| Popis: |
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°C. However, metallurgically, the Sn‐based lead‐free solders cannot approach a melting point as low as 183°C without incorporating a high content of low‐melting elements such as In, Bi 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 In ≤q 6 wt.%, Bi < 4 wt.% or Ga ≤q 0.5 wt.%. This paper summarises the results for one of the lead‐free systems studied within a ten‐year research program. |
