A high-performance lead-free solder - the effects of In on 99.3Sn/0.7Cu

With the established surface mount infrastructure and the temperature constraints of components and printed circuit boards, the melting temperatures of lead-free solder alloys need to be designed as close to 63Sn/37Pb as practical, and not to exceed 215 degreesC. However, metallurgically, the Sn-based lead-free solders cannot approach a melting paint as low as 183 degreesC without incorporating a high content of low-melting elements such as In. Si or Ga. Incorporating such high contents of these elements involves prohibitive mechanical properties and/or cost. Nonetheless, it has been found that a low dosage of one or more low-melting point elements within a well designed alloy composition can achieve superior performance to 63Sn/37Pb without incurring an unacceptable cost. To avoid prohibitive cost and/or mechanical properties, whilst achieving the melting temperature requirement. the thresholds for each of these elements, as examples. are found to be ln less than or equal to 6 wt.%. Si < 4 wt.% or Ga less than or equal to 0.5 wt.%. This paper summarises the results for one of the lead-free systems studied within a ten-year research program.