Roger,
You know with a title like that, we at Concoat are getting a complex....
Anyhow, you could help by advising which polyurethane coating you are using.
There are many different varieties that certainly have different curing
characteristics.
First off, please refer to IPC-CC-830. In it you will find the specified
thickness that should be applied to a flat unencumbered substrate that is
0.002inches +/- 0.001 inches for acrylics and polyurethane's, and 0.003 +/-
0.002 inches for silicones. We generally figure that the coating will be x3
or x4 around the component shoulders.
Contrary to some thoughts, thicker is not better. If you apply the coating
too thick, you may well cause more problems than you solve. These include
shrinking, cracking or crazing and solvent entrapment that manifests itself
as gazillions of bubbles.
When dip coating, which will give you the best coverage under, as well as
over the components, one dip cycle should be all that is necessary. However,
that will depend upon the viscosity of the product you are applying.
Generally we would expect the product to be around 200cps and 40% solids. If
yours is less than these values then 2 to 3 coats may be required to achieve
the desired thickness. Consider also that applying multiple dip coats will
also look awful because the second coat will partially eat into the first
coat.
Hope this helps, let us know if you need more
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 Roger M. Stoops
Sent: Friday, March 02, 2001 04:29
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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