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Old 29th Oct 2014, 10:17 am   #1
Al (astral highway)
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Default Silver soldering/ CU tips please

I've watched a couple of you tube vids on this but I need expert advice from real people!

I am experienced in brazing cu to cu, as in making good plumbing joints with Yorkshire fittings. However, my attempt to silver solder two piece of copper busbar was a non-event. I am making a DC power bus for a high current on a switching high frequency circuit

I have a powerful blowtorch and an appropriate flux, but the silver solder is just curling up and not flowing, and then the metal is oxidised. Hmm.

ANy tips gratefully received.
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Old 29th Oct 2014, 11:01 am   #2
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Default Re: silver soldering/ cu tips please

I don't think you brazed* Yorkshire fittings, as they are pre-loaded with soft solder.

When silver soldering the parts to be joined must be thoroughly clean. Mix the flux immediately before starting the job and apply BETWEEN and around the joint surfaces. Heat the end of the silver solder rod and dip it in the flux.

It's important to heat the joint quickly and evenly, otherwise you'll just end up with cooked flux. If the flux retreats away from parts of the joint apply more. Observe the flux. Initially it will just dry out as you boil away the water, but eventually it will melt to a liquid state and start to look "active" (that's the best way I can describe it). Then is the time to apply the solder. It should immediately melt and flash round the joint. If the job's a big one move the flame round a bit and repeat. Always heat the job, never the solder.

* There's some debate as to what brazing means. To me it's hard soldering using Brass spelter. Others use the term to refer to all forms of hard soldering including silver soldering.
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Old 29th Oct 2014, 12:48 pm   #3
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Default Re: silver soldering/ cu tips please

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I don't think you brazed* Yorkshire fittings, as they are pre-loaded with soft solder.

* There's some debate as to what brazing means. To me it's hard soldering using Brass spelter. Others use the term to refer to all forms of hard soldering including silver soldering.

Thanks for the Ag soldering tips Graham. I thought it would be easy for someone with plumbing experience, but not so. One of the problems is the work moves apart, hard to keep the pieces together.

And I didn't know brazing was exclusively used in the way you're suggesting - so now I do. Anyway, it's definitely what we're talking about here with Cu/Ag soldering.
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Old 29th Oct 2014, 12:56 pm   #4
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Default Re: silver soldering/ cu tips please

There's no reason why you shouldn't clamp the joint. You can use a home made 'G' cramp made from a piece of steel bent into a 'G', a screw and a couple of nuts. It helps if the steel is rusty so it won't take solder. Alternative lightly rivet the joint with copper rivets.

It also helps to make several centre punch marks on the joint surfaces, so the when clamped up there's a gap for the solder to flow into.

Make practice joints using odd bits of copper before you do the job proper.
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Old 29th Oct 2014, 12:56 pm   #5
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Default Re: silver soldering/ cu tips please

Brazing is a much higher temperature process than soldering, using brass rod and an oxy/gas flame. From my memory of using this process at school, steel would be at red heat for the brass to melt and flow. It's the process once used for making lugged motorcycle and bicycle frames.
From how you are describing the problem you are having with silver soldering, it seems that the parts to be joined are becoming too hot before the solder is applied.
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Old 29th Oct 2014, 1:05 pm   #6
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Default Re: silver soldering/ cu tips please

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Brazing is a much higher temperature process than soldering, using brass rod and an oxy/gas flame.
Quite so. At my school we used coal gas fired torches and an air blower, although one torch had an old pair of hand driven bellows. I can remember doing a "foreigner", brazing up the broken frame of my younger sister's Triang solid tyred two wheeler.

Nowadays, for copper at least, the self blown propane torch is almost universal. Oxy-acetylene can be used, but it's a skilled job and you need to preheat the copper with propane first.
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Old 29th Oct 2014, 1:08 pm   #7
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Default Re: silver soldering/ cu tips please

Yes, it seems that and Mapp gas is even replacing Oxy/Acet.
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Old 29th Oct 2014, 4:09 pm   #8
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Default Re: silver soldering/ cu tips please

Why silver soldering, what is wrong with soft solder for electrical connections?

