Fundamental Study on Reflow Mechanisms of Sn and Sn Alloys for Fine Bump Pitch Scaling

Plated tin solder bumps with various alloy overlayer thickness were reflowed using formic acid vapor and traditional flux to characterize the process window of tin alloying solders that sscould improve the performance of small pitch FLI bumps or provide lower melting temperature. The results show that the presence of secondary metal overlayer plating increases the complexity of reflow and requires re-definition of reflow key process parameters (KPP) and process optimization. A mechanism to explain the reflow behavior of tin alloy solder bumps is proposed to understand the interaction between metal oxide and formic acid vapor.