Effect of aluminum concentration on the microstructure and mechanical properties of Sn–Cu–Al solder alloy

• Microstructure of Sn–Cu–Al is refined by Al, and Cu6Sn5 IMC is suppressed by Al.
• IMC varies from Cu6Sn5 in Sn–Cu–(0.01–0.025)Al to Al2Cu in Sn–0.7Cu–(0.05–0.075)Al.
• Sn–0.7Cu–0.075Al alloy exhibits the superior wettability and tensile property.
• Stress concentration in the Sn–0.7Cu alloy occurs along the IMC-eutectic boundary.

The effect of aluminum on the microstructure and mechanical properties of Sn–0.7Cu–xAl (x = 0–0.075) solder alloy is investigated. The results show that the microstructure of Sn–Cu solder alloy is refined by Al. The intermetallic compounds (IMC) in the Sn–Cu–Al solder alloy is varied from Cu6Sn5 in the Sn–Cu–(0.01–0.025)Al to Al2Cu in the Sn–0.7Cu–(0.05–0.075)Al. The volume fraction of the IMC and eutectic is raised with increasing aluminum concentration. The wettability of Sn–Cu–Al solder alloy is improved by aluminum. The spreading coefficient of Sn–0.7Cu–0.075Al is reached 70%. The tensile strength of Sn–Cu–Al alloy is decreased after aluminum adding to 0.025 wt.% and then is increased after aluminum adding to 0.075 wt.% due to the IMC variation and the volume fraction increasement of Al2Cu/Sn eutectic with high microhardness. The superior mechanical properties of Sn–0.7Cu–0.075Al are guaranteed by the refined microstructure, the dispersed IMC and the uniform stress distribution. The Sn–0.7Cu–0.075Al is an optimized lead-free solder alloy for the copper substrate.