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.
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3\$\begingroup\$ Just use a few surface-mount resistors, just be careful of heat cycling cracking the solder joints over time. \$\endgroup\$– John UNov 1, 2013 at 14:42
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\$\begingroup\$ What mechanism will be controlling your heater, before MC starts (-60C - -40C)? For heater, see this link - Heated-bed \$\endgroup\$– Oleg OlivsonNov 1, 2013 at 14:51
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1\$\begingroup\$ I'd love to see an internal layer being used as a heater. You'll want to meander the trace to get it nice and long, but all the calculations for power dissipation and temperature rise of internal copper traces are readily available on the internet. Is power consumption not an issue? If it is, you can use a RTG and kill 2 birds with 1 stone :> \$\endgroup\$– HL-SDKNov 1, 2013 at 14:51
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\$\begingroup\$ Try this: pcbco.com.au/tracecalc.html \$\endgroup\$– HL-SDKNov 1, 2013 at 14:53
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1\$\begingroup\$ What about some sort of temperature sensor, or a small thermistor connected to a switching circuit? \$\endgroup\$– John UJun 19, 2015 at 9:13
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2 Answers
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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.
answered Nov 1, 2013 at 14:58
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1\$\begingroup\$ is it wise to place traces on a internal layer becouse of the low thermal conductivity of FR4 epoxy? \$\endgroup\$ Nov 4, 2013 at 13:27
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2\$\begingroup\$ @zontragon: It really doesn't matter all that much. You're not trying to move a lot of heat, like you are with a heatsink, just raise the temperature of the system overall. \$\endgroup\$ Nov 4, 2013 at 14:19
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\$\begingroup\$ Why would you rather to use pcb traces instead of using external flexi heater? What advantages does it have? \$\endgroup\$ Nov 4, 2013 at 14:45
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\$\begingroup\$ @zontragon: Because it's basically zero incremental cost -- included in the cost of the PCB. \$\endgroup\$ Nov 4, 2013 at 21:55
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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.
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\$\begingroup\$ Nice design! But if the system sits below -60 degree and is shut down, how to start the system without running MCU under 60 degree? \$\endgroup\$ Jun 19, 2015 at 2:12
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\$\begingroup\$ Could you please introduce how to wake up/close the temp sensor? \$\endgroup\$ Jun 19, 2015 at 14:58
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\$\begingroup\$ @richieqianle it was all done in analogue with a comparator as I remember. \$\endgroup\$– Andy akaJun 19, 2015 at 15:23
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\$\begingroup\$ To avoid starting the other part of the circuit(except the heating part) at low temperature, what was done? \$\endgroup\$ Jun 20, 2015 at 2:20