I am trying to route a differential pair with track width 0.168 mm and gap 0.3 mm.
The differential pairs are defined in the “Board Setup” → “Pre-defined Sizes” and “Net classes” as follows. However, the routed differential pair still has track width of 0.2 mm and 0.25 mm gap.
I am using KiCAD 8, and have successfully routed differential pair with desired properties before. But this time I am not sure if there is anything mis-configured. I also upgraded to KiCAD 9 and the problem persists.
Much appreciate any help.
There are two columns (DP width, DP Gap) at the end of the netclass property setting. I updated them to the desired values (0.168 mm for track width and 0.3 mm for gap). Then the routed differential pair has 0.2 mm of track width and 0.3 mm of gap! The gap value is CORRECT but the track width still not!
I changed the DP width of other netclasses to 0.168 to make sure they will not override my desired netclass value while still get this 0.2 mm of width. Also in the top menu bar I also choose track width to 0.168 mm, still get 0.2 mm of width. Not sure how this 0.2 mm comes from.
I can tweak the DP width and gap values in the netclass property to make the resulted routing has my desired properties. However, this does not sound elegant.
Thanks,
How familiar are you with the settings indicated by the three arrows in the screenshot below? If there is any doubt, then read the manual of how these work.
@paulvdh, thanks for the answer which help me fixed the problem! Obviously I am not very familiar with these settings and this is only my second project of using KiCAD. I used the settings “use netclass width” and “use netclass sizes” as indicated in your post. Which still give me 0.2mm instead of 0.168 mm. I then tried manually set the track width to 0.168 mm but still get 0.2 mm.
The actual reason (after reading the manual) is that this value will be overridden by the board minimum width, which I set it to 0.2 mm. After modified that value then I can get correct behavior.
To make a solution and close the topic, here is the summation:
- Board setup → design rules → net classes: create a differential pair property with the desired values of DP width and DP gap, assign the desired net classes with this differential pair property.
- Notice the DP width and gap properties might be overridden by the minimum clearance and track width in the Board setup → design rules → constraints. Modify them if necessary.
1 Like