What flux are you using?
 
Old 29th Oct 2014, 5:03 pm   #9
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Default Re: silver soldering/ cu tips please

Mapp gas has rather taken over for hard soldering and can do rough-ish brazing.

Oxy-acetylene doesn't just give a hotter flame, it is more tightly focused. I've done quite a bit of brazing and silver-soldering with an oxy-acetylene flame and I found it much easier than with a hot blowlamp. I could use temperature gradient to select where the flux came to active temperature and where capillary action would taake the bronze or silver solder.

Because of safety concerns, acetylene use has been falling for years and then we were down to only one acetylene manufacturing plant in Britain. Then it blew up. It was left in limbo with doubt over whether it would be licenced to restart if it was ever re-built. Gas was imported from Holland for a while but I found the cost had multiplied dramatically, and smaller size cylinders than the very largest weren't available for more than a year. I don't know the current status.

With the fears over acetylene raised by an incident under the elevated M1 motorway closing it, then several factory/garage fires etc, I decided to stop using it and laid out some serious money in moving to TIG.

To silver solder two chunks of copper together with 'blunt' heat, you will need to bring the whole thing up to soldering temperature, wait until you see the dried out flux activate and then feed in silver solder, watching wetting and capillary action draw it in until you have a bead built up to the extent you want.

I mentioned TIG welding earlier, and this is what has taken over for a lot of copper vessel fabrication.... and many other metals.

It's arc welding with the arc from a non-consumed tungsten electrode against the parent metal. Argon gas is blown on to drive oxygen bearing air away, and your spare hand feeds in copper from a filler rod. You aren't just melting rod, you are melting into both pieces of parent metal and establishing a weld-pool, you work the pol along the joint, feeding in filler as needed, leaving a bead behind and a penetration zone.

This gives you a copper-copper join with only copper as the filler.

The parent metal and the filler rod need to be shining clean before you start. This is the lowest resistance joint possible. Quite a high level of skill is required.

I think Al is building a Tesla coil, rather a large one by the sound of it.... either that or he's going directly for the positronic ray machine. I can't remember what year Barbarella is going-to-be-destroying it in.

David
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Old 29th Oct 2014, 7:23 pm   #10
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Default Re: silver soldering/ cu tips please

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Why silver soldering, what is wrong with soft solder for electrical connections?
Silver soldering because:
1) The pieces are large (3mm busbar, self-supporting)

2) I can't have any parasitic resistance. The lowest measured resistance in a large physical joint made with soft solder is too high.

I am using some kind of specialist silver flux that you mix from a powder with water. It's from Johnson Matthey, I think.
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Old 29th Oct 2014, 7:25 pm   #11
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Default Re: silver soldering/ cu tips please

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I think Al is ... going directly for the positronic ray machine. I can't remember what year Barbarella is going-to-be-destroying it in
David, shtttt, you weren't supposed to tell everyone that. Now I'll have to go under the radar for a bit...
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Old 29th Oct 2014, 8:19 pm   #12
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Default Re: silver soldering/ cu tips please

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The lowest measured resistance in a large physical joint made with soft solder is too high
With a close fitting join the solder won't make any difference to the bulk resistance, e.g. copper has a resistance of about 1.7 e-8 and lead 22 e-8 (the units don't matter it's the comparison) tin is about 10 e-8, so even saying that solder is as 'bad' as lead a joint of 0.1mm length would be the same as making the copper 22/1.7 times 0.1mm longer, about 1mm. It won't matter.

It is nice to be 'perfect' but engineering is about being good enough.
 
Old 29th Oct 2014, 8:34 pm   #13
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Default Re: silver soldering/ cu tips please

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It is nice to be 'perfect' but engineering is about being good enough.
I'll learn, eventually. I constantly struggle with this in all areas, not just here. Not the first time I've been told that!
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Old 29th Oct 2014, 10:04 pm   #14
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Default Re: silver soldering/ cu tips please

Hi

If you are concerned about the physical strength and to prevent it from moving during soldering, why not drill the joint and bolt it together with a nice clean brass bolt, then flux and soft solder the whole lot.

