Electrical Contact Resistance

[hkymstr]

Here is the issue. We are currently applying chem film, Class 3 to parts that need low electrical resistance. Our supplier applies chem film to our parts and our receiving and inspection department will only accept a part if they can "see" the chem film. This results in parts with much to high electrical contact resistance due to much chem film on the part. The part is then rejected and the chem film reapplied, yes, a very inefficient way of doing things. I am trying to identify a process that will allow inspection to pass a part by a quantitative number rather than an subjective opinion on whether or not the chem film has been applied.

My question is, is there any way to measure the contact resistance of the chem film, other than that in the MIL spec 5541 and 81706? Going by MIL-C-5541 and MIL-C-81706 there is a method, but very impractical for manufactured parts.

 

[MikeHalloran]

That process is not inefficient, it's insane.

Do you allow materials suppliers to deliver certifications with their material, rather than lot- by- lot test results? Most outfits that are still in business do. Would your inspecton department be okay with having your supplier certify, order by order, that the film was in fact applied?

The other option is to help the inspectors 'see' better. Surely the film is detectable by means other than contact resistance.







Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[VEBill]

Use C of C (certificates of conformance, or compliance) instead of inspection. Request changes to the (written?) RI procedure. Specify (on the spec) that the the proper film tickness is, by the way, not visible.

Notice that the pen is mightier than the sword (all the above suggestions relate to written documentation). Stop talking and start filling in Change Request forms.

 

[buzzp]

All the certificates in the world are not good enough for some equipment. Depends on how critical this is. I can see the need to at aleast spot check and in some cases verify all of them, again depending on what its used for. Since MIL specs are mentioned it could be this is for something critical.
There are ways to measure this using a test bed. The methods vary from simple Kelvin connections (measuring resistance from test points on the PCB using 4 wire connections) or injecting current and measuring the voltage drop. All can be done relatively quickly from a testing standpoint. I believe with careful calibration of the test bed and a properly made test bed, the added resistances of the connections/solder could be calibrated out. Of course, this all depends on the contact layout and where the test fingers can be applied.

 

[VEBill]

"All the certificates..." The more critical something is, the more paperwork. Even simple testing cannot be done "relatively quickly". There's drafting the Test Plans and Procedures, Test Data Sheets, QA and endless other approvals, etc.

The OP's problem is (so far as I can see) a paperwork problem. Fight it ~within the system~ by changing the paperwork. Ideally, he can present the RI people with their new, fully approval, RI procedures (that he help to draft).

 

[buzzp]

The point is all the paperwork in the world may not be enough, testing may still be required.

His issues could be a paperwork issue but depends on what his end product is. If its used in defense then I would like to think that each one would be tested regardless of what the mfg says about its quality. Just good practice. Sure paperwork is in the equation but so is, at least, some random lot testing.

 

[hkymstr]

Yes, this is a defense application and the amount of chem film is critical. The problem is not so much in the application of the film but rather the inspection. The way our system is setup now is, if inspection cant "see" chem film on our parts it is rejected. Problem is, at the point you can see the chem film on a part is when you have applied to much making your electrical resistance higher than the bond resistance test calls out for. If we have our supplier apply the proper amount of chem film to begin with and have a test done at the inpsection end, then we can have a finite result. Either way it is done paper work will be involved, thats just a given.

 

[buzzp]

There are a couple of issues here, one has already been identified - the supplier is not supplying what you need. The solution is to talk with the supplier and get some reassurance (whether paper trail or other) that the future parts will have better quality control on their end.

The other issue is your inspectors - why would they only approve parts that you already know are bad (can see film)? This needs to be changed. After all of this, I would still want to randomly sample a box of these for conformance to your needs. You may even consider testing all of them, electrically (sounds like this is what you want to do). With that, I would follow up on the suggestions given above in my last post for testing. If this is mounted on a PCB then test fixtures can be built to accomplish what you need. If its not on a PCB a test fixture could probably still be viable.

 

[MikeHalloran]

Ah. The paperwork is not the problem.

The supplier is presumably capable of applying the correct, invisible, amount of film, but has been induced, apparently by the inspection department, to produce parts that are absolutely known to be unsatisfactory.

Apparently the inspection department is running the company, and the inspection department is being run by idiots.

Have you got a President there?

You have a leadership problem.





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA