The microstructure and shear behaviors of the solder joints with decreasing diameter in WLCSP

The micro solder joints serve as mechanical supporting and electrical connections in assembly of WLCSP devices on board-level. As the requirements of miniaturization, weight reduction in modern electronic devices, the dimension of solder joints is decreasing as small as 100um due to higher-density connections and smaller chip size. The integration of the solder joints is one of the key problems related to the reliability of assembly devices. In this study, the WLCSP samples with micro SAC(Sn-Ag-Cu) solder joints which diameters from 100um to 300um were prepared. The IMC thickness and shear behaviors of the solder joints with different dimensions were investigated. The IMC thickness was studied by cross-section observations when samples were subjected to solder reflow or not. By the single ball shear testing, the failure modes of solder joints were summarized. The relationships between IMC thickness and shear failure modes for the decreasing solder joints´ diameters were discussed.