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In the process of determining the track width/separation of differential pairs for a board design

that needs to route usb2, pcie(90R) , gigabit ethernet(100R), CSI/DSI(100R), HDMI

im looking a a four layer board with a stackup of

GND
PWR/Signal
PWR/Signal
GND

euro circuits does an impedance controlled stackup, based on is400, of

copper
prepreg PR2116 0.12mm
copper (35 um)
Core fr4-improved 1.2 mm
copper (35 um)
prepreg PR2116 0.12mm
copper

i can calculate the pairs using https://saturnpcb.com/pcb_toolkit/

i can't find a second calculator for (internal) asymmetric edge coupled microstrip or the actual equations for them

im trying to understand why when moving a track from surface to internal the track separation has to increase to get the same differential

e.g

Target 100R

surface W = .125 S = .2 H = .12 zDiff = 95.457

internal W = .125 S = .787 H = .12 h2 = 1.2 zDiff = 95.463

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  • \$\begingroup\$ Internal traces couple to two planes, one above and one below., while a trace on the outer layers only couple to one return plane. This reduces the impedance of each trace, and so the diff impedance. \$\endgroup\$
    – SteveSh
    Feb 22, 2021 at 19:01
  • \$\begingroup\$ Usually, the signal layers are not in the middle, but on top and bottom, so why do you want to do it like that? Also, are you sure 4 layers is enough? \$\endgroup\$
    – Justme
    Feb 22, 2021 at 19:08
  • \$\begingroup\$ ive been doing some research and on a four layer board and people like Rick Hartley say that its better for both emi and routing (no components in the way), i could go to six layers but world rather not if i don't have to \$\endgroup\$
    – fireblade
    Feb 22, 2021 at 23:46

1 Answer 1

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the calculations for internal trace impedance is less precise than for external traces, which is why they are often ignored. I've built boards like this before for EMC reasons (in radios) so it is workable. but the strip-lines are best on the surface with GND underneath) since the matched pair will couple to each other. The other thing to do is talk to the PCB manufacturer, often they can tell you the trace width to use for a particular stackup. With the frequencies you are talking about, it would be a good starting point as they might recommend certain prepregs to make it easier. see also https://www.eeweb.com/tools/edge-coupled-stripline-impedance/

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