Multi-stranded copper tin electrical solders that break?

[Christophe]

I found that some electric solders (on multi-stranded cable) became brittle after a few weeks: the copper core became more fragile and eventually broke.

I am on a project of electric propulsion since 3 years, I made hundreds of welds, the problem appeared on 2 or 3 welds it is little but it is too much and I would like to understand why?

a) Soldering iron too hot?
b) Quality of the tin (I have several Chinese coils ...)?
c) Bad tinning product (it's still an old bottle that must have been 10 for )?
d) I do not know how to weld?

The cables in question are not mechanically constrained, the last ones were even completely protected by a heat-shrinkable sheath so they remained almost immobile the problem is not mechanical.

Also I also respect the instructions of amperages: the welds which break do not have too much heat ...

20191017_111720 (1) .jpg (49.68 KIO) Accessed 6364 times

20191017_111729 (1) .jpg (63.46 KIO) Accessed 6364 times

It looks like the break is after the tinning zone ...

[phil53]

In general, when vibrating material is used, lugs or crimped ends are used.
The solder creates a rigid area and where the copper becomes flexible it breaks.

[Christophe]

There was a little vibration but not that much ... especially that the heat-shrinkable sheath (which I cut out on the picture) kept the cables still ...

How do you put a lug on the electrical circuit in question?

Maybe I have to put less tinning product then? And that I limit it to the level of electronic plate?

[izentrop]

It breaks at the junction soft wire / solder because of vibrations.
We must clamp the wires closer to the weld, either with fasteners or with a coating resin http://www.resilec.fr/enrobage/
Otherwise, never tried but the hot glue should do the trick locally

[Christophe]

Ah ... good if 2 answers speak of vibrations is that it must be that ... But it is not too vibrating where it is mounted ...

Ok, I will "secure" the welds with rislin collars!

I'll keep you informed!

Thank you friends!

[phil53]

For a proto it can be enough but if you give a suite, it is essential to go crimping. And no matter how, there are sections to respect for it to be reliable

[Christophe]

Yes it's proto there.

Ok for crimping with male / female lugs ...

[GuyGadebois]

Ah ... good if 2 answers speak of vibrations is that it must be that ... But it is not too vibrating where it is mounted ...

Ok, I will "secure" the welds with rislin collars!

I'll keep you informed!

Thank you friends!

Indeed, subject to repeated movements or vibrations, the metal cracks and eventually breaks. Fold and fold a piece of wire (it's a bit longer with the copper), it will heat to the fold, become harder and will break.
Of course, for the tin solder, it is necessary to heat as little as possible because the tin becomes brittle if it is heated too much. In addition, it binds intimately to the metal, weakening it.

[Christophe]

Yes except that there are not so many mechanical constraints ...

So according to you it breaks in tinning?

Because for me it is at the tinning junction multi-strand that it breaks ... the proof in image of the connector side (same thread as the first 2 photos), it remains 1 strand surviving witness!

20191017_161019.jpg (45.73 KB) Viewed times 6313

So I wonder if there is not (also) a reaction that weakens this junction (electro-chemical ??)

[GuyGadebois]

Yes except that there are not so many mechanical constraints ...

So according to you it breaks in tinning?

Because for me it is at the tinning junction multi-strand that it breaks ... the proof in image of the connector side (same thread as the first 2 photos), it remains 1 strand surviving witness!

20191017_161019.jpg

So I wonder if there is not (also) a reaction that weakens this junction (electro-chemical ??)

Yes, it's the weak point. On a rigid side because of the weld, and at the junction it becomes soft, it is there that concentrates the vibration, making work the metal which finally breaks.