Liquid-state interfacial reaction of Sn-10Sb-5Cu high temperature lead-free solder and Cu substrate

Sn-Sb alloys are potential solders for replacement of high-Pb solders because of their high melting temperature in lead-free solders. However, Cu substrate is extremely dissolved by the Sn-Sb binary alloy during the high temperature soldering process, which will cause serious reliability problem of the solder joint. Based on this critical issue, we designed a new high temperature lead-free Sn-10Sn-5Cu ternary solder to prevent the dissolution of Cu substrate. In this study, liquid-state interfacial reaction between the high temperature lead-free solder and the Cu substrate was investigated. The liquid-state interfacial reaction of the solder on the Cu substrate was carried out at the different temperature of 280degC, 320degC,360degC and 400degC, and the reaction time was 1min, 10mins, 30mins and 60mins, respectively. Microstructure of the Sn-Sb-Cu bulk solder and the solder joint was observed by scattered electron microscope (SEM). The identification of phase composition was determined by Energy Dispersive X-ray Detector (EDX) and electron probe microscopy analysis (EPMA). During the four reaction temperatures, the interfacial reaction products included a scallop Cu6Sn5 intermetallic compound (IMC) layer and a flat Cu3Sn layer adjacent to Cu substrate. IMCs thickness with the reaction time was measured by the area of interface IMCs layer divided by the interface length. The IMCs thickness increased with the reaction temperature and reaction time, and the relationship between IMC thickness and reaction time was linear with square root of time, which signified that the IMC growth dynamics was diffusion controlled. The diffusion coefficient was calculated by the IMC growth rate, which increased with the higher temperature, corresponding to be 2.30 times 10-14, 6.84 times 10-14, 1.63 times 10-13, 1.99 times 10-13 m2/s for the temperatures of 280degC, 320degC, 360degC and 400degC, respectively. And then the diffusion activation energy was determined to be 57.8 KJ/mol by fitting the four diffusion coefficients at various temperatures, which indicated that the diffusion mechanism was grain boundary diffusion. Between lower temperature of 280degC and higher temperature of 400degC, huge differences existed on the microstructure of IMC in the interior solder of the solder joint.