How to thermally couple SMT NTC?

[prasimix]

I'd like to measure temperature on the heatsink populated with THT MOSFETs. I'm aware of NTC sensors that are also THT and comes with mounting lug (with or without preassembled JST connector) like shown on the picture below.



SMT NTCs are in general less expensive and easier to assemble but question is how to thermally couple them with nearby heatsink. I came to the following idea and would like to hear your opinion about it: make a small cut on the heatsink that is larger then SMT NTC that NTC is not mechanically stressed and fill the gap between NTC and heatsink with thermal glue (e.g. this one) or thermal paste.

[Giaime]

I would use thermal paste, not glue: the different thermal expansion coefficient could ruin your NTC solder joint.

Take also care about safety insulation (creepage, clearance) between heatsink (might be PE - earthed) and NTC circuit (might be SELV - insulated from PE).

Usually due to insulation the THT NTC with mounting lug is the best solution...

[Siwastaja]

IMO a great idea, things to watch out for:

1) manufacturing tolerances on the heatsink and its mounting, so that you guarantee no mechanical stress to the NTC. Also, I'd consider a thermal pad material instead of glue or paste.

2) If you use thermal paste or pad material, how to guarantee it stays in place

3) Make the thermal pad / paste transfer heat not only to the body of the component, but to the pads and solder joints, as well. Use thin traces off the pads so that they don't carry the heat away. If it is a 4-layer (or more) design, add a small cut to the plane beneath - these closeby layers can suck surprising amount of heat away since thin (100-200 µm) layer of FR4 conducts heat to the copper plane quite well, whereas thicker FR-4 is a fairly good insulator.

[brabus]

I'm afraid that the thermal paste will get dry in some years, hence measuring a wrong (lower) heatsink temperature.

I would mount the NTC Close to the heatsink, but not in contact with it, just some milimeters away. I would then run the NTC contacts via two wide tracks under the heatsink, without the soldermask. Mount the heatsink in close contact with the PCB, insulate the naked tracks with a layer of silpad and you are done.

The heat will be spread from the heatsink to the wide copper pads underneath, which will then quickly transfer it to the NTC via the soldering contacts. Keep the tracks as wide as possible and as close as possible to the heatsink until you get to the NTC.

[prasimix]

I agree that heatsink paste is not a good solution. Therefore I've suggested thermal glue, but it can easily damage NTC surface if heatsink has to be removed for any reason.

I like what Siwastaja mentioned and I completely forgot: thermal pad. I believe it could stay securely fixed for prolonged period of time if cut/hole is e.g. 1.5-2 mm and 3 mm thick thermal pad is used. It's also good for its electrical strength.

[EmmanuelFaure]

It will never be reliable, for the reasons explained above. On all the common 3 legged transistor cases, the die is glued to the middle pin (And connected to the lateral pins only with very thin bond wires). Locate your smd thermistor next to the middle pin, directly on the PCB. Even better, solder one tab of it to the pin, if possible.

[prasimix]

Thanks, please explain "reliable": does it mean that measured temperature cannot be accurate, or that huge variations can be expected, etc.
I cannot solder it directly on middle pin without extra tricks since it is drain and it is on the high voltage (up to 375 Vdc). This temperature measurement will be used as precaution of extreme heating that shouldn't be reached during the operation despite of load. For example, if I expect that under max. load and ambient temperature of up to 40 oC heatsink cannot be hotter then 75 oC  then tripping point will be set to about 85 oC.

[2N3055]

Isolation is problem, also how do you plan to connect to it? Solder wires to SMD component?

There are also NTC with M3 screw that you simply screw in in a threaded hole. Or just use glass encapsulated teardrop NTC, drill a shallow hole, and glue it in...

SMD is usable only if you plan to make mini PCB on aluminum substrate, solder NTC and connector(wires) to it and mount that mini PCB to heat sink.

Price is not only price of NTC.  It is price of NTC, mounting solution, and work needed to make and mount it.

Regards,

Sinisa

[Gyro]

If your intention is an over-temperature trip then you might want to consider using a traditional heatsink mounted thermal trip switch. That would take care of both the supply interruption and the insulation requirements.

