Author Topic: low thermal EMF solder  (Read 17992 times)

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AnonUser

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low thermal EMF solder
« on: May 06, 2014, 06:47:53 am »
Hello - I am looking for a source to supply low thermal EMF solder - can anyone provide pointers ?

In the past a cadmium - lead alloy was used for this, but these are banned (RoHs) ... - the aim is to minimize thermo-couples on a very sensitive 4 layer printed circuit board - 0.1ppm / K on a 10V full scale - so every 100nV we can reduce is taken under consideration (preventive actions)

The assembly factory is located in Belgium.

Thank you for all feedback !

« Last Edit: June 24, 2018, 11:02:30 am by EEVblog »
 

Online Dr. Frank

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Re: low thermal EMF solder
« Reply #1 on: May 06, 2014, 07:53:48 am »
Just to finish your search quickly:
We habe discussed that on volt-nuts already,  Cd based solder really is not available any more, and nobody found an alternative, either.

Frank
 

Offline Vgkid

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Re: low thermal EMF solder
« Reply #2 on: May 06, 2014, 07:54:34 am »
http://www.kappalloy.com/tec-solder.php
kapptec solder google it.
I cant find any vendors, most others are hard solders, so that is a sign.
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Offline SArepairman

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Re: low thermal EMF solder
« Reply #3 on: May 07, 2014, 12:25:04 am »
how difficult is it to craft this solder? I assume the lower melting point materials are melted first, then the silver metal dissolves in them due to solubility rather then temperature.

Both elements can be bought in high purity fairly cheap, what are the ratios?
Is anyone interested in buying some home made Cd-silver solder?

Sure, I cannot offer you a flux core, but it should still be highly usable. I assume it is only used around references ETC, not the whole PCB?

it could be easily melted under an argon shield aswell if oxide contaminants are a concern, with metal pre-treated with mineral acids to remove oxides.

how hard is this stuff? can it easily be cast? what is the maximum solder diameter that you would consider useable? how much would you be willing to pay by weight?
« Last Edit: May 07, 2014, 12:36:20 am by SArepairman »
 

Offline SArepairman

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Re: low thermal EMF solder
« Reply #4 on: May 07, 2014, 05:18:34 am »
well I am interested in this stuff now too and I want some, I will make it for myself if there are no sources. Cadmium rod is damn cheap on ebay...


for production what do you need, solder paste? I think home made solder rod would be appropriate for through hole, but SMD thin solder would probably require some kind of special extruder machinery, unless you are like my friend and some how manage TSSOP with "pipe solder" and a 10$ soldering iron (while I need a nice iron and 0.7mm or less solder)

I imagine that you are probably using through hole technology if you are worried about thermal EMF, due to strain and whatnot.

But I do wonder if you could melt the alloy then grind it up in a chilled ball mill and put it through mesh sieves and mix it with flux... i bet robrenz would be interested in this ...
or is it produced through atomization and cooling (for spherical particles), not that hard a task for low melting point solids..

do you have any literature about the chemistry of these low emf solders?

i think realistically you need to contact a solder paste manufacturer and ask them if they can do a special batch for you, you could try a kick starter or something like this when you get a quote for low EMF solder production to manage the upfront costs, I'm sure some people might be interested.

i wonder if you can mix up the alloy yourself and then just mail it to some kind of factory to save money, i wonder how hard it is to make an alloy
« Last Edit: May 07, 2014, 05:50:36 am by SArepairman »
 

Offline Conrad Hoffman

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Re: low thermal EMF solder
« Reply #5 on: May 07, 2014, 09:21:45 pm »
The old L&N low thermal solder was (according to one reference I found) 70% Cadmimum and 30% Tin. Should be trivial to make, but you probably couldn't legally sell a product using the stuff in most places.
 

Offline SArepairman

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Re: low thermal EMF solder
« Reply #6 on: May 08, 2014, 12:39:09 am »
rohs compliant unless proven otherwise
 

Offline rexxar

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Re: low thermal EMF solder
« Reply #7 on: May 09, 2014, 01:36:30 am »
rohs compliant unless proven otherwise

Cadmium is banned by RoHS.
 

Offline zapta

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Re: low thermal EMF solder
« Reply #8 on: May 09, 2014, 05:56:23 am »
Hello - I am looking for a source to supply low thermal EMF solder - can anyone provide pointers ?

What is a low thermal EMF solder?
Drain the swamp.
 

