I’m routing QFN package and I’m using net class for signal with higher clearance to avoid crosstalk etc. My track width is 0.15mm and clearance 0.3mm. Near the QFN component, I’m not able to route the signal because tracks are violating clearance rules. I can allow smaller clearance near the component.
What is the best way to handle the situation? I’m using walk around mode when routing. Should I change routing mode to highlight collisions and allow DRC violation? Or is it possible to define some kind of area, where smaller clearance is allowed?
Try Board setup > Design Rules > Custom rules. Top RH corner of newly opened window is “Syntax help”
At the bottom of this help, if there is nothing to suit or modify in the list, is a link to the full documents.
“BGA neckdown” in the syntax help: condition "A.intersectsCourtyard('U3')".
See also PCB Editor | master | English | Documentation | KiCad in the full help (BTW, “master” in the link refers to the ongoing development version, the link was in v9.0RC). enclosedByArea('x') is another option.
The difference between “intersects” and “enclosedBy” is important. There’s no way to “slide” from one clearance to other. Clearance is always strictly between two whole objects, and the rule engine decides which rule exactly is in force between two objects under consideration.
“Intersects courtyard” means that if two track segments cross that same courtyard line, the specified (smaller) clearance is applied to those segments all the way, even outside the courtyard.
If the condition is “enclosedByArea” you have to draw a rule area explicitly. Then the rule – smaller clearance – is applied to two track segments which are totally inside the area, and they must end within that area. The normal larger clearance is forced to the segments intruding the area.
So, after creating the rule you can’t just continue with the router and hope it does everything automatically as if it were an AI. You have to have a strategy, how the track segments from outside the area are connected to the tracks from inside, or vice versa.
I am presently using EnclosedByArea in v9 for this since the courtyard rule option is too close to the pins and you need to fan out. Now I am using a 3W rule to minimise crosstalk (not 2W like the OP) so maybe they can fanout
in v8 there is only intersectsArea, which means you need to be careful and work on sections but it does work - see below