Thermistors are good for things like intelligent sensing and fan speed control, but for 'should never happen' situations, the simplicity of an electro-mechanical trip is hard to beat.

[prasimix]

Isolation is problem, also how do you plan to connect to it? Solder wires to SMD component?

There are also NTC with M3 screw that you simply screw in in a threaded hole. Or just use glass encapsulated teardrop NTC, drill a shallow hole, and glue it in...

SMD is usable only if you plan to make mini PCB on aluminum substrate, solder NTC and connector(wires) to it and mount that mini PCB to heat sink.

Price is not only price of NTC.  It is price of NTC, mounting solution, and work needed to make and mount it.

Regards,

Sinisa

That's right, and I believe that having SMT NTC soldered on PCB beneath heatsink as shown in my picture could be more price effective then using THT with connector or M3 thread (shown below) which also require adding of connector and wire insulation.

[prasimix]

If your intention is an over-temperature trip then you might want to consider using a traditional heatsink mounted thermal trip switch. That would take care of both the supply interruption and the insulation requirements.

Thermistors are good for things like intelligent sensing and fan speed control, but for 'should never happen' situations, the simplicity of an electro-mechanical trip is hard to beat.

Great to remind me of that solution. Are you aware of some tiny thermal trip switch? I don't have a lot of space for it.

[EmmanuelFaure]

Thanks, please explain "reliable": does it mean that measured temperature cannot be accurate, or that huge variations can be expected, etc.

Any stress put on the heatsink will partly be transmitted to the thermistor. At least, the thermistor will give biased values. At worst, it will crack, in a mechanical meaning.

[prasimix]

Ok, but it does not touch directly heatsink and in case of using thermal pad it will be additionally guarded (all its three exposed sides) against mechanical stress.

[Gyro]

If your intention is an over-temperature trip then you might want to consider using a traditional heatsink mounted thermal trip switch. That would take care of both the supply interruption and the insulation requirements.

Thermistors are good for things like intelligent sensing and fan speed control, but for 'should never happen' situations, the simplicity of an electro-mechanical trip is hard to beat.

Great to remind me of that solution. Are you aware of some tiny thermal trip switch? I don't have a lot of space for it.

If you don't have the space for the traditional circular 15A rated ones with two hole fixing, then this 2.5A type might suffice (I haven't done an exhaustive search)...
https://uk.rs-online.com/web/c/fuses-sockets-circuit-breakers/fuses/resettable-thermal-fuses/?redirect-relevancy-data=636F3D3126696E3D4931384E53656172636847656E65726963266C753D656E266D6D3D6D61746368616C6C7061727469616C26706D3D5E2E2A2426706F3D31333326736E3D592673723D52656469726563742673743D43415443485F414C4C5F44454641554C542673633D592677633D4E4F4E45267573743D746865726D616C2074726970267374613D746865726D616C207472697026&r=f

Some of them include an internal heater that latches them open until mains power is removed. Just a thought anyway,

[T3sl4co1l]

Another option is to add a flex or riser board, which contains the thermistor and brings it up close to the heatsink, preferably mounting to it.

So, you could have a (rigid) PCB that slots into the main PCB, back facing the heatsink (with thermal goop, and maybe a screw to hold it there -- BTW, mount it before soldering, to minimize stress), thermistor on the front.  Downside: FR-4 is a terrible conductor, thru-plane, so this will not respond quickly, or particularly accurately (especially if there is much airflow around the thermistor!).

With flex, you could put down a layer of tape (to insulate the heatsink), then a dollop of paste, then the thermistor (on flex, facing the heatsink), then another piece of tape to hold it there.

I've done a variation on this, where I tack a chip thermistor (e.g. 0805 size) between two wire leads, dip the assembly in epoxy, and use epoxy and tape to mount this in the same way.  Mind, this is a tedious method (and not particularly reliable, if the wires stress the component and it cracks), which I only do for prototypes, out of laziness for actually buying a proper mountable one.

It's too bad the lug-type thermistors are all on the expensive side (usually $5+, probably can get lower from other mfgs/sources), but they are handy and save a lot of assembly labor!

Tim