Just talked to Molex about a process they have for making 3 layer boards. The center layer is stamped copper, so can be as thick as you want. (Could be .1" copper if you wanted that thick a materials.) The top and bottom layer are more conventional plated copper. This seems like it would be great for motor inverters; drain and source go to copper bars, and everything else is a plated trace.
Paper on this:
Paper on this:
