Influence of Bi Contents and Soldering Temperature on the Microstructure and Reaction Behavior of Low-Temperature Solders on ENIG Finish

Low-temperature solders (LTS) have attracted significant attention as key materials capable of achieving low temperature interconnection, minimizing thermal damage to heat sensitive substrates, and enabling environmentally sustainable manufacturing process. With the rapid expansion of flexible substrates in electronic components and the increasing size of display and semiconductor packages, controlling thermally induced deformation (warpage) had become a critical challenge. Conventional lead-free solders, such as Sn-Ag-Cu (SAC) systems, require a relatively high reflow peak temperature of around 245°C, often resulting in severe thermal expansion mismatch (CTE mismatch) and warpage in polymer-based or flexible substrates.</br>The Sn-58Bi solder had the low melting temperature as 138°C. This reduced melting temperature could effectively suppression of substrate warpage by reduced reflow peak temperature from 245 to 180°C. The solder alloys with varying Bi contents were reflowed at different peak temperatures to clarify the combined influence of alloy composition and reflow peak temperature conditions on solder joint formation. In the melting temperature, liquidus temperature was increased when Bi contents was reduced from 58wt% to 30wt%. However, joint strength was increased because off-eutectic composition as low contents of Bi did make the solid-solution strengthening. Moreover, joint strength was increased when reflow peak temperature was increased in off-eutectic LTS compositions except for Sn-58Bi solder.</br>Optimizing both Bi composition and reflow temperature was found to be critical for achieving a balance between mechanical reliability and stable interfacial reactions. This study provided insight into the design of reliable low-melting-temperature solder systems for ENIG finished substrates under low-temperature reflow conditions.