**Shock and Thermal Cycling Synergism Effects on Reliability of CBGA
Assemblies**

Reza Ghaffarian, Ph.D.

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

818-354-2059

Reza. Ghaffarian@JPL.NASA.Gov

### Abstract

Ball Grid Arrays (BGAs) are now packages of choice especially for higher
I/O counts for commercial applications and are also being considered for
use in military and aerospace. Thermal cycling characteristics of BGA
assemblies have been widely reported including those by the JPL-led
consortium. Thermal cycling represents the on-off environmental
condition for most electronic products and therefore is a key factor
that defines reliability. As a results, much data available for
accelerated thermal cycle conditions, but very limited data is available
on vibration and shock representative of aerospace applications. Test
vehicles with daisy chain plastic and ceramic BGAs (CBGAs) ranging from
256 to 625 I/O count were subjected to random vibration/shock
representative of a spacecraft launch environment. The effect of board
rigidity on behavior was also investigated by adding strips to or
bonding of board to an Aluminum plate . This paper compares accelerated
thermal cycles-to-failure data under four temperature ranges before and
after thermal random vibration for CBGAs with 361 and 625 I/Os. Stress
and strain projections by finite element analysis are also presented.

# Table of Contents

# 1. Accelerated environmental test

# 2- Test procedures

# 3- test results

# 4- Conclusions

# 5- References

# 6- Acknowledgments

# 7-Biography

# 1- Accelerated environmental test

### Introduction

There are many reason to perform an accelerated environmental
verification and testing program for electronics assemblies including:

-   Qualification of design for in-service conditions

-   Modeling of in-service condition or to project life

-   Definition of manufacturing variables and their effects

-   Screening for manufacturing defects

-   Demonstration of quality and reliability of a design

-   Demonstration of suitability for the intended use

    For electronics in commercial applications, commonly, thermal
    cycling tests are performed to simulate on/off condition. However,
    most electronic systems are exposed to other environments including
    mechanical fatigue and random vibration. For example, vibration
    occurs during transportation and mechanical fatigue occurs by
    repeated use of key punching for portable electronics. Occasional
    high shock could occur due to accidental drops. Drop test and
    mechanical fatigue are beginning to be considered for qualification
    electronic assemblies especially for newer chip scale package
    assemblies.

    In addition to much harsher thermal requirement for aerospace and
    military applications, generally assemblies are required to meet
    sever dynamic loads and vibration fatigue cycling. Therefore there
    is a strong need to understand assembly behavior under such stress
    conditions. Very limited data are available for vibration and
    especially shock behavior of BGA assemblies. To understand behavior
    for space environment, several test vehicles with plastic BGAs
    (PBGAs) and CBGAs with 1.27 mm pitches were subjected to dynamic
    testing representative of a launch, 3 minutes at 3 axis
    vibration/shock. Each test vehicle had four package types which were
    commercially available and had I/Os from 250 to 625.

Synergistic stresses including thermal cycle representative of the
following environmental conditions were considered:

-   Prelaunch thermal cycling due to manufacturing, repair, screening,
    storage, and transportation

-   Dynamic testing representative of the launch environment included
    sinusoidal vibration, transient vibration, pyroshock, and acoustic
    vibration

-   Thermal cycling representative of internal and external temperature
    change of a spacecraft

    Effects of several test variables were considered.

a)  Thermal cycling before vibration/shock and after

b)  Increase in rigidity of board by addition of thin rigid strips in
    one case and bonding to a rigid plate in another case

c)  Change in thermal history condition by exposure to thermal cycles
    before vibration/shock

This paper presents experimental results as well as analyses for two
CBGA assemblies (361 and 625 I/Os) subjected to either or both thermal
cycling and vibration. Refer to the JPL BGA Packaging Guideline
(Reference 1) for details on design, package daisy chain, board layout,
manufacturing processes, inspection results, and thermal cycling
behavior of ceramic and plastic package assemblies.

