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 150A degrees C and 200A degrees 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.