Effect of surface modification by citric acid on fluxless vacuum bonding of Cu with Sn-Cu alloy

The effect of surface modification of Cu and solder foil by citric acid was investigated on the joint strength and the microstructure of the fluxless vacuum bonded Cu joint. In the joint with surface modification, the joint strength increases with increasing bonding temperature and time. The deviation of the joint strength in the joint with surface modification is very lower compared to that of the joint without it. The rugged joint interface forms in the joint without surface modification. On the contrary, the relative smooth joint interface forms in the joint with it. It was clarified that surface modification of Cu and solder foil by citric acid has an effect to stabilize the reliability of the joint.