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

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.