### Literature Review

Cycles-to-failure after vibration of CBGAs and column (CCGA) were
presented by M. Cole of IBM (Reference 2). Balls and columns were 90
Pb/10 Sn with 0.89 mm (.035 inch) in diameter for CBGA and for CCGA 0.5
mm (.02 inch) in diameter 2.2 mm (.050 or .087 inch) in height. Packages
were assembled on FR-4 boards with single test specimens mounted on 110
x 90 mm board, clamped by screws at 101 x 75 mm locations. Mil-STD 810E
were used to generate impact and vibration data for their test vehicle.

No failure of CBGAs and CCGAs was observed after vibration with heatsink
of 73 g. CCGAs with heatsink weights of 100 and 150 grams failed whereas
CBGAs did not. Cracks were induced in CBGAs in the eutectic solder
either in package or board sites when subjected in 20-2000HZ with 7.73
grms. It was reported that for CBGAs, crack initiations were similar to
those of accelerated thermal cycling (ATC), but with no deformation
typically present in ATC. Also, thermal mismatch induce both shear and
tensile, but vibration induce primarily tensile and did not cause
deformation.

It was observed that CBGA assemblies with heatsinks of lower weight that
than 150 grams which were subjected to shock and vibration did not show
any degradation in thermal fatigue life, no statistical differences
between those with initial shock and vibration and those without any a
priori test were observed.

In another study (Reference 3), CBGAs with 256 I/Os, 625, and 1089 I/Os
were subjected to thermal cycling and vibration evaluation. The 1089 was
fabricated with balls rather commonly supplied in column since they were
used on metal matrix restraining core rather than polymeric board such
as FR-4 . For restrained board with a CTE (6 ppm/°C) close to the
ceramic package (7.2 ppm/°C), local mismatch between solder to
package/board considered to play a primary role rather than global
mismatch. Assemblies on restrained board were subjected to thermal
cycling in the range of -55°C/125°C (a 5 °C/min ramp and a 30 minute
dwell at each extreme). They reported no failures to 1,500 cycles.

They also performed vibrations on three CBGAs at three levels:

-   Level I was the military avionics equipment workmanship vibration
    level of Environmental Stress Screening (ESS)

-   Level II was an enveloped subassembly based on several avionics
    programs

-   Level III was the same as level II with power spectral density (PSD)
    increased by 3 dB.

Test results are listed in Table 1. All assemblies passed the level I
whereas only CBGA with 254 I/O passed the Level II vibration. Finite
element prediction for level I agreed with the experimental results, but
for other levels had a mixed agreement.

# 2. Test Procedures

### Test Vehicle

The test vehicles in this investigation included both plastic and
ceramic packages on either FR-4 or polyimide printed circuit board (PWB)
with six layers, 0.062 inch thick. Ceramic packages with 625 I/Os and
361 were included in our evaluation. Solder balls for CBGAs had high
melt temperature composition (90Pb/10Sn) and about 0.035 inch diameters.
The high melt balls were attached to the ceramic package with eutectic
solder (63Sn/37Pb). At reflow, package side eutectic solder and the PWB
side eutectic paste will be reflowed to provide the electro-mechanical
interconnects.

The CBGAs had internal daisy chains which made a closed loop with daisy
chains on the PWB enabling the monitoring of solder joint failure
through continuous electrical monitoring. Daisy chains for CBGA 625 were
in the ring form center to peripheral in order to identify failure sites
with increased in thermal cycles. The first failure is known to occur
from the peripheral ring in the corner solder joints with the maximum
distance to neutral point (DNP). To improve assembly reliability, the
supplier had removed package internal daisy chain connections among a
few corner balls; excluding them from assembly failure detection during
electrical monitoring. This means that cycles to first failure data
cannot be directly correlated to package diagonal dimension which
usually are assumed to be equal to the maximum DNP.

### Thermal Cycling Conditions

Four different thermal cycle profiles were used. These were:

-   Cycle A: The cycle A condition ranged from -30 to 100°C and had an
    increase/decrease heating rate of 2 to 5°C/min and dwell of about 20
    minutes at the high temperature to assure near complete creeping.
    The duration of each cycle was 82 minutes.

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-   Cycle B: The Cycle B condition ranged from -55 to 100°C with long
    time duration. The heating and cooling rates were 2 to 5°C per
    minute with an oven dwell setting of 45 minutes at the two extreme
    temperatures. The duration of each cycle was 246 minutes.

-   Cycle C: The cycle C condition ranged from -55 to 125 °C with
    2-5°C/min heating/cooling rate. Dwell at extreme temperatures were
    at least 10 minutes with duration of 159 minutes for each cycle.

-   Cycle D: The cycle D condition ranged from -55 to 125°C, the same as
    condition C, but with very high heating/cooling rate It could also
    be considered a thermal shock since it used a three region chamber:
    hot, ambient, and cold. Heating and cooling rates were nonlinear and
    varied between 10 to 15 °C/min. with dwells at extreme temperatures
    of about 20 minutes. The total cycle lasted approximately 68
    minutes.

The criteria for an open solder joint specified in IPC-SM-785, Sect.
7.8, were used as guidelines to interpret electrical interruptions.
Generally, once the first interruption was observed, there were many
additional interruptions within 10% of the cycle life. This was
especially true for ceramic packages.

### Vibration

Three test vehicles with three levels of rigidity representing three
loading conditions were stacked with spacers and subjected to random
vibration. The bottom TV was bonded to an aluminum plate, the middle had
center and edge stiffener strips, and the top had no stiffeners. The
test vehicles were clamped from the two sides of the PWB, the sides with
no connectors as shown in Figure 1. The block of three stacked test
vehicles were mounted on a very stiff thick Al plate on the shaker table
with their natural frequencies well above the test vehicle assembly
range.

Initially, a very low vibration spectrum was sweeped to determine
natural frequencies for the test vehicle. The board were subjected to a
vibration spectrum in the range of 200-2000HZ.

# 3. Test Results

### Natural Frequency Measurement/Projection

Plots for natural frequencies were generated. A simplified finite
element analysis was performed to predict natural frequencies and
stress/strain condition at the solder joints (Reference 4). Ten percent
model damping was used for the random response calculation. Projections
for the 1^st^, 2^nd^, and 3rd natural frequencies are compared to
measured values as listed in Table 2.

### Damage Induced for Thermal Cycling Alone 

Both board and package interface cracking was observed with increasing
number of thermal cycles. Figure 2 shows typical failures for the two
cycling conditions, A and D. Failure under the A conditions were
generally from the PWB and for the D conditions from the package sites.
Failure mechanism differences could be explained either by global or
local stress conditions.

Modeling indicates that the high stress regions shifted from the board
to the package themselves when stress conditions changed from the global
to local. For the A cycling, with slow heat/cooling ramping, which
allowed the system to reach uniform temperature, damages could indicate
a global stress condition. For the D cycle with rapid heat/cooling,
damages could indicate a local stress condition.

### Damage Induced with a priori vibration

Damage induced by vibration was different in some cases as shown in
Figures 3-4. Appearance of tensile deformation from the center of high
balls as shown, was significantly different from those observed for
thermal cycling condition. Similar to thermal cycle condition, damage
was more dominant for the balls with higher distance to neutral point
(DNP), especially for the corner balls. However, additional microcracks
in eutectic solder joints, different from the norm for thermal cycling,
were induced by tensile and shear load during random vibration.

### Cycles-to-failure for Thermal Cycling Alone

Figure 5 shows Weibull plots of cycles to first failures for CBGA 625
and CBGA 361 under four different thermal cycle conditions. To generate
plots, cycles-to-failures for a population were ranked from low to high
and failure distribution percentiles were approximated using median
plotting position, F~i~ = (i-0.3)/(n+0.4) (e.g., see Kapur,1977). Then,
two-parameter Weibull distribution was used to characterize failure
distribution. The Weibull cumulative failure distribution was used to
fit cycles to failure data. The equation is

F(N) = 1- exp (-(N/No)^m^ )

where

F(N) is the cumulative failure distribution function

N is the number of thermal cycles

No is a scale parameter that commonly is referred to as characteristic
life, and is the number of thermal cycles with 63.2% failure occurrence.

The m is the shape parameter. A large m is approximately inversely
proportional to the coefficient of variation (CV) by 1.2/CV; that is, as
m increases, spread in cycles to failure decreases

This equation, in double logarithm format, results in a straight line.
The slope of the line will define the Weibull shape parameter.

### Synergism of Vibration and Thermal cycling

Cycles-to-failures for three assemblies with levels of rigidity and a
priori vibration condition are shown in Table 3. Two test data for
cycles-to-failure after vibration for the most sever condition, i.e. the
test vehicles with no rigidity enhancement, were marked in Figure 5 for
comparison only. It is difficult to draw a statistically meaningful
conclusion because of insufficient sample size.

# 4. Conclusions

-   A near-thermal shock in the range of -55°C to 125°C induced the most
    damage on CBGA assemblies, up to 50 percent reduction in cycles to
    failure compared to a thermal cycle condition.

-   Assemblies with three level rigidity passed a launch random
    vibration/shock condition. However, cycles-to-failures after
    vibration were affected by the rigidity of the board, a significant
    reduction (\>50%) for a less rigid and minimum reduction for highly
    rigid assemblies after vibration.

-   Failures for thermal cycled CBGAs were either from board or package
    sites in the eutectic solder joints. Those subjected to vibration
    showed tensile deformation in high melt balls and tensile and shear
    failures in eutectic solder.

# 5. References

1.  Ghaffarian, R. \"Ball Grid Array Packaging Guidelines," distributed
    by Interconnect Technology Research Institute (ITRI), August 1998,
    [http://www.ITRI.org](http://www.ITRI.org/)

2.  Cole, M., Kastberg, E., Matin, G., "Shock and Vibration Limits for
    CBGA and CCGA", Surface Mount International Proceedings, 1996,
    pp.89-94

3.  Marsico, J.W., AIL, private communications.

4.  Pitarresi, J., University of New York at Binghamton, private
    communications.

5\. Kapur, K.C., Lamberson, L.R., Reliability in Engineering Design,
John Wiley & Sons, 1977

# 6. Acknowledgments

The majority of research described in this publication was carried out
by the Jet Propulsion Laboratory, California Institute of Technology,
under a contract with the National Aeronautics and Space Administration.

I would like to acknowledge the in-kind contributions and cooperative
efforts of JPL BGA Consortium on test vehicle design and assembly and
those who contributed to the program. Also, thanks to Dr. James
Pitarresi, University of New York at Binghamton for performing
analytical modeling.

# 7- Biography

*Dr. Reza Ghaffarian has nearly 20 years of industrial and academic
experience in mechanical, materials, and manufacturing process
engineering.* *At JPL, Quality Assurance Office, he supports research
and development activities in SMT, BGA, and CSP technologies for
infusion into NASA's missions. He has authored nearly 100 technical
papers and numerous patentable innovations. He received his M.S. in
1979, Engineering Degree in 1980, and Ph.D. in 1982 in engineering from
University of California at Los Angeles (UCLA).*\
![](media/image1.wmf){width="0.8125in" height="0.9479166666666666in"}

**Table 1 Vibration and thermal cycling for CBGAs**

+---------+---------+---------+---------+---------+---------+---------+
| **Vi    | **D     | **Input | **R     | *       | *       | *       |
| bration | uration | level   | esponse | *CBGA** | *CBGA** | *CBGA** |
| level** | (min)** | (gr)**  | (gr)**  |         |         |         |
|         |         |         |         | **256   | **625   | **1089  |
|         |         |         |         | I/O**   | I/O**   | I/O**   |
+---------+---------+---------+---------+---------+---------+---------+
| I       | 10      | 6.06    | 21      | Pass    | Pass    | Pass    |
|         |         |         |         | (4/4)   | (2/2)   | (3/3)   |
+---------+---------+---------+---------+---------+---------+---------+
| II      | 60      | 12.8    | 37      | Pass    | Pass    | Pass    |
|         |         |         |         | (4/4)   | (0/2)   | (0/3)   |
+---------+---------+---------+---------+---------+---------+---------+
| III     | 120     | 18.07   | 53      | Pass    | NA      | NA      |
|         |         |         |         | (1/4)   |         |         |
+---------+---------+---------+---------+---------+---------+---------+

