Recently someone asked me about reworking a SAC305 solder joint with a SnBi solder. The reason to consider SnBi was that the lower melting point of SnBi would make the rework process easier. The main concern the person had was what the properties of the reworked solder joint would be. Unfortunately no one can tell them. Note that I didn't say that there would be a problem, I just said that the properties of the resulting solder would be unknown. Let's see why.
The photo above shows a SnBiPb ternary phase resulting from mixing leaded and Bi containing solders. This phase has very poor mechanical properties as can be seen in the photo, which is taken after thermal cycling. The photo is from Zequn Mei, Fay Hua, and Judy Glazer, "SN-BI-X SOLDERS", SMTA International, San Jose, CA, Sept. 13-17, 1999.