Investigation of the Effects of Surface Finish and Reflow Conditions on the Microstructure and Mechanical Properties of Sn-based Solders

Abstract

To minimize solder joint cracking caused by the the polymorphic transformation in Cu6Sn5, the effects of surface finish and reflow conditions were investigated. It was found that interrupted-cooling in reflow profiles minimized cracking in the interfacial Cu6Sn5 intermetallic compound layer of Pb-free solder joints but slower cooling enhanced cracking. The interrupted-cooling reflow condition resulted in improved resistance of the reflowed solder ball to failure in high-speed impact shear tests. In Sn37Pb solder joints, the use of this interrupted-cooling condition did not significantly alter the microstructure or shear strength of the solder on either Cu-OSP or ENIG substrates. The solder joints subject to an interrupted cooling cycle are likely to have the best reliability.