Fatigue life of solder bumps in a system in package: relating power cycling to thermal cycling

Usually the mechanical reliability of electronic packages is qualified by temperature cycle tests (TCT). However this is a more or less isothermal test, while in operating condition temperatures can have a non-uniform distribution, because thermal transient effects are present. This effect can play an important role in packages that generate heat, like the module with power amplifier that we studied in this work. The transient thermal loading in the active areas in the flip chip power amplifier causes thermo-mechanical stresses that may cause fatigue fractures in the solder bumps and eventually failure of the module. In this paper we simulate the operating conditions and a TCT. For this purpose a FEA model was created with a new way of working to handle major challenges regarding the large differences in length and time scale. The required element size in the bumps was approximately a factor 200 smaller than that required in the PCB. Furthermore the combination of the heat penetration depth of the short (milliseconds) power pulses together with the heating time of approximately half an hour for the complete module makes conventional simulation methods very time consuming. The new method showed to be accurate and efficient. The simulation results show that in TCT the loading is very different than in power cycling. The critical bump was in power cycling in the middle of the heating area, while in TCT is located at the corner. We have also calculated an acceleration factor for the TCT test