The supercomputers (all?) used wirewrap rather than PCBs. I heard a story once about someone coming in for a demo of a supercomputer and Cray realized there was a bug in the hardware during the demo and while the potential customers were at lunch, he rewired the machine to fix the bug.
Right. Seymour Cray said that the two big problems in supercomputing were "the thickness of the mat" (of wires on the backplane) and getting rid of the heat.
I wonder if any of his machines ever caught fire, causing an localized out-of-season aurora borealis. On the other hand, Cray machines are never mentioned in the "Halt and Catch Fire" machine code instruction humor.
