Microstructural Variation and Phase Evolution in the Reaction of Sn-xAg-Cu Solders and Cu-yZn Substrates During Reflow

This study aims to investigate the reaction of Sn-xAg-0.5Cu (x = 1.0 wt.% and 3.0 wt.%) solders on Cu-yZn (y = 0 wt.%, 15 wt.%, and 30 wt.%) substrates at 250°C for 0.5 min, 2 min, and 10 min, respectively. Cu and Zn atoms dissolve from the Cu-yZn substrates into the molten solders during reflow, leading to variation of the solder composition. It was revealed that such composition variation altered the microstructure of the solders. The coarsening of the eutectic region and the decrease of large-sized Cu₆Sn₅ compounds inside the Sn-1.0Ag-0.5Cu solder on both Cu-15Zn and Cu-30Zn substrates were correlated with this elemental redistribution. In addition to the solder matrix, the interfacial reaction was also affected by Zn dissolution. For a Zn concentration of 15 wt.% to 30 wt.% in the Cu-Zn substrate, formation of Cu₃Sn was suppressed. An increase of the Zn content in Cu₆(Sn,Zn)₅ at the solder/Cu-30Zn interface resulted in the formation of a new Cu(Zn,Sn) phase. It was demonstrated that the microstructural variation and the phase evolution in the solder joints were controlled by the reflow time and the Zn concentration in the Cu-yZn substrate.