Browsing Wikipedia (as one does), I have stumbled upon this picture of a circuit board¹:
Now, when you look at it, there are several things that stand out as being interesting and different to today's commonly used PCB manufacturing techniques.
I don't have access to the board pictured, but here² are some more pictures of it. However I do recall seeing other circuit boards using these techniques; especially on lower-cost devices from the late 80s/early 90s and remote controls.
So, here is what I'd like to find more about:
There seem to be at least two levels of soldermask. One of which is visible only around the vias, and the other opaque one covers and conceals stuff further.
What's the purpose of this second, very opaque layer?
To hinder reverse-engineering of the PCB layout. This however doesn't make that much sense to me, since the PCB pictured comes from an MSX computer, and AFAIK the MSX architecture was pretty much an open standard. Also, this additional opaque layer is absent on the bottom side.
Or is this not just an opaque soldermask, but that and another (rough) conductive layer over another (what you'd think of as traditional) soldermask; which would make some sense here too, perhaps for EMI shielding. Is this it? Another thing going for it is the large clearance around the vias.
Or is it something else entirely?
Don't the vias look interesting? How were those made? It seems that they did not use (now ubiquitous) plated through-holes manufacturing this board.
But what is this that they used instead and how were those made? Is this some sort of conductive epoxy?
The (relatively) large via size could be justified by it. It also seems like the material over the vias is rather concave with likely designed-in overlap over exposed copper around the hole. Were those filled-in one-by one?
Now, why did they use this method instead of the "modern" PTH? I suspect it must have been significantly cheaper for small enough number of holes. What would this method be called?
Searches for epoxy-filled vias do yield results, but of different-looking ones to this.
This is my first question here after all this time; I know however that having multiple sub-questions is sometimes frowned upon, but I hope this still counts as being reasonably related enough. Thank you for your answers.
I also hope some others find this as interesting, with the epoxy via technique perhaps finding another life in my homemade PCBs.
²) MSXinfo.net article on Panasonic FS-A1WX