# "Silver solder" questions



## graham-xrf (Apr 14, 2021)

Apparently, the term is discouraged. One should be saying "silver brazing" instead.
The aim is to get a very low electrical resistance joint on 22mm copper tubing that might carry high currents (200A).
The recommended stuff is something (probably tin) with 55% silver in it.
I understand that it melts at 650°C , which is 1202°F. That would be sort of reddish hot.
One hopes the butane can torch normally used for copper plumbing joints with tin/lead solder will get it there, but I can augment it with a butane gas ring if need be.

Is Borax OK to use as a flux? I have a tub of Borax.


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## RJSakowski (Apr 14, 2021)

I had occasion to take apart some silver brazed copper plumbing.  I used a MAPP gas torch and had to build an improvised oven with firebrick  to do the job.  The problem with larger pieces of copper is that it sucks the heat away so efficiently.  I have used the same torch for brass brazing though.

For the silver soldering that I have done,  I used a flux designed for it.  I have used borax for forge brazing of iron with brass brazing rod but I don't know if it would work with silver braze.


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## JPMacG (Apr 14, 2021)

Silver solder used to mean silver brazing.  Then plumbers stopped using tin-lead solder and started using solder without lead but with a small amount of silver and everyone started calling that silver solder.  So now "silver soldering" could mean silver brazing or soft soldering with low-temperature solder containg a small amount of silver.

Solder is generally not a great conductor - it has high resistance compared to copper.  In order to minimize the resistance of the solder joint you would want to keep the film of solder as thin as possible and make the surface area of the joint as large as possible.

Interesting thing about metal alloys - the alloy of two good conductors can be a poor conductor.  So I would not count on silver brazing being a good conductor just because silver is a good conductor.  You would need to look up the conductivity of the particular alloy.


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## graham-xrf (Apr 14, 2021)

JPMacG said:


> Silver solder used to mean silver brazing.  Then plumbers stopped using tin-lead solder and started using solder without lead but with a small amount of silver and everyone started calling that silver solder.  So now "silver soldering" could mean silver brazing or soft soldering with low-temperature solder containg a small amount of silver.
> 
> Solder is generally not a great conductor - it has high resistance compared to copper.  In order to minimize the resistance of the solder joint you would want to keep the film of solder as thin as possible and make the surface area of the joint as large as possible.
> 
> Interesting thing about metal alloys - the alloy of two good conductors can be a poor conductor.  So I would not count on silver brazing being a good conductor just because silver is a good conductor.  You would need to look up the conductivity of the particular alloy.


Yes - solder is not a great conductor when it is a low melt temperature tin/lead combination, like used for soldering electrical circuits. Now, we go lead-free, where the alloy is about 3% silver, and 0.5% copper, with the rest being tin. Some include zinc or antimony to broaden the range between solid and melted, but all these melt between 200°C and 240°C

The 55% silver braze, is altogether different. Much higher temperature required, much different strength joint, and with that much silver with tin, also a high conductivity.


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## graham-xrf (Apr 14, 2021)

RJSakowski said:


> I had occasion to take apart some silver brazed copper plumbing.  I used a MAPP gas torch and had to build an improvised oven with firebrick  to do the job.  The problem with larger pieces of copper is that it sucks the heat away so efficiently.  I have used the same torch for brass brazing though.
> 
> For the silver soldering that I have done,  I used a flux designed for it.  I have used borax for forge brazing of iron with brass brazing rod but I don't know if it would work with silver braze.


I am happy with the conductivity of the 55% silver braze onto the copper. Those little rods are quite expensive, but I won't need much.
I searched on silver solder fluxes, and it seems they are borax based.
One recipe I found is to use one part borax, + 1 part potash + 1/2 part table salt, used dry, or with just enough water to make a paste.
A variation if one does not have potash available is to just leave it out. Apparently, the flux still works.

I have never silver soldered before. I know it is used for brazing steel, but I have no idea how it goes on copper.
Your point about the copper soaking the heat away fast is exactly what I feared.
It's easy enough to get the end of the rod to melt in the blue flame. I am thinking getting the end of the copper pipe up to that temperature may be a different matter! I may be getting in some MAPP gas now.

