Twisting conductors or not?

[Big Tone]

Yes, the annealing process as it’s known. I have a feeling it’s better in designs where the screw or thread has been manufactured at an angle, effectively forcing the wire further into the hole.

I’m convinced the problem is worse where it’s just straight, as in perpendicular to the wire. (Not sure if I could have explained that better, but you know what I mean).


I have a theory about this. (Method in their madness).

 

[stillp]

Solder doesn't bond molecularly with the copper and can be peeled off. There is some molecular bond between the tin and copper in tinned copper wire which makes solder joint on tinned copper wire stronger ( mechanically ) than on plain copper wire.

If done properly, solder forms a molecular bond (an intermetallic layer) with copper. If it can be peeled off you're not doing it properly.
The "tin" on tinned copper wire is usually a tin-lead alloy very similar to solder.
The main reasons why I wouldn't solder wires that are going to be connected in a compression joint such as a screw terminal or crimp, is that over time, solder can oxidise, forming a high resistance layer on the surface, and also because over time and temperature cycling it will crystallise, leading to increased resistivity and brittleness.

 

[Big Tone]

Good point, although from my electronics background the purpose, (or one other purpose), of the flux in solder used in electronics is to prevent that oxidisation. I’ll try and expand on my screw theory now if I may..

By having it at an angle I can see how it would, in effect, act more like a vernier adjustment thereby reducing the length or travel in the vertical plane dictated by the thread pitch.

I don’t know how to explain this better but hope it makes sense. To my mind, this would give an advantage to the force needed to tighten the screw as well as splaying the copper better/differently.

 

[rebuke]

As I think has been established, don't twist wires - otherwise in extreme cases you can end up with things like this which I discovered a while ago:

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• rebuke
• 29 Jul 2013
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(Yes, those are brown and white wires (long story to do with previous uses of the cabling for 110V stuff), and the CPC that looks like it's heading into the backbox was in the socket, but popped out as I opened it - now it's all properly re-terminated and sleeved etc!)[/img]

 

[stillp]

Good point, although from my electronics background the purpose, (or one other purpose), of the flux in solder used in electronics is to prevent that oxidisation.

The flux is normally to prevent oxidisation of the copper, not of the solder, and should (except for some specialist fluxes) be cleaned off after the joint is made, hence the reason why we clean PWBs after soldering.

 

[Big Tone]

That was a contentious issue when I was working in America. One company wanted their PCBs cleaned whereas others did not. At the time, (for a contract with JPL), I had to use solder with a water soluble flux.

When I was working for Eddystone radio over here, many years ago, the equipment we built for the military were never cleaned as the flux was viewed as a protective coat.

You're right of course about it preventing the copper from oxidising, if it looked like I was saying differently. Anyway, I didn't mean to topic drift soz...

 

[Big Tone]

Incidentally, if you have never seen the effect that the moisture from your hands does to some metals you would be amazed!

When we sent metal sheets off to be ‘zinc plate and passivate’ they’d come back looking brilliant – literally! However, if one came back for repair or recalibration years later and you saw them it looked like fingerprints of acid had been stamped all over.

I mention this because the same will happen to the copper when you touch them. This is another case, I think, for never twisting or touching the copper wires. (Never did look up what the passivate bit was...?)

 

[bernardgreen]

From memory of lectures decades ago and more recent experience..

The temperatures required to get a tin-copper molecular bond cannot be achieved when soldering components on a PCB without risk of damaging components and/or the PCB. When soldering cables the heat conducted away by the cable makes it difficult to achieve the required temperature without damaging the insulation either side of the joint.

Copper wire tinned during manufacure is either

[a] hot dipped in tin at a temperature high enough for molecular bonding to occur.

electro-plated with tin which creates molecular bonding. I seem to recall this produces an interface layer of a tin-copper alloy.

 

[stillp]

Bernard, I don't know exactly what you mean by a molecular bond. When I was researching this subject for aerospace and automotive applications the term "intermetallic layer" was used, and that can certainly form during normal soldering using tin-lead (or the modern lead-free alternatives) at temperatures around 180 C.

 

[JohnW2]

There is a difference between twisting two wires together and wrapping them round each other. I feel wrapping the CPCs together before they go into the terminal provides a fall back area of contact should ( when ) the screw in the terminal becomes loose. Thus the CPC continuity is maintained.

What do you mean by 'wrapping' - do you mean 'wind round and round' (as in wire-wrap connections)?

Kind Regards, John

 

[bernardgreen]

Twisting .. gripping both wires in a pair of pliers and rotating the pliers such that the individual wires are twisted along their length as well as being twisted together

Wrapping .. holding the wires lightly in the fingers as they are wound together, the individual wires are not twisted as they are free to rotate between the fingers. This method wastes an inch or so of wire as the end has to be long enough to provide leverage the fingers to wind the wires around each other

 

[JohnW2]

Twisting .. gripping both wires in a pair of pliers and rotating the pliers such that the individual wires are twisted along their length as well as being twisted together ... Wrapping .. holding the wires lightly in the fingers as they are wound together, the individual wires are not twisted as they are free to rotate between the fingers.

I'm really not convinced that there is much material difference - even with the 'pliers' method one will not get much axial twisting of individual conductors (unless one applies the pliers to a single straight conductor, which one would not do). With both methods, one, if not both, wires must end up with a helical configuration - which I would call 'twisting'.

Kind Regards, John

 

[bernardgreen]

Bernard, I don't know exactly what you mean by a molecular bond. .

Again from memory.

Molecular as in the formation of an alloy layer between the solder and the copper. Lead-copper alloy or tin-copper alloy ( or both ? ) where the molecules of the alloy were strongly attached to both the atoms of the copper and the atoms of the other metal. Provided the molecules of the alloy were strong the two metals would be strongly "glued" to each other.

I recall a lot of testing on surface mount soldering where the alloy formed at lower temperatures was brittle and sheared when there was any mechanical stress on the joint. Thermal expansion created as much stress as flexing the PBC. Using higher temperatures produced a different alloy which was stronger but that temperature damaged component. I am guessing but probably there was a tin-copper alloy while at lower temperatures it was a lead-copper.

I will have to dig out the notes from the archives. ( or google )

 

[stillp]

Bernard, thanks. I'm also working from memory of a long time ago, but we were sectioning solder joints, polishing and etching the surface, and examining with a scanning electron microscope. From what I remember the intermetallic layer was one in which the crystals of the lead-tin alloy were interlocked mechanically with those of the copper. I don't remember different alloys at different temperatures, it was all-or-nothing in that the joint was either good or not. The solder remained an alloy.
The intermetallic layer did not form if the copper temperature was significantly below the melting point of the solder.

I wish I still had those photographs!

 

[maltaron]

Does anyone remember the sockets that had 2 terminals for each pole. I can't remember the make but they saved all the problems of twisting or parallel. On another point, when I was in the sound installation industry it was quiet amazing how screw terminals miraculously came loose due to vibration at audio frequencies, the same can happen at 50 Hz.