Stability of AuSn eutectic solder cap on Au socket during reflow

The stability of Au20Sn eutectic cap formed during reflow of Sn/Au bump was studied under multiple reflow cycles. For a 5 mu m thick Sn layer, the eutectic caps formed for the bumps of 60 and 40 mu m in diameters were quite stable up to 10 reflows and the zeta compound layer at the interface was a good barrier layer to prevent the exhaustion of the eutectic solders. However for 20 mu m bumps, the relative larger eutectic volume resulted in side wall wetting. Therefore only small part of eutectic alloy was left on the top of the Au socket after one reflow. After three reflows Au socket was almost covered by a single zeta compound layer. The results identified that the maximum cap height formed during reflow should be smaller than half of the bump diameter to maintain it on the Au socket.