Strong effect of Cu concentration on the reaction between lead-free microelectronic solders and Ni

A slight change in the Cu concentration in the Sn-Ag-Cu solders can make a big difference in the strength of a solder joint in microelectronic packages. It was shown that (Ni1-xCux)(3)Sn-4 formed at the interface when Sn-3.9Ag-0.2Cu (wt%) solder was reacted with Ni and (Cu1-yNiy)(6)Sn-5 formed when Sn-3.9Ag-0.6Cu was reacted with Ni. The industry therefore must impose a very strict control over the Cu concentration to produce the desired intermetallic compound.