Temperature sensor with very low height but thermally conductive - Page 1
Forum section: electronics / projects

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: March 29, 2024, 02:58:48 am
content: FriendsI have an IC package which has an exposed die on the top side. The top side die connects to a heatsink using a TIM (Thermal Inteface material) which is about 0.2mm thick. I need to measure the temperature on the top side of the die. For that I need to place a very thin thermocouple between the die and the TIM or between the TIM and the heatsink.Something like the picture hereCan anyone recommend a suitable temp sensor or thermcouple which can do this job effectively. I need the temp sensor to be thermally conductive so that it does not create any thermal issues for the IC.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 29, 2024, 01:56:49 pm
content: Which IC? Are you sure it doesn't have integrated temperature sensor, because that's how this problem is usually solved.

author: ConKbot
author_rank: Super Contributor
timestamp: March 29, 2024, 02:11:27 pm
content: https://www.omega.comomega engineering has fine wire thermocouples. They are down to 75um diameter, so you could lay it down before you apply the pad and heatsink. Being a thermocouple, it's all metal so still reasonably thermally conductive.Not a good solution for something you ship, but for one off measurement, to qualify something like heatsink temp vs die temp, and make it so you only have to probe the heatsink in the future, it may be acceptable.

author: peter-h
author_rank: Super Contributor
timestamp: March 30, 2024, 10:27:16 am
content: IR camera.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 30, 2024, 12:34:09 pm
content: That processor range has internal temperature sensing. From the datasheet"The temperature sensor has to generate a voltage that varies linearly with temperature. Theconversion range is between 1.7 V and 3.6 V. The temperature sensor is internallyconnected to the same input channel as VBAT, ADC1_IN18, which is used to convert thesensor output voltage into a digital value."

author: peter-h
author_rank: Super Contributor
timestamp: March 31, 2024, 05:48:50 am
content: https://www.eevblog.com/forum/microcontrollers/stm-32f4-reading-cpu-temperature/msg4980826/#msg4980826

author: inse
author_rank: Super Contributor
timestamp: March 31, 2024, 11:55:12 am
content: Why not drill a hole through the heatsink and push the thermocouple right down to the exposed pad?

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: April 04, 2024, 06:11:12 am
content: Quote from: inse on March 31, 2024, 11:55:12 amThats a neat idea. Will try that...

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: April 04, 2024, 06:12:05 am
content: Quote from: Siwastaja on March 29, 2024, 01:56:49 pmThe processor does have an internal temp sensor. But it is not very accurate. Kinda +/-7degC error. I need the temp to be within 1degC error.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: April 04, 2024, 06:13:03 am
content: Quote from: peter-h on March 30, 2024, 10:27:16 amThe problem is the heatsink on the top covers any thermal image of the die. I need to be accurate to within 1deg. The heatsink on the top defeats this idea.

author: johansen
author_rank: Super Contributor
timestamp: April 04, 2024, 02:56:12 pm
content: Quote from: EEEnthusiast on April 04, 2024, 06:13:03 amYou can get that level of accuracy if you deposit a 100-ish ohm platinum resistor on the die, and then put the effort into calibrating your custom resistorAlternatively, grind away the epoxy around the die and put the thermocouple right next to it on the side. Cut away the thermal film so the thermocouple bead does not touch the thermal film so it is not cooled by the heatsink.You cannot put the thermocouple between the die and the heatsink because it will drastically change the thermal conduction, and you are still going to read the intermediate temp between the two. So if your chip is 60C the heatsink 55, the thermocouple will read 57.A copper spacer with a thermocouple inside it would require two layers of your 0.2mm thermal film, which means you are reading the temp half way between the die and the heatsink.I will have to confirm the details but a friend of mine had to send a dummy chip with a thermocouple epoxied in it, to an independent cal lab to prove that a 10 C temperature discrepancy (at -40C) was not imaginary.Since i dont know the wattage i cant give you an order of magnitude estimate as to how difficult it becomes to get 1C actual accuracy.You can do the math yourself, its actually really simple. But proving your 1.8mwk thermal film was actually 1.3... not so easy

author: Siwastaja
author_rank: Super Contributor
timestamp: April 06, 2024, 03:43:27 pm
content: Why is the exact and accurate die temperature so important? Even if it is, do you realize there is no such thing as uniform die temperature, but local hotspots anyway because power dissipation within die is not uniform and coupling to the exposed pad not perfect. BTW this is also one of the reasons why the internal temperature sensor is so "inaccurate". It measures a certain spot within the die. How "accurate" it is depends what you are comparing it against.

author: MarkT
author_rank: Frequent Contributor
timestamp: April 06, 2024, 08:34:40 pm
content: Quote from: inse on March 31, 2024, 11:55:12 amBecause it will measure mainly the heatsink temperature due to conduction along the thermocouple wire?