HDMI Layout Notes for EOMA68 Cards by Richard Wilbur Thu 3 Aug 2017
Recommendations for this Layout
Source (processor) end:
Could we shrink the via pitch between constituents of a differential pair (bring the vias of a differential pair closer to each other) and combine the anti-pads into an oval shape on each layer? This reduces fringing fields and thus parasitic capacitance.
I like the ground vias close to the signal vias between differential pairs. It would be lovely to get a ground via close to the via on HTX2P and possibly move the one between HTX2N and HTX1P closer to the signal vias (if the signal vias of pairs can be moved closer with combining the anti-pads).
What is the intra-pair skew from the processor lands to the first signal vias? I wonder if we could move the vias on the short lines a little further from the processor and make up some of the skew in that segment before we leave it?
Sink (connector) end:
Same thing for differential pair vias--HTX0 and HTX2--it would be lovely to shrink the via pitch and combine anti-pads (if possible). Again, I like the ground vias close to the signal vias HTX2P, HTX2N, and HTX0P. It would be lovely to be able to either put a new ground via closer to the signal via on HTX0N or move the one on the ground shield trace closer.
I like what you were showing on the video with the signal vias at the connector lands: putting a neck on the trace between the via and the land should dampen the spirits of the solder but not the signals.
If we could reduce the signal via pitch by combining anti-pads at the connector, we might be able to move the HTX1P and HTXCN signal vias to the other side of the lands next to the other side of the differential pair, thus equalizing the skew on the segment between the ESD chip pads and the connector pads. If that worked the final touch might be to add a ground via between DC3 pin 10 (GND) and the board edge for return current paths.
Other than that, I would try and move as much of the skew compensation close to the source of the skew as possible.
I'm not sure what I'm looking at as you mentioned the ground reference planes were solid under the HDMI differential pairs, but it looks like they have voids under the signals in the pictures. Am I seeing a negative, that there are only little strips of conductor in the ground reference plane directly under the high-frequency lines? Neither of these interpretations is very satisfactory, nor do they seem to represent reality.
Please let me know what can and can't be done and I will adjust recommendations accordingly.