How to Solder a Thermocouple

Soldering a thermocouple differs from soldering anything else, mainly in the solder type and the amount of heat required for the task.

From an engineering standpoint, thermocouples are fascinating devices brimming with potential. A thermocouple relies on one of a number of thermoelectric effects (most notably the Seebeck effect) to produce a voltage when exposed to heat. All metals and alloys produce a certain amount of voltage when one part of the metal is hotter than another part. This voltage can trigger valves to open or close according to thermal load, and can help to protect computer circuit boards in the event of a power overload. Soldering the thermocouple to the extension wire or circuit board is one of the best ways to conduct heat quickly and accurately through the thermocouple.

Things You'll Need

• 400-grit carbide sanding paper
• Rubbing alcohol
• Adjustable, high-temperature soldering iron
• High-temperature soldering paste

Instructions

1. Scuff the soldering pad on your circuit board or the extension wire tip with 400-grit carbide sandpaper. You only need to scuff the pad or wire tip until it becomes bright and shiny. Repeat this procedure with the input wires on your thermocouple. Wipe the scuffed components with a lint-free rag soaked in rubbing alcohol to eliminate the sanding dust.

2. Set your soldering iron temperature to about 200 degrees past the thermocouple's maximum range and allow it to heat up. Lay the thermocouple sensor wire on top of the soldering pad and lightly press it down with the soldering iron tip to heat the thermocouple sensor wire and pad.

3. Remove the soldering iron and apply a dab of soldering paste. The paste will instantly melt and wick into the sensor wire/solder-pad interface, quickly hardening and bonding the two together. Allow to cool. If the soldering bead keeps its "wet" sheen, you have a good joint and proper soldering iron temperature. If the solder quickly dulls, the iron was too cold. You can attempt to melt the solder with the iron and use a soldering wick to draw the solder away, but thermocouples don't tend to do well with re-soldering. If you need to re-solder, play it safe and use a new thermocouple.

Tips & Warnings

• Soldering a thermocouple isn't very different from soldering anything else; the trick is in choosing the right solder temperature range (composition) and iron temperature. Choose a solder based upon its liquidus temperature, that is, the temperature at which it becomes a liquid. The liquidus temperature should always be at least 100 degrees higher than the highest temperature that the thermocouple will experience. Set your soldering iron at about 100 degrees past the solder's liquidus temperature.

• There's a very thin line between soldering and brazing, but a joint is usually considered brazed if the process occurs at more than 350 degrees Celsius.

• Alternatives to soldering include using aluminum tape combined with Kapton tape, which is completely reversible and about as reliable as soldering. Kapton tape by itself will work at lower temperatures, but tends to get hotter faster than solder and to lift when temperatures rise. Electrically-conductive adhesive epoxy is the easiest to use, but its thermal conductivity is the least consistent of the three and it can fail after a single heat cycle.

References

• KIC Thermal: A Comparison of Methods for Attaching Thermocouples to Printed Circuit Boards for Thermal Profiling
• Microwaves 101: Solder for Microwave Assemblies
• Finishing: Solder Constantan Wire (Nickel/Copper Alloy) for Thermocouples
• Custom Thermoelectrics: History of Thermoelectrics