IIRC, copper conducts heat well, but doesn't shed it as well as aluminum. And it oxidizes. And don't dissimilar metals have different expansion rates? I would think that would lead to seperation. In practice, all that's needed is a slightly larger heatsink, anodized to reduce oxidation. Either that, or simply don't run it that hard. On second thought, all we need are FETs with extremely low rdson values and switch instantly on and off with no rise/fall time. Heatsinks would be a thing of the past.
