I'm currently busy routing a high speed design containing MIPI signals @ 2,5Gbit / lane to a connector on an 8 layer board. As MIPI also contains a clock pair, it is advisable to match the length between pairs as best as possible.
I've come pretty far so far with matching <0,05mm, but I'm wondering what is the best option to extend the length of a trace, see below picture:
Some information: Blue = layer 1 Green = layer 3 Purple = layer 6
Layer 2 and 7 are ground layers
Option 1 is extending the traces on the inner layer and running them back on top to the connector. To me, this seems like an excellent option, as it gives me maximum spacing towards other traces. However, I'm wondering how much of an influence the traces running back towards the connector exactly on top of the layer 3 traces has. It is a differential pair, and there is a ground/reference layer in between, so I would think this would allow only minimal crosstalk or other effects.
Option 2 is the traditional serpentine tracing. This would be my least preferred option, but I don't have much choice because I can't extend the traces behind the connector.
However, if option 2 is preferred for whatever reason, I could change the option 1 by retracting the traces back towards the connector as much as possible, and add serpentine tracing on layer 3.
So my question, which of these 2 options would give best signal integrity and/or EMI compatibility?