Hi there. Noob trying to design something probably too complex for themselves.
I’m trying to wire up a TQFP-100 package using the KiCAD library
Package_QFP:TQFP-100_12x12mm_P0.4mm footprint.
I’m only using the default net class, which I’ve set to:
Clearance: 8 mils
Track Width 10 mils
Via Size: 32 mils
Via Drill: 16 mils
For the TQFP-100 I’ve set the local pad clearance to 0.01 mils, but I still can’t get 8 mil traces to connect to the pads.
What am I doing wrong?
Thanks,
Glenn
It works if I set the global clearance to 7, but I’d prefer to keep it at 8 except where you can’t route otherwise.
You would need to set clearance to more so like 0.1 mm (4mil), maybe 0.15 mm (~6mil) at the most. Otherwise you won’t have much luck.
0.4 mm pitch is 15.7 mils so 8+8 is too big, but 7+8 is OK. You can use 7 mils track + 8 mils clearance or 8 mils track + 7 mils clearance. As I know (not 100% sure) in KiCad you can’t set smaller clearance locally to allow it being smaller only here and keep it bigger elsewhere. As I have seen such solution is taken into account so may be in V6.
When I moved to use KiCad I also decided to use mm. So in your case I would use 0,2mm track + 0.2mm clearance. I have checked (at V4.0.7 level) that to work it that way I should set clearance to not 0.2 but to 0.199. Didn’t tested it in V5 but probably is the same.
I guess my question is:
What is the purpose of the local pad clearance on the footprint if it doesn’t override the global clearance of 8?
You can set it to a larger value than global (DRC takes the maximum set by the netclass or pad/footprint and zone settings)
If two clearance values are competing for one item, it’s logical that the higher value wins. That way both are satisfied.
If I have understood correctly a system with local values - for example tightly routed MCU packages - is planned, see https://gitlab.com/kicad/code/kicad/-/wikis/KiCad-6.0-Roadmap#constraint-management-system.
I was under the impression that the search order was pad->footprint->global with the lowest setting taking precedence and a 0 in the setting indicate use the settings from the next level up.
Constraint management does sound good.
Defining lower value for a footprint or pads helps with pad-to-pad clearance inside that same footprint. But a track which comes to pad naturally have the clearance value of the netclass, not that of the pad. So pad-specific clearance value serves a different purpose.
Ah, that makes more sense. Thank you.
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