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Take Care Mixing Solders in Rework


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.

Think about what is going on in reworking the solder joint with the above parameters. An unknown amount of SAC305 solder is being reworked with an uncontrolled  amount  Sn Bi solder. The resulting solder joint with be a mixture of the two solders with unknown percentages. So it is not possible to predict the properties of the reworked joint. I suppose if one knew the properties of all mixes of SAC305/SnBi from 0 to 100% you could at least bracket this performance. However, considering that the industry is crying out for more reliability data on SAC305 itself, it is unlikely that such data exists.
So the bottom line is to proceed with caution when mixing solders in rework. You will likely end up with a solder joint with unknown properties.


Dr. Ron

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.