Comment by bsder
18 hours ago
> The PCB construction is curious (which you say is multi-layer) - why use a grid of 0.1" holes?
The claim is multi-layer, but I seriously doubt that. I suspect that these are two-layer boards.
And if that's the case, the pattern is most likely because the holes precede the etch. And possibly precede the copper deposition so that the copper deposition can coat the insides of the holes.
And the holes are in a regular pattern because CNC simply wasn't a thing yet. You probably had some fixed array of drill bits that were used to make the holes in a very strict fixed automation fashion.
Why do you doubt that these are multi-layer boards? IBM was making four-layer boards exactly like these in 1964 for the System/360 (signals on the top and bottom layers, power and ground in the middle layers).
IBM was using flip chip in their mainframes in the same timeframe, too. That doesn't mean usage was widespread. This was 1970-1975, after all.
In addition, 2-layer has some big advantages over 4 layer for reliability (won't delaminate under launch vibration, for example)--which is an issue in aerospace.
And, to my eye, these boards simply don't look like the have 4 layers nor are the laid out like that: https://upload.wikimedia.org/wikipedia/commons/7/7a/Mitra_15...
Besides, even if it were 4 layers, the issue is still that drilling holes in a non-regular pattern simply wasn't something that could be done easily 1975.
I don't want to be rude, but I am not following your argument here. a) The computer that I'm examining is from 1980, not 1970, so I don't know why you're changing the time period. b) I have the circuit boards in front of me. They have more than two layers. c) You linked to a photo of a completely different computer. d) Drilling PCB holes in a non-regular pattern was trivial in 1975 and how most PCBs were built. Look at the S-100 boards from the Altair 8800 (1974) or look at the Apple I circuit board (1976) for instance.
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