Problems de-soldering copper pipes (lead free solder)

[vwhammer]

First a little background.
We are soldering some copper tubes on a thermo-siphon used in an ultra low temp freezer.
The solder we are using is a lead free mix.
It consists of 96% tin and 4% silver non-flux core.

We then use Rubyfluid liquid flux.

In case anyone thinks it is important we are using R508b as the refrigerant and the tubes do get down to -110c and sometimes lower.

The problem we are having is that the tubes will not de-solder when we need to take something apart for repair.

We heat them until the solder should melt and it does not.
Then we heat them more sometimes until it is glowing red and the joint will not come undone.

The really bizarre thing is the fact that there are two joints and they are right beside each other.
Sometimes one joint will come loose and the other will not.

If it was not for the fact that we will be selling in Europe I would switch back to lead solder.(We never had this problem before the switch)

Does anyone have any input or more importantly a solution to our problem?

 

[Eltron]

Are you sure the lines are completely dry?

Dan

http://www.eltronresearch.com

Dan's Blog

 

[ivymike]

when I have trouble desoldering pipes, I make sure that I'm applying the heat to the female part and I apply linear and torsional vibrations to the joint. wear safety glasses.

 

[btrueblood]

I've been using a tin/antimony solder that seems to work well, not had too much difficulty de-soldering...though I try to avoid having to do so.

 

[vwhammer]

Yes the joint is dry.
There is never any liquid in them until it is at much lower temps.

We are working on the system at room temps with the system discharged so no liquid is present.

I don't believe any of the normal soldering problems apply to this situation.
Something is happening that is causing the solder to not melt or crystallize.

You can see the solder at the joint and it simply does not melt.

 

[swall]

Lusterous, silvery tin changes into a gray allotropic form at temperatures below 13C. This could effect the melting behavior of the solder. Another possibility is that the soldering process itself caused the silver to alloy with the copper, raising the melting point of the solder. I would first see if the solder supplier can supply any technical assistance. If that fails, you will need to consult a metallurgical engineer to conduct the necessary investigation and analysis.

 

[hacksaw]

excessive heating results in alloy changes with increasing melting point, you can add more solder to re-balance the alloy,but watch the heat

 

[moltenmetal]

I was thinking the same thing: excessive heat (i.e. the red heat you've described) for an extended period and you've likely made a copper/tin/silver "bronze" now. .

You can try to dilute/dissolve the crud in the joint with more tin solder, but with a capillary joint like that you're probably going to have a tough time getting anything in there.

 

[vwhammer]

I have tried adding more solder to the joint and as expected that did not work so well or at all for that matter.

There is a possibility that we are overheating the material during the soldering process.
I will explore that a little further.

I will be running a couple of test next week to try to come to some conclusion.

 

[btrueblood]

Agree with hacksaw and molten, again - your best bet is to change alloys to something without silver in it (silver is a lovely liquid metal solvent).