Comment by GlibMonkeyDeath
1 day ago
The PCB construction is curious (which you say is multi-layer) - why use a grid of 0.1" holes? Is that so it could be easily jumpered? Can you tell if the traces run through the holes or between them?
I don't have the patience to reverse-engineer these types of boards, but I do find them really interesting to think about. CAD was just getting started (I just looked up that Gerber format was released in 1980) so I wonder if the masks were hand-drawn.
The grid is 0.1" holes because that's the spacing of most components; it's a bit like perfboard. If you're asking why they used a grid instead of a normal-style PCB, I'm not sure. It probably makes manufacturing the boards much easier since you can drill the holes with an assembly line rather than one at a time. The traces go between the holes; the traces are very narrow, so two traces can fit between a pair of holes. That's probably the tradeoff, that your traces need to be very precise and you probably need more layers because of all the holes in the way. The layout was probably done with CAD; PCBs with CAD go way back. IBM was doing circuit boards with CAD back in the early 1960s, using a flying spot of light to draw out the PCBs on photoresist.
>The traces go between the holes
That's what I was curious about - otherwise there would be effectively be through-vias along the traces (so traces could be probed between devices.)
You can see how the traces go between the holes if you zoom in on this photo: https://static.righto.com/images/cimsa/board-back.jpg
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> 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.
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