STM32 TSC, silk screen as "panel"

[straubm]

Hi,

long time, no see, so greetings to all!

When implementing touch sensors, can I use the PCB silk screen as "panel", i.e. omit the glass/plexiglass panel?

It is for a private project, the unit will be infrequently used in lab environments.

Obviously there are issue like silkscreen and silkcreen print wear (which might be disregarded for the intended use case), but would it basically work?

Any experiences or caveats are highly appreciated.

BR Martin.

[berendi]

Yes, it should work. Omitting the glass layer and putting the finger directly on the PCB actually makes it easier to detect touches. You might have to adjust some threshold values and that is all.

[OlivierR]

Keep in mind ESD side effect (carpet, synthetic,...).

Best is to put a resistor between mcu io's and copper-sensor/finger. Details in AN3960.

Rgds.

[straubm]

Thx. I'm aware of the AN and have resistors designed in.

Btw, how stringent is the 2(4-5)mm requirement between traces connected to touchpads and e.g GND on short distances (see pic attached. The gap is 0.8mm, will that work?). The board is 4 layer with a GND plane as 2nd layer.

[OlivierR]

This will increase parasitic capacitance and reduce sensitivity.

Nevertheless, on MCU side, R15/18/19/21 gnd plan is too close from signals in 4th places.

See modification proposal in light-blue.

For R15/R18:

For R19/R21:

[straubm]

Ok. Got it. Thanks for your effort. Highly appreciated!​