coaxial cable with ring terminal fails after being in oven

Hello Guest,

A) I presume it is the copper conductor which oxidizes until it fails.

B) Advise if it is the silver solder joint which fails.

If the problem is (A) above, you could do what has worked fore us: Using PTFE tape - the sort used by plumbers for screw thread joints in metal pipeware, wrap around the silver-soldered joint and conductor.

This tape we used is Brand "Ceelon" and has no adhesive coating, but can make an airtight wrapped joint, which both strengthens the joint mechanically, and keeps the oxidation problem to a minimum..

Reply advising progress or lack of it, with thank you.

Kind Regards....

Sparkstation

We are not sure if it is oxidizing or because of the high temp (190 degree C) the core wire is getting brittle.

We do not see any clear signs of oxidation even under magnification.

The failure is where the end edge of the terminal ring meets the core wire.

Do you know if the PTFE tape brand "Ceelon" will hold up at the high temps (190 degrees C); in the past when we have used teflon (in other areas of the oven) we have seen that the teflon gets soft almost "liquid". Seems like we are working at the outer limits (high temp) of teflon.

Thank you for you input.

Why not use silicone hi voltage wire, [which is steel] with stainlees steel ring connectors & add braid if the shielding is required.

or here's a different style electrode which can be used with a burner control to spark & detect flame http://www.flynnburner.com/flynncd/english/bakeryelect.htm, flynn also makes an older style that has a porcelien insulators & a stainless steel tube for shielding.

Unfortunately, as there is stress concentration where the wire contacts the edge of the terminal, that doesn't really give extra information.

Some general comments:

If there is sufficient vibration, that may weaken the joint over time without requiring any more complex process

In the absence of air, copper wire will normally soften during storage at 190-degrees, rather than becoming brittle. However, if it is in contact with other metals intermetallic diffusion can cause it to become brittle. Tin is particularly problematic in this regard. Other problems can include the copper diffusing into a porous connector, leaving a very narrow section at the interface. Intermetallic diffusion can be avoided by suitable choice of connector (pure copper is one such)

Note that if the copper is porous (either in its initial construction or as a result of intermetallic diffusion) oxidation can cause significant weakening before it becomes visible.

In the absence of specific information on the materials I can't say anything categorical. I have no experience of PTFE tape under these conditions, but pure PTFE should be fine up to at least 250 degrees (at least some tapes described as PTFE are deliberately softened, so might not be suitable). However, my related experience is that it can be sufficient to exclude water vapour, and this can be achieved using a silicone coating directly on the copper (N.B. it needs to be a true coating directly on clean metal, as silicone itself is not impervious to water - the barrier layer is formed by the chemical bond between the silicone and the copper). If the silicone layer is thin, the effects of vibration can be ameliorated by covering with a suitable heatshrink tube.

Clamp the teflon somewhere to prevent movement or vibration from mechanically wear the relatively weak center wire.

Does the postmortem show any signs that the center conductor (core wire) might have been nicked during the stripping operation? That will accelerate breakage at your ring terminal. As a company that uses hundreds of teflon based coaxial assemblies, we've seen the problem occur too often. If the cable assembly is made "in house", it would be worth witnessing the build from start to finish, if it's purchased, sacrifice one and dissect before use. Please let us know what you find, it's always good to hear the solution to these "mysteries".

Tom

YOU could also consider skipping the terminal & forming a small loop & silver soldering that. this would reduce the stress to the wire from the action of crimping.

You can also try to crimp the wire onto an all metal lug, solder the wire onto the lug, and insulate it with shrink sleeving.

Generally, standard solders are not such a good idea at 190^OC

Hello again Guest,

A) Perhaps the problem break of the wire is occurring because of the alloy junction at the silver-soldering joint.

B) It may also be caused by thermo-expansion/contraction, resulting in embrittlement of the alloy in the joint.

C) If you are able to form a small circle into the wire, before it is silver-soldered, then that would eliminate 2(B) above.

D) A further thought would be to use Self fusing Silicone Rubber Tape, good to 200 Celsius continually, to insulate the joint, if that helps.

Advise your progress, or lack of it.

Kind Regards....

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