There are several methods to attach thermocouples to PCBs, some better than others. A popular study, “A Comparison of Methods for Attaching Thermocouples to Printed Circuit Boards for Thermal Profiling,” was conducted by KIC in 1999, looking at the various methods such as described below. Since this report, means for finding non-destructive methods of thermocouple attachment, such as is required when measuring complex masses such as BGAs has been required. The Center for Electronics Manufacturing and Assembly of the Rochester Institute of Technology conducted a study, “Evaluating the Accuracy of Non-Destructive Thermocouple Attach Method for Area-Array Package Profiling,” exploring non-destructive methods of TC attachment that could accurately measure components such as BGAs. This ground breaking study found that aluminum tape to attach a TC directly onto the top of the BGA provides a good approximation of the temperature readings under a BGA. Furthermore, this offset can be calculated with a reasonable level of confidence by using a formula developed in this research.
A free tool was developed that can be download here.
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Epoxy is a common method of thermocouple attachment, though it has the pitfall of being destructive. Be sure to check the specification of your epoxy since they can be either insulative or conductive. An insulative epoxy of course would give you poor results. Keep in mind if you choose epoxy to not vary the amount of epoxy used for attachment.
High Temperature Solder
High temperature solder is a common method of thermocouple attachment, though it has the pitfall of being destructive, like the case of epoxy. KIC has developed an application note that walks you through the process. Keep in mind if you choose high temperature solder to be sure to not vary the amount of solder used similar to the case of epoxy for attachment. The mass of solder does have an influence on the reading of a thermocouple under certain conditions. You want to minimize this impact by using only what is needed and keeping this amount consistent from TC to TC.
Kapton® tape is widely used for thermocouple attachment. It is popular for its ease of use, low cost and non-permanence. Its major draw back is its tendency to lift off a PCB with one or more profiles.
Low density aluminum tape is conductive and can be applied as a measured layer, not suffering some of the variability issues of high temperature solder and epoxy. Furthermore, like Kapton® it is not permanently attached to a printed circuit board. Aluminum tape by itself does not adhere to a PCB so a solution is required with takes us to a combination of Kapton® and aluminum tape.
Aluminum Tape Used Along with Kapton®
Kapton® and Low Density Aluminum tape when used together produce the least invasive methods of attachment. Kapton® tape is used to hold the aluminum tape in place and secure the thermocouple to a PCB. A common method of TC attachment is called Window-Paning. Use Kapton® around the aluminum tape as an anchor, while keeping only the area of attachment in contact with the aluminum tape. This gives your PCB many more uses before it needs to be re-taped.