How to Set Different Clearance Between Pads and Traces (Same Net) in KICAD 8

[rayan]

Hi everyone,

I’m working on a layout that includes multiple differential pairs (like MDI1 and MDI2), and I’m trying to maintain proper spacing between these different pairs to minimize crosstalk—similar to the layout shown in the attached image.

To achieve this, I set a clearance value between the pairs (for example, 5W which is .7mm). But the issue is: when I apply this clearance, I’m not able to route traces out of the pads—the clearance rule blocks it due to tight pin spacing.

Is there any way in KiCad to apply a different clearance near the pins (so routing is still possible) while keeping a larger clearance between the differential pairs in the routed area? Basically, I want more spacing between different pairs, but looser rules near the component pads.

Spacing provided for the differential pair

The clearance around the pad makes routing at the pad impossible.

Any advice or workaround would be really appreciated.

Thanks in advance!

[amiller112]

Hi, for some reason, KiCAD is set up that the clearance is also the distance between the differential pairs, so it has to be set at or less than the DP Gap setting to allow routing.

One way round it is to set the clearance as or less than the DP Gap and use a custom rule that also sets the clearance between differential pairs as bellow:

(version 1)
#LVDS Clearance Between Diff Pairs
(rule "LVDS Clearance"
	(condition "A.hasNetclass('LVDS') && B.hasNetclass('LVDS') && !AB.isCoupledDiffPair()")
	(constraint clearance (min 0.7mm)))

Im not sure how to disallow it just for the pads.

Edit: the class I was using was called LVDS, change this to your class name.

[rayan]

Thank you for your response. I truly appreciate your help. If you don’t mind sharing, may I ask where you found this design rule?

To set the clearance for the pad in the footprint, I added this, and it worked.
(version 1)
(rule clearance_U12
( constraint clearance (min 0.1mm))
( condition “A.insideCourtyard( ‘U12’)”)

[rayan]

(rule “DPMDI Clearance”
(condition “A.hasNetclass('DPMDI ') && B.hasNetclass('DPMDI ') && !AB.isCoupledDiffPair()”)
(constraint clearance (min 0.7mm)))

showing error

[paulvdh]

Custom Design rules Examples

Reduce clearance inside courtyards. Tracks are often narrower then pads, and this limits the maximum clearance that can be used for tracks, while wider clearances for tracks are often preferred to reduce capacitive coupling between adjacent tracks. Especially for fine pitch footprints it is nice to fine tune this. This can also help if you prefer to use a combination of track width + clearance that fits on your grid. (rule clearance_U1 ( constraint clearance (min 0.2mm)) ( condition "A.insid…

[eelik]

@rayan

showing error

…aaaand KiCad version?

[rayan]

KiCad version is 8.0.4

[rayan]

(rule clearance_J9
( constraint clearance (min 0.2mm))
( condition “A.insideCourtyard( ‘J9’)”)
)
I tried creating a design rule for pad spacing, and it’s working. Is it also possible to write a rule to enforce a specific clearance between MDI1 and MDI2, as shown in the image?

[eelik]

V8 doesn’t have “hasNetClass”. Use either KiCad v9 or documentation for v8.

[paulvdh]

You should not be using KiCad V8.0.4. Update to V8.0.9. It has a lot of bug fixes.

In general, when there is a mayor update (I.e. KiCad V9) it starts with quite a lot of (usually small, but sometimes still irritating) bugs, and the most problematic are ironed out by the time the 2nd or 3rd bugfix release (the third number in the version number) is reached. For this reason it’s relatively common to stay with the older mayor version for a few months. But apart from that, updating for bug fix releases should always be done (If you’re very cautious, wait a week after a new bug fix release is made, because sometimes *&^%$#@! happens, but this gets fixed in a few days).