> I want to make a PCB to fit in an existing enclosure with existing
> buttons, and wondering what I need to do for the PCB side

One project I had was to retro-fit a new PCB to existing buttons so
I used low-profile tact switches. They were mounted on the back of the
PCB with the legs bent upwards for soldering, so that only the very tip
(about 1mm) of the actuator poked through the board. A bit labour-
intensive and fiddly though

> They always seems to be black, and IIRC this is a carbon coating...?

http://www.coates.de/ccp/connections/carbon-inks-pcb.pdf

You mind find a source for a small quantity of this ink to DIY. I'd
wonder if you could try your own formulation - it's only a binder and
graphite (pencil lead scrapings ?). or eg the meerest smear of silicone
or contact adhesive on the PCB, dusted with graphite

> Can I use regular HASL for this?  Or will it tarnish over time?

I'd not use bare solder as a contact surface, it soon oxidises

> I expect  getting PCB's with non-standard coatings will be cost-
> prohibitive I suspect

A jeweller (friend of a friend of a friend) gold-plated some small ball
bearings for me, to be used in a novel tilt-switch. It cost only a couple
of dollars when thrown in the basket with all the other jobs

Joe
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