Good morning Barbara,
You have been given some pretty good comments on conformal coating so far.
As suggested, IPC-HDBK-830 is a good reference to have if you wish to
become your local coating guru. Rockwell uses all five coating types
(acrylic AR, urethane UR, Silicone SR, Epoxy ER, and Parylene XY) on
products here. Which one you select depends on the nature of the end use
environment, the length of field service, reworkability needs, and the
consequences of failure. Here is the short primer that I use to give our
managers an overview.
1. Polyparaxyxylene (Type XY), trade name Parylene. We use Parylene C. A
batch process involving gas-tight masking on areas to be kept free of
coating. Thicknesses of 0.1 mils to 0.5 mils. Coats all surfaces easily
and uniformly. Bulletproof against the environment. Very difficult to
rework in house or in the field. Can burn through with soldering irons or
can media blast to remove. Of the five coating types, the most expensive
and the less amenable to manufacturing. Specialty Coating Services in
Indianapolis wrote the book on Parylene coating.
2. Epoxy (Type ER). Typically a two part system with set pot life after
mixing. Can be spray or hand applied. Used more to repair circuit board
surfaces than as a conformal coating. Thicknesses of 1-5 mils typical.
Highly resistant to end use environment. Difficult to work with in large
volumes without waste. Very difficult to rework as you often can't tell
where the epoxy coating ends and the epoxy laminate starts. Not as
expensive as Parylene, but usually more expensive than one part systems.
As a rigid coating, may crack in high thermal transition rates.
3. Silicones (Type SR). Typically a one part system with long working
life. Can be spray or hand applied. Used when the end-use environment is
chemically hostile, very hot, or liquid water. It's chemical resistance
makes it difficult to rework. It's rubbery nature makes it difficult to
media blast. Silicone coatings are good against liquid water, but not
water vapor. Sulfur based vapors also tend to go right through this
coating. Silicones do not play well with other materials and will outgas
onto nearby surfaces. Expense is similar to epoxy but more amenable to
production automation, but with long cure times. Very flexible coating.
We use Dow Corning 1-2577 on about 2-5% of product.
4. Urethanes (Type UR). Typically one part systems in a solvent carrier.
Can be sprayed or hand applied. More economical and works well in
production. Does have VOC emission considerations. Good barrier against
the environment, but with some chemical susceptibilities. Tends to darken
with time and can make part markings illegible with time. Can be
chemically stripped, but requires aggressive solvents. As a rigid coating,
may crack in high thermal transition rates. We use Conap (now Cytec) CE
1155 on a few legacy boards.
5. Acrylics (Type AR). One part systems in a solvent carrier, which may
have VOC emission considerations. Can be sprayed or hand applied. Good
for protection against humidity and liquid water. Has the least chemical
resistance and least heat resistant of the 5 coatings. Tends to be more
flexible in thermal transitions. Very reworkable locally and in the field.
Most economical of the 5 types. We have used Humiseal 1B31 for 25 years
with good success. This goes on 80% of our products.
Now, before people start disputing this opinion or that opinion, this is
intended to be a GENERAL overview, so don't bust my chops if your pet
coating does not appear in the best light.
Barbara, I hope this helps. If you would like more guidance, contact me, I
would be happy to help.
Off to find some Diet Dew................
Doug Pauls
Barbara Burcham
<bburcham@FAIRFIE
LD.COM> To
Sent by: TechNet [log in to unmask]
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Subject
05/16/2005 04:55 [TN] Conformal Coating of Assembled
PM Boards
Please respond to
TechNet E-Mail
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<[log in to unmask]>
; Please respond
to
Barbara Burcham
<bburcham@FAIRFIE
LD.COM>
Well, my company now has an application requiring conformal coating on
some small assembled boards. We need to mask off 3 connectors and coat
everything else.
I have some information on a product called Parylene applied by Parylene
Coating Services, Inc. in Katy, Texas.
We did a small lot with a simple spray-on process, but that is not the
way to do production runs, of course.
What would you recommend?
Thanks,
Barbara
Barbara J. Burcham, C.I.D.
Fairfield Industries, Inc
281-275-7687
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