Authored By:
Jianbiao Pan
California Polytechnic State University
San Luis Obispo, CA
Summary
Many researchers have used different failure criteria in the published solder joint reliability studies. Since the reported time-to-failure would be different if different failure criteria were used, it would be difficult to compare the reported reliability life of solder joints from one study to another. The purpose of this study is to evaluate the effect of failure criteria on the reported thermal fatigue life and find out which failure criterion can detect failure sooner.
First, the application of the control-chart based method in a thermal cycling reliability study is described. The reported time-to-failure data were then compared based on four different failure criteria: a control-chart based method, a 20% resistance increase from IPC-9701A, a resistance threshold of 500 degrees, and an infinite resistance. Over 3.5 GB resistance data measured by data loggers from a low-silver solder joint reliability study were analyzed.
The results show that estimated time-to-failure based on the control-chart method is very similar to that when the IPC-9701A failure criterion is used. Both methods detected failure much earlier than the failure criterion of a resistance threshold of 500 degrees or an infinite resistance. A scientific explanation is made of why the 20% increase in IPC-9701A is a reasonable failure criterion and why the IPC-9701A and the control-chart based method produced similar results.
Three different stages in resistance change were identified: stable, crack, and open. It is recommended that the control-chart based method be used as failure criterion because it not only monitors the average of resistance, but also monitors the dispersion of resistance in each thermal cycle over time.
Conclusions
In this study, the application of the control-chart based method to detect solder joint failure in a thermal cycling reliability study is presented. In the control-chart based method, the thermal fatigue failure of solder joints is defined as the mean or range of resistance when the thermal cycle increases significantly, measured by k sigma of the natural variation purely due to thermal effects. Note that the variation from gauge repeatability and reproducibility is not considered here because it is typically much smaller than the variation due to thermal effects.
The reported cycles-to-failure data based on different failure criteria were compared. The results show that the reported cycles-to-failure from the control-chart method is very similar to that when the IPC-9701A failure criterion is used. Both IPC-9701A and the control-chart based method can detect failure much earlier than the failure criterion of a resistance threshold of 500 or an infinite resistance.
A scientific explanation is made of why the 20% increase in IPC-9701A is a reasonable failure criterion and why the IPC-9701A and the control-chart based method produced similar results. From the physics of the metal's temperature dependence of resistance, the range of resistance purely due to thermal effects is calculated as the percentage of average resistance.
Three stages of resistance behavior are identified: stable, crack, and open. In the stable stage, cracks could have initiated and propagated but have not reached a full crack. In the crack stage where the increase in resistance is small (typically less than 1), at least one solder joint has propagated to almost a complete crack, or a full-crack has occurred but the failed solder joint was compressed by the surrounding good solder joints in a daisy-chain. In the open stage, a complete crack has occurred and the gap of the crack is large. Partial cracks are difficult to be detected by the electrical continuity measurement method due to limited resolution of commercial equipment. The duration of the crack stage depends on the severity of the test conditions.
It is recommended that the control-chart based method be used as failure criterion because it not only monitors the average of resistance, but also monitors the dispersion of resistance in each thermal cycle over time. 4 to 6 readings are suggested to be collected per thermal cycle. In addition to monitoring the resistance value, one could conclude that an interconnection may have failed if the range of resistance is significant larger than the theoretical range in a thermal fatigue reliability study. Monitoring the dispersion of resistance over time gives another way to detect failure of solder joints.
Initially Published in the IPC Proceedings
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