Sporadic Early Life Solder Ball Detachment Effects on Subsequent Microstructure Evolution and Fatigue of Solder Joints in Wafer-Level Chip-Scale Packages

The combination of continuous miniaturization of electronics and the demanding reliability requirements for industrial and automotive electronics is one big challenge for emerging packaging technology. One aspect is to increase the understanding of the damage under environmental loading. Therefore, the solder joints of a wafer-level chip-scale package assembled on a printed circuit board (PCB) have been analyzed after a temperature cycling test. In the case of the investigated package, a limited number of joints did not form a proper mechanical connection with the PCB copper pad. Although not intended in the first place, these circumstances cause a detachment of those joints within the first few thermal cycles. However, this constellation offers a unique opportunity to compare the solder joint microstructure after thermomechanical loading (connected joints) with pure thermal loading (detached joints) located directly next to each other. It is shown that microstructure aging effects can be directly linked to regions in the joint with increased loading. This is particularly the case for detached joints, which could almost retain their initial microstructure up to the effect of the high-temperature part of the thermal profile. By means of finite element simulation, it is further possible to quantify the increased loading on adjacent joints if isolated solder balls detach from the board. In one case presented, the lifetime of the corner joint was calculated to reduce up to 85% only.