surface/external edge coupled microstrip vs (internal) asymmetric edge coupled microstrip

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

• 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. Feb 22, 2021 at 19:01
• 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? Feb 22, 2021 at 19:08
• 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 Feb 22, 2021 at 23:46