Electronics > Metrology

low thermal EMF solder

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Herewith another necropost!  :horse:

I am in the middle of purchasing a Vishay HZ Series (Model HVA) resistor for a standard and am contemplating how to connect the leads to binding posts. In this pursuit, I was given the following information from a Vishay Applications Engineer who has had 'considerable experience' with hand soldering VHA resistors in custom decade boxes:

'[Given that] Cd-Sn solder is essentially impossible to buy today because of the very toxic nature of cadmium, the recommend solder to use for Sn-Pb leaded VHA resistors is Sn60Pb40. A better and safer solution is to use a razor blade or X-ACTO knife to carefully scrape the solder off of the ends of the resistor leads until there is nothing but bare copper (i.e., no solder coating). Then tightly wind the "no-solder" wire around the test terminal and solder with regular Sn60-Pb40. Thermal EMF (Seebeck effect) is created by heating the junction of dissimilar metals. Eliminating the solder layer between the copper lead and terminal will provide a very low thermal connection.' -- Vishay Applications Engineer with 30+ years experience.

A similar wire-wrapping example can be found with this link (https://youtu.be/a6AIoEj5Bzw?t=1016) from the YouTube channel 'theBreadboard' where he inspects a Vishay-made custom reference box. The reviewer does not allude to low EMF attributes, but he does suggest that the 'quality of the connection' is greatly improved as a result of wrapping the leads around the bus connection.

From SvanGool's previous post, I examined these cadmium-free options to solder precision resistor leads:

Cu conductor to Sn5/Pb92.5/Ag2.5: 3.02 µV/°C, 296 °C melting point (highest melting point)
Cu conductor to Sn95.5/Ag3.8/Cu0.7: 3.22 µV/°C, 219 °C melting point (poor wetting)
Cu conductor to Sn96/Ag4: 3.33 µV/°C, 223 °C melting point (poor wetting)
Cu conductor to Sn60/Pb40: 3.34 µV/°C, 190 °C melting point (best compromise to avoid heating the resistor)

The choice of solder will ultimately depend on your application.  Obviously, there are other properties to consider besides liquidus temperature.  :-/O

Ultimately, the reduction of EMF can be achieved by making good contact with the copper conductors and avoiding temperature gradients at dissimilar metal junctions while under test. This rule has been mentioned already but should be emphasized.

Good luck!

Dr. Frank:
low e.m.f. (solder) is required ONLY for voltage measurements!
For reistors, any e.m.f. can be cancelled by using the Offset Compensation method.



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