Electronic – Warming PCB in a low temperature environment

1. The sensor part you suggest is only rated to work to -55°C, not below. It probably will work somewhat below that but if you really need it .. that part is not guaranteed. There are PTC heater elements that can be used that don't require active circuitry.

2. Number and rating of resistors (or other heater) depends on the physical arrangement. It's not easy to calculate. You could to a heat flow FEA or build a prototype and test it (you don't necessarily have to go that low for testing the heat flow).

3. Covered by 2.

4. You mean insulation? Engineering textbooks have thermal resistance figures. Materials commonly used that have good insulation usually have some trapped air in them. You may have other requirements (fire rating, toxicity, cost) that will limit choices. Urethane foam is one common one. There are much more expensive insulations that perform even better, but you don't necessarily want a really good insulation. If you have a super-insulation your circuit may overheat during normal operation in a warm environment.

So I think your design process would be to determine the mechanical design and insulation you want to keep the internal temperature to an acceptable level with a hot environment (and covering the other factors), then estimate the power required to increase the internal temperature from the lowest ambient at least to operating temperature. You can also decide where to turn the heater off at- if the internal power is less than the heater power you may end up with the heater turning on and off (or if you control it smoothly the power will decrease as the ambient temperature increases).

You also need to check that the storage temperature of all the parts in both circuits includes the lowest possible ambient and that every part in the temperature control circuit will operate at the lowest possible ambient (or if not operate, at least not shut off).

Some consideration may have to be given to frost and condensation, but that's outside of the scope of this question.

There are silicone based conformal coatings, available as spray. They should bear this temperature range easily. Polyurethane based sprays may be used, too. We had tests running with PU. It proved usable up to 90°C. But all depends on the actual use. You can buy those materials manufactured by several companies like CRC industries, electrolube etc.

If you want to increase creep resistance by using a conformal coating, you will have to do several tests anyway.