On the top of that UR CC takes a lont longer to cure than AR CC.
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Amol (349)
Sent: Tuesday, June 20, 2006 1:24 PM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coat
Wow Richard! That was some story! Another thing I would like to add is that UR coatings are very hard (almost like glass) and are less forgivable for expansions and contractions as a result of temperature extremes occurring due to the altitude change. This can result in cracked components due to stress. AR coatings are more flexible that UR coatings.
The Mil-SPEC 46058 recommends a nominal thickness of 2 mils with a tolerance of + - 1 mil. When the coating is too thick, it may not cure properly and can skin or cloud over causing cosmetic issues to say the least. Plus that change will not go over well with Boeing as that violates the mil spec.
My advice, don't do this unless you have a very good reason (documented board failure due to CC failure)
Amol Kane
M.S (Industrial Eng.)
Process Engineer
Harvard Custom Manufacturing
941 Route 38 Owego, NY 13827
Phone: (607) 687-7669 x349
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, June 20, 2006 12:47 PM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coat
As with any other change made to avionic equipment no matter how small it is, a change like this should be qualified prior to being put into flightworthy aircraft. There is always some risk.
One thing I can think of that the coating thickness alone may change is the dielectric properties of the assembly. CC does affect it, and it could affect the electrical performance.
Changing materials or types (such as from AR to UR) should always be qualified.
As an example that I used here on the forum a few years ago, in a previous life someone once changed the method of fabrication of a simple FR-4 insulator used under a metal-shroud shielded 9-pin D-Sub connector in a power supply module. The old method of fabrication from Vendor #1 was to machine out the insulator and drill the 9 holes. The new vendor found a way to laser-cut the insulator and cut the holes using laser also.
The price difference was like $.05/per insulator. The buyer incorporated the new vendor without any change in the part number and without any ECN that would have notified anyone of a change being made (and why not, it met all of the print specs, right?).
Over time, with 50V applied, the current began to leak through the laser-carbolized (burnt) FR-4 edges of the holes. This in turn led to further heating/carbolization, and made the burnt edges of the FR-4 insulator holes that were in full contact with the connector pins even more conductive, until catastrophic shorting took place and the insulator actually began smoking and the power supply fuses blew out.
Of course, when this happened it was assumed that it was due to a solder short or partial short under the pin-through D-sub. Finally, after several through-hole process engineers and wave-solder technicians were badly beaten up only to see the problem continue, X-ray of the connector was added as a process step to catch the "solder shorts". Needless to say, the problem continued, and no shorts were ever found.
Using a hi-pot tester, I found the "bad" boards had lower resistance readings through the connector, but after two days of probing, I still could not figure out what the difference was between the older "good" assemblies and the new ones. I finally unsoldered and removed the connectors and noticed that when probed from the connector holes in the board, the "bad" boards became "good". But there were no solder anomalies. I could not see any difference whatsoever in the connectors when I tested them by themselves. When I switched the connectors to other assemblies, good boards turned bad and vice-versa (because I inadvertently mixed the insulators, how the heck would I ever have thought of switching them also?).
Finally, after one of the formerly good power supplies burnt up while I was testing it, and I sat at the bench ready to cry, with the "good" connectors and "bad" connectors and "good" assemblies and "bad" assemblies and all kinds of insulators laying all over the bench, I put my head in my hands and tried to think my way through the problem. As I was staring at all of the parts in front of me, I noticed the slight brown discoloration along the outside edges of the insulator and inside of the holes. On a hunch, I put the hi-pot tester on each end of an insulator that was slightly brown around the edges, and the resistance reading was significantly lower than one that had no brown edges. The readings were something like 50k versus 15 meg on the "old" (machined) insulators. Upon further investigation, the SQE went to the old insulator vendor ready to beat the stuffing out of them. The old vendor noted that they had not provided any insulators for 14 weeks, and the owner showed that all of their insulators were machined out, and did not look brown around the edges. Then it was discovered that a second source had been added to the Approved Vendors Listing for the insulator.
Guess who had to go out and look for a new job the following Monday? Not me, man. I was a hero!
The point of all this is, don't ever change anything on military or avionics electronics until it has been fully qualified.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bremer, Gary
Sent: Tuesday, June 20, 2006 10:35 AM
To: [log in to unmask]
Subject: [TN] Conformal Coat
Engineering is recommending that we increase the thickness of the AR coating to .005 ± .002 to seal the boards against moisture. These boards are used on Boeing aircraft, my thought is to use UR coating due to the environment these boards are used.
Is the Acrylic conformal coating the best choice for this application?
Are there any problems with increasing the thickness of the Acrylic conformal coating?
This box is open to the environment and with my knowledge of the mounting location the sanitary lines could be located above the electronics.
Thanks
Gary Bremer
Senior Manufacturing Engineer, CIT
Curtiss-Wright Controls Inc.
Embedded Computing -Subsystem
28965 Avenue Penn
Santa Clarita, CA 91355-4185
Main: 661.257.4430 extension 2170
Fax: 661.257.4782
e-mail: [log in to unmask] <mailto:[log in to unmask]>
Web: www.cwcembedded.com <http://www.cwcembedded.com>
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