Graham,
I did encounter certain cases of application/coverage issues but the
vast majority were
(>75%) were due to degradation/failure of the coating to maintain
effective barriers.
The issue I have is that these problems generally occur after the
"warranty" period has expired.
There are no readily available test methods or equipment capable of
determining the effectiveness
of coatings after they have been in the field for extended periods.
Most problems were due to :
1. Melting/evaporation of Ar with low Tg (15 degrees C) when exposed to
internal forced air cooling.
2. Permeation of AR coatings by foreign material (dust and particles)
when Tg was exceeded
3. Cold flowing of AR into places it was not suppose to be when Tg was
exceeded.
4. Apparent permeation of SR by chemical contaminants, causing color
changes and increase sensitivity
to moisture.
5. Delamination of UR coatings from circuit traces when exposed to
several years of
temperature cycling combined with high moisture/condensation conditions
(this resulted in corrosion of the circuitry).
Do coatings degrade/wear out? YES!
How long does this process take? It depends on the operational
environment, type of coating,
application
process, and circuitry design.
Is there any methodology which can predict or verify optimal coating
performance for extended periods? Not currently.
I have written a paper about this titled "Avionics Systems, Reliability,
and Harsh Environments".
It was presented May 9th, 2001 in Washington D.C. at the 57th annual
American Helicopter Society
Meeting.
You may also get a copy of the following paper "Are Lead-free Assemblies
Especially Endangered by Climatic Safety?" presented by Zestron at this
years APEX meeting. They have covered many of the same issue I did.
David A. Douthit
Manager
LoCan LLC
Collins Graham wrote:
>David
>Were the 100+ failures due to a failure of the conformal coating
>material, or are they attributable to another issue - contamination
>under the coating, poor application, bad design? The best coating in
>the world is only a band-aid on a poorly designed or built product.
>
>And on a less serious note, to the other Graham: OK, one should not
>drink Skydrol, but what aviation hydraulic fluids would you normally
>imbibe??? I'm sticking with the G&T.
>
>regards,
>
>Graham Collins
>Process Engineer,
>Northrop Grumman Canada Corporation
>Halifax
>(902) 873-2000 ext 6215
>
>
>
>>>>[log in to unmask] 03/10/04 08:45AM >>>
>>>>
>>>>
>Graham,
>
>For the record, acrylic coatings have been in use in the aerospace
>industry,
>on flight systems of all types and locations, for over 30 years now. We
>have
>even seen circuits running with water as they pass the dew point, no
>failure
>recorded.
>
>I take exception to that!
>I have personally documented over 100 failures due to moisture in
>aircraft systems!!
>
>David A. Douthit
>Manager
>LoCan LLC
>
>Graham Naisbitt wrote:
>
>
>
>>Folks
>>
>>For the record, acrylic coatings have been in use in the aerospace
>>
>>
>industry,
>
>
>>on flight systems of all types and locations, for over 30 years now.
>>
>>
>We have
>
>
>>even seen circuits running with water as they pass the dew point, no
>>
>>
>failure
>
>
>>recorded.
>>
>>Their chemical resistance is excellent (MIL Spec term) but they do
>>
>>
>have
>
>
>>selective solvent resistance including moisture (remember that water
>>
>>
>is a
>
>
>>solvent) but can easily be removed using most chlorinated or
>>
>>
>fluorinated
>
>
>>solvents.
>>
>>Urethanes will provide better solvent resistance hence they are more
>>difficult to rework / repair.
>>
>>I was once asked: "does you coating survive exposure to "Skydrol", an
>>aviation hydraulic fluid that should NEVER be drunk!!! (Worse than
>>
>>
>Mountain
>
>
>>Dew I am told!)
>>
>>I asked why, and was informed that it was: "in case the circuit gets
>>
>>
>sprayed
>
>
>>by this stuff whilst in flight."
>>
>>My reaction: "This is a circuit in the cockpit of the aircraft, and
>>
>>
>you are
>
>
>>worried about the effects of a hydraulic leak of Skydrol spraying onto
>>
>>
>a
>
>
>>circuit in a sealed housing - why? There ain't no pilot still in
>>
>>
>there...."
>
>
>>"Er, good point. Hadn't really thought that one through...."
>>
>>They used an acrylic!
