A self-heating board is more efficient than heating an entire enclosure. A sensitive circuit or part could use this trick to efficiently keep key componants warm on demand without an entire external heating system. And dont let Louis Rossmann about this. Inbuilt desoldering circuits underneath key componants might make at-home board repair too easy even for his tastes.
We had this use-case back when I worked for a pro-audio company building analog synthesizers.
One of the components of a voltage controlled oscillator (VCO) is a linear to exponential converter, that converts the 1V per octave control voltage to a per-hertz reset pulse for the sawtooth waveform.
Such components can be done in various ways, but one of the most traditional is to use the exponential part of a transistor's voltage-to-current transfer function (not sure what the actual term is here). The issue is that this function is very sensitive to temperature, and so fluctuation in temperature on this part can easily detune the VCO by a very audible amount. Fluctuation is less of an issue at higher temperatures (around 70-80°C), so the solution was: heat!
The trick we implemented in the synth was to use a transistor array in a chip: use two transistors for the Lin/Exp converter, another as a heat source (we nicknamed it "the oven"), and another as a heat measurement device, to get some sort of PID feedback loop. I believe there were five transistors in the chip, so there was a leftover that got used for another purpose.
Surprisingly efficient, though we could still hear some pitch drop when blowing on the transistor array chip. Once in the enclosure, we figured only applying some ice on the front panel would have an effect.
This "heating trace" in the PCB would have been a good alternative, as some of those transistors arrays were particularly hard to source (we needed a particular transistor type, can't remember which).
Oven-stabilized crystal oscillators are a time-honored technique for precision electronics.
It turns out that no matter how carefully you stabilize a circuit for temperature changes, it gets much more stable when held at a controlled temperature.
Arrangements for this go from simple (gluing the component to a PTC thermistor driven at constant voltage) to elaborate (thermocouple-controlled PID heater in multiple styrofoam insulating enclosures). There are some pictures at https://en.wikipedia.org/wiki/Crystal_oven
I've never seen PCB traces used for this, but I've always wanted to try making a clock from a piece of LCD color-change material laminated to a PCB with traces in seven-segment patterns.
If it's for an art project (where power consumption, noise, product lifespan and general sanity don't matter as much) just put it in an enclosure with a fan blowing across the digits programmed to turn on once a minute.
You can install toaster coils too. A pcb heater layer directly under a temperature-sensitive chip is much more direct, and thinner. I see a cold-weather cellphone with a touchscreen that can get up to workong temperature quickly.
A self-heating board is more efficient than heating an entire enclosure. A sensitive circuit or part could use this trick to efficiently keep key componants warm on demand without an entire external heating system. And dont let Louis Rossmann about this. Inbuilt desoldering circuits underneath key componants might make at-home board repair too easy even for his tastes.