Conformal coatings for PCBs have been mentioned here in passing but a look
back through recent history doesn't show any detailed discussions.

I'd be interested in knowing what people use, why, how good it is (pros and
cons) and what it costs.

I've thrown in a few thoughts of my own below and a number of references as
'my contribution'. Hopefully others will add usefully.
Mention of Dow Corning below more than any other single firm does not imply
my endorsement or recommendation of their products (for what such would be
worth :-) ) but I seem to  encounter their products in a number of differen=
t
areas than most others and they are generally competent and well represente=
d
internationally..

Conformal coatings provide some or all of: protection against moisture or
environmental contaminants (eg "tropicalisation"),  shock and stress
reduction, component ID obscuring, ... .  I'm mainly interested in the firs=
t
area.



                 Russell McMahon


________

MANY  potential coatings exist - acrylics, silicones, epoxy resins,
 urethanes, ...
Curing may be air cure, thermal, UV and/or visible light cure, 2 pot or som=
e
combination.
Application may be spray, brush, dip, or vapour deposition.

eg a UV cure product may have a secondary atmospheric cure mode to allow
material which is shielded from direct UV access (eg under components) to
cure.

Curing times range from near instantaneous for eg light cured, a few minute=
s
for high temperature cured or extreme cases of most other types, through
hours (or even days by design).

Secondary aspects may include such as organic solvent content, generation o=
f
methanol as a curing byproduct (5% to 10% typical where applicable),
generation of toxic fumes where solder through is attempted inappropriately
and more.

Arguably the "best" CC (Conformal Coating)  is Parylene, which is a family
of vacuum deposited thin film coatings based on decomposed xylene  xylxylen=
e
products .
Performance in a range of areas is superb but the processes involved make i=
t
somewhat expensive in terms of processing times and required equipment.
Coating thicknesses of a few nanometres can be effective (!!!).

    http://en.wikipedia.org/wiki/Parylene
     http://www.conformal-coating.com/parylene_coating.htm

There are many excellent proprietary coatings.
One example which I consider a good example of a top class product that
requires a relatively low setup effort and cost (but which still has its
issues) and which
is as good as most is Dow Corning 1-2577, which has a high Toluene content,
making it less desirable than some from health &safety aspects, also
available in "low VOC" (low Volatile Organic Compound versions). 1-2577 and
friends forms an essentially environmentally impervious coating when applie=
d
as thin as 0.1mm (100 microns). Application can be by dip, spray or brush.
Cost per volume is high compared to some more utilitarian alternatives but
the low coating thickness means costs per PCB are usually acceptable.
Coating is transparent and contains a UV tracer that allows inspection for
defects using a UV light. The coating is solderable through and repairable
by eg brush overcoating areas where eg soldering has been carried out.

Towards the other extreme is "polyurethane varnish" having the main
advantages of being low cost, readily available, low cost, easy enough to
apply and low cost.

I've used both.
_____________________

Basic but good Dow Corning Tutorial


http://www.dowcorning.com/content/etronics/etronicscoat/etronics_cc_tutoria=
l.asp?DCWS=3DElectronics&DCWSS=3D

NASA how to .. 60 pages
NASA-STD-8739.1 August 1999
     http://www.goes-r.gov/procurement/flight_documents/NASA-STD-8739-1.pdf

Dow Corning CC product finder.
Note that 2620 provides the same finished coating material as 2577


http://www.dowcorning.com/applications/search/products/default.aspx?Ne=3D42=
94965941+4294963250&N=3D4294965929+4294956824&bhcp=3D1

Dow Corning CC product comparions
    http://www1.dowcorning.com/DataFiles/090007c8801bfd2b.pdf

  3-1744
http://www.dowcorning.com/applications/search/default.aspx?R=3D706EN&DCCSF=
=3D1538EN&DCCSF=3D21EN

Dymax light curing
Overviews and links
http://www.dymax.com/products/electronic/conformal_coatings/
         Selector guide
http://www.dymax.com/pdf/literature/lit204_conformal_coating_selector_guide=
..pdf
          Rework
http://www.dymax.com/pdf/literature/lit078_rework_removal_of_uv_conformal_c=
oating_tb.pdf
           Magazine article / case studies / comparisons
http://www.dymax.com/pdf/articles/lit034_conformal_coating_made_easy.pdf


MG Chemicals CCs    http://www.mgchemicals.com/products/protcoat.html

Electrolube CCs   http://www.electrolube.com/docs/conformal.asp

Humiseal   http://www.humiseal.com/

Diamond Systems   http://www.diamondsystems.com/services/conformalcoating

Masterbond http://www.masterbond.com/produse/produse_cc.html
  Selector guide - 2 part  http://www.masterbond.com/sg/masterbond_ccsg.pdf

_____

Wikipedia       http://en.wikipedia.org/wiki/Conformal_coating

Simple tutorial
http://www.electronics-project-design.com/ConformalCoating.html
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