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May 21, 2012

Wire Tinning Dispute

Originally I worked for NASA, now I am an avionics instructor for American Airlines. Why is there a difference between standards for tinning wires. Most standards specify a gap between the insulation and where the tinning starts on the wire.  When I worked for NASA I had to make sure there was no gap. The solder had to flow under the insulation, but could not deform the insulation.

On aircraft we are told the gap is for vibration isolation so the wire will not break inside the insulation. At NASA we were told the added strain relief of the insulation over the tinning was the highest standard.

What is best, to gap or not to gap?


Experts Comments

I'm not sure of the exact answer, but I would guess it probably has to do with the type of flux being used. At NASA the flux most likely has been defined and it corrosiveness has been decided, so any flux beneath the insulation would not be a problem. Whereas in the commercial business as well as some military contractors, it is stated within the IPC standards, that high activity fluxes are not to be used to tin wires as the corrosivity of the flux will impact the strands in the wire. Hence to prevent any flux from going beneath the insulation, the tinning cannot go up to and beneath the insulation.  

I will be looking at the responses for this one as I'm interested in the other comments which will provided for this answer.
Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
NASA-STD-8739.4 says that tinning all the way to the insulation is preferred, but it is acceptable for there to be a gap. While there are no specific limits defined, NASA-STD-8739.4, Appendix A, Figures A-19 through A-21 show a range from "tinning all the way to the insulation, but wicking is minimal" (Preferred Figure A-19), to "Length of tinning is determined by type of termination; however, it should be sufficient to prevent separation of strands when wire is wrapped around a terminal." (Acceptable Figure A-21). Again, no specific dimensions are given.

NASA-STD-8739.3 does not address tinning clearance, only that "The portion of stranded or solid conductors or part leads that will eventually become a part of the finished solder connection shall be tinned with solder and cleaned prior to attachment (Requirement)."

Note that both standards have defects for birdcaging and the closer the solder is to the insulation, the opportunities for birdcaging are greatly reduced.

I have never heard of a wire fracturing at the insulation because it was tinned too close to the insulation (seems it would have to be an awfully rigid insulation material). Whether the wire breaks under or outside of the insulation seems to be a moot point anyway the circuit goes open in either case. In an environment where vibration is expected, wires (and the solder connections they are attached to) should always be prevented from movement by design, whether it be by cable clamps or potting for connectors or mechanical/adhesive support for wires on a circuit board.
Garry McGuire
Sr. Engineer
NASA/Marshall Space Flight Center
Garry McGuire is a manufacturing process engineer and Chair of the IPC J-STD-001 and IPC/WHMA A-620 Space Addendum committees.
When tinning wire it has always been my understanding that no solder should "creep" under the insulation. Solder flowing under the jacketing on a wire can cause the material to swell & become dry & brittle. Brittle insulation will have the tendency to crack & flake off.
Jerry Karp
JSK Associates
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.
Reader Comment
How would this apply to the tinning of a motor winding wire? In other words, a single stranded wire coated with a varnish. Tinning is typically performed directly after removal of the varnish and without the use of external flux.

Since I am dealing with a pure copper surface and tinning immediately, the copper does not have a change to oxidize prior to tinning. However, I have not come across a specification or standard that details this process.
Mark Jeddy, Stryker Global Quality & Operations, USA
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