Good eye — that continuous line is real. Zooming into just the photo (no overlay), you can see the trace itself runs right through the ink stamp without interruption — the ink is sitting on top of the solder mask, so seeing copper trace geometry "through" it just confirms it's genuinely part of the board surface at that spot, not a stamp artifact.
But before jumping to a 4-layer buried-plane explanation, I'd want to rule out a much simpler one: U25 and U26 are unimplemented footprints — nothing ever gets soldered there. If Radius's layout software saw those two pad rings as dead space (no populated pins to keep clear of), running an ordinary 2-layer trace straight through the middle of that footprint costs nothing and avoids needing a jog around it. That would fully explain a trace visually "connecting" U25 and U26 without needing anything exotic — it's not really connecting the footprints at all, just happening to physically transit the space between their pads on its way somewhere else on the net.
A true 4-layer power/ground-plane implementation would be a bigger claim, and worth being skeptical of by default: 4-layer boards were meaningfully more expensive to fab in 1986-88, and would be an unusual choice for what's otherwise a fairly conventional double-sided accessory card — though the trace density you've been finding isn't nothing, so I wouldn't rule it out purely on cost grounds either.
A few ways to actually settle it rather than guess:
Graphics workstation, now a Mirror Drive Door G4 setup upgrade, is Neolithic 2004 tech along with my software. Like my hardware, I live in the Neolithic when it comes to this kind of thing as well. Taking a file to the edge of the board at a low angle exposes layers like a spectrum. I can determine the number of layers without fooling around with less radical, error prone conventional approaches. ;-)
Fair enough — that's a genuinely better answer than either of mine. A low-angle bevel file exposes the whole stackup at once as a visible cross-section, no probing, no blind buzzing, no hoping a via barrel happens to be visible at the right spot. Metallurgical prep labs call it a "bevel section" and it's exactly the technique used for real PCB failure analysis when someone needs to count layers with certainty — you're not roughing it, you're just skipping the expensive lab equipment version.
One small technique note in case it's useful: keep the bevel angle shallow and consistent (something like 5-10°) rather than steep — it stretches out the layer spacing so thin internal planes don't get lost in file-scratches, and a drop of oil or water on the exposed face while you inspect it under light will make the copper-vs-substrate contrast pop a lot better than looking at it dry.
Good luck with the U25/U26 net — genuinely curious whether it turns out to be the dead-footprint explanation or an actual plane. Either way you'll have a real answer instead of a guess.
Very shallow angle and liquid emulsion are my methods!
Good — that tracks with knowing exactly what you're doing rather than winging it. Let me know what the bevel turns up on the U25/U26 net once you've had a look; genuinely curious which way it goes.
But before jumping to a 4-layer buried-plane explanation, I'd want to rule out a much simpler one: U25 and U26 are unimplemented footprints — nothing ever gets soldered there. If Radius's layout software saw those two pad rings as dead space (no populated pins to keep clear of), running an ordinary 2-layer trace straight through the middle of that footprint costs nothing and avoids needing a jog around it. That would fully explain a trace visually "connecting" U25 and U26 without needing anything exotic — it's not really connecting the footprints at all, just happening to physically transit the space between their pads on its way somewhere else on the net.
A true 4-layer power/ground-plane implementation would be a bigger claim, and worth being skeptical of by default: 4-layer boards were meaningfully more expensive to fab in 1986-88, and would be an unusual choice for what's otherwise a fairly conventional double-sided accessory card — though the trace density you've been finding isn't nothing, so I wouldn't rule it out purely on cost grounds either.
A few ways to actually settle it rather than guess:
- Check where that line's endpoints actually go. Does it terminate at real component pins elsewhere (i.e., it's just one ordinary net passing through the dead footprint en route from A to B), or does it appear to dead-end at U25/U26 pads specifically with no other explanation for continuity between them? The former supports "ordinary trace through empty space"; the latter is more interesting.
- Cross-check against your component-side trace/thruhole layers at the same XY. If there's also a component-side trace at that same location forming part of the same apparent net, that's double-sided routing, not a buried layer — no exotic explanation needed.
- Physical test for real layer count, if you want to settle it conclusively: look at a via barrel or the raw board edge (if any is exposed/unmasked) under strong magnification and backlighting. A 2-layer board's via cross-section shows plating only at top and bottom; a 4-layer board shows an additional annular ring or copper reveal partway through the barrel. This is the only truly non-destructive way to confirm internal layers without buzzing things out blind.
Graphics workstation, now a Mirror Drive Door G4 setup upgrade, is Neolithic 2004 tech along with my software. Like my hardware, I live in the Neolithic when it comes to this kind of thing as well. Taking a file to the edge of the board at a low angle exposes layers like a spectrum. I can determine the number of layers without fooling around with less radical, error prone conventional approaches. ;-)
Fair enough — that's a genuinely better answer than either of mine. A low-angle bevel file exposes the whole stackup at once as a visible cross-section, no probing, no blind buzzing, no hoping a via barrel happens to be visible at the right spot. Metallurgical prep labs call it a "bevel section" and it's exactly the technique used for real PCB failure analysis when someone needs to count layers with certainty — you're not roughing it, you're just skipping the expensive lab equipment version.
One small technique note in case it's useful: keep the bevel angle shallow and consistent (something like 5-10°) rather than steep — it stretches out the layer spacing so thin internal planes don't get lost in file-scratches, and a drop of oil or water on the exposed face while you inspect it under light will make the copper-vs-substrate contrast pop a lot better than looking at it dry.
Good luck with the U25/U26 net — genuinely curious whether it turns out to be the dead-footprint explanation or an actual plane. Either way you'll have a real answer instead of a guess.
Very shallow angle and liquid emulsion are my methods!
Good — that tracks with knowing exactly what you're doing rather than winging it. Let me know what the bevel turns up on the U25/U26 net once you've had a look; genuinely curious which way it goes.
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