Can I use an internal layer as a heat bed if I want to warm my multilayer pcb?
Is it proper to use the internal layer as a heat bed? If not, any disadvantage?
You can not take away any copper that you don't need for heating, that'll help a lot, and if you can connect those to a safe ground trace or plane: First of all that will conduct the heat away to that plane, which helps, but it also avoids resonances and such when you start PWM-ing the heater.
I'd use a cheap SMT thermistor for the sensor assuming you only want to control at one temperature. Just stick it in and it will work.
You can shield electrostatically with a ground plane, but the magnetic field will go right through everything, so if you have sensitive circuits you might need to filter the PWM to something more like DC.
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