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first post here.

I'm currently making a set custom sleeved/crimped cables for my computer's modular power supply, and my crimper isn't very good, so for extra security, I've soldered all the crimps, using flux so that they flow properly. They are male-and-female type connectors, and carry high current (~5A) 12V DC, as well as some slightly lower current 3.3V and 5V DC.

After soldering, I proceeded to clean them in ethanol/alcohol, however I'm pretty sure they still have a thin layer of flux in them since it's hard to clean inside the female connectors.

I've done testing with a multimeter by first inserting a male connector, then measuring resistance and diode checks from the inserted male connector to the other end of the wire, and they check out (~0.003 ohms, the same as an un-crimped wire, the resistance is probably caused by the multimeter probes anyways, I've checked the probe-to-probe resistance to be the same).

Now for my question: If my resistances check out, and Diode check passes, will I experience unstable current under high loads? (e.g 8 Wires for a 150W graphics card)

Thanks!

Samuel H.
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    What do you mean by 'unstable current?' – alex.forencich Nov 22 '16 at 01:24
  • eg a weak connection, with very short disconnects/reconnects and/or fluctuation, in computers, when you have unstable current, you can glitch your hardware out and result in artefacts on the screen or freezing. – Samuel H. Nov 22 '16 at 01:29

3 Answers3

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There are two problems from the flux (see Why do we need to remove flux from circuit boards?)

  • Electical conductivity -- this does not apply in your case. A couple of megaohms, or even a couple of kiloohms, will not affect PC PSU in any visible way.
  • Flux reactivity -- this may apply in your case, but it is unlikely. Again, the PC wires are usually very thick, so it will be many years before corrosion becomes a problem.

What you should be worried, however, is that if the connectors that were designed for crimping were soldered, the wires can break off much easier. A proper crimp holds the wire by insulation, and soldering most likely has destroyed this. Make sure not to bend wires too much once you have installed them.

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theamk
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  • I crimped them, then soldered them, because only crimping wasn't enough. – Samuel H. Nov 22 '16 at 01:59
  • @SamuelH. If the crimp is equal on insulation and wire ( i.e. 2 stage crimp) that will be rugged, for Ohms you can expect 5 mOhm max. on properly crimped sockets and more on the cable. Dropping a few Amps of measured current and measuring voltage drop will give you micro-ohm resolution on measurements. Tie up wires with plastic ties or floss. to restrain motion. Use multiple source wires and returns equal or more from ATX PSU. – Tony Stewart EE75 Nov 22 '16 at 02:15
  • @TonyStewart.EEsince'75 thanks for the reply, unfortunately I am not too familiar with this area, and do not have the equipment to perform measured current + volt drop tests (not that i know of). I am not worried about the wires causing too much resistance, but more of the possible flux residue on the connector terminals. If their resistance values pass when testing, I should not experience connectivity problems under load, is that right? – Samuel H. Nov 22 '16 at 02:29
  • You can target 1% loss in your cable & connection150W @12V is 12.5A thus 120mV drop is 0.12V/12A = ~0.010 ohms which gives your 0.003 ohm per contact (+/-) and 0.002 ohm per DC line (+/-) while sleeve contacts are 0.001~0.002 ballpark... to get cable resistance down to 0.002 at 1 ft each you need at least 5x 20AWG or 2xAWG16 so if you have more your single contact is the weakest link and should be doubled. – Tony Stewart EE75 Nov 22 '16 at 02:57
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Your application of the soldered connections raises a number of concerns.

1) When you solder crimps the wire will easily break at the boundary of where the stranded wire has soaked up the solder between the strands. This can happen up into under the insulation and into the portion of the wire that had the insulation crimp. Use great care with handling your cable and do not bend the wires at the connectors.

2) One hopes that you used a proper electrical soldering flux (rosin or organic type) instead of acid based flux. If you used acid I suggest you start over and build cables with the proper crimping tool and avoid the need to solder all together.

3) Use of flux that possibly got down inside the connector contacts could very well lead to intermittent connections when the cables are mated to the circuit boards. You will need to clean out the inside of each contact. The best way to do this is to use a spray can of flux remover. These cans come with a short plastic spray tube that can get inside female contacts and clean out all the residual flux.

Michael Karas
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  • The crimps are 2 stage crimps, where a part of the insulation is also clamped onto separately. The cables are rarely bent, and never move once in place since they're plugged in and then left there. As for flux, I know the paste flux I used was non-acidic, while the solder wire had a rosin core. I have just purchased a can of flux-off, and will proceed to cleaning the connectors one by one, thanks! – Samuel H. Nov 22 '16 at 05:59
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If you can see a nice shiny, smooth fillet of solder inside the joint, then there is no chance of residual flux causing any problem with the soldered connection.

Richard Crowley
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  • I am referring to the flux flowing to the inside surface of the connector and insulating the connector from its mating end. – Samuel H. Nov 22 '16 at 02:00