Can one MIG weld copper with copper wire electrode? (Asked in ignorance - please forgive if it's a stupid notion).


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## jwmelvin (Apr 14, 2021)

One can TIG copper easily enough but I don’t know about MIG.


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## graham-xrf (Apr 14, 2021)

jwmelvin said:


> One can TIG copper easily enough but I don’t know about MIG.


That is great to know!
My new welder is still in storage, and temporarily unreachable. I can hardly wait to get to play with it! It does do "Lift TIG", as well as MIG etc.

One thing I do need to invest in is the TIG torch, the gas tube, and some "get started" consumables. At this stage, there is a whole lot I need to know before I end up buying stuff I will regret. It requires a type "17V" torch with a gas valve in the handle.
Also, I would be needing a bottle of pure Argon. The bottle I do have has 5% CO2 in the Argon, intended for MIG.


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## WaneB (Apr 14, 2021)

I have used jewelry silver solder for soldering sterling (.925%), and fine silver (.999%).  The solder I used had hard, medium, and easy varieties, hard melting at the highest temperature, then medium, then easy.   These *were* expensive, kind of like sterling expensive, but you don’t need much. I have used propane, acetylene air, and oxy acetylene torches successfully. I used boric acid powder mixed with alcohol as a flux. I also have used white-out or yellow ochre where I didn’t want solder to flow.
I have also joined copper, brass, and occasionally steel with the same materials.  I once made a t-handled nut driver by cutting off the plastic handle and silver soldering a steel rod to make the “T” part.  Turned out to be an incredibly strong joint.


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## RJSakowski (Apr 14, 2021)

Ersin used to make a soft solder for electronic purposes with 2% silver under the trade name Multicore SavBit.  The silver content successfully  prevented erosion of the soldering tip.  Composition was 62% Sn, 36% Pb, and 2% Ag.  Currently, they market a product with the silver replaced with copper although the silver bearing alloy is still available through Newark/Farnell.  For many years, my soldering iron was in 24/7/365 at a fairly high temperature and tips lasted for months.

Alpha Metals and Kester also make a 62/36/2 alloy.


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## jwmelvin (Apr 14, 2021)

Oh one thing to keep in mind with copper TIG is that it’s the most current-hungry material (due to the thermal conductivity) so machines quickly run out of oomph as the thickness increases. I only did it briefly as an experiment; my machine is 250A.


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## pacifica (Apr 14, 2021)

Silver solder with 30-45 % silver,copper zinc and 20% cadmium wets and flows much better than non-cadmium. Don't inhale the fumes. 
Harris black stay-silv flux works very well for higher temps, ssbf1.
 HVAC pros use a rod that is 15% silver, 5 % phosphous(fluxes) and 80% copper-like sil-sol 15.


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## benmychree (Apr 14, 2021)

Another possibility is SilPhos, it contains 15% silver, I think and copper and phosphorus; it does not require flux on a copper to copper joint, it is used extensively for refrigeration work, as without having to use flux there is no chance of contamination in the closed system, for copper to brass, flux is required.


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## aliva (Apr 14, 2021)

I've silver soldered 1/2" copper pipe with MAPP gas. On a 22mm piece it may not be enough, give it a try. And it was Silver Solder flux coated rod.
Expensive, about 90$ for 10 rods 14" long. I would  stick to the proper flux. it's cheap Harris makes a good product.






						GSParts Harris Stay-Silv Silver White Brazing Flux Paste for Hi Temperature, SSWF7 - - Amazon.com
					

GSParts Harris Stay-Silv Silver White Brazing Flux Paste for Hi Temperature, SSWF7 - - Amazon.com



					www.amazon.com


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## graham-xrf (Apr 14, 2021)

jwmelvin said:


> Oh one thing to keep in mind with copper TIG is that it’s the most current-hungry material (due to the thermal conductivity) so machines quickly run out of oomph as the thickness increases. I only did it briefly as an experiment; my machine is 250A.


That's not such good news.  
.. and, if MIG is possible with copper, the same thing would apply.

The 22mm pipe is 22mm OD, and 20mm ID, so apparently about 1mm thick wall. That's about 40 thousandths (0.0394").


