Asymmetrical solder microstructure in Ni/Sn/Cu solder joint

Asymmetrical solder microstructure was observed in the Ni/Sn/Cu solder joint. The asymmetrical solder microstructure resulted from the Cu concentration gradient along the Ni/Sn/Cu solder joint. Mechanical tests showed that the mechanical property of the Ni/Sn/Cu solder joint is highly correlated with the asymmetrical solder microstructure.