The effect of reflow temperature on time at the end of gravity zone (TGZ) of Sn-3.8Ag-0.7Cu solder alloy

Abstract

The reflow time for solder until the end of the gravity zone (T<inf>gz</inf>) is considered to be the optimum reflow time for obtaining high mechanical performance from lead-free solders. In the present work, the effect of reflow time and temperature on T<inf>gz</inf> of Sn-3.8Ag-0.7Cu (SAC387) lead-free solder alloy reflowed on the copper substrate has been investigated. The evolution of interfacial microstructure and solder bond shear strength under different reflow temperatures and time was assessed. Solder balls weighing 0.08 ± 0.01 g were reflowed at 260°C, 280°C, and 300°C for reflow times of 30 s, 60 s, 120 s, and 240 s. Times at the end of the gravity zone for SAC387 solder were obtained as 110 ± 5 s, 55 ± 5 s, and 23 ± 3 s for reflow temperatures of 260°C, 280°C and 300°C, respectively. The contact angle for SAC387 solder on the copper substrate at T<inf>gz</inf> was found to be 25.5° ± 0.2° for all reflow temperatures. Scanning electron microscopy revealed the formation of a Cu<inf>6</inf>Sn<inf>5</inf> intermetallic compound (IMC) layer at the interface. The IMC layer thickness increased with increase in reflow temperature and time. Maximum solder joint strength was obtained at T<inf>gz</inf> reflow times for all reflow temperatures. Microstructures of samples reflowed beyond the gravity zone showed secondary Cu<inf>6</inf>Sn<inf>5</inf> precipitation in the solder bulk. The present investigation reveals a reduction in T<inf>gz</inf> reflow time for SAC387 lead-free solder at higher operating reflow temperatures. © © 2020 by ASTM International