Intermetallic compound formation and solder joint reliability of Sn-Ag-Cu solder ball on Cu-Zn substrate

We recently developed Cu-Zn alloy wetting layer for the SAC solder balls to improve the reliability. In this paper, IMC formation and solder joint reliability have been investigated in the solder joint between SAC solder balls and Cu-Zn alloy pads, and Cu pads were also used as a reference. Sn-4.0Ag-0.5Cu solder balls were used and the substrates were Cu and 80 wt%Cu-20 wt%Zn pads which were defined with solder masks. The SAC solder balls on Cu or Cu-Zn pads were reflowed and the solder bump specimens were aged at 150 degrees C up to 1000 h. The CU6Sn5 and large Ag3Sn plates in the solder ball were formed at the SAC/Cu interfaces after soldering. The well-known double layer structure of Cu-Sn IMCs (CU6Sn5 and CU3Sn) and Kirkendall voids formed in the SAC/Cu interfaces during aging. On the SAC/Cu-Zn interfaces, CU6Sn5 formed after soldering and thickened with aging. However, CU3Sn fMC was not formed on SAC/Cu-Zn interfaces after aging. Also, the growth rate of IMCs at the SAC/Cu-Zn interface was much slower than that at the SAC/Cu interface. The ball shear test was conducted after soldering and aging to evaluate solder joint strength. The fracture mainly occurred in the solder and the shear strength value decreased with increasing aging time on both SAC/Cu and SAC/Cu-Zn. The shear strength of SAC solder on Cu substrates was lower than that of SAC solder on Cu-Zn substrates during aging.