Low-temperature solders (LTS), with reflow peak temperatures of 200°C and below, have been intensively studied since the 2017 iNEMI Board and Assembly Roadmap forecasts up to 20% adoption of LTS for board assembly by 2027. BiSn solders are the leading candidates in these studies because their low eutectic temperatures of 138°C allow the reflow peak temperature to be as low as 165°C to 170°C. However, their low melting temperature may also limit their operation temperature, which restricts the product application with a compromised reliability relative to SAC305. In addition, the intrinsic brittleness of Bi phases renders poor drop-shock performance. A Sn-rich In-containing solder paste - SnInAgCuX, developed on basis of a mixed solder powder technology reflowable at 200°C and above, was designed for excellent drop-shock performance with acceptable thermal fatigue behavior. The In-containing powder in SnlnAgCuX will melt first around 118°C to spread and wet. Then, the Sn-rich powder (melting point ＞217°C) will keep dissolving into the molten solder during reflow. After reflow. the solder joint has a remelting temperature around 189°C. The drop shock failure number of SnlnAgCuX (reflowed at 200°C peak temperature) is around two-orders-of-magnitudes higher than that of BiSnAg eutectic solder and another ductile BiSnAg solder. By optimizing the reflow profile, the drop-shock performance of SnlnAgCuX is superior to SAC305. Thermal cycling tests (-40 to 125°C. 10min dwelling) on BGA192s and chip-resistors (1206. 0805 and 0603) demonstrated the dependence of the characteristic life on the reflow profiles. A plateau type profile is preferred for excellent drop shock resistance and acceptable reliability performance.
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