Solidification Behavior and Mechanical Properties of Sn-2.5Ag-0.8Cu-0.05Ni-1Bi and Sn-0.75Cu-0.065Ni-1.5Bi Solder Alloys, and Microstructures in Joints Formed Using Them

To form excellent solder joints in both thermal cycling and drop tests, Sn-2.5Ag-0.8Cu-0.05Ni-1Bi and Sn-0.75Cu-0.065Ni-1.5Bi composition solder balls were developed. In this study, undercooling and solidification characteristics of the alloys, resulting microstructural changes, the solid solution effect of Bi, physical properties, and interfacial reaction properties were investigated and compared with existing solder compositions of SAC305 and SAC1205N. The Sn-2.5Ag-0.8Cu-0.05Ni-1Bi and Sn-0.75Cu-0.065Ni-1.5Bi solders were found to have large undercooling of 38.36 degrees C and 33.38 degrees C, respectively. As a result, the Sn-2.5Ag0.8Cu-0.05Ni-1Bi solder ball had the smallest average size of Sn grains, and the eutectic structures between Sn grains formed relatively small areas and were observed to solidify into fine and uniform structures. Consequently, the total area of the beta-Sn phase decreased, while the total area of the eutectic structure relatively increased. Using XRD and STEM analysis, we observed that the addition of a small amount of Bi resulted in a solid solution of the beta-Sn phase, which increased the interplanar spacing of certain crystal planes, and contributed to the improvement in mechanical properties such as the hardness of the beta-Sn phase. When using the Sn-2.5Ag-0.8Cu-0.05Ni-1Bi solder ball, the intermetallic compound (IMC) layer at the bottom Cu pad interface of the solder joint was relatively thin from right after reflow soldering and maintained a thin thickness throughout the thermal cycling test. The growth suppression property of the IMC layer by Sn-2.5Ag0.8Cu-0.05Ni-1Bi composition was also confirmed in cases where the paste of this composition was applied to the existing solder ball.