Effects of Zn and Ga Additions to Suppress PdSn4 Growth at a Solder/Pd Interface Under Current Stressing

PdSn4 is the major phase formed in the interfacial systems of Pd with lead-free solders. In this study, we examined the interfacial behaviors of the Sn/Pd/Sn system under current stressing of 5000 A/cm(2) at 180A degrees C. The results revealed that the electromigration effect on the PdSn4 was insignificant. However, the PdSn4 growth was very fast, similar to that without current stressing. The thickness of the PdSn4 layer was similar to 44 mu m after only 6 h of current stressing, and further increased to be similar to 83 mu m after a 24-h test. The PdSn4 phase had a superior electromigration-resistance, but the fast growth was a major concern for the interfacial reliability of the solder joints. To suppress the PdSn4 growth, minor amounts of Zn or Ga were added into the Sn-based solder to investigate the solder/Pd couples under current stressing. Remarkably, only minor 0.5 wt.%Zn addition effectively inhibited the PdSn4 growth under current stressing. After 6 h of current stressing, the thickness of the PdSn4 layer was reduced to similar to 17 mu m. In addition, minor Ga addition had a similar inhibiting effect. In the Sn-0.5 wt.%Ga/Pd couple, the PdSn4 layer was only similar to 12 mu m thick even after current stressing of 24 h. The inhibiting effects of Zn and Ga on the PdSn4 growth under current stressing and the related possible reasons are further discussed.