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.
Paul Austen
Senior Project Engineer, Electronic Controls Design Inc
Paul has been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 45 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes as well as moisture sensitive component storage solutions.
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."
Richard Henrick
Quality Assurance/Regulatory Compliance Manager, Sanmina Corporation
Richard has 18 years experience in the medical electronics industry at both a contract manufacturer and OEM. His experience includes PWA and finished device manufacturing as a Manufacturing Engineer and during the past 7 years as plant Quality Assurance/Regulatory Compliance Manager. He holds 5 American Society for Quality Certifications and is a Certified IPC 610 Trainer.
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.
KN Murli
Head-Quality, Astra Microwave Products, Hyderabad, AP India
Holds Degree in Engineering, started off as Scientist/Engineer in ISRO (Indian Space Research Organization) in Quality Assurance of Space hardware Electronics Production. Worked in the area of Parts, Material and Process; DPA, FA and Process Qualification for space and ground hardware. Later moved into Private sector and worked in the area of Quality Management Systems & ISO 9001 certification. Currently hold a position as Head-Quality in RF/Microwave Product manufacturing for Defense and Aerospace segment.
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.
Kris Roberson
Manager of Assembly Technology, IPC
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.
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.
Peter Biocca
Senior Market Development Engineer, Kester
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.
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