Thermocouple Attachment Review

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall of ITM Consulting here with Board Talk, and we're coming to you today from the roof of the ITM building in beautiful downtown Mount Rialto.

Jim
For those of you who are regular listeners to Board Talk, you will know that very early in our presentations we talked about attaching thermocouples to printer circuit assemblies for the purpose of profiling a reflow machine or a wave machine.

We got on one of our sacred, holy soapboxes and said we don't like tape and we like to glue down thermoucouples using high-temperature solder because it's more permanent. 

We got a number well thought out responses, people saying, "Hey, we can't do that in our environment. We can't sacrifice a board. We've got to use tape." A lot of people are saying, "We like aluminum tape." And so we just thought we would weigh in and review.

With any temperature measurement, the key element is to get the thermocouple itself in good contact with what you're trying to measure and to do so in a way that does not modify the area with a lot of extra mass or material that's going to give you an inaccurate thing. So if you have to use tape, make sure that the thermocouple is in good contact with what you're trying to measure, be it a material or a solder joint or whatever. Try to strain relief it with some other tape before you put the tape over the thermocouple so there's no pressure on the tape holding down the thermocouple.

So when the tape does heat up and the adhesive may get a little weak, there's no pressure to lift that thermocouple up. Be careful that the location, the shape and the size of the piece of tape you use do not affect the heating of what you're trying to measure.

Phil
Right, and that same principle applies to if you are indeed using the preferred method of conductive epoxy or high-temp solder in that you don't use an inordinate amount of quantity or volume of solder or conductive epoxy. Because for that same reason Jim just outlined, you're going to affect the readings in a bad way.

Jim
You want to measure the temperature of the solder joint, not the temperature of a big piece of tape sitting on top of a circuit board, not the temperature of a big blob of high-temperature solder sitting on the circuit board or a big blob of adhesive.

Phil
Okay, that's it for today's Board Talk. Remember whatever you do, wherever you go ...

Jim
Don't solder like my brother.

Phil
Don't solder like my brother.

Jim
Keep the kids away from the flux pot.

Here are the results of the RIT study in a nutshell. The results confirm much of what Phil and Jim have been saying. I was somewhat surprised that aluminum tape clearly beat out Kapton "thermally" aside from the obviously problem a TC has staying in place with Kapton as we all know when we see saw tooth zig-zagging of TC readings.

I was really surprised high temp solder performed so poorly! After this study I don't really see a situation where it should ever be used?

Supporting evidence can be found at the following link ahead of a formal publishing of the data.

http://profilingguru.com/reflow/measure/thermocouple-attachment-results-are-in/

Hey guys, great response to your earlier session. I posted on my blog comments to this discussion at: http://profilingguru.com/reflow/measure/thermocouple-attachment-discussion/

I have a link to a conference paper that studied all of these different materials for TC attachment, included in the blog, but it is 10 years old. Currently RIT is re-conducting a similar study to test if our findings are still up-to-date.

Hope to have some results by SMTAI. Great stuff again, love it! Hey you guys ever think about a Podcast? Wouldn't mind taking both recordings and adding to my profiling guru podcast.