Any ideas on mask inversion or additive mask editing?

Layout Footprints
#1

Hi all,
I am currently creating a layout with RF elements. On my RF traces I want to remove the soldermask. I found a helpful solder mask expander plugin (“RF Tools for KiCad”), but that removes all solder mask - but at least between the pads of a series-cap, I want to have a little strip of mask remaining, as a trade-off between RF and solderability.
I understand how the mask works - you can remove it by adding polygons - so right now I only see two methods, and I’m wondering if anyone has a better idea. Or if anyone knows a helpful plugin, or any upcoming feature I could wait for.

My Method 1 - add many separate polygons
Benefit: does what I want
Drawback: tedious, as many hand-drawn polygons are needed

My Method 2 - solder mask expansion with plugin
Benefit: easy
Drawback: no way to get a thin strip of solder mask back at specific places (under the series cap)

smask4
smask4724×529 95.8 KB

Thanks a lot!

*Edit: swapped method 1 and 2 to match the graphic I posted…

#2

Not a solution, but a possible workaround:
The “Solder Mask Expander” script apparently works quite well for you.
As an extension to this, you can :

  1. Copy (at least the most commonly used) symbols to personal library.
  2. Dedicate one of the free user layers to “solder mask rectangles”, and then add some graphics in the footprints themself.
  3. Update the Footprints on the PCB with your modified footprint (Change links in Eeschema).
  4. Use some kind of script to copy the graphics on your dedicated user layer to the paste layer.

KiCad V5.1.x is a bit *&^%$#@! because it does not have named user layers for front and back. Maybe you can use Eco1.User and Eco2.User.

KiCad-nightly V5.99 has the ability to add new layers and also give them user defined names.

#3

Sorry, I was referring to mask, not to paste. Also, could you please elaborate why to use an ECO-layer in the footprint? I can directly draw to mask in the footprint. But I think I don’t fully understand your suggestion.

However, it gave me an idea: I add solder mask polygons in the footprints - thank you for that hint - and then let the solder mask expansion tool do the rest. Not ideal, but probably a huge improvement over my current process (especially with curved traces the manual process is tedious).

#4

I am not sure how your script works. Solder mask is traditionally a “negative” layer, which means that anything you put on it will be removed (instead of added) to the solder mask.

My idea / workaround was to first let your script do it’s thing, and then put some solder mask back from the graphics you put on another layer (Eco.User was just an example). By “temporarily” parking the graphics on another layer, it does not get modified by your script.

Unfortunately I’m afraid it will not work, or at least not in a simple way.
I think you can not first create a cutout in the solder mask with your script and then add some graphics by copying it from another layer (or embedding it directly in the footprint).
The only way is to modify the polygons created by the script to go around the areas you want to keep the soldermask. I do not know if that script can do that.