This is subjective, but I am looking for other people's experience.

If I am going to screw eight wires into a 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.

  • \$\begingroup\$ 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. \$\endgroup\$
    – AndrejaKo
    Apr 13, 2012 at 17:14
  • \$\begingroup\$ 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. \$\endgroup\$
    – kingchris
    Apr 13, 2012 at 17:34
  • 3
    \$\begingroup\$ @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. \$\endgroup\$ Apr 13, 2012 at 17:35
  • \$\begingroup\$ @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. \$\endgroup\$
    – AndrejaKo
    Apr 13, 2012 at 18:01

5 Answers 5

  • 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.

  • \$\begingroup\$ 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. \$\endgroup\$
    – kingchris
    Apr 13, 2012 at 17:28
  • \$\begingroup\$ 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. \$\endgroup\$
    – akohlsmith
    Apr 14, 2012 at 18:37
  • \$\begingroup\$ "that your days may be long on the face of the land" -- and may your loins be full of fruit. (Terry Pratchett) :) \$\endgroup\$
    – user98663
    Jan 26, 2017 at 8:07
  • \$\begingroup\$ Do you have references to any of these regulations? NFPA 70/NEC preferred, but any others would be fine. \$\endgroup\$
    – Paul
    Nov 22, 2019 at 3:52
  • \$\begingroup\$ @Paul I wrote that 7+ years ago. I stand by what I said but cannot lay my hands on relevant regs at a quick search. I'm in New Zealand. I'm absolutely certain that NZ (and Australian) regs cover non soldering of connections to be used in a screw down connector - and I have personally seen on a number of occasions failures that occurred when this was done. \$\endgroup\$
    – Russell McMahon
    Nov 23, 2019 at 11:38

If you want to do your installation a favor, use wire ferrules:

Wire Ferrules

Image source: http://de.wikipedia.org/wiki/Aderendh%C3%BClse?uselang=de

Do not, ever, tin the wires. A tinned wire will slowly give way to the screw's pressure and eventually become loose. The contact resistance will rise. In the worst case, your contact will become hot and cause fire.

If you don't have wire ferrules at hand, just twist the stranded wire before putting it into the terminal block. While not recommended because some strands might cause shorts to neighboring terminals, this will still be far better and reliable than a tinned wire.

  • 2
    \$\begingroup\$ These are the types of termination I specify for production. Gives a much cleaner finish and doesn't lead to wire fraying over time with insertion/removal cycles. \$\endgroup\$
    – akohlsmith
    Apr 14, 2012 at 18:38

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.

  • \$\begingroup\$ Doesn't the copper surface oxidize and form a resistive barrier. Or is this negligible ? \$\endgroup\$
    – kingchris
    Apr 13, 2012 at 17:31
  • 9
    \$\begingroup\$ @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. \$\endgroup\$ Apr 13, 2012 at 17:33

I have learned my lesson on soldering a stranded wire (1950's thinking)exiting a transfer switch for better contact in screw terminal. Was under constant 25-30 amp load and finally blew out and burned wires right off at that screw terminal. Other similar treatment showed loose wire screws after 3 years in my RV.

I have just now instead used a single 6" copper 10ga wire in and out of the screw terminals on transfer switch and the pigtailed with twist-on connector to the stranded 10 ga. Could be the new push in flip down tab connectors would have been more positive connection than the twist-on but even those only "bite" into a small portion of the inserted wires.


According to https://www.ipc.org If you go here - IPC-A-610E-2010, Acceptability of Electronic Assemblies, April 2010 Supersedes IPC-A-610D.
On page 4-10 under section Hardware Installation – Threaded Fasteners - Wires

Near the bottom which states "tinned wires" as a defect.

  • \$\begingroup\$ Nice to see it in black and white. But it is interesting that it says under that point."Missing solder or adhesive as required per customer requirements". So we hope the customer didn't want solder on his multi strand flex. (A little joke.) \$\endgroup\$
    – kingchris
    Mar 1, 2020 at 14:15
  • \$\begingroup\$ Broken link updated September 2023. \$\endgroup\$
    – Russell McMahon
    Sep 19 at 5:57

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