Agree. We have written and signed papers that forbid us to make changes like
the one described. There are three levels, and change coating would belong
to a level that makes us responsible to all damage and accidents that could
follow a change that is not agreed by the customer.
Acrylic coating has been used here for military aircrafts for two decades
with good result. Just works fine.
Ingemar Hernefjord
Ericsson Microwave Systems
----- Original Message -----
From: "Stadem, Richard D." <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, June 20, 2006 6:47 PM
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>
NOTICE: This e-mail communication is private and confidential and is
intended for the addressee only. The email may only be used for conducting
business with VISTA Controls Inc., dba Curtiss-Wright Controls Embedded
Computing Subsystems. If you are not the intended recipient, please notify
us by replying to this email followed by deleting the email and all
attachments from your system. Anyone other than the intended recipient or
those with a need to know within the recipient's organization is strictly
prohibited from using, printing, distributing or disseminating the email or
any information or attachment contained therein.
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e To
unsubscribe, send a message to [log in to unmask] with following text in the
BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet
NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send
e-mail to [log in to unmask]: SET Technet Digest Search the archives of
previous posts at: http://listserv.ipc.org/archives Please visit IPC web
site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
for additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
