|Ask the Experts
July 12, 2023 - Updated
November 19, 2013 - Originally Posted
Twisting Multi-strand Wires
Should multi-strand wires be twisted before they are tinned with solder? What are the benefits of twisting prior to tinning?
|Expert Panel Responses
Twisting wires after tinning can cause cracks to form exposing the base metal. This can promote oxidation and therefore solderability. It also adds internal stresses on the tinning and this increases the risk of tin whiskers potentially, especially with high tin alloys.
So in most cases, wires are twisted and tinned using no-clean fluxes or RMA low solids fluxes to avoid corrosion issues.
Senior Market Development Engineer
Most multi-stranded wires come twisted from the factory. If they are not twisted, it's possible that the wires were un-twisted when the wire was stripped. To answer your question directly, yes, the wires should be twisted before tinning.
According to the IPC-WHMA-A-620B, section 3.3, "Disturbed wire strands should be restored to approximate their original lay." It's also called out as a defect for a class 2 and 3 assembly if the general lay of the strands has not been maintained.
One word of caution, if you use bare hands to twist the wires back to their original lay, be sure to clean the wire well before attempting to tin.Your bare hands will leave oils and salts which will interfere with a good solder joint.
Manager of Assembly Technology
Multi storied wires should be restored to the original twist and should not be twisted beyond.
Tinning takes place through capillary action the solder moves up the strand. It is a good practice to use heat sink to prevent the solder getting into the insulation.
There should be un-tinned portion of at least 1 D where D is overall wire diameter to the end of the insulation.
The tinned portion of the wire becomes solid and un-tinned portion should provide the flexibility to give stress relief to prevent failures.
Astra Microwave Products, Hyderabad, AP India
No, twisting is generally not necessary. However, IPC-A-610 states, "Wire strands disturbed during insulation removal process should be restored to approximate the "original lay."
Quality Assurance/Regulatory Compliance Manager
To twist or not to twist? Looks like you should always "...restore the original lay..." before tinning per IPC standards as mentioned by several of the expert replies. An other question is why tin? If you are pre-tinning before soldering into a thru-hole in a board, it is not really needed, the solder process will tin and solder it just fine. If prior to crimping or installing into a screw tightened connector, I do not recommend tinning. Solder is soft and will yield under pressure, relaxing the previously tight connection and become loose. Best to use ferrules to capture the strands before installing into/under a screw tightened terminal/lug.
Senior Project Engineer
Electronic Controls Design Inc
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