In-situ observation of material migration in flip-chip solder joints under current stressing

This investigation studies how electron flow distribution and the vacancy concentration gradient affect the diffusion of solder atoms in a flip-chip solder joint under current stress. The migration of materials was traced by monitoring the positions of 21 Pb grains of the eutectic PbSn solder joint. Experimental results indicate that the displacements of the Pb grains were not uniform along the electron flow direction. Additionally, certain Pb grains exhibited lateral displacements. The nonuniform material migration is attributable to the combined effect of electromigration and the vacancy concentration gradient, which was caused by electromigration. By measuring the displacements of the Pb grains, we estimated that the DZ* value of Sn in eutectic SnPb solder was 5×10 −10 cm 2 /s.