0
\$\begingroup\$

I bougth some 4mm male banana jacks today with the purpose of making myself some more test leads. They have screw connection, similar to the ones in this picture (from the farnell product page):

Screw terminal banana jacks

I wondered if I should stick to the screw connection, or if I should fill the hole with solder and insert the cable into that. The guy in the store said they solder all theirs - just get rid of the screw and fill it up with solder.

My first idea would be that "solder is clearly better, it's a pure metal connection!" but I know that this is likely false - crimp connectors are much more reliable than solder joints. Is this also the case here? I'm worried that since it will be moving a lot, the edge of the screw pin might cause a cutting to occur.

I'm going to be working with quite thin cables (0.5mmsq) because the other end is going into a micrograbber style connector, that can only accept this thickness. I mention it because I don't know if this changes things.

\$\endgroup\$
6
  • \$\begingroup\$ Solder is for getting a good electrical connection. Screws/Crimps are for getting a good mechanical connection. \$\endgroup\$
    – PlasmaHH
    May 8, 2017 at 14:00
  • \$\begingroup\$ I tin the leads and then screw them. Zero scientific data to back that up though. \$\endgroup\$
    – Wesley Lee
    May 8, 2017 at 14:10
  • 4
    \$\begingroup\$ @WesleyLee Tinning the leads and then screwing them makes the leads brittle. Eventually, they tend to snap just at the point where the tinning stops. \$\endgroup\$
    – Simon B
    May 8, 2017 at 14:13
  • \$\begingroup\$ Yes but it also prevents the copper from splicing. It works ok for me since I never rely on the conducting metal for strain, but as I said, zero scientific data do back that up. \$\endgroup\$
    – Wesley Lee
    May 8, 2017 at 14:17
  • \$\begingroup\$ If you want this to last, you might look at attaching the thin wire to something thicker, and then doing a staged sleeve to gently transition the "bend impedance" with multiple sizes of heat shrink. Or you can do it crudely and plan to re-do it every once in a while. \$\endgroup\$ May 8, 2017 at 16:17

3 Answers 3

4
\$\begingroup\$

There are four issues with soldering the connectors.

  1. You have to get the connector and wire VERY HOT to get a good solder joint. That means the solder will wick up the wire a long way and the insulation will likely melt back on you.

  2. Where the flexible wire meets the stiff solder, an interface is generated that becomes a stress riser where the individual wire segments will break easily.

  3. The heat from soldering itself can damage the properties of the connector and make it rather hard to repair or re-use.

  4. Connectors, and crimps, are normally plated with materials, like chrome, which are not compatible with standard solder and flux.

As such I recommend the following options.

1. Tight Fit Wires Tinned End.

enter image description here

Trim back sufficient insulation to allow you to insert the wire so the insulation butts to the shell. Tin the end of the trimmed wire to just past the screw location and screw it in place normally. Try not to allow the solder to wick too far up past the screw down point.

2. Medium Fit Wire : Bend back and shrink fit.

enter image description here

Medium sized wires should be trimmed sufficiently to allow you to bend the wire back on itself and the bent part tinned and screwed. This time add two inches of heat shrink tubing to the wire so it fits snuggly within the housing hole.

2. Fine Wire : Slug and Double Shrink.

enter image description here

With really fine wires you can not expect to use the lead screw to tighten down on the wires. As such you need to use some thicker grade wire to make a "slug" that will fit in the connector to take the screw force. The end of the wire should be looped and soldered to that slug.

This time you should use two layers of heat-shrink tubing for bending strain relief. If it will fit, allow the original insulation to be inside the connector.

With fine wires additional strain relief will be required if heavy usage is expected on the connector.

Adding the Cap

IMPORTANT Remember to slide the connector cap onto the wire before you do any of the above. When finally sliding it into place and screwing it down be careful to ensure that the cap does not cause the wire to twist relative to the connector or it may break off inside.

Added Adhesives And Strain Relief

You can buy banana connectors that actually have strain relief features built into them. However the following measures can be taken to improve matters if those are unavailable or cost prohibitive.

