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Having one of those moments again where you feel you are missing something seemingly obvious.

In multilayer PCB stacks, you have alternating layers of copper and substrate (either prepreg or core). The question is, does this result in voids?

Obviously the copper has some thickness, so if you pattern the copper, and then attach a layer of dielectric (e.g. FR4, rogers, etc.), then surely the areas where there is no copper would now be voids in the structure. Unless of course the dielectric is a conformal layer or the voids are in some way filled?

As an example. Take a design with a 6-layer stack up and say you want to do impedance controlled traces - is it possible to put a reference place on, say, layer 3, and the signal on layer 1 - assuming of course a gap is left in the copper on layer 2. If there are in fact voids caused by the thickness of copper on layer 2, then this wouldn't be possible because there would be a gap in the dielectric. I'm not planning on doing this, it's just a curiosity.

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    \$\begingroup\$ PCBs are definitely porous on a microscopic level, so there are voids. Your question seems to be more if there is something like an epoxy leveling the traces on the internal layers - it seems logical that there could be both intentionally and as an artifact of gluing the substrates together. That epoxy could ideally have similar dielectric properties to the binder in an epoxy glass board, but it is probably not perfect. \$\endgroup\$ – Chris Stratton Aug 21 '16 at 16:23
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typically, multi-layer boards consist of single-sided or double sided PCBs glued together. So that glue/lacquer fills these voids – the typical few µm aren't a problem to fill.

Now, I haven't thought about that, but obviously, a board manufacturer would choose a glue/lacquer that has comparable dielectric properties as your substrate. For FR4, that might be quite possible; whether that works with more exotic materials such as ceramic substrates would be something I'd definitely have to ask.

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  • \$\begingroup\$ Yep, thanks, knew I was missing something obvious - glue. It would certainly be interesting to know if the epoxy is chosen to match the dielectric characteristics of the substrate material, will do some research. \$\endgroup\$ – Tom Carpenter Aug 21 '16 at 16:29
  • \$\begingroup\$ @TomCarpenter my guess is that, yes it is, probably pretty close to the epoxy used to laminate the glass fiber layers in the substrate. That would also make sense, because it would minimize stress due to different thermal expansion. \$\endgroup\$ – Marcus Müller Aug 21 '16 at 16:31
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    \$\begingroup\$ Thinking about it further, it should be pretty much the same. Prepreg layers in multilayer PCBs are effectively substrate material impregnated with uncured resins, so should cure to match the core layer. Looking at the Rogers 4350 core and 4003 prepeg they are of similar Ek. \$\endgroup\$ – Tom Carpenter Aug 21 '16 at 16:47

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