Electronic – Four layer PCB stackup with thick prepreg layer: how is it useful

pcbstack up

I've been working on a four layer board with 100Ω differential pairs. Prototypes were built, impedance was measured, things were fine. But then as I tried to move the production to a different facility, I discovered that some PCB fabs use a much thicker prepreg layer (dielectric between top layer and inner 1 copper).

If you look at specs from MacroFab (https://s3.amazonaws.com/mfprodpublic/datasheets/MacroFab+Stackup+Report.pdf), they use a 0.23mm prepreg layer. OSHPark has an even thinner 0.17mm (6.7mil) prepreg (http://docs.oshpark.com/services/four-layer/). I've seen similar numbers in standard stackups from Chinese manufacturers as well.

But then I encountered a local fab that specified 0.36mm, and looking around I saw that the Eurocircuits standard 4 layer build has a 0.36mm (14mil) prepreg layer (http://www.eurocircuits.com/images/stories/ec09/ec-std-buildups-0-8-layers-english-4-2010-v2.pdf).

I am puzzled as to how this kind of stackup can be useful.

Assuming 0.1524mm (6mil) trace separation, 35µm copper, with a 0.23mm prepreg, I calculated 0.233mm width for 100Ω differential pair (differential microstrip) traces. That's about 9mil, and it's perfectly fine.

But again with a 0.36mm prepreg (same 6mil trace separation) I end up with my differential pair traces having to be 0.32mm wide — 12.6mil! That seems too wide to be useful, you can't route those traces to 0.5mm pitch ICs. And things get even worse if you need 90Ω (USB).

So, what am I missing? From this point of view, the thick-prepreg stackups of some manufacturers are useless. But they exist (and in fact are standard!) for some reason. How do people use them?

Best Answer

So, what am I missing? From this point of view, the thick-prepreg stackups of some manufacturers are useless. But they exist (and in fact are standard!) for some reason. How do people use them?

I think what you're missing is the fact that majority of four-layer boards will not have ANY controlled impedance lines. Therefore, any old prepreg thickness will do.

Other than controlling trace impedances, four layer boards are useful for improving EMI performance by providing better grounding, allowing more dense component placement and simplifying routing.

It would be interesting to know what factors determine the PCB stackup in these cases, though. Maybe it is the ease of manufacture or cost of material?

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