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Mechanical properties of soft solders.

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Robin Graham15/10/2020 23:53:33
867 forum posts
258 photos

For reasons too tedious to go into I am trying to keep a pair of wire-framed specs functional for another month or so by soft soldering the fragile join between the bridge and the lens frame. Initially I used standard 60/40, which didn't work well - about 4 days MTBF. Then I found a reel of lead-free (95.5 Sn, 4.0 Ag, 0.5 Cu) in my soldering drawer, tried with that, and it's held up a treat.

It set me wondering if I could use the stuff for a project involving multiple 'mortice and tenon' joints in a pendulum system I'm making in brass. I had been planning to silver solder, but soft soldering would make the whole thing so much easier. Preliminary experiments make me think that the lead free stuff will be more than adequate - perhaps even 60/40 leaded would be OK. But it set me thinking.

I've tried, but failed, to find to find data on the mechanical strength (specifically shear modulus, which is what mainly concerns me) of soft solders. I have found vague statements suggesting that 'the more tin there is the stronger it is' then 'the silver is added to increase the strength'. Hmm. What does that mean? Is a 99.5 Sn/ 0.5 Cu 'stronger' than the one with the silver?

Most of the information I've found about the properties of these alloys is focused on the mass production of electronic components, where mechanical strength isn't a major consideration.

Any light that can be shed into this fog would be most welcome!



Edited By Robin Graham on 15/10/2020 23:57:06

Jeff Dayman16/10/2020 00:31:22
2167 forum posts
45 photos

Hi Robin, I don't have any data on 60/40 or lead free solders, but from my early work days I have the following about 50/50 tin lead solder. The average shear strength of joints soft soldered with 50/50 tin lead solder is 4130 psi, according to the 1975 report on brazed and soft soldered joints from the American Welding Society. This is the ref document we used when I worked at Honeywell as a design guideline for soldered fittings and valve bodies for steam and hot water service for residential heating. The AWS may have some more modern data on 60/40 and lead free solders if you Google them a bit.

Steviegtr16/10/2020 00:34:05
2225 forum posts
311 photos

Well i was brought up using tinmans solder. Which is what you used 60/40 resin cored. I have had quite a bit of experience using plumbers solder to wipe immersion heater brass bosses into copper tanks. I also used the same type of solder to sweat electrical lugs onto large cables, in the days before crimp lugs were available.

For the life of me i cannot remember the constituency of the plumbers solder.

Be careful with silver solder as you may break your lenses due to the heat required for it to flow.


not done it yet16/10/2020 07:05:18
6284 forum posts
20 photos

I don’t suppose there is that much of a great difference - the main problem is that of trying to make butt joints for these repairs. Adding an long over-lapping support, while looking naff, is the way to go...

Keith Hale16/10/2020 09:12:35
316 forum posts

Hi Robin

Mortice and tenon joints - ideal as any load will be held in shear.

Will the joint carry the load? I'm sure it will as long as the joint is a good one! Remember the basic principle of soldering and brazing - CAPILLARY FLOW.

Make a sample joint to set your mind at ease.

If you want a black joint use tin/lead. If you want a white one use tin/silver. The joint strength of both will be adequate for what you want.


Howard Lewis16/10/2020 13:14:14
5238 forum posts
13 photos

Purely a guess, but would expect soft solder to be weak in tension, but much stronger in shear.

Anyone who knows like to comment?


Steve Skelton 116/10/2020 15:46:47
120 forum posts
3 photos

In the industry in which I worked a number of manufacturers of printed circuit boards moved over to all tin solder in their fabrication and we experienced nothing but problems. The soldered joints were forever cracking as the boards were subject to vibration with regular very expensive failures. Based on this I would suggest that Tin/Lead solders are much more forgiving and less brittle (but maybe less strong).


Keith Hale16/10/2020 16:44:21
316 forum posts

Hi Howard,

Does 50+ years in this business count as someone who might know?

Of course joints in tension rely on the bulk strength of the solder and are relatively weak. Joints in shear are much stronger. It is the reason that soldering and brazing have been such popular means of joining metal components for the last 5000 years.

No guesswork involved.

Mortice and tenon joints are ideal for exploiting this fact.

A more comprehensive answer and the reasoning will be found in my book!


roy entwistle16/10/2020 21:09:26
1403 forum posts

A word of caution. I tried to silver solder a spectacle frame. The frame melted before the solder. crying

duncan webster16/10/2020 22:02:46
3460 forum posts
63 photos

This gives shear strength of 60/40 solder as 5660 psi


Robin Graham16/10/2020 23:44:25
867 forum posts
258 photos

Thanks for replies.

On the specs front, I am quite pleased with my repair:


as was my optician. He hadn't noticed it when he measured the optical properties of the lenses - he obviously needs better glasses! I suppose I could put a bar across the top, aviator style - seems a bit 70's, but it perhaps the style will come back, as styles do. And I'll be a trend setter.

It turns out that frames of this type are silver soldered, so had I gone along that route for the repair the specs might have fallen apart completely. I wonder what Roy's frames were made from? Aluminium?

Returning to my main question, I made a test join using the Sn/Ag/Cu alloy:


You can see the solder line, which would probably be invisible with Ag/Cu, but it's livable with. That's a 3/16" (4.76mm) 'tenon' in a 5mm 'mortice'. So about 0.12mm gap. Maybe it should be tighter. Anyhow, after destructive testing I'm now sure the joins will hold up. I still don't know what the silver does though!

Keith - what book? I'd be interested. If forum rules don't allow you to promote your work here, please PM me.






Edited By Robin Graham on 17/10/2020 00:05:38

roy entwistle17/10/2020 11:37:07
1403 forum posts

I think from the colour of the resulting puddle that the frames were brass. I think my blow torch could have been a little fierce.


Keith Hale18/10/2020 11:22:06
316 forum posts

Hi Robin

The success of brazing and soldering is covered in BS EN 14324 available from BSI at £254!

Alternatively a more readable book based on the BS is currently undergoing another print run and drawing on 50+ years in the business.

See pm for more information.


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