If the terminations on your components are not barrier plated, you must be very careful about the amount of time you stay above liquidous. As you'd expect, longer is not better. The longer you stay at liquidous, the more of the component termination plating that leaches into the solder. If you stay too long, this leaching process consumes the termination plating and makes it look like the termination has dewetted.
As a tent stake in the sand, with barrier plated terminations, you can stay at temperature about ~4X the time you can stay with non-barrier plated terminations.
Beyond the barrier plating, some shops use a silver loaded solder to help reduce the degree to which the silver plating is leached from the termination. [We don't really know if this is true or not, but we do it. Probably one of those urban soldering myths. We used to believe that Ag2 would help reduce embrittlement caused by gold in solder connections, but at least we don't bite on that one any longer. We're making progress!!! ;-)]
For more, check with your component supplier. For instance: http://www.microcapacitors.com/pdf/multipart3.pdf
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