Demonstration and Characterization of Sn-3.0Ag-0.5Cu/Sn-57Bi-1Ag Combination Solder for 3-D Multistack Packaging

A combination solder of Sn-3.0Ag-0.5Cu (numbers are all in weight percent unless specified otherwise) wrapped by Sn-57Bi-1Ag was tested for application to three-dimensional (3-D) multistack packaging. The experimental variables controlled were the reflow peak temperatures (170, 185, 200, and 230°C), the reflow cycles (up to four times), and the mask which controls the amount of Sn-57Bi-1Ag solder paste (two sizes). We demonstrate and evaluate the combination solder structure, focusing on microstructural changes and the shear strength. The degree of mixing in the combination solder, which is enhanced by an increase in the reflow peak temperature, is independent of the number of reflow cycles. The ball shear strength and the lab shear strength both increased with increases in the reflow peak temperatures. This behavior is explained by the amount of the brittle Bi phase that constitutes the eutectic Sn-Bi phase.