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Why Solder To Steel When You Can Solder To Copper?

Okay, this has no use for semiconductor packaging that I know of, but it is a neat trick that I discovered in the lab a couple days ago. If you clean a piece of steel with HBF4 that has copper dissolved in it, the copper will lightly ‘plate’ onto the steel and passivate it. Copper is much easier to solder to, and typical fluxes should work (otherwise you need a very specialized iron flux). This opens up a world of possibilities for those rusty old steel surfaces.