The transition from tin-lead to lead free soldering in electronics manufacturing industry has been in steady progress for many years all over the world. However, this fundamental change in soldering technique is by no means an achieved task and there are still some critical issues waiting to be resolved. At the same time, an interim stage characterized by mixed application of tin-lead and lead free solders has been widely acknowledged within the industry.Both benefits and uncertainties involved in this interim stage have been gradually investigated and discussed. One critical issue that causes contradiction in study is the compatibility between lead free solders (especially Sn-Ag-Cu alloy system) and tin-lead solders. For example, area array components (BGA/CSP) with lead free SAC solder balls are often assembled to printed circuit boards using traditional 63Sn-37Pb solder paste. Some studies suggest improved reliability of solder joints, whereas others indicate inferior performance. This contradiction may be caused by various reasons, but the lack of data on the mechanical and physical properties of this Sn-Ag-Cu + Pb alloy formed at solder joints is the problem that needs to be addressed in the first place.In current study, the physical and mechanical properties of mixed formulation solder alloys have been explored. Seven different mixture ratios of 63Sn-37Pb and SAC305 solder alloys have been formed, which include five carefully controlled mixtures of the two solder alloys (by weight percentage) and the two extreme cases (pure Sn-Pb and pure SAC305). For the various percentage mixtures, the melting point, pasty range, microstructure and mechanical properties (modulus and strength) have been characterized. Aging effects have been explored in detail. The results show a very complicated nonlinear dependence of properties on the mixture ratio.
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