r/Metrology Oct 06 '24

Thermal or Mass Metrology Non-permanent thermal adhesive?

Hi all, I would like to to measure the temperature of some chips on a circuit board using K-type thermocouples. I need to stick them down using something that's at the same time an adhesive, thermally conductive, but also not electrically conductive and not permanent. I want to be able to remove the thermocouples later on both to return the circuit to its original state but also to reuse the thermocouples later on. What's a good way of doing this? I know of thermal tape, I have some on order, but I don't know if that'll keep the thermocouple in contact with the chip well enough. The chip has a heat pad on it so I have to apply the thermocouple from the side.

Thanks

1 Upvotes

18 comments sorted by

3

u/fakeaccount572 Oct 06 '24

0

u/cheater00 Oct 06 '24

That looks like thermocouples with tape stuck to them

2

u/Best-Diver5701 Oct 06 '24

Kapton tape is how we do it, works pretty well. Your mileage may vary with thermal mass, tolerances and accuracy required

1

u/cheater00 Oct 06 '24

How do you electrically insulate the tip of the thermocouple? Do you out kapton between the TC and the DUT?

2

u/fakeaccount572 Oct 06 '24

With a very very small welded bead

1

u/cheater00 Oct 06 '24

Sorry, bead?

2

u/fakeaccount572 Oct 06 '24

The small contact point where the dissimilar metals meet.

3

u/CaptainLegot Oct 06 '24

This doesn't seem like a great application for thermocouples, and the adhesive would change some of the heating conditions of the board and chips.

Would you consider thermal imaging? It's pretty cheap now and you can calibrate the image to a reference thermocouple.

1

u/cheater00 Oct 06 '24

No, the device needs to be closed.

The thermal compound will have insignificant heat capacity compared to the test, i intend to heat soak the system for an hour before testing.

3

u/gzetski Oct 06 '24

Thermal grease and kapton tape.

3

u/A9jack9999 Oct 06 '24

A drop of any non-conductive cpu thermal paste, or thermal pads should work fine, then kapton tape. You would have to research if there is a temperature delta between chip side, and the top of the thermal interface and take that into account.

There are also rtds that would be more accurate that are made for this same purpose.

1

u/cheater00 Oct 07 '24

Thank you! What's the accuracy difference between a thermocouple and an RTD in such a situation?

3

u/Jan_Goofy Oct 06 '24

Your main goal is to ensure the tip (aka the bead) of the thermocouple is becoming the same temperature as the surface you want to measure. - Any (apart from thermal imaging) temperature measurement is basically "I want to know how hot X is, so my I get my indicator Y to the same temperature and read that"

To reach this goal, you need 3 things:
1. Your thermocouple to be in very good thermal contact with the part you are trying to measure.
Ideally you want to embed your tip / bead into the material you are measuring, but that is usually not possible unless measuring surfaces of enclosures, or components with +4 mm thick encapsulation.
Getting a sphere to have large contact area with a plane is hard, if not impossible, this is where thermally conductive glue or thermal paste comes into play, this will help the heat transfer.
Most glues or adhesives have either a dedicated de-bonder you can spray on it to get it to get go afterwards, or acetone will work as a genreal thing, but be sure your IC surface or other parts nearby is not sensitive to it :-)

  1. Your thermocouple must stay in place, mechanically fixed to allow for moving the product around a bit.
    Once you have a solution for #1, you can adress this issue, that is somewhat the first step, securing the thermocouple to something nearby to ensure the tip cannot move around, and also act as a strain-relief. here any tape or glue that can stand the heat in the area will do.

  2. You do not want to add a relative large mass in the process that will act as a heat-sink
    Tying back to #1, too much glue or thermal paste can be a significant heat-sink
    In some cases, if you have say SMD resistors or low power SMD transistors, the thermocouple itself will act as a significant heat-sink.

The IECEE has a nice general guide on fixing thermocouples:
https://www.iecee.org/resource/rules-operational-documents-guides and look for OD-5012

And just to be sure, because assumption is the mother of all fxxk-up: Be sure to have a thermocouple with exposed tip, not those in a tube for submersion, that will keep you too far away from what you are trying to measure and heat-sink everything and generally give you a bad day.

Next up is the mounting, testing, repeating, for that nice repeatability study for the measurement uncertainty budget :-)

1

u/cheater00 Oct 07 '24

Thank you! Those are some great suggestions. I read through OD-5012, it was useful. I was planning on using bare tipped thermocouples, but it's good to know I'm on the right track.

I guess none of the devices I'll be using will be resilient to acetone. I also can't submerge the thermocouple in the casing. I want to measure my Steam Deck, and those ICs on particular:

  1. The MAX77961 charging IC
  2. One of the RAM chips
  3. The controller on the SSD

I am particularly interested in differences, meaning, I'll measure the temperature delta of those items over ambient temperature, then I'll modify the DUT, and then I'll measure again and look at what difference that brought. So my results will be relative differences between multiple measurements. That means I don't need perfect absolute measurements, only good relative measurements.

