RE: solid insulation vs coating

[email protected] Wed, 11 Aug 2004 10:30:20 -0400

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Rich,

Take a look at IEC 60664-3: 2003-02 "Insulation coordination for
equipment with in low-voltage systems - Part 3: Use of coating, potting
or moulding for protection against pollution."

This standard covers the topic very well and is referenced by IEC
61010-1: 2nd Ed. 2001-02.  I expect (or hope) more of the modern
revisions to other safety standards will begin to reference this
document as well.

You will find that there are two types of coatings recognized by IEC
60664-3.  One type simply provides pollution degree reduction and the
other is considered solid insulation.  The type that is solid insulation
has a rigorous set of tests it must pass.  So far, in my company, we
have not achieved the second type because it is very difficult to
provide a quality control check that assures proper application of the
coating.  Also, repair of PWBs becomes an issue.

-doug



From: [email protected]
[mailto:[email protected]] On Behalf Of Rich Nute
Sent: Tuesday, August 10, 2004 1:13 PM
To: Ralph McDiarmid
Cc: [email protected]
Subject: Re: solid insulation vs coating

http://www.ieee-pses.org/symposium
          http://www.emc2004.org/


Hi Ralph:

>   Why is conformal coating not considering solid insulation, but
rather only
>   providing a reduction in pollution degree?  If the coating were of
>   sufficient thickness and free of trapped air, then how can creepage
or
>   clearance exist under it?

Conformal coating MUST be considered solid insulation
because it is a solid material between two conductors,
and it must be an electrical insulator.

A conformal coating necessarily prevents pollution
>from accumulating on the conductors beneath the
coating.  As such, a conformal coating lengthens
the path between two conductors on which pollution
can accumulate.  The pollution accumulates on the
conformal coating instead of directly on the PCB.

Clearances and creepages relate to air insulation.
When two solid insulations butt up to each other,
clearances and creepages continue to exist at the
joint.  A coating applied to a PWB does not
automatically mean the joint is free of air and
therefore would have no clearance or creepage.

To be considered a continuous solid insulation,
the construction of the joint between the two
solid insulations must exclude air throughout the
distance between the conductors.  So, there are
two factors, one being the construction of the
joint, and two being the consistency of the
construction of the joint throughout the joint.

The construction of the joint must include a
cement or similar mechanical adherence such as
an amalgam.  This would be evaluated by means of
a peel strength test.

Then, the cement or amalgam must be uniform
throughout the area of the joint.  This is much
more difficult to evaluate.

So, a simple coating does not necessarily conform
to the construtional requirements of a contiguous
solid insulation.  Indeed, unless proven as
contiguous, conformal coatings are treated as
having air between the two solid insulations.

>   Some standards even consider encapsulation or potting for providing
a
>   reduction in pollution degree, even where it provides void-free
moulding
>   over the part.  Isn't encapsulation the same as the moulding on a
plastic
>   integrated circuit?

Yes and no.  Encapsulation or potting is
evaluated the same way... the joints must exlude
air and be contiguous.  Not all encapsulations
or potting processes exclude air in the joints
between two solid insulations.  The joint must
be cemented, or the two materials must form an
amalgam or equivalent at the joint.


Best regards,
Rich



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This message is from the IEEE Product Safety Engineering Society
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Title: RE: solid insulation vs coating

http://www.ieee-pses.org/symposium
http://www.emc2004.org/
--------------------------------------------------

Rich,

Take a look at IEC 60664-3: 2003-02 "Insulation coordination for
equipment with in low-voltage systems - Part 3: Use of coating, potting
or moulding for protection against pollution."

This standard covers the topic very well and is referenced by IEC
61010-1: 2nd Ed. 2001-02. I expect (or hope) more of the modern
revisions to other safety standards will begin to reference this
document as well.

You will find that there are two types of coatings recognized by IEC
60664-3. One type simply provides pollution degree reduction and the
other is considered solid insulation. The type that is solid insulation
has a rigorous set of tests it must pass. So far, in my company, we
have not achieved the second type because it is very difficult to
provide a quality control check that assures proper application of the
coating. Also, repair of PWBs becomes an issue.

-doug

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Rich Nute
Sent: Tuesday, August 10, 2004 1:13 PM
To: Ralph McDiarmid
Cc: [email protected]
Subject: Re: solid insulation vs coating

http://www.ieee-pses.org/symposium
http://www.emc2004.org/
--------------------------------------------------

Hi Ralph:

>   Why is conformal coating not considering solid insulation, but
rather only
>   providing a reduction in pollution degree? If the coating were of
>   sufficient thickness and free of trapped air, then how can creepage
or
>   clearance exist under it?

Conformal coating MUST be considered solid insulation
because it is a solid material between two conductors,
and it must be an electrical insulator.

A conformal coating necessarily prevents pollution
from accumulating on the conductors beneath the
coating. As such, a conformal coating lengthens
the path between two conductors on which pollution
can accumulate. The pollution accumulates on the
conformal coating instead of directly on the PCB.

Clearances and creepages relate to air insulation.
When two solid insulations butt up to each other,
clearances and creepages continue to exist at the
joint. A coating applied to a PWB does not
automatically mean the joint is free of air and
therefore would have no clearance or creepage.

To be considered a continuous solid insulation,
the construction of the joint between the two
solid insulations must exclude air throughout the
distance between the conductors. So, there are
two factors, one being the construction of the
joint, and two being the consistency of the
construction of the joint throughout the joint.

The construction of the joint must include a
cement or similar mechanical adherence such as
an amalgam. This would be evaluated by means of
a peel strength test.

Then, the cement or amalgam must be uniform
throughout the area of the joint. This is much
more difficult to evaluate.

So, a simple coating does not necessarily conform
to the construtional requirements of a contiguous
solid insulation. Indeed, unless proven as
contiguous, conformal coatings are treated as
having air between the two solid insulations.

>   Some standards even consider encapsulation or potting for providing
a
>   reduction in pollution degree, even where it provides void-free
moulding
>   over the part. Isn't encapsulation the same as the moulding on a
plastic
>   integrated circuit?

Yes and no. Encapsulation or potting is
evaluated the same way... the joints must exlude
air and be contiguous. Not all encapsulations
or potting processes exclude air in the joints
between two solid insulations. The joint must
be cemented, or the two materials must form an
amalgam or equivalent at the joint.

Best regards,
Rich

-------------------------------------------

This message is from the IEEE Product Safety Engineering Society
emc-pstc discussion list.

IEEE PSES Main Website: http://www.ieee-pses.org/

To post a message send your e-mail to [email protected]

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List rules: http://www.ieee-pses.org/listrules.html

For help, send mail to the list administrators:

Ron Pickard: [email protected]
Dave Heald: [email protected]

For policy questions, send mail to:

Richard Nute: [email protected]
Jim Bacher: [email protected]