Doug and Jim
In respect to the suggestions Doug makes, could you just be sure that you
are manufacturing assemblies that are broadly in-line with the "coupon
design" outlined in J-Std001/004/TM650 etc..
Latest research suggests that "older" designs might yield grossly misleading
reliability data (SIR results) - you really must test to conditions to which
your finished assembly shall be subjected.
"Cleaning" to a value of less than 2 megohm/cm also means that it is
acceptable to leave up to that amount of "salt" on every square cm of the
assembly, but fine-line; fine-pitch; COB, BGA etc., might well demand a far
lower value. I know of many that work to less than 0.2 megohm/cm for this
very reason.
Regards, Graham Naisbitt
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of
[log in to unmask]
Sent: Monday, December 10, 2001 02:12
To: [log in to unmask]
Subject: Re: [TN] Conformal Coating Process
I'm trying to qualify a new cleaning process for electrical assemblies
prior
to conformal coating (Military boards). The old MIL Specs had as
cleanliness requirement of 2 meg-ohms minimum, is this all I need to do,
verify cleanliness to 2 meg-ohms? What about subjecting the assemblies to
10-day elevated temp/humidity (steady state)? Any recommendations?
*Jim, to your first question, Yes. When changing a cleaning operation, and
not changing fluxes, MIL-STD-2000A only required Appendix C testing, which
was the standard ROSE test. The 2 megohm-cm requirement translates to the
more familiar 10.06 micrograms sodium chloride equivalence per square inch,
whatever that becomes translated to the specific ionic cleanliness tester.
We all know what a crock that test is, but it still exists, especially in
military contracts.
Two questions you should be asking: What data does my customer need to
see? and I just trying to meet contractual requirements or REALLY show that
the change is not detrimental?
If the customer leaves it totally to you, then I suggest that you do both.
The ROSE test, bogus as it may be, is very easy to do and satisfies the
contractually focussed types. As to a longer temperature-humidity
exposure, that is a fairly good idea. What tests to you perform now to
tell you if product is good or bad? Do that same test but extend the test
time about 50% and run on hardware cleaned with the new method. At
Collins, our products go through -55 to 125C cycling to catch infant
mortality types of failures, er...ahhh... not that we EVER see such
failures, mind you....., and when we have a process change, we use the same
environmental screen.
Doug Pauls
Rockwell Collins
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