Low thermoelectric coefficient solders

[Nemo]

I noticed more drift with temperature than I expected in a circuit with
a DC gain of over a billion, and wondered if a contributory factor could
be the type of solder used. I put it together with lead-based solder for
convenience, but realise in hindsight that modern components are adapted
for lead-free soldering. Searching the Web and this newsgroup, I found
some oblique references to low-TC solder, and some exotic solders which
I don't think will be part of a normal assembly line (we get our small
batches of PCB's made by external assembly houses). Can anyone advise on
whether solder type is likely to be a significant cause of drift over,
say, a 30C swing; and what types are best for this kind of work?

Obviously I can take other precautions like a large thermal mass in the
high gain area to equalise temperature, insulation in the critical zone
to prevent air circulation, minimising the heat dissipated in that area,
careful board layout to minimise differentials, etc.

Thanks,

Nemo

 

[Fred Bartoli]

Stephan Goldstein a écrit :

Standard lead-tin solder makes a thermocouple with copper of some
tens of uV/C. Sorry I don't recall the precise number, it's been a
long time since I faced this problem (mid-1980s). My solution at the
time was to use Cadmium-Tin solder, which was about an order of
magnitude better. It required a special flux and was incompatible
with a soldering-iron tip used for standard solders. These days it
may not even be available because of its cadmium content.

With a DC gain of a billion, everything is a thermocouple, even for
a small delta like 30C. 1uV/C * 30C * 1e9 = 30mV. Do you really

Better make that 30kV!

 

[Grant]

Standard lead-tin solder makes a thermocouple with copper of some
tens of uV/C. Sorry I don't recall the precise number, it's been a
long time since I faced this problem (mid-1980s). My solution at the
time was to use Cadmium-Tin solder, which was about an order of
magnitude better. It required a special flux and was incompatible
with a soldering-iron tip used for standard solders. These days it
may not even be available because of its cadmium content.

With a DC gain of a billion, everything is a thermocouple, even for
a small delta like 30C. 1uV/C * 30C * 1e9 = 30mV. Do you really
need such a high DC gain? Is there any way you can chop? I'd
think your signal would be lost in a billion times the thermal
noise...

Micro spot welding? At least you reduce and control different metals
involved. Do that high gain circuit on ceramic, like RF modules?
Then put module on PCB with other stuff not have the gain issue.

Grant.

 

[Nemo]

DC Gain of over 10^9, Wow! How much temperature drift did you
expect? Can you tell us what you are measuring?

Hah! Right, I see I worded the query poorly. There's a (photodiode amp)
TIA with a gain of a few hundred thousand at the front end. In this
current gain configuration, its offset voltage is only amplified by
unity, of course. The DC gain AFTER that is about 3,000. I assumed the
drift was coming in at the front end but after you guys tossed some
numbers around there, I see TE effects are larger than I thought and, if
they ARE the cause, could be coming in after it. So I shall tie some
things to 0V, and build a version with lead-free solder, and see where
it's really coming from.

Cadmium-tin solder sounds like nasty stuff, but at least I know there's
one option if it turns out that lead free solder isn't good enough!
Thanks guys.

 

[Uwe Hercksen]

I noticed more drift with temperature than I expected in a circuit with
a DC gain of over a billion, and wondered if a contributory factor could
be the type of solder used. I put it together with lead-based solder for
convenience, but realise in hindsight that modern components are adapted
for lead-free soldering. Searching the Web and this newsgroup, I found
some oblique references to low-TC solder, and some exotic solders which
I don't think will be part of a normal assembly line (we get our small
batches of PCB's made by external assembly houses). Can anyone advise on
whether solder type is likely to be a significant cause of drift over,
say, a 30C swing; and what types are best for this kind of work?

Hello,

there are a lot more unwanted thermocouples in your circuit than the
solder joints. The IC leads are another metall than the traces of the
PCB, inside the case there are bonded gold wires, the silicon is covered
with aluminium traces. The resistors are also a combination of different
metalls.

Bye