Reliability study of new SnZnAl lead-free solders used in CSP packages

We further mounted SnZnAl solder balls onto Cu/Ni(nickel)/Au(gold) plated polyimide substrate at a joining temperature of 215 °C for CSP applications. It was confirmed that the joint interface between soldered ball and substrate at the initial stage was made of 2-layered compounds, i.e., AuZnSn on soldered side and ZnSnNi on substrate side. After 1000 h aging at 150 °C, the two layers compounds become one layer of ZnSnNiAu compound. No strength deterioration of Cu/Ni/Au/SnZnAl after the 1300 h shelf test was detected in the ball shear strength compared with the initial value. This paper describes the characteristics of the new lead-free solder and the results of a study on its solder ball CSP package.