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Old 31st Oct 2014, 11:38 am   #15
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Default Re: Silver soldering/ CU tips please

The problem with silver solder is that it doesn't fill gaps, the joint must be close fitting over the whole area. If you have two bus bars then they must be clamped, either with a clamp or put a brass bolt through and solder that in as well. A torch isn't hot enough to silver solder, must use propane.

Silver soldering, brazing, bronze welding and welding are all completely different operations, they are not necessarily interchangeable.

As mentioned, silver solder really isn't needed, soft solder is fine BUT there must be no physical stress on the joint, soft solder is not a mechanical joining method.

Best method is to simply drill a hole and put a bolt through, but even this has problems. I have dismantled lots of high current PSUs and the general quality of design and construction is awful. The only joints that were still tight were ones using Belville washers, these are slightly domed and maintain a pressure on the joint. Copper is soft and will creep using standard washers, antivibration washers are exactly that, they are not joint tensioning washers. As to the people who tap one bus bar and then screw a bolt in to clamp the joint, well, what is there to say?
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Old 31st Oct 2014, 1:10 pm   #16
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Default Re: Silver soldering/ CU tips please

When I used to be involved in telephone exchange 48VDC systems, bus-bar joints were overlapped and clamped together rather than drilled/bolted, or soldered/brazed/welded.

The clamp-together method is universally used on building lightning-discharge straps too. See http://www.cablejoints.co.uk/upload/..._Catalogue.pdf
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Old 31st Oct 2014, 1:44 pm   #17
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Default Re: Silver soldering/ CU tips please

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then flux and soft solder the whole lot
Probably the best method, for flux use Fluxite spread on all joining surfaces before assembly, place some (you won't need much) solder wire at the corners of the join and heat up. There will be a bit of fizzing and all of a sudden the solder will flow into the join no matter how hard it is clamped. All this can be done on a gas cooker, the advantage is you can leave it on there to cool.

Re-reading the original post I don't think you got the metal hot enough, for silver soldering it is a dull red heat.

Happy jointing!
 
Old 31st Oct 2014, 2:59 pm   #18
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Default Re: Silver soldering/ CU tips please

I do a lot of silver soldering in my day job. Very clean close fitting joints are good, I hold the parts together with iron wire, just wrap a loop around and twist it to tension the loop. Use a borax based flux on both parts to be joined and on the solder wire. Cut small pieces of solder off and place them on the joint. Heat the joint, propane is hot enough, and the flux will dry out and bubble. This is the time to watch that your solder is still on the joint, a cocktail stick is useful to reposition the solder pieces if they move. When the joint is hot enough the flux becomes a viscous liquid and not long after that the solder will flow.

I would suggest cutting some practice pieces first. Silver soldering is fairly easy once you know how to do it!

I often use three temperature grades of silver solder while assembling parts, use the hard one first, followed by medium then easy. That way you can do subsequent soldering operations without disturbing the first, but that's probably a bit more information than you need right now!
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Old 1st Nov 2014, 12:41 pm   #19
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Default Re: Silver soldering/ CU tips please

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Re-reading the original post I don't think you got the metal hot enough, for silver soldering it is a dull red heat.
That'll be it! It wasn't nearly this hot! I've got my propane torch ready now and am going to do some test pieces soon.

Quote:
Originally Posted by mole42uk View Post
I would suggest cutting some practice pieces first. Silver soldering is fairly easy once you know how to do it!
Good tips there, great to have insider advice! It may be 'easy' after a while, but I can just see ££ of silver curling up in front of my eyes while I'm getting the knack!!!
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Old 1st Nov 2014, 2:09 pm   #20
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Default Re: Silver soldering/ CU tips please

Quote:
but I can just see ££ of silver curling up in front of my eyes
|Get the fluxed joint red hot first and then dab on the solder.
 
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