Specification for Damaged Shielding

I was a member of IPC for a number of years. If your company is a member of the IPC they will help you.

Let me ask you this. Which side do you represent?

Why all the fuss over a single nick? Is it expensive?

What is your incoming inspection acceptance criteria?

What is your internal inspection acceptance criteria?

Is this cable being built for spacecraft?

What is the customer acceptance criteria?

Lyn, Thanks for your reply. I am a Manufacturing Engineer with a degree in Electronics Engineering. I work for an aerospace company that builds test equipment. I know and understand the difference in coax and twisted shielded. I worked in the RF industry for many years. We currently have our engineering manager stating that there can be up to 25% strands cut/missing without degradation to the function of the shielding. I would agree with this because we terminate everything with hermetically sealed solder sleeves and drain wires. The solder is the electrical connection to the shield from the drain wire. On the other side of me is the QA dept which are not electrical/electronic engineers who are looking at the IPC standard for Twinax and wanting to hold to that. Thus I am in the middle (closer to engineering) but have been tasked to "Find" an industry accepted standard. The following are answers to your questions.

#1 No our company is not a member of IPC. So far I have been unable to "justify" the expense to the upper management.

#2 In the middle.

#3 QA wants justification for the standard, not just an "opinion".

#4 We do all stripping in house and it then goes to our QA inspectors.

#5 Same as above.

#6 NO, it is used in testing of critical systems on aircraft (not in flight) to resolve intermittent and hard failures.

#7 We are (the customer) in that we (engineering) do all design work and testing for form, fit, and function.

Engineering designs a product to fit a particular purpose. It is handed over to the M.E. group to design build processes and QA Standards. We have recently been in a growing spurt which has caused QA dept to become more aggressive in seeing industry accepted standards. Realizing that this type of wire is used in many different and diverse products and with the lack of any other standard the QA dept wants to use the coax/Twinax standard, which in the opinion of Engineering and ME Group is to tight to hold. We recently purchased some new tooling that will make it easier, but they still want an industry ACCEPTED standard for this product. In our company structure M.E.s are responsible for creating the build procedures which include the standards for QA.

You have my deepest sympathy.

Imagine submitting something to QA people who can only blindly follow written specifications for NASA spacecraft equipment. I've been there. It's no fun.

I don't suppose there's any chance you could write the acceptance criteria for the cables??? Probably not.

Not sure I can help. I'll root for your side.

Thanks Lyn. I have a great deal of respect for you and others on this blog. We are currently trying to work out an internal solution, but if there is some industry accepted specification on this it would make my job easier. Do you have an opinion on this matter?

There are others here much better able to address specifics. I was a mechanical/materials/packaging engineer when I dealt with those NASA types.

Good luck.

Because unlike Twinax and Coax where the shield is part of the circuit and is therefore critical to the signal carrying capability and impedance of the cable, whereas the shield for twisted pair cable is there as an electrostatic drain and to reduce electromagnetic noise pickup. You can see the difference in the terminations, Coax and Twinax cables use connectors that are part of the circuit and are grounded at both ends, while twisted pairs can be terminated in connectors, barrier strips, headers, etc. but with only one end of the shield/drain connected to ground. Also commodity shielded twisted pair cable can have many different wire sizes, numbers of pairs, shielding configurations, etc When you specify RG-8/U Coax you get a certain diameter conductor made of certain materials, with a set of known electrical characteristics, especially its characteristic impedance (50 ohms); if you specify 8 twisted pairs of 24 gauge wire and an overall shield, you don't.

I suspect that you will not find anything that allows you carte blanche with any particular number of strands since the strands are providing multiple potential functions.

As conductors, you would be interested in the lost cross section and also potentially the localised resistance at the damage.

As a shield you might be more concerend about the area of unprotected surface to enable EMF influence on the internal conductors.

As an armour you might be concerned about the loss of protection from a future event.

As a strain relief you might be concerned about local deformation due to stretching.

Depending on the signal frequency, the cut ends themselves could become antennas.

Cut shielding also implies that the outer sheath is damaged and so environmental factors of corrosion and such also come into play.

(Or really any combination of these.) No standard could cover the potential permutations of all these factors for all potential uses of the cable.

The design target should (obviously) be nill damage and your end use will determine what is tollerable (not really acceptable) in the specific situation you face.

Ask the person/department/customer that specified the cable to understand their reason/need for the shielding.

Down Under,

Please See comment 4 to Lyn. Thanks for your reply.

Thanks for the acknowledgement.

You might like to contact KOMAX and other companies that specialise in cut/strip/terminate equipment as they will be able to provide details of teh acceptance criteria that they need to meet for each industry and each wire type.

I know that SUBA Engineering are their agent in Australia, but there would likely be an agent local to the USA that you could consult.

I see that I had left out the fact that we are stripping and terminating the shielding. The termination points are in solder sleeves and the solder sleeves are inside metal back-shells where required.

I would suggest that you contact Anixter (a major supplier of electronic & communications wire & cable). They have a free Technical Information Handbook that is available as a download at:

http://www.anixter.com/AXECOM/US.NSF/TECHNOLOGY/TheWireandCableTechnicalInformationHandbook_WIRECABLE?OpenDocument&Division=DivTab5

See chapter 11, which lists all sorts of industry standards. You could also call them at 1-800-ANIXTER and ask for Tech Support. They may have some guidance for you.

I spoke with Anixter about this. They do not have a specification on the wire they handle for the termination of the shielding. They were helpful in that they would look further into it and are very willing to sell us wire. We will see where we go from here. Thanks for the sugestion.

Here this some of the information you seek maybe there. TIA/EIA-568-C