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meet , 03-11-2019, 03:57 AM

I am designing PCI express interface PCB, which will be used as card to insert in PCIe slot. Differential pair signal appears on both side of PCB (TOP and BOTTOM Layer). one differential pair is on TOP and another is on Bottom layer, so I have to use Via for anyone of the pair. the Diff. pair which is on TOP side don't need any via, as this is one to one connection.

I am planning a four layer board, as there is not much components. I am planning 2 power planes and TOP-Bottom for Diff pair routing.?

please suggest what should I follow..

Should I go for 6 layer and all signals in inner layer. In this situation, via Stub could be challenge...

Meet

Paul van Avesaath , 03-12-2019, 03:54 AM

when routing a diffpair from bottom to top or vice versa you do not have a stub..
you can design the via to be virtually 50 Ohm and SI wise you wont even know it is there...

so a 4 layer board would make sense.. keep the power planes solid underneath your traces and you should not have a problem.
make sure you have enoug clearance between via and internal planes to comply with the 50 Ohm and place simmetrical GND via's next to it.

robertferanec , 03-15-2019, 02:44 AM

I would also add to @Paul van Avesaath's answer, if only one of your Power planes is solid GND, then I would probably change the PCIE signals layer as soon as possible, so all PCIE signals will use the GND plane as their reference plane as much as possible.