solder wicking up wire insulation?

[coppercone2]

What is the deal with solder wicking behind the insulation if you are doing a wire splice (say with heat shrink?)

Other then being stiff in an area that you don't anticipate it to be stiff, how does the solder joint react to vibrations if the solder goes in under the insulation? I thought I read some where your not supposed to let it happen , but I am not sure why. Let's say its teflon and does not give a shit about heat. I have trouble understanding explanations like 'stress concentration'. Can someone explain graphically?

And can someone throw a number on this. Say you do a really bad job and it wicks up to 1 joint length under the insulation (so 2 joint lengths too long). And one wire that does not wick up at all. If they were put on some kind of vibration tester thing, how much faster would it fail?

For a wire joint is there a formula for solder joint length vs wire gauge vs strand count for a optimization of electrical and mechanical vibration resistance properties?

[helius]

Visualize a strain-relief boot in reverse. A strain relief distributes the stress forces across a larger section of the wire, making the strain (displacement) less at any single point. The effect is that there is a gradual change in modulus as the wire emerges from its fixed point and towards the free end.
When solder wicks up into a stranded wire, the modulus becomes a step function (stiff to suddenly soft) and that concentrates the strain at a single point.

[Psi]

When doing a mid-wire join/splice it's less of an issue because it doesn't bend at that point.

It's when the solder wicks up the wire at each end that you get problems, since wires usually get bent at the termination ends under the weight/inertia of the wire vs the rigid mounted pcb.

[coppercone2]

I thought it would be better because what comes to my mind with the soft wire is like a lasso tip. To me it seems better if there is soft wire inside of insulation going into hard wire inside insulation going to hard wire without insulation. If it smacks into something won't the overall bend at the junction be less because there is insulation?

I understand the math of what your saying with the functions but I just can't visualize it. It just seems to me like if you have two dissimilar materials bonded to each other, having a support around them makes it stronger.

Doesnt the solid bit covered with solder want to twist around its own axis and yank on the wire? If there is insulation around it, I imagine the twisting is dampened (the force goes into stretching the insulation) so the overall displacement is less so there is less force on the solder junction.

[coppercone2]

I drew a diagram, please help me figure out why this view is not correct?

[vk6zgo]

You can get little devices which look like mutant alligator clips, which are called "anti-wicking tools".

[silverback]

As an apprentice some 40 odd years ago I was tort to apply methylated spirit to the wire so it wicked up the insulation. This evaporated when the solder was applied reducing the wicking of solder up the wire, also reducing shrink back of PVC insulation.