Correlation Between the Growth of Voids and Ni3Sn4 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 Ni3Sn4 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 Ni3Sn4 at Ni/Sn-3.5Ag interfaces, under the same conditions. The growth of Ni3Sn4 layers thicker than 5 μm was correlated with the formation of voids in the solder near the Ni3Sn4 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 Ni3Sn4 layer, in direct proportion, consistent with a dominant Sn vacancy diffusion mechanism in the growing Ni3Sn4 layer (the ratio of average Ni3Sn4 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.