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

Abstract

  The effect of formic acid surface modification on the bond strength of the solid-state bonded interface of tin has been investigated by SEM observations of the interfacial microstructures and fractured surfaces. Formic acid surface modification was carried out by boiling a tin surface (finished by electrolytic polishing) in formic acid for 750 s. Solid-state bonding was carried out in a vacuum chamber at a bonding temperature T_j of 383-453 K and a bonding pressure P_j of 7 MPa (bonding time=1.8 ks). Because of surface modification, bonded joints were obtained at a bonding temperature that was ~50 K lesser than the typical temperature required, and the bond strength was comparable to that of the base metal. When the joint strength was much lower than the base-metal strength, a number of fine inclusions were found to be distributed in the interfacial region. As the joint strength increased with bonding temperature, these inclusions were coarsened, and their distribution density decreased. The increase in joint strength with bonding temperature corresponded well with the observed change in the size and density of the inclusions.