Tapeout: Better bumps
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[This is more of an FYI than an issue but I'm filing it here so we have a record of what's happening. The assignees are not being asked to do anything---I'm pretty sure I have it under control---- this is just a way to notify the interested parties.]
Mark sat down with me to look at bump routing. We decided that we could do better in at least two areas
- more power-pad connections per signal pad;
- straighter / more direct routes from power bumps to pad rings.
In particular, according to Mark, the important thing is to have plenty of grounding bumps/pads/wiring near signal output pads. So we looked at the lower left corner of the chip, where there are sixteen outputs. In the "before" image below, you can see that there are just seven power/ground bumps distributed amongst the 16 outputs. Six power bumps are available but unconnected because that's just how we did it last time, no one seems to know why.
In addition, you can see some wonky looking roundabout power routing e.g. the VDD and VDDPST pads near the center of the image, both of which loop up and around before finally coming back down to the pad ring. This is an artifact of how I decided to route the bumps, giving preference to signal over power lines and not trusting the tool to find good routes on its own.
So anyway we decided that I would try and
- include all the power bumps in the edge region, including e.g. the six that are unconnected in the example section;
- try a different routing strategy so that the power-to-pad-ring connections take more direct routes.
This second image shows more what we are targeting.
Of the twelve power/ground bumps in the original image, all are connected now, instead of just six. In addition, two of the three power/ground bumps along the left edge, not shown in the "before" picture, connect to the bottom ring---originally they connected to the left edge, farther away from the signal pads.
Also, most of the power bumps connect more directly to the pad frame, with fewer jogs and roundabout loops.