A comparative study of laser soldering and reflow soldering using Sn-58Bi solder/Cu joints

In this study, the bonding reliability and characteristics of laser soldering and reflow soldering using a low-melting temperature Sn-58Bi solder/organic solderability preservative finished Cu joints were compared and analyzed. Because laser soldering uses high density energy, it enables faster and more localized soldering compared to conventional reflow soldering. As a result of laser soldering, thin and small intermetallic compounds (IMCs) were formed under the condition of applying lower energy, but thick and large IMCs were formed under the condition of applying higher energy. As the bonding time increased at the same laser power, a relatively thick and long Cu6Sn5 was formed at the interface. To evaluate long-term reliability, a high temperature storage test (HTST) was performed at 110 degrees C, and Cu3Sn IMC formation was observed at the interface after HTST for 100 h under all laser soldering conditions. After 2000 h of HTST, the shear strength tended to decrease and the thickness of the interfacial IMC tended to increase. Initially, ductile fracture occurred, and the brittle fracture proportion increased as the HTST time increased. The joints formed under the laser soldering condition showed relatively higher shear strength even after long-term heat treatment compared to the reflow soldering joints. From these results, it was confirmed that the laser soldering process is superior in terms of long-term reliability of Sn-58Bi/Cu joint compared to the reflow soldering process.