Introduction
Positioning of a thermocouple should always be such that the thermocouple has the best thermal contact possible to the object that should be measured. Further it is important that the thermocouple is not thermally obscured e.g. by a dot of glue or a thermal conductive tape as these materials may slow the reaction speed of the thermocouple. This is especially important in processes with a fast dT/dt, like in wave soldering if the object contacts the solder wave. The mass of the thermocouple itself should have a minimum effect on the mass of the component to be measured, but also to assure a fast dT/dt response if this is necessary.
Wave soldering
If we want to measure the temperature at a solder joint, the best way to connect a thermocouple is to 'hook' the weld against the protruding wire and as close to the board surface as possible. This position should be fixed with a dot of glue over the thermocouple wire, placed at a distance of a few millimetres from the joint to be measured. In this way the spot to be measured is free accessible for the wave and the spot is free from glue.
A thermocouple mounted in this way will directly respond to the spot/lead temperature.
Since the metal of the lead, the solderspot and the solder are all contacting the uncovered thermocouple wires, one has to realize that the measurement can be affected by "parallel welds" created by all these metals. So the real temperature can be somewhat higher than the registered temperature by the thermocouple. This is a slight drawback that is however mostly well compensated by the fact that this set-up gives reproducible measurements. This is important when comparative measurings have to be made using the same measuring positions over again.
If one has to measure at a board or component surface it is important that a good thermal contact between the thermocouple and that surface is assured. In most cases a Kapton-tape can be used to fix the thermocouple to the surface. Also aluminium foil tapes are often provided to improve a good thermal contact between the foil, the thermocouple and the object to be measured. This metal foil has however also its own mass that may affect the correct measurement, as this mass might act as a heat sink.
Finally the contact with aluminum in a solder wave should be prevented to avoid possible contamination of the solder by aluminium.
Reflow soldering
In reflow soldering where the dT/dt is over the total process much lower than in wave soldering, we can often refrain from the restrictions that are necessary for not to affect the dT/dt of the measured spot.
This means that often it can be accepted to glue the thermocouple directly to the spot that must be measured. The glue, although a thermal barrier, will in this case due to a slower process with a lower dT/dt have less effect on the accuracy of the measurements. Also the restrictions on the use of aluminium foil tape, that may be necessary for wave soldering, may be less stringent in reflow soldering for the same reasons.