RE: Potted components

[email protected] Fri, 07 Jan 2005 18:02:58 -0400

This is a multi-part message in MIME format.

Brian,

You are absolutely correct, once you have a reliable encapsulant then
the spacings problem becomes one of solid insulation or distance through
insulation.  The trick is, as you mentioned, is the reliable adherence
to the PWB itself.


As it turns out a "pancake void" along a surface is the worst kind and
if you are dealing with any appreciable voltage at all, then you may
have a significant problem corona in the circuit.  I've seen problems
manifest even after 6 months of operation.  The test is the same for
solid insulation, Partial Discharge and/or Impulse testing.

If PD becomes a problem, a possible fix is to specify a bead-blast (no
sand-blast please) of the surface prior to mounting parts and no solder
mask.  This removes any release compounds or contaminants that may still
be present on the surface of the laminate.  Naturally if you fully
encapsulate a PCB then there is no need for component designators or
silkscreen.

There is also the issue of doing repairs and restoring the encapsulant
to its previous capability.

-doug



From: [email protected]
[mailto:[email protected]] On Behalf Of Brian O'Connell
Sent: Wednesday, January 05, 2005 9:53 AM
To: [email protected]
Subject: Re: Potted components

For UL I.T., the materials must be approved for the operating
conditions,
IP, and WV.

IEC 60950-1, 2.10.8 Spacings filled by insulating compound
Where distances between conductive parts are filled with insulating
compound, including where insulation is reliably cemented together with
insulating compound, so that CLEARANCES and CREEPAGE DISTANCES do not
exist,
only the requirements for distance through insulation of 2.10.5.1 apply.

luck,
Brian

> -----Original Message-----
> From: Doug Beckwith [mailto:[email protected]]
> Sent: Wednesday, January 05, 2005 8:44 AM
> To: [email protected]
> Subject: Potted components
>
>
> Hi Everyone,
> A happy new year to you all.
>
> Now for my question. I am looking at a unit that has mains and
> SELV circuits that is encapsulated (potted). Am I correct in
> assuming that creepage and clearance distances on the PCB for a
> potted unit become the same as that for distance through
> insulation, as when the traces are buried in a multilayer PCB?
>
> Regards
>
> Doug
>
>
> ______________________________________________________________________
> Post your free ad now! http://personals.yahoo.ca
>
> ----------------------------------------------------------------
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This message is from the IEEE Product Safety Engineering Society
emc-pstc discussion list.    Website:  http://www.ieee-pses.org/

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

Instructions:  http://listserv.ieee.org/listserv/request/user-guide.html

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

For help, send mail to the list administrators:

     Scott Douglas             [email protected]
     Mike Mcantwell            [email protected]

For policy questions, send mail to:

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

All emc-pstc postings are archived and searchable on the web at:

    http://www.ieeecommunities.org/emc-pstc

Title: RE: Potted components

Brian,

You are absolutely correct, once you have a reliable encapsulant then
the spacings problem becomes one of solid insulation or distance through
insulation. The trick is, as you mentioned, is the reliable adherence
to the PWB itself.

As it turns out a "pancake void" along a surface is the worst kind and
if you are dealing with any appreciable voltage at all, then you may
have a significant problem corona in the circuit. I've seen problems
manifest even after 6 months of operation. The test is the same for
solid insulation, Partial Discharge and/or Impulse testing.

If PD becomes a problem, a possible fix is to specify a bead-blast (no
sand-blast please) of the surface prior to mounting parts and no solder
mask. This removes any release compounds or contaminants that may still
be present on the surface of the laminate. Naturally if you fully
encapsulate a PCB then there is no need for component designators or
silkscreen.

There is also the issue of doing repairs and restoring the encapsulant
to its previous capability.

-doug

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Brian O'Connell
Sent: Wednesday, January 05, 2005 9:53 AM
To: [email protected]
Subject: Re: Potted components

For UL I.T., the materials must be approved for the operating
conditions,
IP, and WV.

IEC 60950-1, 2.10.8 Spacings filled by insulating compound
Where distances between conductive parts are filled with insulating
compound, including where insulation is reliably cemented together with
insulating compound, so that CLEARANCES and CREEPAGE DISTANCES do not
exist,
only the requirements for distance through insulation of 2.10.5.1 apply.

luck,
Brian

> -----Original Message-----
> From: Doug Beckwith [mailto:[email protected]]
> Sent: Wednesday, January 05, 2005 8:44 AM
> To: [email protected]
> Subject: Potted components
>
>
> Hi Everyone,
> A happy new year to you all.
>
> Now for my question. I am looking at a unit that has mains and
> SELV circuits that is encapsulated (potted). Am I correct in
> assuming that creepage and clearance distances on the PCB for a
> potted unit become the same as that for distance through
> insulation, as when the traces are buried in a multilayer PCB?
>
> Regards
>
> Doug
>
>
> ______________________________________________________________________
> Post your free ad now! http://personals.yahoo.ca
>
> ----------------------------------------------------------------
> This message is from the IEEE Product Safety Engineering Society
> emc-pstc discussion list.    Website: http://www.ieee-pses.org/
>
> To post a message to the list, send your e-mail to [email protected]
>
> Instructions:
> http://listserv.ieee.org/listserv/request/user-guide.html
>
> List rules: http://www.ieee-pses.org/listrules.html
>
> For help, send mail to the list administrators:
>
>      Scott Douglas             [email protected]
>      Mike Mcantwell            [email protected]
>
> For policy questions, send mail to:
>
>      Richard Nute:           [email protected]
>      Jim Bacher:             [email protected]
>
> All emc-pstc postings are archived and searchable on the web at:
>
>     http://www.ieeecommunities.org/emc-pstc
>

----------------------------------------------------------------
This message is from the IEEE Product Safety Engineering Society
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To post a message to the list, send your e-mail to [email protected]

Instructions: http://listserv.ieee.org/listserv/request/user-guide.html

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

For help, send mail to the list administrators:

Scott Douglas [email protected]
Mike Mcantwell [email protected]

For policy questions, send mail to:

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

All emc-pstc postings are archived and searchable on the web at:

http://www.ieeecommunities.org/emc-pstc