How to add additional solder stop/coverlay layers for mid-layers

[thunderslug]

What I'm trying to do is design a stack of three two-layer PCBs that are soldered together. I want to design as a single 6 layer PCB so that I can see the layers on top of each other while I'm working. Maybe there's a different way to do this?

Is it possible to add additional solder stop/coverlay layers that correspond to mid-layers, in order to add pads to mid-layers and have them cut out the corresponding solder stop/coverlay?

I have tried using the Layer Stack Manager and Custom Coverlays, but it seems like I can only add a "dielectric" layer, and there is no way to get the pads in the mid-layer to make cutouts in the dielectric. Also I don't know how to get a gerber output for a dielectric layer.

[ahbushnell]

why not just build a 6 layer board? 

[thunderslug]

There are many reasons. However, they aren't relevant to the question.

[T3sl4co1l]

What's it matter?  Sounds like a normal board stack design where you only have mechanical constraints between boards, and presumably some board-to-board connectors.  Just plop in 3D models of the other board designs and keep them updated (link to file) as you build.

Tim

[thunderslug]

Thanks Tim,

You are correct, you could also think of it as a board stack design in which there are no conventional connectors, only openings in the dielectric layers.

I guess we could make some footprints for the different layer interconnections, and use mechanical layers for the additional coverlays. Will try that.

[T3sl4co1l]

So, not even components, just soldered as LGAs..?  Will that work? -- PCB is notoriously nonconductive thru-plane so it'll take forever to melt paste inside a sandwich of it.  Castellated side connections are fine, or vias/thru holes of course, but those also sound awfully more conventional than what you must be after.

And how is that different from a multilayer board with buried vias?  Why can't you route that on a single say 8-layer board, or optimize it down to 6 or 4 without stacking?  A single board will be stronger, too, and not left with leaky (flux residue) gaps between pads.

You can throw whatever you like on mechanical layers, but it all has to be done manually, so you don't get any advantage from placing pads (pad stacks only generate graphics on the hard-wired layers).

Tim

[free_electron]

inject a plane. the planes print as negative so they can be used as 'mask'
top soldermask
top1
bottom1
plane1 (fakemask)
top2
bottom2
plane2 (fakemask)
top3
bottom3
bottom soldermask

plane1 and plane2 have their clearance set to 4mils ( just like soldermask )

when you print those planes they will look just like soldermask.

so your first 'board is
top soldermask
top layer 1
bottom layer 1
plane1 used as mask

second board is

plane1 used as mask
top layer 2
bottom layer 2
plane2 used as mask.