Lead, antimony, rosin, and high vacuum!

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Greg Courville
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Lead, antimony, rosin, and high vacuum!

Post by Greg Courville »

There are a few joints in my system, on the high-vacuum side, which I plan to soft-solder. I purchased some 3.5%Ag-96.5%Sn solder, but failed to realize that it was flux-cored. Based on the color and smell of the residue, the flux appears to be rosin. Something tells me a mess of burnt rosin will not do me any good when it comes to high vacuum work.
In the interest of saving precious time (only a few more days before school starts up once again -- not enough time to wait for a shipment) and money (silver solder is expensive, and I'm broke!), I would like to try to make use of one of the numerous rolls of tin-lead and tin-antimony solders that I already have in my lab.
I am told that tin-lead solder works fine for stainless steel given that a proper flux is used. Although lead has a somewhat higher vapor pressure than tin or silver, it actually doesn't even reach 10^-8 Torr until almost 350°C, which is a far greater temperature than many of my chamber components will withstand anyway. The joints in question will be highly unlikely to exceed 100°C, barring some flagrant lapse of judgement on my part while operating the device, and both are shielded from direct electron or ion bombardment. I am aware that tin-lead solders are clearly inferior to silver soldering/brazing alloys in terms of tensile strength, etc.; however, given that the majority of the force on these areas will act to push the pieces against each other rather than apart, I am inclined to think that an extremely strong joint is not absolutely necessary.
Surely there are issues here of which I am not aware. A search of the Web turned up very little on this subject. Would someone care to enlighten me as to the disadvantages of a tin-lead soldering alloys for high vacuum? What about tin-antimony, or tin-lead-silver?
Chris Trent
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Re: Lead, antimony, rosin, and high vacuum!

Post by Chris Trent »

As I recall, most rosin core solder residue can be cleaned off the joint with Isopropyl alcohol. I've seen much worse cleaned off of wafer fab equipment so you should be fairly safe if you should choose to use it anyway. Just clean it well.

I can't speak for the strength of the joint, or the suitability for stainless steel though.
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Richard Hull
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Re: Lead, antimony, rosin, and high vacuum!

Post by Richard Hull »

Soft soldering stainless is very difficult. I would use a Tin-silver formulation. Antimony is allowed in vacuum work with no problem. As for the flux that will allow the joining, regardless of what it is, you will need to meticulously clean the internal joint very well, indeed.

As you note all soft solders are weak by nature, but a good pretinned joint will warrant a lot of strength in sliding, insert or large contact area butt joints.

As I have always at least silver soldered, (brazed) SS, but prefer hard TIG weldments, I have zero first hand knowledge of the specific fluxes needed to soft solder SS.

Richard Hull
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Carl Willis
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Re: Lead, antimony, rosin, and high vacuum!

Post by Carl Willis »

Hi Greg,

You bought the best kind of soft solder alloy for vacuum purposes, but unfortunately the rosin IS going to get in the way. First of all, it does not flux stainless steel. Second, it will coat the surfaces of the parts anyway, and prevent the proper liquid acid flux from being usable. So it's going to be very frustrating.

Stainless steel is actually a piece o' cake to solder, provided you use acid flux in liberal quantities. Look on McMaster-Carr for it. The recipe is also online in places. It contains concentrated HCl, some zinc chloride, and some ammonium chloride. These components create a highly oxidizing / dissolving environment that strips the protective oxides off the metal. Use sandpaper to pre-roughen the joint areas. Heat the parts. Spray on flux, trying to control the extent it covers (or the solder will follow it all over your parts). Use glass wool or a stainless needle as an applicator-nothing organic or you'll find it makes a tarry mess. Add solder to the joint. Usually it helps to re-flux the joint a few times after the solder is molten, just to help homogenize and clean the joint. As soon as the solder sets, immediately wash the parts in hot water to get rid of the corrosive flux residues. Further cleaning in CLR (phosphoric / oxalic acids) will remove residual oxidation from the surface of the steel and return it to its usual stainless color. Do not use acid flux indoors or near metallic objects you care about.

I would recommend you avoid lead and antimony. Granted, the vapor pressures are not immediately suggestive of trouble. But if you have a situation where an electron or particle beam may impinge on a surface like in a Fusor, these high temperatures can locally develop on a surface while bulk temperature remains low. Check your hardware store for lead-free plumbing solder, which often is a tin-copper alloy. It doesn't have quite the wetting power of Ag-Sn and melts at a higher temperature, but from experience I will say it works great in high vacuum.


-Carl
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