I have to develop a microcontroller circuit that will run at low temperatures (as low as -60C). I want to heat the FR4 PCB board until it reaches the commercial temperature range above -40C. I found some flexi heaters to heat the PCB. What other options do I have? Is it possible to use a PCB layer as heater? I couldn't find any information about using a layer as a heater.
I would be inclined to use traces on or in the PCB as a direct heater, as you suggest. Obviously, you'd start by insulating the board as much as possible to minimize the energy required to maintain temperature. Pay particular attention to external electrical connections, which can also be good conductors of heat. Extra lengths of wire buried in the insulation will help quite a bit.
I would put the trace(s) mostly around the edge of the board, generally keeping them away from other heat-producing components in order to keep temperature gradients across the board to a minimum. I would design a current source that has temperature feedback to drive the trace, with a "maximum" setting that's guaranteed not to burn it out.
The flexi heaters you found are essentially doing the same thing, but they probably consider the applications data somewhat proprietary. You'll have to work out the details yourself; things like trace widths and thicknesses, current levels and temperature rise, based on the basic resistivity of copper, which is 1.68×10-8 Ω-m @ 20°C.
I had to design a 400MHz radio receiver data-demodulator that could sit down at -60 ºC and I put a bunch of surface mount resistors in a matrix formation underneath the PCB on the bottom trace (4-layer board). The matrix meant if one resistor failed it didn't kill the whole heater idea.
I used a temperature sensor to control the heating and at round about +10 ºC this turned off because the same module could also sit out in the baking heat all day. The only problem that the unit initially had was when heating back up through 0 ºC - condensation formed on the circuit board and for 5 minutes the radio didn't operate as expected. This was cured with selectively lacquering vulnerable areas of the PCB.
There were still a few noticable glitches as it rose back through 0 ºC but these were very short-lived. The unit was designed for an aero engine test cell - not a nice place to be when the engine was on! The radio receiver was collecting data from the engine.