Solid State Bonding of Tin and Copper by Metal Salt Generation Bonding Technique Using Citric Acid

In the previous studies, the surface modification effect of formic acid on the bonding strength of tin and copper has been investigated. As a result of previous investigations, it was found that the lowering of the bonding temperature is achieved by removing the oxide film on the bonding surfaces with formic acid, forming a metal salt film, and thermally decomposing the film. However, it has been pointed out that formic acid is toxic and irritating and difficult to handle. Therefore, in this study, we decided to investigate the effects on the removal of the oxide film on the bonding surface and the bonding strength using citric acid, which is relatively harmless to the human body. In addition to observing the fractured surfaces and the bonded interfaces of the solid-state bonded tin and copper by SEM, thermal analysis of the compound produced by surface treatment with citric acid was carried out. The metal salt coating treatment using citric acid was carried out by boiling the bonding surfaces of tin and copper in citric acid for 300 s. Solid-state bonding was performed in a vacuum chamber under the following conditions; 383-473 K for bonding temperature, 7 MPa for bonding pressure and 1800 s for bonding time. Regardless of the metal salt coating treatment, the bond strength of the joint increased with the increase of the bonding temperature. When the metal salt coating treatment using citric acid was applied, the bonding temperature was reduced by 70 K to fabricate a joint with the base metal strength of tin. However, when the metal salt-coated surface was exposed to the atmosphere, the bonded surface was again covered with an oxide film and the bond strength decreased.