adhesive for thermocouple

Roger Saunders, President Saunders Technology, Inc wrote a fairly unbiased paper that compared different thermocouple attach methods. You can probably find it on their site. In it, about gluing thermocouples, he says:

There are two general classes of material commonly used to adhesive bond thermocouples. A thermocouple is properly installed with a small amount of adhesive.

One is accelerator or UV activated adhesives that set in seconds, but are only rated for around 120*C. They do not hold up well in reflow, and are more commonly used for wave solder applications.

Special high temperature, two part epoxies are rated up to 260*C, but require several hours at elevated temperature to cure. PRO: � These products are easier to use than high temperature solder. � The accelerator and UV activated products set quickly, to speed installation. � High temperature, two part epoxies will withstand reflow temperatures for many cycles. � Both products can attach thermocouples to unsolderable surfaces such as plastic or ceramic components or FR4 board. CON: (Accelerator or UV activated adhesives) � Their low temperature rating makes them prone to popping off during reflow, where peak temperatures are around 210*C. � Their relatively poor thermal conduction makes it essential that the thermocouple junction be held securely against the surface to be measured while the adhesive is activated. � They are usually easy to remove by "popping " them off with a knife. However, they do leave a film-like residue that is noticeable on FR4 and dark components, and can be difficult to remove. The most effective solvents, like acetone, also dissolve plastics, and can therefore damage circuit boards. CON: (Epoxies) � High temperature epoxy requires an oven cure of several hours. This is inconvenient, especially for quick trouble shooting. � It also requires careful fixturing to insure that the thermocouple junction stays in contact with the surface to be measured, throughout the oven cure cycle. � It is difficult to remove this material cleanly, without damaging the board or component. � Quick curing epoxies like "5 minute" epoxy are rated in the range of 130*C, so they often pop off during reflow.

Search the fine SMTnet Archives for discussion on thermocouple attach. For instance: http://www.smtnet.com//forums/index.cfm?fuseaction=view_thread&CFApp=1&Thread_ID=6929&#Message27727

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Hello Fredrick,

I have used several methods of TC attachment including quick set adheasives (not good at reflow temps as Dave suggests) long set two part epoxies (work well if you have the time to wait for a sufficiant cure) temprobes, tape (kapton and aluminum) Of all methods I have found the aluminum tape to work the best for me. It has pretty good bond strength and introduces minimal thermal diffences. It also cleans up fairly easily.

I hope this helps.

Good Luck

Chris

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