Can't route 0.4mm CSP [solved]

I’m a fairly experienced KiCad user, probably at least 50 boards under my belt. I just put down a footprint for a CSP package with 0.4mm pitch, and I can’t route it. I have the pad size at 0.25mm, mask 0.02 (over), and I’ve got the Pad Clearance down to 0.06mm.
I can’t initiate or complete a trace to any of the pads, using 0.18mm trace width (I plan to use 0.2 or wider but I changed down to 0.18 in an attempt to route).
There is something setting a larger keepout but I can’t figure it out.
FWIW, I have another BGA on this board, with 0.5mm pitch, and I can route that one just fine.
Any ideas?

Can you share the project or create an example project which shows the problem?

0.40 Pitch minus:
0.25 Pad size, minus:
0.12 2x pad clearance.

That leaves 0.03mm for the trace.
What happens if you try to route a trace with a width of 0.01mm. Does KiCad accept it or not?

Pretty recently we had a very similar question and it turned out there simply was not enough room to place the traces.

As a test you can connect all pins of a schematic symbol for your CSP footprint together in Eeschema, and then try to route the CSP footprint in Pcbnew.
Then you will probably see that it does not fit.

Thanks - I found out what was wrong. The default net class defined Clearance of 0.2mm. Routing with a 0.2mm trace, the trace was already 0.175mm away from the adjacent pad.I changed the default to 0.1mm and it routed. That is not clear, at least not to me. I would have thought that by changing the “Local Clearance and Settings” in the footprint properties it would over-ride the global settings. That’s what the “local” settings are for after all. But the Default Net Class settings seem to still have highest priority. Lesson learned.
And yes, 0.4mm pitch means clearance between pads is only 0.15mm, not much. But not trying to route between the pads of course. This chip is layed out so that you can “escape” without vias - pins are grouped together that way.

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It is not that one clearance has “priority” over another.
There are different clearances, and all the clearance rules must be met at the same time. (Or you can disable DRC)

Right, it’s like the guarantee policy of your car, it’s either n miles or m years, which one comes first :slight_smile:

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