|Ask the Experts|
May 21, 2012
Wire Tinning Dispute
Originally I worked for NASA, now I am an avionics instructor forAmerican Airlines. Why is there a difference betweenstandards for tinning wires. Most standards specify a gap between the insulation andwhere 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 vibrationisolation so the wire will not break inside the insulation. At NASA we were told the added strain reliefof the insulation over the tinning was the highest standard. What is best, to gap or not to gap?
|Expert Panel Responses|
I'm not sure of the exactanswer, but I would guess it probably has to do with the type of flux beingused. At NASA the flux most likely has been defined and it corrosiveness hasbeen 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 statedwithin the IPC standards, that high activity fluxes are not to be used to tinwires as the corrosivity of the flux will impact the strands in the wire. Henceto prevent any flux from going beneath the insulation, the tinning cannot go upto and beneath the insulation. I will be looking atthe responses for this one as I'm interested in the other comments which willprovided for this answer.
Vice President, Technical Director
NASA-STD-8739.4 says thattinning all the way to the insulation is preferred, but it is acceptable forthere 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 tothe insulation, but wicking is minimal" (Preferred Figure A-19), to "Length oftinning is determined by type of termination; however, it should be sufficientto 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 notaddress tinning clearance, only that "The portion of stranded or solidconductors or part leads that will eventually become a part of the finishedsolder connection shall be tinned with solder and cleaned prior to attachment(Requirement)." Note that both standards havedefects for birdcaging and the closer the solder is to the insulation, theopportunities for birdcaging are greatly reduced. I have never heard ofa wire fracturing at the insulation because it was tinned too close to theinsulation (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 mootpoint anyway – the circuit goes open in either case. In an environment wherevibration is expected, wires (and the solder connections they are attached to)should always be prevented from movement by design, whether it be by cableclamps or potting for connectors or mechanical/adhesive support for wires on acircuit board.
NASA/Marshall Space Flight Center
When tinning wire it hasalways been my understanding that no solder should "creep" under theinsulation.Solder flowing under thejacketing on a wire can cause the material to swell & become dry &brittle.Brittle insulation will have the tendency tocrack & flake off.
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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