Indium Blog

Pb-free Thermocouple Fix

A set of wires prepared for connection

A set of wires prepared for connection

Properly joined thermocouple wires

Properly joined thermocouple wires

Thermocouples do a lot for us.  We use them for profiling reflow ovens, checking material temperatures, and a host of other temperature related measurements.  They see a lot of abuse and they are bound to break with enough rough use.  So how do we fix broken thermocouple wires?  I have a method that works very well.  It may not be the cheapest way to fix the wires but it has the following advantages:

  • Pb-free joint

  • Highest thermal conductivity of any current method

  • Good for use up to 280°C

  • Strongest connection (40,000psi tensile strength)

  • Requires no specialized equipment


This method can also be used to convert leaded thermocouple wires to pb-free.


Here’s how:

1) Clip the ends of the thermocouple wires so they are even

2) Strip the sheathing back as shown in the picture (1/4inch)

3) Use a razor blade or emery paper to scrape the oxide layer off the wires, then twist the ends together

4) Put a very thin (~.001”) layer of NC 506 flux on the surface of a ceramic coupon and the exposed thermocouple wires

5) Place an 80Au/20Sn preform or a sphere(s) of the correct volume on the flux layer

6) Place the coupon onto a hotplate set to 400°C

7) Bring the wires over to the Au/Sn (which should now be molten)

8) Dip the wires into the solder

9) The solder should wick onto the wires, when it does – remove the wires.


You can leave the no-clean flux residue on the wires, or wipe it off using a solvent and rag.  You now have a high-temp pb-free thermocouple.


If you'd like to discuss this with me, click here or just give me a call @ (315) 853-4900 x-7592. 

Authored by previous Indium Application Manager Jim Hisert