Effect of solder volume on interfacial reactions between Sn3.5Ag0.75Cu solder balls and cu pad

This study focused on the effect of solder volume on the interfacial reaction between Sn3.5Ag0.75Cu solder balls and Cu pads on PCB after various reflow soldering times. The diameters of the solder balls were 200, 300, 400 and 500 μm, respectively, and the opening diameter of the Cu pads was 250 μm. The solder volume ratio for the 200, 300, 400 and 500 μm balls was approximately 1:4:8:16. The interfacial intermetallic compound (IMC) was common scallop-type Cu₆Sn₅. The thickness of the interfacial Cu₆Sn₅ IMC layer was obviously thicker for the smaller solder ball after the same reflowing times from 1 to 5. After the first reflow, the thickness of the interfacial Cu₆Sn₅ IMC layer for the 200 μm solder ball was 4.39 μm compared to that of 3.15 μm for the 500 μm solder ball. The average diameter of the interfacial Cu₆Sn₅ grains for the smaller solder ball was significantly larger than that for the bigger one after the same reflowing times from 1 to 5. After the first reflow, the average diameter of the interfacial Cu₆Sn₅ grains for the 200 μm solder ball was 5.23 μm compared to that of 4.22 μm for the 500 μm solder ball. Both the thickness of the Cu₆Sn₅ IMC layer and the average diameter of the interfacial Cu₆Sn₅ grains increased with the increasing reflow time. The ratios of the average diameter of Cu₆Sn₅ grains to the thickness of the interfacial Cu₆Sn₅ IMC layer for all the four kinds of solder balls varied from 1 to 1.5, which showed a vivid growth of the interfacial Cu₆Sn₅ grains from the perspective of 3 dimensions. Based on the data from the experiment, the dissolution kinetics of the Cu pad can be simulated.