Effects of Bi contents in Sn–5Ag lead-free solders on mechanical properties and morphology of IMC

This study explores the effects of adding different contents of bismuth (Bi) to Sn-5Ag lead-free solder on its microstructure, mechanical properties and formation of intermetallic compounds (1M C). As electronic products develop towards high density, miniaturization and high performance, the demand for lead-free solder becomes increasingly urgent. Sn-Ag alloy is the first choice for lead-free solder due to its excellent thermal stability and mechanical strength, but during the reflow soldering process, large brittle flaky Ag3Sn IMC is formed, which may cause the integrity of the solder joint to be damaged. This study aims to improve the morphology of IMC and enhance the mechanical properties of the solder by adding O.5wt. % and 1 wt. % bismuth to Sn-5Ag solder. In the experiment, SAC1205, Sn5Ag, Sn5AgO.5Bi and Sn5Ag1Bi solder balls were used for reflow soldering on the ENEPIG substrate, and three different cooling rates (O.5°C/s, 1°C/s and 2°C/s were used)). The morphology and distribution of IMC were observed by optical microscopy (OM) and scanning electron microscopy (SEM), and the mechanical properties were measured by tensile test. The results show that adding bismuth can effectively reduce the size and thickness of Ag3Sn IMC, forming a more uniform and compact microstructure. The increase in cooling rate further improves the structure of the IMC, resulting in a more uniform grain distribution. Mechanical property testing results show that the addition of bismuth increases the modulus of the alloy (up to 47 GPa) but reduces the ductility, showing a trade-off between strength and flexibility. Overall, the findings of this study indicate that by optimizing the bismuth content and cooling rate, IMC formation can be improved while improving mechanical properties, thereby enhancing the reliability and performance of lead-free solders in electronic packaging.