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In this paper, the effect of gold amount on the gold embrittlement of solder balls in wafer-level chip-scale package (WLCSP) CSP 80 is investigated. First, 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 h of thermal aging at 150 degC, the solder balls still fail in the bulk solder. When Au is incorporated, brittle failure occurs after 1000 and 200 h for CSP 80 with 0.3 wt% and 0.5 wt% of Au, respectively. Detailed inspections on the microstructure reveal that, after the reflow process, fine AuSn4 intermetallic compound (IMC) particles disperse uniformly in the bulk solder. After 200 h at 150 degC, the (Cu,Ni)6Sn5 ternary compound transforms to Cu-Au-Ni-Sn quaternary compound and the IMC at the interface of solder and under bump metallization (UBM) becomes a completely continuous layer. After 1000 hours at 150 degC, two IMC layers, light gray and dark gray, forme at the interface. EDX analysis confirms that the light and dark gray IMC layers are the (Cu,Ni,Au)6Sn5 compound. However, the light gray (Cu,Ni,Au)6Sn5 compound contains a higher amount of Au and a lower amount of Ni, while the dark gray (Cu,Ni,Au)6Sn5 IMC contains a lower amount of Au and a higher amount of Ni. After 2000 h at 150 degC, the two IMC layer structures still exist.