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This is a followup to an earlier question.

In my experience, PCB fab houses always default to using prepreg and foil on the surface layers. For example, on a six-layer board, the stackup would be PCPCP:

enter image description here

However, it is possible to use a CPCPC, and one EE.SE user mentioned using PPCPP .

What are the considerations when choosing between these types of stackups?

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  • \$\begingroup\$ A symmetrical build to avoid PCB distortion \$\endgroup\$ – Claudio Avi Chami Aug 20 '16 at 1:22
  • \$\begingroup\$ Not a full answer, but the reason we went for PPCPP was to have four impedance controlled layers with reference planes on layers 2 and 5. The prepreg substrate being a rogers type (was a JESD204b design @ 5Gbps). Could have done 8 layers, but rogers is expensive stuff! The high speed signals were on 1 and 6, with the inner two layers being lower frequency impedance controlled signals and power lines. \$\endgroup\$ – Tom Carpenter Aug 20 '16 at 1:22
  • \$\begingroup\$ @TomCarpenter Thank you, but I'm not sure I'm getting it. Are you saying that you used rogers prepregs over a non-rogers core? \$\endgroup\$ – bitsmack Aug 20 '16 at 1:32
  • \$\begingroup\$ @bitsmack yes, using different dielectrics for the layers to prioritize high speed signals is definitely a thing. \$\endgroup\$ – user2943160 Aug 20 '16 at 1:34
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    \$\begingroup\$ @TonyStewart -- don't forget about embedded capacitance layers \$\endgroup\$ – ThreePhaseEel Aug 24 '16 at 0:00
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Generally Core is more reproducible than Prepreg, for both thickness and dielctric constant. It is made in a factory making core to a specific thickness.

When prepreg is assembled into the final board, which usually has a finished thickness specification, the thickness of the prepreg dielectric will have an additional variation due to the variations of all the other layers. Prepreg results in a wider range of dielectric constant on the board, because the prepreg supplier is only specifying the raw material, not controlling the assembly process.

This means that controlled impedance layers should ideally be core material. However, there are different specs for controlled impedance. Logic signals do not need as much control as printed microstrip filters, so using prepreg for the former need not be an issue.

When making our 6GHz microstrip 1.6mm thick board, we used rogers 4350 core for layers 1-2 and 9-10, and FR4 with whatever mix of core/prepreg the fab house were happy with inside.

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