>>
>>--
>>Regards Graham Naisbitt
>>
>>[log in to unmask]
>>
>>Cell: 079 6858 2121
>>Office: +44 (0)1252 813706
>>
>>Concoat Limited - Engineering Reliability in Electronics
>>
>>NEW Additional Web address: www.conformalcoating.com
>>
>>www.concoat.co.uk www.concoatsystems.com
>>
>>
>>
>>
>>
>>
>>>Hans, All,
>>>
>>>We use both AR and UR in different applications on Class 3. It
>>>
>>>
>really
>
>
>>>depends on your end use and fitness for use, and in particular the
>>>
>>>
>unit or
>
>
>>>requirement specification you are trying to meet. AR is easier to
>>>
>>>
>rework
>
>
>>>etc, bit that also means it can be less effective when it comes to
>>>protection... tougher coating = generally harder to remove.
>>>
>>>If you have a requirement to protect against aircraft fluids etc,
>>>
>>>
>then AR
>
>
>>>will be a poorer choice. If you want to use it in a telephone box, AR
>>>
>>>
>will
>
>
>>>do a great job. AR does however have better moisture properties than
>>>
>>>
>UR.
>
>
>>>None of course are truly hermetic, but they will do well in a
>>>condensation/evaporation scenario.
>>>
>>>AR may be good for rework (if/when it is needed), but it also needs
>>>
>>>
>to be
>
>
>>>good for service, which may be decades.
>>>
>>>Clive ff.
>>>
>>>
>>>
>>>-----Original Message-----
>>>From: Hinners Hans M Civ WRALC/LUGE
>>>
>>>
>[mailto:[log in to unmask]]
>
>
>>>Sent: 09 March 2004 15:00
>>>To: [log in to unmask]
>>>Subject: Re: [TN] Pros and Cons on Type UR vice Type AR Conformal
>>>Coating
>>>
>>>
>>> *** WARNING ***
>>>
>>>This mail has originated outside your organization,
>>>either from an external partner or the Global Internet.
>>> Keep this in mind if you answer this message.
>>>
>>>I've approved switching from UR to AR at the request of the repair
>>>
>>>
>line. The
>
>
>>>assemblies are IPC-A-610 Class 3, on aircraft,crew accessible cargo
>>>
>>>
>area.
>
>
>>>Hans
>>>
>>>~~~~~~~~~~~~~~~
>>>Hans M Hinners
>>>AC-130 Systems Engineer
>>>
>>>WR-ALC/LUGE
>>>Special Operations Forces / Combat Search & Rescue
>>>System Program Office
>>>226 Cochran St
>>>Robins AFB, GA 31098
>>>DSN Prefix: 468
>>>Com: (478) 926 - 5847
>>>Fax: (478) 926 - 4911
>>>mailto:[log in to unmask]
>>>
>>>
>>>-----Original Message-----
>>>From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
>>>Sent: Tuesday, March 09, 2004 8:15 AM
>>>To: [log in to unmask]
>>>Subject: Re: [TN] Pros and Cons on Type UR vice Type AR Conformal
>>>
>>>
>Coating
>
>
>>> One thing to know is that AR can be coated to patch UR but not
>>>
>>>
>the
>
>
>>>other way around. Patching AR with UR creates a mix that takes too
>>>
>>>
>long to
>
>
>>>dry, stays soft. So to switch from UR to AR is recommended.
>>> Regards,
>>> Ramon
>>>
>>>
>>>
>>>
>>>
>>>>-----Original Message-----
>>>>From: - Bogert [SMTP:[log in to unmask]]
>>>>Sent: Monday, March 08, 2004 4:55 PM
>>>>To: [log in to unmask]
>>>>Subject: [TN] Pros and Cons on Type UR vice Type AR Conformal
>>>>
>>>>
>Coating
>
>
>>>>As a Class 3 customer we are considering mandating transition from
>>>>Type UR
>>>>
>>>>
>>>>
>>>>
>>>to Type AR conformal coating, to make it easier to replace failed
>>>components. Are there any Class 3 users of Type AR coating? What is
>>>
>>>
>track
>
>
>>>record for using Type AR? Are there any compatibility issues with
>>>
>>>
>certain
>
>
>>>types of solder masks? Any input you have would help. Another
>>>
>>>
>advantage
>
>
>>>with AR would be for returned equipment. The OEM could remove all
>>>
>>>
>the
>
>
>>>coating from an assembly using alcohol, then perform
>>>
>>>
>>>
>>>
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>>>>Search the archives of previous posts at:
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>>>>
>>>>
>>>>
>>>>
>>>for additional information, or contact Keach Sasamori at
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>
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>>>>
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