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"Chem Film Surface Resistance Measurements"
February 12, 2010
Q. Are you aware of any Surface Resistance Testing of products certified to Chem film per
Mil-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil] Class 1A ?
Our customer has flowed down this finish requirement plus a resistance requirement of
< 0.1 ohms.
Product Designer - Budd Lake, New Jersey, USA
January 17, 2013
Q. Hi everyone,
I apologize for the length, but I'm trying to convey the entire situation.
I am a materials engineer working on an electrical box in a space application. Currently, we are having difficulty understanding the electrical resistance performance of MIL-DTL-5541, Class 3 coatings.
In this specific box, we have attach an electronic piece part to the chem film surface with adhesive and mounting hardware (screw, threaded through holes, nut and washers). Prior to bonding, the surface is prepped using an Acetone wipe. The post adhesive cure (24 hours at room temperature) requirement is to have contact resistance between the piece part and the panel conversion coated surface of <10 milliohm.
Up until one year ago, we were able to consistently meet this requirement; however, recently, we have begun to see issues. The grounding path is through the hardware assembly and into the panel. It has been verified by destructive physical analyses that no adhesive is in the fastener assembly, and torque is verified, so we know there should be solid electrical contact.
Perhaps most interesting, is that if the resistance measurement is taken during the cure, we pass the requirement. As time passes, the resistance becomes worse, eventually to a failing state.
As I mentioned, we have verified that there is no adhesive in the screw assembly, so it should not be affecting the electrical path. Partially due to lack of other sources, we are not looking into the chem film surface itself.
Are there any solvents that could cause the electrical resistance to increase? Primarily, they may come into contact with Acetone, IPA, and MEK. I am currently working with the adhesive vender to understand if there are any curing solvents that might be a concern.
Are there any other post processing issues that could increase resistance? We do not heat the assemblies before the testing, so I don't see that as an issue.
Thanks for your help!
- Denver, Colorado
A. I do not understand most of your writing, however I do want to comment on the following sentence:
"Perhaps most interesting, is that if the resistance measurement is taken during the cure, we pass the requirement. As time passes, the resistance becomes worse, eventually to a failing state."
Apparently the chromate conversion was, in this case, much too thin; and, unprotected, the base aluminum continued its tendency to oxidize in open air.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina