**THERMAL CYCLING EFFECTS ON A**

**JPL FLIP CHIP PRINTED CIRCUIT BOARD**

Test Report

Richard Patterson

NASA Glenn Research Center

&

Ahmad Hammoud

QSS Group, Inc.

NASA Glenn Research Center

Cleveland, Ohio

June 15, 2002

**THERMAL CYCLING EFFECTS ON A**

**JPL FLIP CHIP PRINTED CIRCUIT BOARD**

**Background**

Printed circuit boards employing flip chip technology are finding wide
spread use in many industrial and aerospace applications. NASA is
currently exploiting the use of such boards in electronic systems
designed for future space missions. In addition to being lightweight and
efficient, the boards must be able to withstand operation in harsh
environments. These include severe temperature swings that are typically
experienced in planetary exploration and deep space probes.

In a collaborative effort between NASA GRC and JPL under the NASA
Electronic Parts and Packaging (NEPP) Program, a flip chip printed
circuit board was provided by JPL for in-house evaluation under thermal
cycling to extreme temperatures. The Flipchip Technologies, Inc. board
(FB250/500 REV-A) was populated with a total of ten chips and had a
20-connector termination. A photograph of the flip chip printed circuit
board is shown in Figures 1. The effect of thermal cycling on the
board's electrical continuity and physical integrity was investigated.

![](media/image1.jpeg){width="4.991666666666666in"
height="2.9756944444444446in"}

Figure 1. Top view of the flip chip printed circuit board.

**Test Procedure and Setup**

The flip chip board was characterized in terms of electrical continuity
of the various circuit traces between the connector terminals 1 through
20. The board was examined for physical integrity and condition prior to
any testing. Then resistance measurements were made in the temperature
range of +90 C to --125 C. The printed circuit board was checked again
for changes in physical integrity, such as delamination, warping, and
discoloration. Next, the board was subjected to thermal cycling for a
total of 10 cycles between +90 C and --125 C. A temperature rate of
change of 10 C/min was used throughout the cycling activity with a soak
time of 10 minutes allowed at the extreme temperatures. After completion
of the thermal cycling, the board was evaluated again in terms of
electrical continuity (a resistance measurement) and structural
integrity. The electrical continuity/resistance tests were performed
using a Keithly 237 Source Measure Unit and an HP 34970A data
acquisition/switching unit.

Test Results

Changes in the resistance, i.e. trace continuity, between terminals 1
and 2 before the thermal cycling is shown in Figure 2 as a function of
temperature. It can be clearly seen that the resistance changes in
direct proportion to the test temperature. Figure 2 shows the same
effect experienced by the trace resistance between terminals 19 and 20.
In fact, such was the case for all the traces as depicted in table I. No
visible damage was observed to the board due this temperature exposure.

![](media/image2.wmf)

Figure 2. Resistance between terminals 1 and 2 versus temperature before
thermal cycling.

![](media/image3.wmf)

Figure 3. Resistance between terminals 19 and 20 versus temperature
before thermal cycling.

Table I. Trace resistance before thermal cycling.

  -------- ------------ ------- ------- ------- ------- ------- ------- ------- ------- -------
           Resistance                                                                   
           (Ω) between                                                                  
           terminals                                                                    

  Temp (C) 1-2          3-4     5-6     7-8     9-10    11-12   13-14   15-16   17-18   19-20

  90       2.445        2.525   2.525   2.585   2.465   2.445   2.345   2.425   2.365   2.485

  25       1.964        2.024   2.004   2.084   1.964   2.004   1.804   2.004   1.804   2.004

  -125     .862         .842    .882    .902    .862    .862    .822    .842    .842    .862
  -------- ------------ ------- ------- ------- ------- ------- ------- ------- ------- -------

Figure 4 depicts the variation in the resistance between terminals 1 and
2 following the thermal cycling. It is evident that this limited cycling
had little effect on the resistance (continuity) of this trace as the
data did not change from that of pre-cycling condition, as was shown in
Figure 2. Similarly, the circuit trace between terminals 19 and 20 did
not exhibit any changes with cycling as depicted in Figure 5. Needless
to say, all the other traces displayed the same behavior as can be seen
from the data listed in Table II. Following the thermal cycling, the
printed circuit board appeared to undergo no changes in its structural
integrity as no delamination or warping was revealed upon examination
visually or under a microscope.

![](media/image4.wmf)

Figure 4. Resistance between terminals 1 and 2 after thermal cycling.

![](media/image5.wmf)

Figure 5. Resistance between terminals 19 and 20 after thermal cycling.

Table II. Trace resistance after thermal cycling.

  -------- ------------ ------- ------- ------- ------- ------- ------- ------- ------- -------
           Resistance                                                                   
           (Ω) between                                                                  
           terminals                                                                    

  Temp (C) 1-2          3-4     5-6     7-8     9-10    11-12   13-14   15-16   17-18   19-20

  90       2.445        2.525   2.525   2.605   2.465   2.425   2.345   2.425   2.365   2.485

  25       1.984        2.044   2.024   2.104   1.964   1.964   1.924   1.944   1.924   1.964

  -125     .882         .902    .882    .922    .862    .862    .822    .862    .822    .882
  -------- ------------ ------- ------- ------- ------- ------- ------- ------- ------- -------

**Conclusion**

A flip chip printed circuit board was evaluated under extreme
temperatures between +90 C to --125 C. The Flipchip Technologies, Inc.
board was populated with a total of ten chips and had a 20-connector
termination. The effect of thermal cycling on the board's electrical
continuity and physical integrity was also investigated. The results
obtained indicate that this limited thermal cycling produced no effect
on either the physical integrity or the electrical continuity of the
board. Long term thermal cycling and aging as well as comprehensive
testing are required to fully understand the behavior of this flip chip
and other printed circuit boards under multi‑stress conditions to
determine their suitability for use in extreme temperature environments.

**Acknowledgments**

This work was performed under the NASA Glenn Research Center GESS
Contract \# NAS3-00145 and the NASA Electronic Parts and Packaging
(NEPP) Program, Task " Interconnect Reliability of Cold Electronics".