Effect of Formic Acid Surface Modification on Bond Strength of Solid-State Bonded Interface of Tin and Copper

Abstract

The effect of formic-acid surface modification on the bond strength of the solid-state bonded interface of tin and copper has been investigated by SEM observation of the interfacial microstructures and fractured surfaces. Formic-acid surface modification was carried out by boiling a tin and a copper surface in formic acid for 600 s. Solid-state bonding was carried out in a vacuum chamber at a bonding temperature T of 403–473 K and a bonding pressure P of 7 MPa (bonding time: t=1800 s). The bond strength increased with an increase in the bonding temperature, independently of the formic-acid surface modification. Because of the surface modification, bonded joints were obtained at a bonding temperature that was 40 K less than the typical temperature required, and the bond strength was comparable to that of the base metal. When surface modification is applied, the oxide film is destroyed by the improved plastic deformability of the tin. Thus, the tensile strength of the joint is increased. On the other hand, when surface modification is applied, a high-tensile-strength joint is obtained at a low temperature because metallic tin and copper are exposed at the bond interface as a result of the decomposition of formate in the bond interface at a low temperature.