Once joined to the wire the cap should be a fairly tight fit around the insulation or heat shrinking. If not you can use a flexible adhesive, like silicone, to augment the join under the cap.

Alternatively, you can dispense with the cap entirely and make your own from heat-shrink tubing that covers both the wire and the threaded part of the body.

enter image description here

For production cables this last step is normally molded onto the cable assembly.

In Conclusion

As with ALL connectors, mechanical stability is the key factor. The initial electrical connection is actually easy to achieve, but once made, it is important to ensure that any mechanical movement on that connection is kept to a minimum if not eliminated entirely.

\$\endgroup\$
4
  • \$\begingroup\$ Thanks for the detailed reply. I'm using silicon wire, so the insulation doesn't tend to melt on me. However, I didn't think of the fact that the solder will wick up into the cable causing a weak point. I'm deffinetly going to give this a try later today with the folding back method. I was under the impression you shouldn't solder wires for screw terminals because it prevents the cold weld. Is this only the case when dealing with crimp connectors? \$\endgroup\$
    – Joren Vaes
    May 8, 2017 at 17:17
  • \$\begingroup\$ @JorenVaes yes the "heavier" the crimp or connector the harder it is to get a good solder joint. Further, crimps and connectors are usually plated with metals that are not compatible with the solder and flux used. Crimps, despite their appearance, actually form a pretty solid joint if the wires are tinned correctly and the proper tools / machines are used. \$\endgroup\$
    – Trevor_G
    May 8, 2017 at 17:22
  • \$\begingroup\$ @JorenVaes ultimately though, as I said, it's a matter of identifying the mechanical forces you can expect on the join and eliminating them. \$\endgroup\$
    – Trevor_G
    May 8, 2017 at 17:24
  • \$\begingroup\$ @JorenVaes If you don't tin the end of multi-stranded wire when you use a screw terminals the screw tends to splay the wires apart, and you can end up only holding on one or two strands. Obviously that significantly affects the resistance and mechanical strength of the connection. \$\endgroup\$
    – Trevor_G
    May 8, 2017 at 17:27
3
\$\begingroup\$

Since you are working with very thin cables, I suggest a third possibility: insert the wire end into a ferrule, crimp, then screw the ferrule into a jack as you would screw the wire.

First, the ferrule will bite well enough to hold the wire in place, probable even if you use pliers instead of a crimp tool. Next, the flattened ferrule will provide a good surface for the screw to meet. I try to turn the screw harder than usual, so it deforms the ferrule into the shape of the barrel so it doesn't loosen over time.

\$\endgroup\$
1
  • \$\begingroup\$ Thankyou for the suggestion. I will try this in the future! \$\endgroup\$
    – Joren Vaes
    Jul 5, 2018 at 3:51
1
\$\begingroup\$

Because this is a test lead that sees lots of use and flexing, I'm going to strongly suggest that you do NOT completely tin the wire that enters the plug. The reason is that tinning the wire makes it brittle.

What I do is tin only the very tip of the conductors - perhaps the very last 1mm or so. The portion of the wire that is under the screw might have a bit of solder on it but the wire that exits the plug does NOT have any solder.

FWIW - my preferred banana plug is from Johnson - their series 108 connectors are awesomely reliable. Do note that most people don't install them correctly - the proper way to install that plug onto the test cable is to strip about 3/4" of insulation (20mm), insert the wire all the way through the plug, then wrap the wire around the plug in the same direction as screwing the plastic back shell on. Then just screw the plastic piece on - this clamps the wire in place.

Bottom line is that you do NOT want to have solder in the wire strands where the conductor is going to flex. The technique that I described above has proven to be extremely reliable over decades of use.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ On top of making the wires more brittle, tinning the wire under the screw could result in creep, causing the wire to pull loose. en.wikipedia.org/wiki/Creep_(deformation) \$\endgroup\$
    – ks0ze
    May 8, 2017 at 19:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.