# Tinning wires that will be screwed in to a chocolate block/terminal strip

This is subjective per say. But I am looking for other peoples experience.

If I am going to screw eight wires into daughter relay board's terminal block, is there a benefit or advantage to tinning the copper end before?

The environment that the units are to be installed is not near the coast and is not normally damp/wet. Rather dry and hot.

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I've seen ends of copper wires oxidate even in more or less normal environment. How much that would affect your use, I don't know, but I decided to tin all exposed ends of copper wires I use. –  AndrejaKo Apr 13 '12 at 17:14
Yes. I also worry about oxidation. But I don't know long term effects at mains AC voltages. Have to ask the science guys done at the local Uni. –  kingchris Apr 13 '12 at 17:34
@AndrejaKo - That's kinda scary. I've tinned the 1/16th inch of the end of some wires with lots of very fine strands to make them more manageable, but never tin anything that will be under pressure or other stress. –  Kevin Vermeer Apr 13 '12 at 17:35
@Kevin Vermeer♦ Now that I think of it, never did use stranded wires in situation where they will be under heavy pressure. I probably should take some time and hunt down regulations. –  AndrejaKo Apr 13 '12 at 18:01

• You MUST NOT fully tin the copper wires to be inserted into a screw down terminal block - that your days may be long on the face of the land.

• It is permissible to tin the tip to maintain the wire shape.
The minimum possible amount of copper should be tinned.

Any competent regulatory authority will have this requirement as a rule in their system (see below)

The reason for the prohibition is that when you fully tin a multistrand wire fully, the solder wicks between the strands of copper and forms a solid block, part of whose volume is metallic solder. When you clamp the solder and copper bundle you tighten the screw or clamp against the solder block, and in time the solder metal "creeps" under the compressive forces and the join loses tension. The wire can then either pull out or cause a high resistance connection with heating.

This is a genuine real-world issue and is covered by genuine real-world regulations in many countries.

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I thought so. I have never done it as I have seen how the solder deforms and runs over time. A novice client asked why I had not done it and I couldn't tell him. –  kingchris Apr 13 '12 at 17:28
I've always tinned wire for my own prototypes because I'm always pulling the wires in and out and it prevents the wire from fraying or a little "hair" to creep over to where it doesn't belong. I never gave any thought to doing it in production, so it looks like I was doing the right thing simply by way of dumb luck. Excellent answer, +1 from me. –  akohlsmith Apr 14 '12 at 18:37

If they're stranded wires: don't tin them! The tinned whole will be soft and the screw will become loose in no time. Instead crimp a ferrule on them.

Also for solid wire I don't think tinning is needed. Think about it: have you ever seen electrical wiring in a house where the 2.5mm$^2$ or 4mm$^2$ wires were tinned before fixing into the wiring cabinet? Yet these connections are used in all kinds of environments and for tens of years.

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Doesn't the copper surface oxidize and form a resistive barrier. Or is this negligible ? –  kingchris Apr 13 '12 at 17:31
@kingchris - The terminal block, screwed down to a reasonable torque, will pinch the wire and stop corrosion. The appearance of the rest of the wire is negligible, it's the contact surface that matters. –  Kevin Vermeer Apr 13 '12 at 17:33