low thermal EMF solder

[SeanB]

I was able to turn an older ammeter into a thermocouple meter just by heating the one terminal with a soldering iron while repairing a broken wire. When finished soldering the wire back I loked at the front and it was at full scale and slowly coming back down. Basically a copper/constantan thermocouple with a cold junction at room temperature.

[CaptnYellowShirt]

My experiments on the thermal EMF topic a while back here

I had forgotten about that test until you posted it again. Real world data is always the best. Thanks.



So robrenz is seeing EMF's across solder joints in the range of 1's to 10's of nanovolts per deg C.

I've never done the experiments myself, but that's the range I would expect.

From my experience, offsets of 1-100's of microvolts are an issue of the circuit in its entirety -- not just one joint. If you're seeing this type of offset, you have some type of imbalance along the entire path of the circuit -- an odd number of joints or a pair of joints at different temperatures.

Kovar / solder may produce a high off set voltage. But remember, in any circuit you should have at least *2* of these joints which will be opposing one another. The design objective here isnt to minimize the kovar / solder offset voltage. Its to ensure the pair of joints operates at the same temperature.

[David Hess]

The voltage is generated across the length of wire at different temperatures and not the junction itself which explains why extra pure wire is used for runs across large temperature differentials.

[LukeW]

This sounds like a job for... the giant Wikipedia table of solder and solder-like alloys!

http://en.wikipedia.org/wiki/Solder#Solder_alloys

Note Cd70/Sn30 on that table as a low-thermal-EMF solder, or Pb90/Sn10 as a Cd-free alternative.

I haven't checked specifically what the RoHS exemptions list says, but there's probably an exemption for it, because I don't think I've ever heard of a low-thermal-EMF solder which is both Pb and Cd free. I know there's an exemption for high-melting-point Pb solder off the top of my head.

The table says nothing about the thermoelectric properties of Sn95.5/Ag4/Cu0.5

[quarks]

just looked for low EMF solder and as stated in other threads Keithley suggests for 2182 Sn96/Ag4

Does anyone know or where to find ppm/K comparison information for

Pb90/Sn10
Sn96/Ag4
Sn95.5/Ag4/Cu0.5


 

[David Hess]

just looked for low EMF solder and as stated in other threads Keithley suggests for 2182 Sn96/Ag4

You may have just resolved a decades long mystery for me.  I have a spool of Kester solder which is only marked Sn96.  Maybe it is low EMF solder.

[SvanGool]

@quarks

Meanwhile the "volt-nuts" thread on "Thermal EMF" https://www.febo.com/pipermail/volt-nuts/2016-July/004912.html measurements by Andrea Baldoni:

• Copper - Sn96/Ag4 -> 3.33uV/K
• Copper - Sn95.5/Ag3.8/Cu0.7 -> 3.22uV/K

Best "leaded":     Copper - Pb92.5/Sn5/Ag2.5 -> 3.02uV/K
Best "lead free":  Sn95.5/Ag3.8/Cu0.7 -> 3.22uV/K

My personal conclusion would be: avoid soldering and use crimping/clamping of pure copper, possibly with silver or gold plating, without plating use Deoxit to keep it clean from copper oxide.
The best method is, of course, to keep any temperature gradients away from single joints (joints in both wires may compensate each other), by thoroughly isolating them from drafts and other random temperature variations.

[Conrad Hoffman]

From the Department of Useless Trivia- Leeds & Northrup identified the low thermal solder joints in their equipment by painting them green.

[quarks]

@quarks

Meanwhile the "volt-nuts" thread on "Thermal EMF" https://www.febo.com/pipermail/volt-nuts/2016-July/004912.html measurements by Andrea Baldoni:

• Copper - Sn96/Ag4 -> 3.33uV/K
• Copper - Sn95.5/Ag3.8/Cu0.7 -> 3.22uV/K

Best "leaded":     Copper - Pb92.5/Sn5/Ag2.5 -> 3.02uV/K
Best "lead free":  Sn95.5/Ag3.8/Cu0.7 -> 3.22uV/K

My personal conclusion would be: avoid soldering and use crimping/clamping of pure copper, possibly with silver or gold plating, without plating use Deoxit to keep it clean from copper oxide.
The best method is, of course, to keep any temperature gradients away from single joints (joints in both wires may compensate each other), by thoroughly isolating them from drafts and other random temperature variations.

Thanks a lot for the info
I also do prefer crimping whenever possible

[Kosmic]

(sorry for necroposting)


So apparently you can replace cadmium solder by Pb90Sn10. Solders low  in Tin should have the same low EMF properties than cadmium based solders.

Now lead based solders are now hard to find but it's not impossible. I just bought 5 pounds from ebay. I should have enought for a while   

Ref:
https://en.wikipedia.org/wiki/Solder
http://www.analog.com/library/analogDialogue/archives/39-05/Web_Ch4_final.pdf

[cellularmitosis]

(I see no problem with necroposting  keeps the information concentrated)

This may be of interest to this thread (attached).

[RandallMcRee]

(I see no problem with necroposting  keeps the information concentrated)

This may be of interest to this thread (attached).

Agree--necroposting ok, here. Summary would be nice. I think this is it:

The data from this study suggest that the proteinuria
induced by excessive exposure to cadmium
is not reversible even if exposure to the
metal is greatly reduced.

[Magnificent Bastard]

For low thermal EMF's, why not CD weld copper to copper?  A simple capacitor+SCR CD welder is pretty easy to design and build, or you can buy a kit for a more sophisticated model:

http://zeva.com.au/Projects/SpotWelder/
https://www.philpem.me.uk/elec/welder/
http://www.pittnerovi.com/jiri/hobby/electronics/welder/

Kit:  https://www.keenlab.de/index.php/product-category/kspot-welder-kit/

[texaspyro]

For low thermal EMF's, why not CD weld copper to copper?

Welding copper to copper, particularly with a simple welder?  Not gonna reliably happen.  My CD welder is FAR from simple (20,000 amp peak, continuous weld current and pressure monitoring, etc) and copper welds are still crap or non-existant.  Also if the materials being welded are connected to any circuitry the induced spikes will kill it.

Everyone and their mother seems to think you can whack together a reliable CD welder...  nope...

[Kosmic]

(sorry for necroposting)


So apparently you can replace cadmium solder by Pb90Sn10. Solders low  in Tin should have the same low EMF properties than cadmium based solders.

Now lead based solders are now hard to find but it's not impossible. I just bought 5 pounds from ebay. I should have enought for a while   

Ref:
https://en.wikipedia.org/wiki/Solder
http://www.analog.com/library/analogDialogue/archives/39-05/Web_Ch4_final.pdf

So in the end Pb90Sn10 is really a pain to use. Not super liquid and doesn't stick super well.

I don't think I'm going to use it much.

[leighcorrigall]

Herewith another necropost! 

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. 

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]

Hello,
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.

Frank