**Table 2 Measured and Finite Element Projection of Natural Frequencies
and**

**Stress/Strains for CBGA 361**

+--------------+----------------+-----------+-----------+-------------+
| **TV         | **Frequency**  | **Am      | **Maximum | **Maximum   |
| Condition**  |                | plitude** | Stress    | Strain      |
|              | **Mode 1 (mode |           | (Psi)**   | (μStrain)** |
|              | 2,3)**         | **(g      |           |             |
|              |                | ^2^/HZ)** |           |             |
+--------------+----------------+-----------+-----------+-------------+
| Measured     | 356 (688,1700) | 190       | N/A       | N/A         |
| Values for   |                |           |           |             |
| no Stiffener |                |           |           |             |
+--------------+----------------+-----------+-----------+-------------+
| FEA for TV   | 381 (594,795)  | 170       | 945       | 315         |
| with no      |                |           |           |             |
| Stiffener    |                |           |           |             |
+--------------+----------------+-----------+-----------+-------------+
| FEA for TV   | 593 (820,1625) | 43        | 558       | 185         |
| with         |                |           |           |             |
| Stiffeners   |                |           |           |             |
+--------------+----------------+-----------+-----------+-------------+
| FEA for TV   | 3155           | N/A       | 25        | > 8         |
| bonded to Al | (3520,4700)    |           |           |             |
| Plate        |                |           |           |             |
+--------------+----------------+-----------+-----------+-------------+

**Table 3 Cycles-to-failures after random vibration**

+---------------+-------------------------+----------------------------+
| **TV          | **ID- Vibration (3      | **Thermal Cycles to        |
| Condition**   | axes)& Thermal Cycle    | failure**                  |
|               | Type**                  |                            |
+---------------+-------------------------+----------------------------+
| No Stiffener  | #4- Vib. + Cycle B      | #4, Failed between 292 &   |
| Under         |                         | 326 cycles                 |
| Vibration     | #50- Vib. + Cycle B     |                            |
|               |                         | #50- Failed at 400 cycles  |
|               | #34- Vib. + Cycle A     |                            |
|               |                         | #34- No failure to 434     |
+---------------+-------------------------+----------------------------+
| With          | #33- Vib. + Cycle A     | #33, Failed between 292    |
| Stiffeners    |                         | and 326 cycles             |
|               | #13- Vib. + Cycle A     |                            |
|               |                         | #13- Failed at 330 cycles  |
+---------------+-------------------------+----------------------------+
| Bonded to Al  | #31- Vib.+ Cycle A      | > #31- No apparent change  |
| Plate         |                         | > to virgin test vehicles  |
+---------------+-------------------------+----------------------------+

![](media/image2.wmf){width="4.428472222222222in"
height="2.629166666666667in"}

Figure 1 Photos of random vibration test set up, left photo shows two
three-stack of test vehicle on vibration table and the right photo shows
the enlarged test vehicle with the CBGA361 on the top right corner

![](media/image3.wmf){width="3.2631944444444443in"
height="2.5305555555555554in"}

**Figure 2 Cross-sections of failure sites for CBGA 625 after 350 cycles
under A and CBGA 361 under D conditions**

![](media/image4.wmf){width="3.5722222222222224in"
height="2.8472222222222223in"}

**Figure 3 Thermally cycled samples after vibration. Note tensile
failure for the corner ball and minimum damage for the center balls**

![](media/image5.wmf){width="6.43125in" height="2.7305555555555556in"}

**Figure 4 Thermal cycled samples after vibration. Note mechanical crack
propagation at two locations and minimum damage at a center ball.**

![](media/image6.wmf){width="6.647916666666666in"
height="4.5472222222222225in"}

**Figure 5 Cycles to failure data for two CBGAs under four thermal
conditions with two assemblies with a priori vibration condition**