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## graham-xrf (Apr 14, 2021)

WaneB said:


> I have used jewelry silver solder for soldering sterling (.925%), and fine silver (.999%).  The solder I used had hard, medium, and easy varieties, hard melting at the highest temperature, then medium, then easy.   These *were* expensive, kind of like sterling expensive, but you don’t need much. I have used propane, acetylene air, and oxy acetylene torches successfully. I used boric acid powder mixed with alcohol as a flux. I also have used white-out or yellow ochre where I didn’t want solder to flow.
> I have also joined copper, brass, and occasionally steel with the same materials.  I once made a t-handled nut driver by cutting off the plastic handle and silver soldering a steel rod to make the “T” part.  Turned out to be an incredibly strong joint.


Thanks for the flux recipe. I do have some boric acid, as well as the tub of borax.
At least some of these are things I can try out.

One thing - I think the decimal place inadvertently got messed in your sterling proportion. I think you meant 92.5% and 99.9%. I can believe it was expensive. That stuff is also called "bullion"!


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## graham-xrf (Apr 14, 2021)

Looking at "How to weld Copper | TIG Time" from weld.com on YouTube..
Apparently, the "secret to success"  is 100% Helium at 25--40 CFH.
Helium at 40CFH?  - I don't think so!

I don't understand what he means by "Helium has 1.7 x more amperage drive than argon". I suspect he means using Helium works better, but Helium does not, in itself, posses "amperage". Just possibly it might have less ability to carry heat away somehow. In their example, they are using 1/8" thick copper. I suspect I might do better with my 40 thousands. I am going to have to binge-watch YT on copper TIOG before I get the sense of this!






At least I know where I can get copper rod


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## WaneB (Apr 14, 2021)

graham-xrf said:


> Thanks for the flux recipe. I do have some boric acid, as well as the tub of borax.
> At least some of these are things I can try out.
> 
> One thing - I think the decimal place inadvertently got messed in your sterling proportion. I think you meant 92.5% and 99.9%. I can believe it was expensive. That stuff is also called "bullion"!


I should have left out the percentage symbol altogether!  I did jewelry fabrication work for 4-5 years, (about 25 years ago).  I found it to be an interesting field, but didn’t make much Of an income from it.


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## graham-xrf (Apr 14, 2021)

Regarding what is the difference between "soldering", and "brazing".
Here I exclude all the flux-flowed low temperature soldering, such as is done between electrical connections, plumbing pipes, etc.

As I understand it, soldering is where the filler material flows into an overlap between two pieces of material to be soldered together. The offered metal can even seem to flow up into thin gaps by itself.

Brazing, on the other hand, is where a filler material can be built up on its own, at a junction of materials, just like any other welding,  only done at a lower temperature than normally would happen if it were (say) welding steel to steel, or TIG copper to copper.

This begs the question - if "silver soldering" with 55% Ag rod, and either borax, or boric acid type flux. will the metal flow up into the gap?

----
A thing I know about low temperature tin/lead soldered joints on copper. The solder "wets" the copper in a layer of amalgam , where there is a thin tin/lead diffusion into the copper. This amalgam is harder and stronger than either the copper, or the solder by themselves. If the joint is made so close that there is little or none of the pure solder in the gap, just amalgam, the joint is way stronger than one with large dobs of solder in the gap. Anyone who has tried to clean away solder tinning from a copper pipe with wire wool can attest this. I know it from my own experience.

Generally, electrical solder joints should not rely on the solder to make the electrical connection. That is achieved by the cold weld of the wrap on sharp edges, and in various other ways. The solder is only used as a tenacious and durable "glue" to hold the wire terminal joint together. In my case, I could use low temperature tin/lead, or lead free solder. I know that these very high currents in a low temperature soldered joint can actually melt in the joint, and it can fall apart. This especially if it was a poorly cooled crystaline joint solder attempt, also known as a "dry joint".


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## pontiac428 (Apr 14, 2021)

Hard soldering behaves very much like soft soldering, with a lot more heat.  Flux is nothing fancy, I use the same Ruby Red for both.  I would not use the recipe with table salt- why (oh why) would you add a halogen to your melt?  That's just a bad plan.  I also would give you the strongest warning I can to avoid cadmium at all costs.  I won't waste anyone's time here, but look up cadmium poisoning on the web.  Personally, I'd rather be tortured with a pair of pliers than have a good hit of cadmium.  Warnings aside, hard soldering is easier than welding copper.  The result will be more dimensionally true with hard solder.  Welding shrinks gaps and leaves an undercut on the edge of the beads, if that's important to you.

