Shear Strength of the Fluxless Solder Joints Between Sn and In Bumps

Abstract

We developed a new low temperature bonding technique using In bump and Sn bump. When the In and Sn solders are in contact under pressure, two solder bumps are bonded by interdiffusion between In and Sn solders. Interdiffusion of solder materials induces the formation of the intermediate phases and the melting temperature will decrease down to the eutectic temperature. During bonding, the oxide layers on the bump surfaces hinder In and Sn atoms from diffusion each other, so good bonding is difficult. Therefore flux was normally used in bonding process to remove the oxide layers in order to achieve good bonding. But the flux residues left behind after the bonding process can degrade or contaminate the electronic part. Furthermore, fine-pitch assemblies are increasingly difficult to be cleaned. Therefore, we introduce a new low temperature fluxless bonding method using Ag capping layer. In order to prevent the solder bumps from oxidizing, we used Ag capping layer which does not oxidize easily at ambient atmosphere