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Recently it come to my attention that using two different lead-free solder alloys in one process should not take place.

Example: populated PCB is flown on wave solderer with alloy X and then (if required) alloy Y used to rework any nonconforming joints. Is this practice allowed? What kind of electrical/mechanical downsides can be expected from reliability of this joint in long-term situation? Unit is exposed to vibrations and shock in it's life.

Thanks for answers, Seb

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Generally speaking, there is no need to match rework solder to the original reflow material. Not only that, it may be difficult to find out exactly what the original material was. A possible exception would be a board that was assembled using a very low-temperature alloy, while the rework uses a much higher-temperature alloy, and the part spacing is so dense that heat from the rework can affect the joints of nearby parts. This is unlikely, but possible, and would affect rework techniques.

But keep in mind that reflow and rework are not one process - they are two separate processes, so your original caution does not apply.

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