Footprint with copper cutout within a pad (e.g. to make a grille pattern for a membrane button within a pad)

I hereby certify that I am not simply asking someone else to design a footprint for me.

I’d like to design an unusual footprint, and I can’t think of the best method to go about it. The footprint is a square shape with copper gaps. The aim is to use this footprint as a switch contact, and the copper gaps are useful for better electrical contact.

I have tried adding a keepout rule shape within a pad, but that does nothing on KiCad 7. I could try to make an elaborate pad shape by using the Pad Edit mode, but although I could design a comb-like shape with that, it wouldn’t be closed on the end as shown in the jpg sketch.
I can also see myself needing circular shapes that look like donuts, and again it’s a similar issue, of having copper cutouts within a larger shape.
What would be the best way to approach this?

You want it in a footprint, and that is the easiest way to implement it in KiCad.

  1. Start by creating a footprint in the footprint editor.
  2. Draw some graphics (Start experimenting with something simple).
  3. Put a (SMT) pad on the graphics, they must be overlapping.
  4. Enter and exit the “Pad Edit mode” by pressing [Ctrl + E] twice.

the graphics and the existing pad have now merged into a big custom pad.

Also, if you want to use them as a switch contact, you also have to create a cutout in the solder mask. You can use an aperture pad for this.

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Hi Paul,

Thank you for that! I did not know the double-Ctrl-E method, I’d previously always done Ctrl-E on a footprint and then drawn a single polygon!
Following your suggestion, I get this:

I have the aperture pad there too, and also added a keepout rule to prevent copper fill close to it. It looks great:

I’m excited to now spend some time to produce a little library I’ve named Keypads, of footprints useful for membrane style shapes for building keypads (I will of course share it once done). There is a little general discussion here if anyone is interested.

From Element14:

I have not done it myself, but this is quite common for soldering flex/rigid LCD panels to a PCB. The magic keyword to find more is “hot bar soldering”.

This is complete nonsense. These flexible terminations only have a use on FR4 and other rigid PCB’s. When a rigid PCB is bent, then brittle ceramic parts soldered onto them can be subject to large forces and they can break. The whole PCB acts as a lever with a “hinge” under the ceramic part and this gives a great multiplication of force on the SMT part. When these parts are soldered on flex PCB, then just the flex will bend and there just are no large forces on the SMT part at all.

Also, consider dome switches. These domes are available in different sizes and “strengths”. Some manufactures also have kits with a variety of them to try them out.

The (random) picture below looks a lot like standard flex PCB used for switches:

I have not searched long, but there must be plenty of other examples floating around on the 'net.

I just wondered what you are using to interface to this capacitive pad, maybe TTP223 or straight to a microcontroller input or something else entirely ? I’m just being nosey as I have used 'touch switches ’ quite a bit recently and I like to know how people are doing it. BJTs and a hand full of passives will do but I’m curious !

It could be used as a capacitive pad, but in my case I’m hoping to use a similar footprint rotated at 90 degrees, on a second PCB. The first PCB will be a flexible one (they are quite cheap now on J-L-C PCB), and there may be something in between to act as a spacer (it is just a half-baked idea currently, I will need to experiment!).
The hope is that it will then act as a membrane switch, with the mesh of the horizontal and vertical copper allowing good contact pressure. If this works, then the silkscreen on the other side of the flexible PCB could contain the text legend for the button.
The end result could be like the flat buttons on typical microwaves for instance.
A different design could use the metal snap domes, for a tactile feel (I need non-tactile silent buttons for my application).
I would love to use capacitive buttons, I too was interested in TTP223, but I need quite a lot (50-100 buttons, maybe more!) so the cheapest silent button option could be the microwave membrane style. It’s just a hobby project for a non-critical application (kids toy). One slight issue if I use capacitive, is that there may be a delay if I scan a lot of capacitive inputs in some manner; I want to be able to press multiple buttons (up to 10 or so) simultaneously, and have them all detected within a couple of tens of msec or so.
The end toy will be a variation of this:
(the same sort of thing, but I want ideally a hundred animal sounds, maybe more! : )

TTP223 is apparently a whole IC for implementing a single touch button. That is quite bonkers. Microcontrollers with a bunch of capacitive touch button channels are quite common. Capacitive touch buttons can also be put in matrices, and even 30 or more years ago I once took apart a (cheap / low quality) PC keyboard which turned out to have capacitive buttons. The PCB had two half circles (or rectangles) and above each key was a a small piece of foam with aluminium foil on the bottom.

There are versions in that family with more inputs and multiplex capability, but as you say a microcontroller would be the way to go for such an extreme amount of capacitive buttons as I wanted!
I think that could be an interesting project, with a lot more uses than just a kids toy. I might add that feature to a future version of the toy (depending on how well the first version is received by people) or consider making a general capacitive input keypad board, I tried something generic a while back, (a bit smaller, but tesselate-able) but the requirement went away, and it’s still untested (so old it’s EAGLE):

That one uses a TI chip that can sense from quite a distance, so as well as presses, it is possible to detect almost-presses (there are uses for that! : ) and also hand-waving etc.

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