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?