Quality and microstructural analysis of SnBiAg-xTiO2 composite low-temperature solder joints

In the past years, low-temperature soldering has received a high attention due to the increasing number of IC deformation failures during the soldering process and the appearance of biodegradable substrates. In this study, composite low-temperature solders pastes were produced from 42Sn57Bi1Ag alloy doped with TiO 2 nanoparticles between 0 and 1wt%. 0603-sized SMD resistors were soldered on FR4 substrates having Cu wiring covered by ENIG surface finish by vapour phase reflow soldering. The mechanical properties of the solder joints were investigated by shear tests, and the microstructure of the low-temperature solder joints was observed by SEM and EDS. At lower weight fractions (0.125 and 0.25 wt%), TiO 2 could considerably refine the microstructure; the average distance between phase changes of the Sn and Bi phases in the composite SnBiAg solder joints decreased to 68-72% of the same distance in the reference SnBiAg solder joints. A similar 20% relative decrease in the intermetallic layer thickness was also detected in the composite SnBiAg solder joints. In the reference SnBiAg solder joints, large Ag 3 Sn agglomerates have been detected over the intermetallic layer. TiO 2 could suppress the aggregation of the dispersed Ag 3 Sn phases in the solder bulk. The microstructural refinements did not change the shear force (average was 47-48N) but resulted in enhanced ductility of the composite SnBiAg solder joints.