How do I distribute heat from a heating pad to my electronics?
Just use 4 resistors, each rated at TWO WATTS, axial leaded, placed near the edge (4) of your PCB, with the heat conducted thru the leads into your PCB and keeping the various GND and Power planes warm.

I suggest TWO WATT resistors because of the thick leads.

You might also use 4 time 270 ohms, to pull about 0.1 amp from the 117VAC power line, daisy-chained about the PCB periphery; use a SCR or TRIAC with opto-isolator that is activated when below +10 &deg; C.

Make all the "daisy-chain" intermediate-connection traces WIDE and located over the GND plane, so the heat gets dumped thru the FR-4 into the GND plane. Of course, some displacement current coupling will occur from the 117VAC into the Ground plane.

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Can you build the heater into a NEW layer inside the PCB?

Conaider a 4" by 4" layer, with 10 mil traces separate by 10 mils. Thus 50 traces per inch, 200 traces total, of length 4" * 200 =  800 inches.

The Length/Width tells us the # of squares.

800/0.01 =  80,000 square of copper.

At 0.000500 ohms/square (for standard thickness 1 ounce/squarefoot foil), or 1/2000 ohms almost exactly, there are 40 ohms of resistance.

With 5 volts across that 40 ohms, the power = 5 * 5 /40 == 0.625 watts.