BGA fanout with differential pairs coming out of the bottom two rows of balls
https://files.ioc.exchange/media_attachments/files/111/849/229/485/607/934/original/6cdcf3f250f03468.png
The RX pairs are easy to route on the top layer. I wasn't expecting a challenge there.
This is using 200 μm traces with 150 μm gaps: pretty tight coupling but that's necessary to avoid excessively large traces on OSHPark's 200 μm thick FR408HR prepregs. A loosely coupled pair with 50Ω single ended impedance for the legs would be 380 μm trace and space probably of 500μm or larger!
I could plausibly pull the gap all the way down to the fab limit of 125 μm but that felt like it might be pushing things just a bit too far in terms of repeatability. This is already a non-controlled-impedance process where I'm just hoping I get close enough that the SERDES work decently.
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