[IAmBroom]

I thought the problem was primarily galvanic activity between copper and aluminum, not thermal growth. A bit of searching says this is so in environments with any wetting or very high humidity. The increased resistivity of aluminum's oxide layer is also part of it.

Basically: they just don't play well together.

[AnIrishDuck]

Yeah that checks out too. I'm basically regurgitating what I was told when working the electronics aisles at a hardware store.

After reading the wiki page more thoroughly, there were just a bunch of different issues. Goes to show what happens when you make a seemingly benign but fundamental change in a system full of parts designed for something different.