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This is the first time I have designed a 4 layer board and the reading I have done says to use stitching vias for the differential pairs due to the stackup I am using. The stackup is as follows

SIGNALS+COMPONENTS

GROUND

POWER

SIGNALS

I have pasted an image that I used for reference when designing my board. For context the board I am laying out is for a Raspberry Pi compute module 3 IO board. It is a section of pins from the SODIMM connector with traces going to a DSI display connector.

I am wondering if I really need 2 capacitors for each side of a pair or can I just put single capacitors between the pairs due to space limitations (as I have it laid out now). Also should I be getting the vias from the caps a little closer to the signal vias?

The capacitors are all 0402 size with 100nF value (X7R 10%).

Thanks in advance!

enter image description here

enter image description here

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  • \$\begingroup\$ Do you understand what the caps are supposed to do? If not: hottconsultants.com/techtips/pcb-stack-up-6.html That should answer things. \$\endgroup\$
    – DKNguyen
    May 26, 2021 at 23:23
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    \$\begingroup\$ Put GND polygons in the VDD layer where the differential pairs are. Connect them to the GND plane near the signal vias. That way you have minimal discontinuities. \$\endgroup\$
    – Janka
    May 26, 2021 at 23:28

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Given that you also have to connect ground to the numerous ground pins on your connector, it would likely be better to simply move the L4-L1 vias as close as you can to the connector, and then surround them with ground vias connected to L3 ground and the connector ground pins.

This avoids having the return path transition from the ground layer to the power layer, and then have to transition again from the power layer to the ground contacts of the connector.

If you must keep the signal vias at some distance from the connector, you could consider making a split plane on L2 (and connect vias from this split plane to L3 near the signal transition, and from L1 to the split plane near the connector) to allow the signals to remain referenced to ground. Whether this is a good idea depends on where the power needs to flow to.

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