Material behaviour of mixed formulation solders

Abstract

The transition from tin-lead to lead free soldering in the electronics manufacturing industry has been in progress for the past 10 years. In the interim period before lead free assemblies are uniformly accepted, mixed formulation solder joints are becoming commonplace in electronic assemblies. For example, area array components (BGA/CSP) are frequently available only with lead free Sn-Ag-Cu (SAC) solder balls. Such parts are often assembled to printed circuit boards using traditional 63Sn-37Pb solder paste. The resulting “mixed” solder joints contain unusual quaternary alloys of Sn, Ag, Cu, and Pb. In addition, the alloy composition can vary across the solder joint based on the paste to ball solder volumes and the reflow profile utilized. The mechanical and physical properties of such Sn-Ag-Cu-Pb alloys have not been explored extensively in the literature. In addition, the reliability of mixed formulation solder joints is poorly understood.

In this work, the creep properties of mixed formulation solder materials were explored. Seven different mixture ratios of 63Sn-37Pb and SAC305 solder materials 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 SAC 305). For the various percentage mixtures, the melting point, pasty range, and creep curves have been characterized. The variations of the mechanical properties and creep rates with aging at room temperature (25 oC) and elevated temperature (100 oC) have also been measured. Finally, the microstructures realized with the various mixtures have been studied for laboratory specimens and actual mixed formulation BGA solder joints. The results for the mechanical and physical properties show a very complicated dependence on the mixture ratio. In a second study on microelectronic solders, the temperature dependent tensile properties of SAC 305 and 63Sn-37Pb solder have been measured over a wide temperature range from -175 oC to +150 oC. The extracted material properties exhibit a linear variations with temperature, and lead free SAC 305 solder was found more sensitive to variations in temperature than Sn-Pb solder. This data provides the baseline input properties for finite element simulations of electronic packages.