Offline SArepairman

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Re: low thermal EMF solder
« Reply #9 on: May 09, 2014, 05:57:41 am »
Hello - I am looking for a source to supply low thermal EMF solder - can anyone provide pointers ?

What is a low thermal EMF solder?

copper-solder connection makes a thermocouple, http://www.eevblog.com/2013/02/03/eevblog-419-thermocouple-tutorial/
some solders make a weaker thermocouple, so you can have instead of 1uV/degree C 0.1uV / degree C or something like this
 

Offline acbern

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Re: low thermal EMF solder
« Reply #10 on: May 09, 2014, 02:08:17 pm »
keithley recommends Ag silver (4%) for use with their low emf connectors with 2182 nanovolt meters. keep in mind a emf voltage can only be generated if there is a thermal gradient in a solder joint pair. this is the first thing to avoid. given the precision achievable today with e.g. the 2182 and its cable this proofs there really is no need for cadmium solder. if you have a emf problem, it is elsewhere in the design.
« Last Edit: May 10, 2014, 02:47:49 pm by acbern »
 
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Offline Vgkid

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Re: low thermal EMF solder
« Reply #11 on: May 09, 2014, 07:55:57 pm »
That makes since, as it is probally rohs certified.
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Offline SArepairman

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Re: low thermal EMF solder
« Reply #12 on: May 09, 2014, 08:01:09 pm »
i wanna punch rohs in the face until all its stupid solder whiskers fall off  :box:
 

Online Andreas

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Re: low thermal EMF solder
« Reply #13 on: May 09, 2014, 08:22:48 pm »
i wanna punch rohs in the face until all its stupid solder whiskers fall off  :box:

Hello,

I do not think that you really want to have cadmium solder.
During soldering cadmium solder gives very poisonous vapours.
Btw. LT AN86 speaks of 60% Cd + 40% Sn.

With best regards

Andreas
 

Offline Conrad Hoffman

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Re: low thermal EMF solder
« Reply #14 on: May 10, 2014, 01:11:01 am »
You don't need a gradient across the junction to generate a voltage. If it simply exists above absolute zero, you get the voltage. Now, single joints aren't very popular- there's usually another one involved. If you can keep the pairs of joints at the same temperature, no gradiant, the voltages will cancel out and no problem. I think Jim Williams even suggested that an extra "unnecessary" joint could sometimes be incorporated to provide cancellation when necessary.
 

Offline ManateeMafia

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Re: low thermal EMF solder
« Reply #15 on: May 10, 2014, 04:54:51 am »
I would like to know if anyone has any documented study or application note that covers testing of different types of solder and measuring the Seebeck coefficients? You can easily find information on paired materials like copper-gold, but I have not seen too much information about the vast array of solders.

I understand the need for paired joints and thermal equilibrium, but having empirical data would give people the best solution based on what is available to them.
 

Offline CaptnYellowShirt

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Re: low thermal EMF solder
« Reply #16 on: May 10, 2014, 05:33:23 am »
Cadmium solder is to the EE as Loctite is to the ME -- if you have to use it, something is wrong with your design.
 

Offline SeanB

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Re: low thermal EMF solder
« Reply #17 on: May 10, 2014, 05:42:41 am »
Normally Loctite is used because you either do not have the space to put in a split pin or you do not want to use a deformable use once fastener. Plus it is easy to use in production as you can order fasteners in bulk with it pre applied. No extra steps required.
 

Offline mzzj

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Re: low thermal EMF solder
« Reply #18 on: May 10, 2014, 11:06:44 am »
keithley recommends Ag silver (4%) for use with their low emf connectors with 2182 nanovolt meters. keep in mind a emf voltage can only be generated if there is a thermal gradient in the solder joint. this is the first thing to avoid. given the precision achievable today with e.g. the 2182 and its cable this proofs there really is no need for cadmium solder. if you have a emf problem, it is elsewhere in the design.

Second that.
Another trick beside obvious advice of keeping solder joints away from thermal gradients is the joint itself. Last thing you want to do is to "bridge" gaps with the solder.
For example wrap the wires together to get good  thermal coupling and solder the joint after that. and pcb mounted component leads should be bend parallel to traces to maximize the contact area.
 
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Offline SArepairman

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Re: low thermal EMF solder
« Reply #19 on: May 10, 2014, 11:46:27 am »
i wanna punch rohs in the face until all its stupid solder whiskers fall off  :box:

Hello,

I do not think that you really want to have cadmium solder.
During soldering cadmium solder gives very poisonous vapours.
Btw. LT AN86 speaks of 60% Cd + 40% Sn.

