Good evening folks,
I noticed this posting and also received a prompt from Michael Simms at
Trace Labs.
Regarding the use of fumed silica, my observation is as follows:
We do actually produce a "gel" version of some coating materials to help
those who require some additional but minimal mechanical support. Frankly it
is not the ideal solution - please pardon the pun - but we have generally
recommended that it be applied before the actual conformal coating is
applied.
The fumed silica effectively "expands" or "thickens" the material. However,
as the coating usually contains a solvent, this can lead to the formation of
a "honeycomb" type structure that is prone to collapse as the solvent leaves
the coating during curing. This is why we have suggested that the conformal
coating be applied after the fumed silica product.
There is also a technique of using floccules of glass - can he get away with
saying that? - No, floccules! - OK. This was or indeed is, being used to
"fill" a coating to provide a "harder / more durable" finish for some
extreme aerospace applications. It does work and has been in use to my
knowledge for some 15 years.
I would suggest that this be used as a last resort. Staking compounds would
be the preferred choice, even thought these will generally be 2 component
and non-qualified materials - in other words, self qualified.
Hope this helps.
By the way, I will not share with you all, the message from Doug Pauls about
Concoat Thickness and BGA Underfills, Pumping Concoat as suggested by
Ingemar - boy the trolls will get you one day Ingemar!
Regards, Graham Naisbitt
[log in to unmask]
www.concoat.co.uk
Concoat Limited www.concoat.co.uk
<http://www.concoat.co.uk>
Alasan House, Albany Park
CAMBERLEY GU16 7PH UK
Phone: +44 (0)1276 691100
Fax: +44 (0)1276 691227
Mobile: +44 (0)79 6858 2121
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Phil Crepeau
Sent: Friday, March 02, 2001 06:56
To: [log in to unmask]
Subject: Re: [TN] Maximum Thickness of Concoat
hi,
you might consider using a polyurethane coating that has been around for
decades and is still used in aerospace applications. it is very versatile.
cab-o-sil can be added to it to thicken it up which means you could probably
get by with just one application. it is used on assemblies that have glass
diodes with no problem that i'm aware of. the material is solithane 113
(http://www.uniroyalchemical.com/solit113.htm).
phil
-----Original Message-----
From: Roger M. Stoops [mailto:[log in to unmask]]
Sent: Friday, March 02, 2001 8:29 AM
To: [log in to unmask]
Subject: [TN] Maximum Thickness of Concoat
Y'all,
Question engineer asked me today, how thick can polyurethane coating be
applied? We apply three coatings with the following procedure:
1) dip 2) dry for 2-3 hours to eliminate most solvent vapors 3) bake for
4 hours @ 60 deg C 4) repeat until 3 coats. Third coat is also baked.
Question to my able colleagues: Anybody know of any long-term problems
with this method? Any test results?
We need to do this to cover all sharp edges for electrical isolation during
operation in potentially high moisture environment. Specifically, a
housing that can fill with condensation before draining away, and we can't
seal enclosure. Considered paralene, but it costs more to coat the board
than to build it...
TIA
Roger M. Stoops, C.I.D., PCB Designer
Trimble
Engineering and Construction Division
5475 Kellenburger Rd.
Dayton, OH 45424-1099 USA
Ph: +01 937.233.8921 or +01 937.233.4574 ext 288
Fax: +01 937.233.7511
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