Interfacial reaction between liquid-state Sn-xBi solder and Co substrate

Abstract

The interfacial reaction between liquid-state Sn-xBi (x = 0, 1, 5 and 10 wt%) solders and Co substrate at 260 °C for different soldering duration time was explored to investigate the effect of Bi addition and soldering time on the growth of intermetallic compound (IMC) and grains evolution by scanning electronic microscopy in this study. The results of research indicated that the Bi addition into Sn solder had not changed the formation of interfacial IMC and the single CoSn3 IMC layer formed at Sn-xBi/Co (x = 0, 1, 5 and 10 wt%) joints. The addition of 1 wt% Bi into Sn solder suppressed the growth of interfacial CoSn3 IMC, compared to Sn/Co system. Bi segregation occurred in Sn-10Bi/Co couple due to the supersaturated Bi element and would weaken the inhibiting effect on growth of IMC resulted from more Sn and Co atoms activated. The thickness of IMC at the interface of Sn-5Bi/Co at the minimum number was resulted from Bi solid solution strengthening, Bi segregation attributed to Bi supersaturation and gravity subsequently appeared and gradually influenced the IMC growth when the soldering time increased from 20 to 60 min. Accordingly, the law of IMC growth did not meet the linear relationship between the thickness of IMC and soldering time. Additionally, the thickness of interfacial IMC increased with extending soldering time, regardless of Bi content in Sn solder. Prism-like CoSn3 grain formed at Sn-xBi/Co (x = 0, 1, 5 and 10 wt%) joints and the law of grain mean size was in well agreement with the change of thickness of IMC in cross-sectional images.