Study on Void Growth in Micro-Size SnAg Solder Bump

The Sn-3.0Ag solder bumps with size less than 100 mu m were fabricated by electroplating. The evolution and distribution of the void at the SnAg/Cu interface and its influencing factors were studied. The results show that the volume shrinkage during phase transformation was believed to be the main reason for the void formation during multi-reflow, while the Kirkendall effect was believed to be the main reason for the void formation during aging. The void growth in the Cu3Sn (epsilon-phase) was affected by the initial morphology of the Cu6Sn5 (eta-phase), a thick eta-phase and the voids among its boundaries promoting the void growth in the epsilon-phase. The layered epsilon-phase/Cu interface and interconnected voids in the epsilon-phase would be threats to the long-term reliability of solder bumps.