Correlation Between the Growth of Voids and Ni 3 Sn 4 Intermetallic Compounds at SnAg/Ni and SnAgCuBiSbNi/Ni Interfaces at Temperatures up to 200°C

The increased demand for microelectronic devices that function in hotter environments compels the study of Pb-free solders containing solid solution dispersoids (such as Bi and Sb), which are stable at significant concentrations in Sn at temperatures close to 200°C. In this study, the growth of Ni 3 Sn 4 intermetallic compounds was examined at Ni/Sn-3.7Ag-0.65Cu-3.0Bi-1.43Sb-0.15Ni solder interfaces at temperatures up to 200°C, and compared to growth of Ni 3 Sn 4 at Ni/Sn-3.5Ag interfaces, under the same conditions. The growth of Ni 3 Sn 4 layers thicker than 5 μ m was correlated with the formation of voids in the solder near the Ni 3 Sn 4 interface, and an order of magnitude increase in the reaction constant. An almost continuous line of voids formed at the Sn/solder interface, some time after the initial formation of voids in these diffusion couples. Continued heat treatment resulted in continued growth of both the voids and the Ni 3 Sn 4 layer, in direct proportion, consistent with a dominant Sn vacancy diffusion mechanism in the growing Ni 3 Sn 4 layer (the ratio of average Ni 3 Sn 4 thickness to average void thickness was one). At Ni/Sn-3.7Ag-0.65Cu-3.0Bi-1.43Sb-0.15Ni solder interfaces this occurred after only 250 h at 175°C; significant effects on the reliability of such solder joints would be expected.