Intermetallic Compound Formation and Evolution in Solid-State Sn/Immersion-Ag/Cu Trilayer Interfacial Reactions on a Flexible Polymer Board

Immersion Ag is a promising candidate Pb-free surface finish on printed circuit boards (PCBs). For flexible PCB and optoelectronic packaging, solid-state bonding rather than reflow is commonly used to join the chips to the PCB with Sn-based solders, after which the immersion Ag layer remains at the joint interface and participates in the interfacial reactions at the solder joints. Solder joint samples composed of a Sn/Ag/Cu trilayer on flexible PCBs were prepared to study the interfacial reactions at 150°C and 200°C. Three phases, Ag₃Sn, Cu₆Sn₅, and Cu₃Sn, were sequentially formed at the interface. Remarkable change of the morphology of the Ag₃Sn phase was observed during thermal aging. The thickness of the immersion Ag layer was found to have significant effects on the growth rates of the Cu₆Sn₅ and Cu₃Sn phases and the void formation in the Cu₃Sn phase.