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

Abstract

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 Cu 6 Sn 5 . The thickness of the interfacial Cu 6 Sn 5 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 Cu 6 Sn 5 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 Cu 6 Sn 5 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 Cu 6 Sn 5 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 Cu 6 Sn 5 IMC layer and the average diameter of the interfacial Cu 6 Sn 5 grains increased with the increasing reflow time. The ratios of the average diameter of Cu 6 Sn 5 grains to the thickness of the interfacial Cu 6 Sn 5 IMC layer for all the four kinds of solder balls varied from 1 to 1.5, which showed a vivid growth of the interfacial Cu 6 Sn 5 grains from the perspective of 3 dimensions. Based on the data from the experiment, the dissolution kinetics of the Cu pad can be simulated.