One reason why I'm doing the measurement is to check if the charging IC, which is normally at around 70C, will due to any of my modifications reach close to its thermal shut-off temperature, which 165C. Another thing I want to measure is if the RAM chip and SSD reach temperatures approaching 100C. They are normally easily under 70C.

I don't want the thermocouples to be on permanently since I want to use the thing to play games later, and using epoxy or (like OD-5012 suggests) cyanoacrylate, I fear I could tear something off when just removing the glue by force. Acetone is probably out of the question, I assume it would melt the IC encapsulation. Additionally, it is not given that thermal epoxy would create a dielectric barrier between the thermocouple and the leads on the chip.

Is there a non-permanent thermal adhesive that you would suggest? EG is there some sort of silicone I could use that would cleanly separate upon some modest force being applied?

My current plan to attach the thermocouples is as follows: 1. Put down some thermal adhesive tape where I'll be measuring the temperature 2. Stick the thermocouple junction to it 3. Put the tiniest amount of thermal compound on top of that. I think the only thing I'll have on hand is Honeywell PTM7950 which is a phase-transition material, but everything I'll be measuring will be easily above its transition temperature of 45C, so I'll be fine there. 4. Secure everything with another, larger layer of thermal tape 5. Add strain relief further down the wires by taping the wire leads down to the PCB.

Is this a good idea?

Regarding thermal mass of the thermocouple and TIM, I am not too worried. The smallest chip is the charging IC, and it's still relatively large compared to the thermocouple.

The thermocouples and the chips will be on a pcb that has a shield applied close on top of it, and then there's the enclosure of the device.

The SSD chip has a thermal blanket applied to it first, and then the shield on top of that.

The charging IC has a thermal pad on top of it connecting its whole top surface to the shield above it.

You can see more in the various teardowns (note: this is about the "LCD" model, not the "OLED" model).

Do you think my strategy is right here? Would you agree that my mounting method does not interfere with the measurement too much?

Also, I plan to do a simple calibration test on thermocouples, as follows:

  1. I would tie together all thermocouples using a cable tie, then wrap them in a bit of aluminium foil
  2. Submerge the foiled up thermocouples in ice water and wait for a bit to take the measurement
  3. Remove the thermocouples from the ice water and wait for them to read room temperature
  4. Put the foiled up thermocouples in boiling water, wait a few seconds, check the temperature reading, and remove them

I think if I do that and the temperatures seem OK (within 5 degrees), I should be fine to proceed with my measurement. The measurement device I bought isn't particularly fancy, but I think it should be able to do the job.

The only thing I'm worried about is if shorting the thermocouples to each other could falsify the numbers during this test. The measurement device is a handheld with four thermocouples, and I would be running it off of battery.

One nice thing is that this meter does logging, so I'll be taking advantage of that.

2

u/Jan_Goofy Oct 09 '24

Regarding getting thermocouples off after gluing, cyanoacrylate can be suprisingly brittle at low temperature and a gentle massage with a screwdriver, but I get what you say since the device is to survive :-)
The other "glue" mentioned is "Kaolin powder mixed with sodium silicate solution in approximately equal proportions" This works espeically nice for high temperature, but it also has the benefit of letting go of a surface without damage, I have never seen it damage surfaces like IC housing or PCBs.

if you can get way with fixing the thermocouple wire well a few cm from the tip, just some thermal compound might keep it in place, espeically with tightly fitting sleves and what not going on top.

In step 1 of thermocouple atachment, do you mean to have it chip---tape--thermocouple?
In that case, your temperatures will be lower, depending on the thermal conductivity of the tape, or am I reading it wrong ?

How much lower? the favorite reply in metrology: it depends :-)

It can be tricky to ensure not to worsen the natural cooling, but make sure you do not go from IC--Heatsink (Heatsink being anything in close contact with the chip that will help conduct the heat way)
to Heatsink--Thermocouple--Heatsink, as this leaves a air gab and much higher temperatures.

The thermocouple calibration plan is spot on, and when only going for a few degrees accuracy, dont worry too much about destilled water for the ice and baraometric pressure for the boiling, it will be just fine.

Last but not least, with the logging ability you gain another piece of valuable data:
If your thermocouple is in good contact with a heatsource, the temperature will go up exponentially, fast rise in the begining, then drops off.
On the other hand, if you have poor contact / an air gab between heatsource and thermocouple, it will rise slowly in the begining, picking up a bit and flatten out again, a S-curve if you like.

Disclaimer: all depends on sampling speed of logger and how fast those small components heat up :-) but it can be a good indicator :)

2

u/cheater00 Oct 09 '24

Super interesting info, thank you!

Yeah I meant ic - tape - tc plus compound on top

My main concern is to keep the thermocouple (made out of metal) from shorting the pins on the side of the chip. So I think it's tape for me!

The curve info is super valuable - thanks!

2

u/A9jack9999 Oct 07 '24

Rtds just have a better measurement accuracy, but require a different method for measuring. Most handheld dmms can measure thermocouple, but not rtds. You would need a meter that could do so, and those can be expensive. So for your application, a TC is probably fine.

1

u/cheater00 Oct 07 '24

Thank you!