It is important to monitor the temperature during the SMT reflow process, especially for LEAD-Free processes. The components can suffer excessive thermal shock very easily. A thermocouple can be attached to a target circuit board to obtain real-time thermal-time data.
In order to obtain reliable data from a thermocouple, the thermocouple junction must have direct and reliable thermal contact with the PCB surface, otherwise only hot air temperature is measured where there is a 2~10°C difference with actual board temperature.
There are many ways to attach a thermocouple to a PCB such as, high-temperature solder, polyimide Kapton tape, aluminum tape and epoxy.
High-Temperature (HT) Solder
In general, we need the melting point to exceed 290°C so that the solder will not melt (and thereby loosen the thermocouple) during the reflow process. This type of solder has good heat conductivity even when the thermocouple junction is slightly off the surface of the circuit board. It provides rigid mounting performance for the test circuit board.
High temperature adhesive tape, such as Kapton tape, can be easily used on any surface. But the tape will get loose when it attains a high temperature, so it is challenging to attach a thermocouple to board surface rigidly using Kapton tape.
Same as Kapton, but aluminum tape has better thermal conductivity.
Epoxy is very good at securing thermocouple conductors to the profile board, but it is also much more difficult tto remove after the measuring has been completed.
Overall, HT solder and Kapton tape are the better choices.