Gold embrittlement of solder joints in wafer level chip scale package on printed circuit board with Ni/Au surface finish

In this study, the effect of gold amount on the gold embrittlement of solder balls in wafer level chip scale package (WLCSP) CSP 80 is investigated. Firstly, precisely controlled amounts of gold were added artificially to solder balls through a reflow process. It is found that for CSP 80 without Au, after 2000 hours of thermal aging at 150°C, the solder balls still fail in the bulk solder. When Au is incorporated, brittle failure occurs after 1000 and 200 hours for CSP 80 with 0.3wt% and 0.5wt% of Au, respectively. Detailed inspections on the microstructure reveal that, after the reflow process, fine AuSn₄ IMC particles disperse uniformly in the bulk solder. After 200 hours at 150°C, the (Cu,Ni)₆Sn₅ ternary compound transforms to Cu-Au-Ni-Sn quaternary compound and the IMC at the interface of solder and UBM becomes a completely continuous layer. After 1000 hours at 150°C, two IMC layers, light gray and dark gray, formed at the interface. EDX analysis confirms that the light and dark gray IMC layers are the (Cu,Ni,Au)₆Sn₅ compound. But the light gray (Cu,Ni,Au)₆Sn₅ compound contains more Au and less Ni, while the dark gray (Cu,Ni,Au)₆Sn₅ IMC contains less Au and more Ni. After 2000 hours at 150°C, the two IMC layer structures still exist.