCB should be
maintained at 92C to achieve a solder temperature of 117C.
Passing such a qualification test should be equivalent to
passing the stated field use condition.
10,000
Calculated t50% (hrs)
Previous studies on lead free flip chip and WLCSP
assemblies were used to select constants for Black’s
equation. The range of current density exponent, n, in Refs
[3,7,8,9,13,16,19,29,33] was 1.0 to 9.8. We used a value of
2.0 in the present analysis. The current density was
calculated using the UBM pad area. The range of measured
activation energy, E a , in Refs [3,7,9,13,16,19,20,22,29,32]
was 0.5 to 1.6eV. We used a value of 1.0eV in the present
analysis. Based on these constants, the data of Gee et.al.
[20] were used to estimate the pre-factor, A in Eq.(1). The
median failure (t50%) of each test condition in Ref [20] was
used to estimate A as 2.2E-6 hrs/(A/mm2)2.0.
1,000
100
Gee et.al. [20]
7.5um Cu/2.5um Ni UBM
10um Cu UBM
.65um Ni/.70um Cu UBM
10
10
100
1,000
10,000
Measured t50% (hrs)
Figure 12. Correlation of calculated versus measured
electromigration performance.
10,000
Shown in Figure 13 is a plot comparing the worst case t50%
performance for 2um Ni and 50um Cu UBM metal stacks
versus the calculated life using Gee et.al. Black’s constants.
The data points are below the line, which indicates better
performance than the baseline case from Ref [20]. The
minimum estimated pre-factor, A, was calculated for 2um
Ni and 50um Cu UBM pad stacks as shown in Table 3. It is
likely that these pre-factors are conservative, since no actual
failures were recorded.
Electromigration testing is not a well-established method
with respect to typical device qualification. A proposed
accelerated test and qualification test based on an example
Calculated t50% (hrs)
2um Ni UBM
The 2um Ni and 50um Cu UBM metal stacks did not fail
under the test conditions studied here. In order to estimate
the worst case pre-factor, A, it was assumed that a failure
could have occurred at 169hrs of the most aggressive test
condition evaluated. Then, the t50% life was estimated
using a Weibull shape factor of 2.57.
50um Cu UBM
1,000
100
10
10
100
1,000
10,000
Worst Case t50% based on Most
Agressive Passing Test Condition (hrs)
Figure 13. Estimated minimum performance of 2um Ni
UBM and Copper Pillar WLCSPs that had no failures in the
present evaluation.
For the accelerated test, the sample size is 15 units, so the
first failure would constitute a median rank cumulative
distribution of 4.5%. The test duration is 168hrs at rated
current. If 25C of joule heating is assumed, then the PCB
should be maintained at 127C to maintain a solder
temperature of 152C. Passing such an accelerated test
should be equivalent to passing the qualification conditions
or the field use conditions.
UBM Metal Stack
A
n
Ea
(hrs/(A/mm2)2.0
(eV)
Gee. et.al. [20]
2.2E-6
2.0
1.0
7.5um Cu / 2.5um Ni
1.1E-6
2.0
1.0
10um Cu
1.8E-6
2.0
1.0
0.65um Ni / 0.70um Cu
7.0e-6
2.0
1.0
2um Ni
> 1.0e-5
2.0
1.0
50um Cu
> 5.5e-6
2.0
1.0
Table 3.
Recommended Black’s equation constants
assuming only pre-factor, A, varies with the UBM metal
stack.
Field Use
Qualification
Test
77
Accelerated
Test
15
Sample Size
Acceptable
Cumulative
< 0.1
< 0.9
< 4.5
Failures
(%)
Test
Duration
8760
1000
168
(hrs)
PCB
Temperature
60
92
127
[C]
Joule
Heating
21
25
25
[C]
Solder
Temperature
81
117
152
[C]
Table 4. Proposed accelerated test and qualification test
conditions based on example field use reliability
requirement and environment. Assumed that E a = 1.0eV,
Weibull shape parameter (slope) = 2.57, and device is tested
at rated max current level.
CONCLUSIONS
1) Electromigration failure of WLCSP assemblies was
found to occur due to a void created in the RDL trace near
the UBM pad.
2) The failure rate increased with higher temperature, higher
current density, and reduced RDL trace width.
3) A Ni UBM pad and a Cu pillar structure had the best
performance of the metal stacks tested.
4) A failure model based on Black’s equation was
developed from both the experimental data and other
published data. The model was then used to develop
recommended guidelines for accelerated testing and
qualification testing based on representative field use
conditions.
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