With best regards

Andreas

oh shit, i did not realize cadmium boiled at 800c, i assume it gives a fair bit more vapor then lead (boils at 1600c).

the vapor pressure at soldering temperatures is enough to make working with it fairly toxic? sounds like it should be done under a fume hood or outside with a fan.. heh glad i caught that
http://www.powerstream.com/z/vapor-press1.png

ooo, kinda bad, 10^-2 torr @ 300c, compare to lead and tin.. which have > 10^-8

certainly don't wanna do that indoors!

whats the working temperature of the solder?

and so long you are not an idiot about it I think it could be done safely. i would feel way more comfortable with cadmium solders (working with or manufacturing) then say, driving a car down the street, or going for a walk around my block.  :-+ you are in control of your own safety here. it would be one of the safer parts of my day.

compared to the amount of times I had a elevated heart rate from a bad driver to the amount of times I poisoned myself with heavy metals... hmm.. no contest

the only dangerous part of this would be running the electronics and a part running hot, possibly slowly vaporizing cadmium. this is where the danger is. might be worth while to put some thermal fuses on that circuit board! I would not want it running at too much more then room temperature. This could be considered if you have a thermally stabilized voltage reference soldered down with cadmium solder.

*in all likely hood though, you should probably focus your efforts on staying away from cigarette smoke if you are scared of Cd
« Last Edit: May 10, 2014, 12:20:17 pm by SArepairman »
 

Offline CaptnYellowShirt

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Re: low thermal EMF solder
« Reply #20 on: May 10, 2014, 02:49:10 pm »

Another trick beside obvious advice of keeping solder joints away from thermal gradients is the joint itself.


I hear this a lot on this blog. There's this idea out there that thermal EMF is generated *at* a solder junction.

To my understanding, thermal EMF is generated along the wires that bridge areas of different temperatures. Its just that because the process is path independent, the thermal EMF can only be realized at junctions:



Here's a sanity check... how can you get a thermal gradient of any significance across a solder junction? And how do thermocouples work? You place the wires between the two baths, not the junctions.

« Last Edit: May 10, 2014, 02:51:25 pm by CaptnYellowShirt »
 

Online Andreas

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Re: low thermal EMF solder
« Reply #21 on: May 10, 2014, 03:03:31 pm »

ooo, kinda bad, 10^-2 torr @ 300c, compare to lead and tin.. which have > 10^-8


The diagram is only valid for pure metals.
With alloys at least the melting point will go down.

With best regards

Andreas
 

Online Andreas

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Re: low thermal EMF solder
« Reply #22 on: May 10, 2014, 03:15:09 pm »
how can you get a thermal gradient of any significance across a solder junction?

Probably less a problem if you use copper/solder/copper joints. (around 3uV/K)
You will have 2 thermocouples with (slightly) different temperatures.
So they do not cancel out exactly.

The problem arises if you use hermetically tight components.
(Ceramic or metal case e.g. LM399 / LTZ1000)
Soldered on PCB you will get a Kovar/solder/copper joint.
Kovar has 40uV/K against copper.
And if you use a heated reference there will be significant difference
across (+ between different) solder joints of the component.

The clue is: Cd-solder will not help in this case  :wtf:
The reason is clear. It has only 0.3ppm/K against copper and not against Kovar.
The only way to get out is to do a excellent thermal isolation of the cirquit.
(see LM399 thread with the construction of Mickle).

With best regards

Andreas
 

Offline mzzj

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Re: low thermal EMF solder
« Reply #23 on: May 10, 2014, 10:18:25 pm »



Here's a sanity check... how can you get a thermal gradient of any significance across a solder junction? And how do thermocouples work? You place the wires between the two baths, not the junctions.
[/quote]

I agree that thermal gradients over solder junctions are tiny. Unless you bridge large gaps with solder or something stupid like that.

I did quick test, 2 pieces of 0,75mm2 wire soldered together badly end-to-end and connected to Agilent 34401A:
Holding only one of the wires between my fingers from near the solder junction results ~2uV offset compared to +-0,1uV seen otherwise. 
Depending which side of junction I heat with my finger the readings go positive or negative. 

And yes I agree, Connectors and dissimilar wires(ie kovar) are much bigger problem than the solder you use.
 

Offline robrenz

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Re: low thermal EMF solder
« Reply #24 on: May 11, 2014, 01:22:42 am »
My experiments on the thermal EMF topic a while back here


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