This study examines factor(s) behind the formation of primary Cu 6 Sn 5 (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 Cu 6 Sn 5 . 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 Cu 6 Sn 5 . The ENIG finish prevented Cu dissolution and the formation of Cu 6 Sn 5 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 Cu 6 Sn 5 and how it can be suppressed to improve joint strength.

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