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December 2006

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Wed, 6 Dec 2006 08:08:02 -0600
Content-Type:
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Correct.

Think of your electronic assembly inside of a plastic bag. You vacuum
seal the bag. No matter how much vacuum you apply, there are air
pockets. Immersion of the sealed bag with the board into water or other
gas/liquid for a short term results in no gas/liquid making direct
contact with the assembly inside, assuming there are no holes in the
bag. However, some moisture can penetrate through the bag over time when
exposed to air. Upon reaching equilibrium, which all matter tends to do,
there is an equal amount of moisture in the small air pockets inside of
the bag as there is in the air around the bag. With temperature changes,
there can be some small amount of condensation. Due to the inability of
the condensation to evaporate quickly through the bag, the moisture is
now trapped in liquid form inside the bag, but in very small amounts.

Without the bag, any liquids make direct catastrophical contact.
With the bag, there is no direct catastrophical contact, but there can
be small amounts of moisture, over time.

Conformal coating is the equivalent of the bag.

The idea here is that you are preventing the catastrophical contact, and
attempting to prevent as much as possible the amount of condensation
that can be formed from available moisture inside of the bag. The
smaller the amount of moisture inside the bag, the greater the
temperature extremes that must be seen to create significant amounts of
condensed liquids. 
Significant = enough to cause electrical performance issues.
Significant threshhold for a power supply or switching terminal is much
higher than for a high-impedance high-frequency RF transmitter/receiver,
as an example.

Sorry if this is kind of simplistic. I am just trying to portray the big
picture.

The primary reason for cleanliness prior to cc is to promote the
adhesive seal of the cc with the surfaces to be coated. But a secondary
reason why cleanliness is critical for conformally coated assemblies:
the presence of flux entrapped with condensation and a power source is
an incubator for all kinds of unhealthy life forms. 

And it is also why conformally coated assemblies should never be totally
immersed in liquids, as part of the process. The bag usually has some
small holes, and as a result some liquid will be absorbed and trapped
inside. 

Conformal coated assemblies are (usually) housed in a box or cabinet
that is sealed from the elements to prevent any direct immersion.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas O. Pauls
Sent: Wednesday, December 06, 2006 7:00 AM
To: [log in to unmask]
Subject: Re: [TN] conformal coatings as water proofing

What you are hearing is that no organic conformal coating makes an
electronic assembly absolutely bulletproof against the environment.  All
of them pass water vapor, and sometimes other gasses, to varying
extents.  It impedes the flow of moisture but does not completely
eliminate moisture.
Important distinction.

Doug Pauls




             Bill Kasprzak
             <[log in to unmask]
             OM>
To
             Sent by: TechNet          [log in to unmask]
             <[log in to unmask]>
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Subject
             12/06/2006 06:56          Re: [TN] conformal coatings as
             AM                        water proofing


             Please respond to
              TechNet E-Mail
                   Forum
             <[log in to unmask]>
             ; Please respond
                    to
               Bill Kasprzak
             <[log in to unmask]
                    OM>






Good morning,

I am surprised that there are such opposing views on whether conformal
coating is effective as a moisture barrier.

My impression has always been that the primary purpose for coating
boards was to act as a moisture and corrosion barrier. It has dielectric
strength as a secondary property.

So, I am I hearing from Brian that conformal coating is useless as a
moisture barrier on PC Boards?

Bill Kasprzak
Moog Inc
Process Engineer, Electronic Assembly




Brian Ellis <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
12/06/2006 01:31 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to Brian Ellis
<[log in to unmask]>


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Subject
Re: [TN] conformal coatings as water proofing






I disagree with your disagreement. CC CANNOT protect against atmospheric
moisture as they are all as porous as a sieve. The best they can do is
to delay the onset of undesirable effects. CCs SHOULD cover everything
except contacts on 2- or 3-D assemblies and, as such, protect against
the effects of condensation. They do not allow the passage of water in
liquid phase, only in vapour phase.

Brian

Edward Mines wrote:
> Doug Pauls wrote than conformal coatings protect against condensation.

> I
disagree.
>
>   Conformal coatings protect against atmospheric moisture. It is giant
leap of faith that many engineers make that they protect against
condensation. When I left the industry 2 years ago there was no
reference to condensation in IPC-CC-830 or MIL-I-46058 or UL746.
>
>   Conformal coatings do not always completely cover all conducting 
> items
on 3 dimensional boards.
>
>   Doug's employer is not the only one that uses a thicker layer of
conformal coating or similar material to protect against moisture.
IPC-CC-830 (and predecessor MIL-I-46058) specify a maximum coating
thickness for conformal coating. I am sure there are reasons for this.
By putting on more coating (a thicker layer) the assembler is out of
compliance with IPC & MIL
>   conformal coating specs.
>
>   If industry wants to use a material to protect PC boards from liquid
water that material should be called a different name (I called the one
we sold at HumiSeal a selective encapsulant). Tests should be developed
to insure that that material does the job for which it was intended; a
QPL should be established.
>
>   IPC, MIL & UL tests for conformal coating all include  accelerated
aging and thermal cycling tests for conformal coatings within a
specified thickness range. Would qualified coatings pass those tests in
thicknesses outside those ranges? Can the user be certain that conformal
coatings cure properly in thicknesses outside those ranges? Might overly
thick coatings crack if subjected to those tests? Or in the field? If a
board fails because the conformal coating was used incorrectly is the
board manufacturer negligent? I am aware of one instance where this last
issue was seriously addressed in the automotive industry.
>
>   Are the tests in MIL-I-46058 (and grandfathered into IPC-CC-830)
realistic? Are they overkill?
>   Is there a reason for the thickness limitation?
>
>   At HumiSeal I heard of several instances each year where customers
tried to put more conformal coating on a board than specified by
MIL-I-46058 and recommended by HumiSeal (it seems like more would be
better). I know for a fact that some conformal coatings do not cure
properly above a certain thickness.
>
>
>   Edward Mines
>
>
>
>
>
>
>
> ---------------------------------
>
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