Voids between substrate layers

In multilayer PCB stacks, alternating layers of copper and substrate (like prepreg or core) are used. My question is: does this layering result in voids?

Given that the copper has a certain thickness, when you pattern the copper and then add a dielectric layer (such as FR4 or Rogers), it seems like the areas without copper would create voids in the structure. Are these voids somehow filled, or is the dielectric layer conformal?

For example, consider a 6-layer stack-up design with impedance-controlled traces. Is it feasible to place a reference plane on layer 3 and the signal on layer 1, with a gap in the copper on layer 2? If voids are indeed created by the thickness of the copper on layer 2, it wouldn’t work because there would be gaps in the dielectric. This scenario is just a curiosity, as I’m not planning to implement it.

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Lamination voids are possible with multilayer boards. There can be several causes from moisture to asymmetric copper features and other reasons. It’s generally not a common problem as the heat of the lamination cycle makes the prepreg soft enough it flows and fills such gaps. To prevent the problem the fab house will do things like pre-bake the materials to drive out moisture and add copper to problem layers. We manage to ensure lamination voids won’t occur by also using a vacuum system with the lamination press to insure the prepreg flow is sufficient to fill any possible voids.


@allank already answered, but something in your wording reminded me of how I thought when I was starting to learn about PCBs, so … just in case, I want to answer more explicitly. Hopefully the actual experts will correct me on anything I’ve still misunderstood.

Imagine you have a hard wooden table, with some legos on it. Now press a slab of playdough down on top of the legos. The playdough will get a little compacted, but will mostly flow around the legos. The table shouldn’t move (let alone deform), and the legos shouldn’t either, but if you press hard enough, the dough will act almost like water. (I think this is what you meant when you asked if the dielectric layer is conformal – and the answer is “mostly”.)

Afterwards, if you were like me, the legos were poking through. oops. But if you had used enough playdough, then the top of the dough would be fairly flat.

If you pressed hard enough and slowly and carefully enough, then there won’t be many large voids still filled with air next to the legos.

How thick the slab is will depend on how much dough you used, but also on how many legos were also taking up space. When you tip the slab over and try to get the legos out, the parts of the slab right over a lego will be thinner (counting only the dough) than the parts that touched the table.

The core is hard enough to function like that table.

The copper artwork is the legos. Ideally, they’re all the same height with vertical sides, but in reality, some legos are shaped kind of strangely, and the copper doesn’t etch exactly like a rectangular prism.

The prepreg is almost like the dough. The pre-preg’s own copper layer keeps the top of the pre-preg flatter than the top of the playdough. The press itself is flat enough that this mostly holds even when that layer is etched – pre-preg flows up to fill that void, though maybe not perfectly. The final thickness of the prepreg will depend on the copper percentage as well as the initial thickness.

Alas, pre-preg is less consistent than playdough. The resin can flow more easily than the fibers, and they have different thermal and dielectric constants. Different enough to matter sometimes, particularly when a design is sensitive enough that a small air pocket would be a problem. (So the voids aren’t nearly as large as the missing copper, but … the stuff filling them isn’t quite like the rest of the dielectric, which in turn isn’t quite like it was before the press.)

Trying to keep a consistent fiber/resin ratio across the whole board while also keeping the thing flat is one reason to worry about copper balance/copper thieving – if the resin has to flow across the whole board to fill in a void, it has more chances to … not do that.

If your prepreg is thirty times as thick as your copper, then the little bit that filled in a void won’t matter much. If your pre-preg is barely thicker than either surrounding copper layer alone, then … you want the copper balanced pretty well, so no resin has to flow very far, and any resulting change in the effective dielectric or thermal constants is at least uniform across the board, and hopefully something the fab house can predict based on the copper percentage.

And yes, you can remove a layer and use the next layer down as a (more distant) reference layer. The extra distance between them will matter far more than the remains of the gap between the prepreg layers.


Thanks for the insight! It’s good to know that steps like pre-baking and using a vacuum system during lamination help prevent lamination voids in multilayer PCBs.

Thank you for your detailed explanation! Your analogy with the playdough and legos really helped me visualize the process.

Typically, multi-layer boards are constructed by laminating single-sided or double-sided PCBs together, with glue or lacquer filling any voids. These voids are usually only a few micrometers in size and can be easily filled.
Board manufacturers generally select glue or lacquer with dielectric properties comparable to those of the substrate. For FR4, this is quite achievable. However, for more exotic materials like ceramic substrates, it’s essential to confirm the compatibility of the dielectric properties of the adhesive used.

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