I've seen this with the cheaper scientific calculators.  They have to put down
the conductive carbon for the keys with a silkscreen anyway, so why make a
doublesided board (which requires 4-6 manufacturing steps) when you can add only
one step (carbon after the solder screen) and have the same effect.  Since these
are keypads you don't have to worry about the resistance and current for these
traces.  As a bonus, you can use the carbon as a resister for various parts of
the circuit (oscillator, current sense, etc)

-Adam

Richard Ottosen wrote:
> I sometimes buy (cheap, like from McFrugles) small electronic products
> to see how they work. Of most interest to me is how they package high
> volume stuff. I once found a gizmo with a keyboard. The buttons were
> rubber with conductive pellets molded into them. Normally, when I see
> this kind of keyboard, there is what appears to be gold plating on the
> pads for the switches. This device was designed so cheap that they did
> not want to put expensive plating on the PCB.
>
> What they did instead was to silkscreen a carbon-looking coating onto
> the pads for the switches. This was a single layer PCB, as well, and
> they really needed a 2-layer PCB. To get the second layer, they put
> jumpers on the board. These jumpers were the same carbon material
> silkscreened over the soldermask between pads! Obviously these "jumpers"
> were not in high current paths.
>
> (I know this was long winded but I hope it was worth it).
>
> -- Rich
>
> --
> http://www.piclist.com#nomail Going offline? Don't AutoReply us!
> use mailto:listserv@mitvma.mit.edu?body=SET%20PICList%20DIGEST

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu