[volt-nuts] cadmium solder alloy for low thermal emf?

[John Devereux]

"Andreas Jahn" <Andreas_-_Jahn at t-online.de> writes:

> Hello
>
>>I do not imagine cadmium bearing solder being easy to acquire.  The
>> Wikipedia entry for solder says Pb90Sn10 can be used as a replacement
>> for Cd70Sn30 in low thermal EMF applications:
>>
>> http://en.wikipedia.org/wiki/Solder
>>
>> On Thu, 30 May 2013 04:00:19 +0200, Volker Esper <ailer2 at t-online.de>
>> wrote:
>>
>>>
>>>By the way: does anyone know, if Agilent uses special solder alloy? I've
>>>heard that a cadmium containing solder is used to get extremely low
>>>thermoelectric voltages (or voltage differences).
>>>
>>>Is that right? If so, which alloy has to be used?
>>>
>>>Thanks
>>>
>>>Volker
>>>
>
> Within LT AN86 Cd60Sn40 is recommended for a limited temperature range
> of 0 to around 40 degrees.
> http://cds.linear.com/docs/en/application-note/an86f.pdf
>
> But: the thermal EMF is only zero against copper.
> Most precision integrated (hermetical) cirquits use Kovar. (39uV/K
> against copper)
> Relay contacts will be either copper berillium or another material.
> So in most cases a optimized solder for copper/copper connections will
> not be useful.

I have never understood why it matters anyway. The conductors being
soldered together end up in very good thermal contact. So there should
be no thermocouple generated by the solder-conductor interfaces (since
there is no temperature difference between the ends of the wires being
soldered).

So if you have copper-solder-copper, say, then the copper-solder
junction is microns away from the solder-copper junction and is
surrounded by metal. So surely they will be at the same temperature
unless there is a huge heat flow.

The referenced AN86 even suggests introducing balancing
copper-solder-copper junctions, by cutting tracks and bridging with
solder.

[...]


-- 

John Devereux