I am designing a flexible PCB to carry two types of differential signals. One is about 1.2Gbps 100R differential impedance, while the other is only 3Mbps, with much more tolerance on the impedance.
It was very hard to achieve a differential impedance of 100R with such a thin dielectric, so it was suggested that I used a hatched ground plane to increase the impedance. However, it's hard to make the high speed pairs properly balanced on all parts on the hatching, since they go around some odd corners. Furthermore, the outer pairs are unbalanced all the way.
As an alternative, I was wondering about literally placing diamonds along the route of the pairs, like this:
This way I could achieve good balance for the whole route, and I could leave the outer pairs with a solid plane, rather than unbalancing them.
Lastly, I wonder if it's worth using circles instead of diamonds to reduce the stress concentration at the corners.
I wonder if what I am trying to do makes sense?
Best Answer
I think the hatched ground plane is more technological technique. It is used for better adhesion of layers on a flexible PCB or to reduce thermal dissipation in manual soldering.
Sometimes a single ground plane cutout is used to optimizing impedance when a signal meets a discontinuity.
It seems that impedance tunung with multiple cutouts is not a good and widespread practice because of electromagnetic compatibility. The HF-line with ground cutouts strongly resembles a slot antenna.