Reactions between Sn-Ag-Cu lead-free solders and the Au/Ni surface finish in advanced electronic packages

The reactions between Sn-Ags-Cu lead-free solders of various compositions and Au/Ni surface finish in advanced electronic packages were studied. Three solder compositions, Sn-3.5Ag, Sn4Ag-0.5Cu, and Sn-3.5Ag-0.75Cu were used, and their performance was compared. It was found that the Sn-4Ag-0.5Cu solder gave the worst results in terms of shear strength. The poor performance of the Sn-4Ag-0.5Cu solder can be explained based on its microstructure. The types of intermetallic compounds formed at the interface were different for different solder compositions. When there was no Cu the reaction product was Ni3Sn4. For the Sn-3.5Ag-0.75Cu solder, the reaction product was (Cu1-p-qAupNiq)(6)Sn-5 immediately after reflow, and two intermetallic compounds (Cu1-p-qAupNiq)(6)Sn-5 and (Ni1-yCuy)(3)Sn-4 formed after aging at 180degreesC for 250 and 500 h. For the Sn-4Ag-0.5Cu solder, both Ni3Sn4 and (Cu1-p-qAupNiq)(6)Sn-5 were present near the interface right after reflow, and there was a layer of solder between these two intermetallic compounds.