[bsder]

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.

[kens]

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.

[bsder]

I mean, if you have the boards, then you obviously have the definitive answer about layers. Can't argue that.

As you point out, if they designed this thing in the late 1970s, there is no reason for those giant arrays of drill holes. PCB design was definitely past this point by then and it would have been a hideous waste of time drilling all that just to fill them all up with wave soldering. It also blocks your routing terribly.

However, I assumed that this was likely a port of something from much earlier given the enormous lead times that aerospace requires (especially in the 1970s). There is absolutely no good reason to leave those extra holes which can become an assembly mistake otherwise.

"The Mitra 125, sometimes called "Mitra 15M/125" succeeded the Mitra 15 in 1975" That is the design that got used for the Spacelab Metra 125 MS in 1980, right?

I presumed that this was a port of a board which was a port of a board which was a port of a board given that design was obsolete even in 1975 since they apparently switched to the AMD bit slice processors even that far back.

And looking at Altair 8800 boards, you can see that the landing pads were very much NOT trivial, and look like they might even be hand drilled given the poor registration. Excellon/Esterline machines were still not that common outside of very high volume in 1975. By the time the Apple II came online a couple years later, though, the Excellon drilling machines were pretty commonplace.

[GlibMonkeyDeath]

>As you point out, if they designed this thing in the late 1970s, there is no >reason for those giant arrays of drill holes.

Yeah, this was precisely what my original question was about. Weird claims in this thread - we had multilayer boards with CAD layout in 1980, but can't drill through-vias in anything but a complete grid pattern? These are low-volume production boards too, trivial to drill an arbitrary pattern. (Edit: Obviously we did have multilayer CAD designed boards in 1980...)

My own (tongue-in-cheek) guess: someone prototyped this circuit on a perf-board with point-to-point soldering, then handed it off to a too-literally-minded junior engineer to do the PCB layout :) Unless there is some aerospace thing I am missing for having the full hole grid available...maybe they thought it was a weight savings?