Characterization of the melting and wetting of Sn-Ag-X solders

There is tremendous interest at present with Pb-free solder assembly in the surface mount assembly industry in response to recent Japanese and European initiatives and proposed governmental restrictions regarding Pb usage and disposal. Many different solder alloys have been proposed as potential Pb-free solder replacements and the most promising of these fall into the general alloy families of tin-silver (Sn-Ag), tin-silver-copper (Sn-Ag-Cu) and tin-silver-bismuth (Sn-Ag-Bi). Published melting point data on some of these alloys indicates that they should be capable of reduced reflow temperatures relative to the commonly available Sn-3.5Ag alloy, which melts at 221°C. Differential scanning calorimetry (DSC) and reflow visualization was used to characterize the melting and wetting of the Pb-free alloys and generate the practical reflow temperature requirements. This was compared to the DSC data to gain insight on the meaning of the DSC melting data for surface mount applications. The results show that, in general, the wetting performance of the Sn-Ag-Bi alloys are more similar to Sn-Ag and Sn-Ag-Cu than would be predicted by the major onset melting temperature data as measured by the DSC