reading some of the capacitive sensing IC datasheets around, they always recommend an isolation overlay over the sensor pad.
I'm wondering, why not use the actual PCB as overlay?
like this (sorry for the bad photoshop):
Please note that I'm not just going to determine the "touched/untouched" state, but the actual amount of touched area (by measuring the actual capacitance).
I've done some experiment with MPR121 sensor and some copper tape on a blank PCB material, and it works pretty good, but I'd like to hear some opinions before I'm going to make some actual PCBs.
Would that be as good as using a proper overlay?
Update: I've read that FR4 (the PCB material) should provide almost the same isolation as acrylic and other plastics, so I think it could work