Study of Solder Ball Hardness on Wafer Level Chip Size Packages

There are some challenges in electronic industries to achieve robustness in Thermal Cycling performance on board for big Wafer Level Chip Size Packages (WLCSP), though usually no concern or robust in Drop Test reliability, partly WLCSP is typically less ball counts and much smaller package size compared to Ball-Grid-Array (BGA). It is essential to balance the solder joint hardness for the overall robustness in the field application where the WLCSP could be subjected various loading conditions like Thermal Cycling Test on Board (TCoB), Bend, Drop etc. In this paper, solder joint hardness was studied for different solder alloy Sn-Ag-Cu of WLCSPs on PCB board before and after thermal cycling test (e.g. 1000 cycles) to improve the TC on Board performance. Berkovich micro indentation technique was used and optimized for providing precise and consistent measurements of indentation hardness. The experimental results showed that hardness of solder alloy induced by Ag or different % in Bi, solder paste, faster cooling after reflowing, softening during annealing and deterioration during TC, which may eventually affect the TCoB performance. Hardness could be a good, simple, and cost-effective indicator for solder joint reliability. It can be used for alloy selection, reflow profile optimization, and investigating aging impact etc.