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I am an EE that is fresh out of college and and starting at a smaller company, and I have a question about electronic self heating, and how to keep heat from spreading in a PCB to heat sensitive components, such as thermistors that are being used to measure ambient temperature, currently there is issues in a few designs, where the temperature inside the housing increases by half a degree C with traces run directly to the thermistor without thermal reliefs or and care to trace size. The devices I have seen having trouble are usually attempting to read ambient temperature at .25% accuracy between the ranges of -20 to 85c (currently done on some designs by having two separate boards in the enclosure with small wires run between the two)

The "hot" components are mainly the switching supply, and the MCU, in most cases for some designs I am looking at, they run at 10-20 mW, and the PCB size would generally be around 2in x 4in.

Would it be better to run large ground planes underneath these components to try and dissipate the heat on that side of the board? Or would the heat just transfer to the pcb's dielectric and heat up the thermistor that way?

Also, would it be better to run thinner traces with thermal reliefs to the thermistor to cut back on heat transfer through the traces, or would it be better to run wider traces to try and dissipate heat before getting to the thermistor?

The common value of thermistor I am looking at is a 10k-2 NTC.

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  • \$\begingroup\$ Is this SMD or thru-hole? It shouldn't really be an issue with thru-hole. \$\endgroup\$ – DKNguyen Jul 28 '20 at 14:49
  • \$\begingroup\$ I have seen SMD components, and thru-hole thermistors ( both radial and axial) and the issue still persists. The enclosure was designed for air to flow through, wall mounted, and vents on top and bottom, natural convection only. \$\endgroup\$ – James S Jul 28 '20 at 14:53
  • \$\begingroup\$ What is the thermistor attempting to measure? \$\endgroup\$ – Andy aka Jul 28 '20 at 14:53
  • \$\begingroup\$ Ambient Temperature in a room \$\endgroup\$ – James S Jul 28 '20 at 14:56
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    \$\begingroup\$ To get the accuracy on some of the other products, they calibrate every single one before going out the door to get as close as possible to it, and have been successful in some cases, but not others. For these products, I think the company gets custom thermistors made for them. I guess I should rephrase the question to, what methods can I employ in other designs to prevent the heat from traveling to the thermistors through the pcb \$\endgroup\$ – James S Jul 28 '20 at 15:09
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Here is an approach to thermal isolation while maintaining some mechanical strength -(the chip is an ovenized buried zener voltage reference- it would have a urethane foam insulator over it in actual operation): As well as the wide routed slots, notice the thinner ones to the left and right and the slots to the top and bottom.

enter image description here

The copper, even 34\$\mu\$m copper, will conduct a lot of the heat, so narrower and longer traces are called for, as well as limiting the amount of laminate (thermal conductivity is proportional to cross sectional area divided by length).

The thermal conductivity of PCB laminate is anisotropic and is much higher parallel to the surface than through the PCB due to the direction of the weave in the fiberglass cloth. For that reason, rotating the pattern 45° might make a noticeable difference.

But, personally, I would V-groove or mouse-bite the PCB with a thin soldered or socketed jumper over it and then snap it off and assemble it with insulation between the two if accuracy is that important.

Or you could consider just faking it and compensate for the error due to the other circuitry either with a second sensor or a thermal model. That would make an interesting project to burn some time if you're not busy.

P.S. Sometimes you can make use of power dissipation by mounting the hot parts towards the top of a ventilated enclosure so that air at room temperature is drawn in at the bottom, over the sensor. I suspect your power levels are way too low for that to be a useful effect in this case, though.

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  • \$\begingroup\$ Did they use the fancy pinwheel route instead of a square perimeter route with direct, straight bridges to increase the length? \$\endgroup\$ – DKNguyen Jul 28 '20 at 16:19
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    \$\begingroup\$ @DKNguyen I think so, and to keep it supported reasonably well on all 4 sides and allow for the required conductors (9 in total). Or maybe they just liked swastikas. For a temperature sensor some other arrangement might be optimal. \$\endgroup\$ – Spehro Pefhany Jul 28 '20 at 16:42
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    \$\begingroup\$ (note: PCB layout may be illegal in Germany) \$\endgroup\$ – user253751 Jul 28 '20 at 17:08
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From what I've seen in a room thermostat (0.1°C resolution), the traces were as thin as possible. The PCB had numerous air gaps to isolate the sensor from the heat generated by voltage regulator.

The sensor was placed at the corner of the board and there were holes milled into it to limit heat passing through the board. No copper pour in that area. So little material was left that it could break off easily if stressed. There were matching venting holes in the device's case near the sensor to get air sample from the room directly onto the sensor.

Air gap for isolation (for illustration only)

According to the datasheet, the unit still had a factory/user programmable offset (+-3.5°C) to cancel any offset error caused by the device heating up.

It might be also a good idea to only power the sensor up when sampled, to prevent it from heating itself directly.

Image source

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  • \$\begingroup\$ The thermal reliefs are employed when possible, but would large ground planes under the hot devices be counterproductive and just heat the whole board, or would it help dissipate heat? And in cases where thermal reliefs can't be accomplished due to fuller boards, would a thick trace just wick up heat and pass it to the thermistor? I would assume a thin trace would carry heat worse than a thick one, but then I wonder if the current through the more resistive, thin trace, would also heat up more? \$\endgroup\$ – James S Jul 28 '20 at 15:36

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