Design of capacitive touch pad

[sottos20]

Hi,

I am struggling to design a touch pad covered with solder mask like shown in the picture below. I have designed a board using the MPR121 chip but my pads are exposed and I need them to be covered with solder mask. Any help is appreciated

[constantin.popescuXD3CL · Accepted solution]

Hi @sottos20,

This is possible and needs to be done in the Library Editor. You need to open the Library from which you place your components, open the footprint for the components that you need to change the solder-mask for and remove the solder-mask objects (the default is rectangle for rectangular SMD pads) for the pads that you want to be completely covered with solder-mask. Then you need to save the library and then go to board design and Update it from the Library changes. I have tried this with a new design, I opened a Library that I have around contains SMD capacitors and I have update the footprint by removing the created solder-mask openings, I have saved the Library then I have placed the caps in Sch, they where synched to board and there were no objects on the Solder-Mask layer for these components. Then I have generated CAM ODB++ / Gerber and the generated files were correct.

This worflow should help you achieve this.

Please let me know if there is anything else I can help you with.

Kind Regards,

[sottos20]

I appreciate the thorough response, but I cannot seem to get this clever technique to work. Do you mean disabling the solder mask under pad properties? If so, it does not appear to have any impact on the pads in my case. Could you provide screenshots by any chance?

[Pieter.Jan.Van.de.Maele]

After you made the changes to the library, did you run the "Update Libraries" command to update your design to use the latest library? 

[sottos20]

Yes I tried a couple of times just to be sure, but the pads appear to be the same with no solder mask both with the solder mask turned off or on in the pad properties.

[Pieter.Jan.Van.de.Maele · Accepted solution]

Could you share a screenshot of what you are seeing? 

I just tried this workflow and for me it works:

- turn off the SolderMask checkbox in the pad properties of the library

- Save the library to a new version

- Update design to latest libraries

=> SolderMask is gone (both on SolderMask layer and CAM output/preview)

[sottos20]

This is the PCB I have been trying to cover with solder mask:

I indeed checked that solder mask box off in the pad properties, in this picture half of the pads have the box checked off. I also updated the libraries multiple times. Here you can see the footprint and 4 of the pads without solder mask:

It appears the same in my cam preview. How odd!

[Pieter.Jan.Van.de.Maele]

Could you quickly enable the SolderMask layer in your library and check if there is still something there? Sometimes SolderMask gets drawn manually instead of using the auto-generated option (checkbox). In that case, you will have to remove the geometry on those layers. 

[sottos20]

You have solved the problem, thank you! There was a big rectangle on the solder mask top layer covering all the pads. Deleting that fixed the issue and now the pads are covered, fantastic. Thanks again!

[Pieter.Jan.Van.de.Maele]

Great to hear. Hopefully your project works out and please let us know if there is anything else that comes up.