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

With the established surface mount infrastructure and the temperature const raints of components and printed circuit boards, the melting temperatures o f lead-free solder alloys need to be designed as close to 63Sn/37Pb as prac tical, and not to exceed 215 degreesC. However, metallurgically, the Sn-bas ed 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. S i or Ga. Incorporating such high contents of these elements involves prohib itive mechanical properties and/or cost. Nonetheless, it has been found tha t a low dosage of one or more low-melting point elements within a well desi gned alloy composition can achieve superior performance to 63Sn/37Pb withou t incurring an unacceptable cost. To avoid prohibitive cost and/or mechanic al properties, whilst achieving the melting temperature requirement. the th resholds for each of these elements, as examples. are found to be ln less t han or equal to 6 wt.%. Si < 4 wt.% or Ga less than or equal to 0.5 wt.%. T his paper summarises the results for one of the lead-free systems studied w ithin a ten-year research program.