For this project I made foot prints for rubber button contacts by placing multiple long, 0.2mm wide pads in 0.2mm distance. (Seeed design rules allow up to 6/6mil~=0.16mm.)
I didn't really find a more elegant way in KiCAD. There still seems to be solder mask between the pads. I am a little concerned that the thickness will prevent reliable contact...
Finish will be HASL, everything else is much more expensive.
Does anyone have experience with these carbon rubber contacts and can give some advises? I have never used those contacts and would like to avoid throwing away the PCBs because of that.
If you are afraid the solder mask will be a problem, why not remove the solder mask for the button area?
And yes, I know making a footprint like that in KiCad is a bit of a pain but you can do it 
Well I tried to put a circle on the F.Mask layer in the footprint, but it didn't seem to do anything. Any hints how to do that?
As happy as I am with some features of KiCAD compared to other options (how in the name of the FSM do people live without the ability to shove traces???), the limitations with trace geometry and such in footprints seriously pisses me off ...
Problem is that it's hard to add a filled circle in the footprint editor...
So what I did when I needed something like that is to make a simple (gray scale) BMP in pain.net or something. Then I imported that into the Bitmap to component converter and made a component with it (of course, select solder mask). After that I added the rest of the copper I needed.
Or, if it was really complex, I did the same for the copper I wanted. But you can't select the copper layer in the converter. So I just picked one and later opened up the footprint file in a text editor and just changed the drawing to the copper layer (it's all text). And then I combined the two files (one with the solder mask and one with the copper) into a single file to have both the copper and solder mask
Yes, it's a bit of a mess but it's possible. Footprints are all just text/code
Ok, I'll go ahead ...
The other thing is that you really should use gold plated contacts to prevent long term corrosion problems.
Hnm, ENIG ups the price from 10$ to 35$ ... I wonder if there isn't any other conducting corrosion protection ... it is not like I would produce these in series.
Your carbon rubber contacts will become erratic after a few years. Also, the carbon rubber should ground the pad on the circuit board, when pressed, not apply a voltage.
Paul
I agree rubber contacts are pretty crap to press in most situations. Don't feel that nice. But why would it matter if it connects to Vcc or GND?
septillion:
I agree rubber contacts are pretty crap to press in most situations. Don't feel that nice. But why would it matter if it connects to Vcc or GND?
Because it is almost impossible to insulate the rubber from ground, but really easy to insulate the circuit board contacts.
Paul
Paul_KD7HB:
Also, the carbon rubber should ground the pad on the circuit board, when pressed, not apply a voltage.
That was a general recommendation, wasn't it? Because if I understood that correctly, it is what I do in the project.
I agree rubber contacts are pretty crap to press in most situations.
Istn't that how basically each and every remote control and most dumb phone keyboards work?
I still wonder if I can get around ENIG with something ... If it is a matter of years, perhaps just cleaning? With the 35$, import taxes will apply, so I end up at almost 50$ instead of 10 ...
I guess I'll at least order a first batch in HASL, see if it works and such. If they stop working in a few years, I can still get a new one in ENIG.
Have you ever gotten something fro. China with an accurate customs declaration? It's a rarity.
I order $300+ of pcbs at a time. It arrives on my doorstep with a declared value of $8
Some.even ask you want value to declare
Well, true. Still, I will order a HASL batch and see how long it works and especially IF it works. If it works for a few years, fine, if it doesn't, I'll exploit the board and order ENIG. No big loss.
Paul_KD7HB:
Because it is almost impossible to insulate the rubber from ground, but really easy to insulate the circuit board contacts.
Why is that impossible? And are we talking circuit GND or PE? Because in a lot of low voltage applications they are not the same.
ElCaron:
Istn't that how basically each and every remote control and most dumb phone keyboards work?
Never seen a dumb-phone with rubber contacts but a lot of remotes indeed use it. Do you find the buttons on a remote nice to press? I certainly don't...
septillion:
Never seen a dumb-phone with rubber contacts but a lot of remotes indeed use it. Do you find the buttons on a remote nice to press? I certainly don't...
Well, for a keyfob remote, I wouldn't complain. Not trying to built a keyboard for typing.
Would still rather have normal tacktile switches or snappy dome switches (But the last have the same problem with PCB finish.)
But for the contact, yes, gold plated is the best. But I still think tinned will still work okay. It's not as terrible as plain copper.
As I said, for the first test, I'll use HASL. If that stops working after two month, I'll order ENIG. If it stops working after two years, I'll apply flux and some hot air from the rework station to it.
I'll apply flux and some hot air from the rework station to it.
That should kill it compleatly.