Thermal cycling reliability of SnAgCu solder joints in WLCSP

It has been widely reported in the literature that for packages that are required to pass thermal cycling test, the SnAgCu solder joints should have high Ag content. In this study, thermal cycling performance of a wafer level chip-scale package was evaluated with different combinations of high Ag solder (Sn3.9Ag0.6Cu) and low Ag solder (Sn1.2Ag0.5Cu) with thick and thin PCB. It was found that with the low Ag solder ball the package mounted on a thin PCB had better performance. Metallurgical analysis of solder joints, mechanical modeling of the package mounted on boards, and coplanarity measurement of the printed circuit boards were performed to understand the results. Because of the CTE mismatch between PCB and die, PCB warpage resulted in high tensile stress in solder joints in the central area, causing cracking of re-distribution layer Cu. The softer solder alloy Sn1.2Ag0.5Cu helped reduce the stress, leading to better performance in thermal cycling test.