Process variables influencing solder reinforced adhesive (SRA) performance

To improve peel strength of adhesively bonded joints, low melting point solder balls were added to adhesive as a means to bridge metallic substrates. This solder reinforced adhesive (SRA) has a potential to enhance the peel strength and energy absorption of adhesively bonded joints by bridging the two substrates via a metallurgical bond and thereby creating a mechanical obstacle to crack propagation in the adhesive layer. In a previous study, the effectiveness of SRA was demonstrated for joints fabricated with steel substrates in terms of cross tension strength and energy absorption however, the metallurgical connection between the solder balls and steel substrates was degraded owing to residual adhesive between the solder balls and substrates. In this paper, the effect of process variables, e.g., adhesive viscosity and solder ball size, on the residual adhesive layer was studied. Results showed that a low viscosity adhesive and larger solder balls help facilitate elimination of the residual adhesive layer. However, the cross tension strength of SRA joints dropped significantly because of low solder-substrate soldering strength. To enhance the bonding between the solder balls and substrates, the effect of flux and substrate material was investigated. It was found that flux could enhance energy absorption by 16.7% compared with pure adhesive bonding with a steel substrate though was unable to achieve the full theoretical potential because of severe pores in the adhesive layer resulting from the flux/substrate interaction. A switch to copper substrates resulted in an improvement of the joint strength and energy absorption by 10.7% and 36.9%, respectively, despite the existence of pores.