Origin of primary Cu6Sn5 in hypoeutectic solder alloys and a method of suppression to improve mechanical properties

This study examines factor(s) behind the formation of primary CuSn (in the bulk, rather than at the interface) in solder joints, even though solder alloys are hypoeutectic. To understand the contribution from copper (Cu) dissolution from the substrate a Cu-free alloy, tin-3.5 silver (Sn-3.5Ag), was used as a soldered-on copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish substrates. Microstructure observations including in situ synchrotron were used to observe microstructure development real-time and confirm the time and location for nucleation of primary CuSn. High-speed shear tests were performed to determine the solder joint’s strengths. The results confirm that Cu dissolution during soldering is responsible for the formation of primary CuSn. The ENIG finish prevented Cu dissolution and the formation of CuSn resulting in higher solder joint strength for the Sn-3.5Ag/ENIG solder joints. The findings can be used to understand the evolution of primary CuSn and how it can be suppressed to improve joint strength