As for amperage capacity simplified, just calculate the cross sectional area of your copper and compare it to an ampacity table from the electrical code book.  I'm not sure if 40mm dia at 1mm thick will carry 200A, but I'm just guesstimating.


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## graham-xrf (Apr 14, 2021)

@pontiac428 :
Thanks so very much for your contribution. I will not get anything involving cadmium.
I thought the same as you about the salt. 
As to the 200A. the pipe is 22mm diameter, I can figure out the power dissipation, but I assure you the copper pipe will carry that current, though it does get warm to the touch.

It has the 6 metres of length in it to dissipate heat,and the resistivity of copper is 16.8E-9 ohm.m
The mean 21mm diameter has circumference near 66m, and at 1mm thick, gives a 65mm2 conductor. That is way more than needed to run 200A.


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## whitmore (Apr 14, 2021)

graham-xrf said:


> Apparently, the term is discouraged. One should be saying "silver brazing" instead.
> The aim is to get a very low electrical resistance joint ...





graham-xrf said:


> I understand that it melts at 650°C , which is 1202°F. That would be sort of reddish hot.
> One hopes the butane can torch normally used for copper plumbing joints with tin/lead solder will get it there, but I can augment it with a butane gas ring if need be.



Silver braze won't like the relatively low butane flame temperature, and only small joints
get hot enough with propane.   Acetylene-air, or propane-oxygen, should get large parts hot enough.

The flux we used with silver solder was called 'solder salts', and it works with jeweler-supply
silver braze supplies.   I've used it on stainless, too.

For copper joining, an alloy called 'sil-fos' is said to require no flux, just the reducing flame of a
torch is enough to clean the surface.  Alas, when I tried to buy some, I was asked 'how many pounds'...
so it was off to buy jewelry supplies for my project!


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## ericc (Apr 15, 2021)

I like to use sil-fos if thermal conductivity is a criterion.  It is inexpensive (the low silver content ones) and flows well without flux on copper.  I use it to make custom interior inserts to use as backers for TIG welding.  They will not melt, and hold the shape of internal features really well.


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## graham-xrf (Apr 15, 2021)

ericc said:


> I like to use sil-fos if thermal conductivity is a criterion.  It is inexpensive (the low silver content ones) and flows well without flux on copper.  I use it to make custom interior inserts to use as backers for TIG welding.  They will not melt, and hold the shape of internal features really well.


I expect I will be trying out all of these on some practice runs.
What is tricky here is about _electrical_ conductivity, for which we need the best we can get.
Unfortunately, most good electrical conductors are also good heat conductors, which is what makes these operations difficult.

Sil-Fos, AKA "Easy-Flo" is interesting. It is a mix of copper and silver and phosphorus, patented in 1931, but the patent thrown open to all for wartime production. Joints brazed with SilFos can be stronger than the parent materials. While it was directly used in joints for airplanes, ships, etc. the biggest use was in fix-ups repairs. Busted gears, shafts, struts, almost anything could be put back to working state.

The way it works without flux, is the phosphorus reacts with the copper oxides to form phosphorus pentoxide which strips the surface clean. Of course, this effect only works on copper. There are several Sil-Fos mixes with a number ranging from 2 to about 15, this meaning the percentage of silver. One has to carefully select which, depending on the application. e.g Sil-Fos 15 has the lowest percentage of phosphorus at 5%, making it the most ductile, and much liked for refrigeration and air conditioning work.

Sil-Fos 18 has the highest silver content, so has the lowest melting point. I can bet that, like all stuff containing silver, it will be expensive. Small quantity use may well be best sought from jewelry suppliers.

I have to thank everybody for the sheer amount of good information.


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## cathead (Apr 15, 2021)

StaySilv flux has worked well for me and contains both boric acid and fluorides so it wets the work easily.

Read on the label and be sure it is the one with fluorides as there are several different Harris products out there.


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