I want to design a 4 layer PCB with the following voltage levels. GND, 5V, 3.3V and 80V. In the circuit there are some MOSFETs which are driven by 3.3V and MOSFET switch 80V (required current is very low uA level). Which makes overall on the pcb, there are 80V and 3.3V signals close to each other (At some places less than 20 mils).
For the protection I kept 80V at the bottom layer. And the other voltage levels and signals are on the top and second layer. And I keep the third layer completely ground.
I tried to represent the design with the simple picture below.
Now I am worrying about the DC break down voltage somewhere in my PCB. For such a circuit, where one different high and low voltage uses, I have not much experience. I am not sure about my structure, whether it is safe enough? Is there any article or source where I can find some useful information regarding to this issue. Do you have any advice for such a PCB-design ? If there is lack of information required for the question please ask.