Effect of Zn on the intermetallics formation and reliability of Sn-3.5Ag solder on a Cu pad

Varying amounts of Zn (1, 3, and 7 wt%) were added to Sn–3.5Ag solder on a Cu pad, and the resultant solder joint microstructures after a reflow and isothermal aging (150 °C, up to 500 h) were investigated using scanning electron microscopy, energy dispersive x-ray, and x-ray diffraction, which were subsequently correlated to the results of microhardness and drop tests. Zinc was effective in improving the drop resistance of Sn–3.5Ag solder on the Cu pad, and an addition of 3 wt% Zn nearly doubled the number of drops-to-failure (N_f). The beneficial role of Zn was ascribed to suppression of Cu₆Sn₅and precipitation of Zn-containing intermetallic compounds (IMCs). However, the Zn effect was reduced as Cu₆Sn₅and Ag₃Sn precipitated in a joint IMC layer after prolonged aging. The interface between Ag₅Zn₈and Cu₅Zn₈was resistant to drop impact, but two other layered IMC structures of Cu₆Sn₅/Cu₃Sn and Cu₅Zn₈/Cu₆Sn₅were not.