Phil Zarrow and Jim Hall of ITM Consulting have a very good piece on TC attachment on Board Talk hosted by Circuitmart.com.
In their first session, they talk about permanent TC attachment, such as high temp solder and epoxy (click here for a link to their recording). Yours truly left a comment with the boys:
We are a big fan of conductive Aluminum tape, used along with Kapton for strain relief like you mention in your podcast. We talk about high temp solder and epoxy which can work also, but like you said you got to be careful of mass. Lot of times we see unequal amounts applied per TC that can throw your readings. What is your take on aluminum tape, realizing of course it is a non permanent solution?
Well, they came back with a terrific response (click here for a link to their recording), where they make a clear distinction between destructive vs. non destructive methods. Non destructive methods are often the only option, since customers cannot sacrifice a board for profiling.
Phil goes on to say:
any measurement method, the key element is to get the thermocouple in good contact with what you are trying to measure and to do it in a way that does not modify the area with a lot of extra mass or material that is going to give you an inaccurate reading….
Phil talks about using for example Kapton as a strain relief to ensure there are no stresses on the point of TC attachment. I’ve been saying for years to use techniques such as window paning where you apply Kapton around the boarder of your aluminum tape to help keep your TC secure if profiling more than once the same PCB. Make sure not to put Kapton over the bead since Kapton can behave as an insulator.
I think Phil makes a great point on emphasizing the “size” of the tape you are using. Again you don’t want the material’s mass to become an issue. So the name of the game is don’t go overboard. Personally I prefer a 1/4″ square piece of aluminum tape along with 1/4″ Kapton.
Jim Hall makes also an excellent point that the same goes for “destructive” methods when using high temp solder and epoxy. You don’t want to overdo it, or the mass can effect your readings. I would add further that you need to be very careful that the mass be equal from TC to TC. It has been my long held belief that the blob of epoxy or solder if of unequal amounts TC to TC, PCB set up to PCB set up will add variability into your process. Just keep your materials to a minimum to get the job done.
Many of these assertions are currently under review by an RIT study. Hope to have results as early as the end of this month. KIC conducted a study 10 years ago on all the materials mentioned (click here for the report). Since a decade has past, one could assume materials have improved therefore warranting a second look. Stay tuned!