Re: Breakdown on PCB versus air
From: legg (legg_at_nospam.magma.ca)
Date: 12/03/04
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Date: Fri, 03 Dec 2004 11:42:23 GMT
On 3 Dec 2004 01:13:35 -0800, klauskvik@hotmail.com (Klaus Kragelund)
wrote:
>Hi
>
>I'm having a problem with a surge test causing a spark on a PCB I
>testing. The problem is that the voltage from Line/Neutral to ground
>exceeds the dielectric breakdown of the PCB leading to a spark
>
>We cannot increase the distance due to size constraints, but we can
>add a cut-out slot in the PCB to increase the creepage distance.
>
>Normally about 2kV/mm is the breakdown voltage for air. And the
>breakdown for a FR4 board lies at approx. V/mm
>
>So adding a slot would help us. But ofcourse one needs to be aware of
>polution/humidity of the air and polution of the PCB
>
>I'm looking for numbers of the breakdownvoltage for both FR4 and air
>versus polutions degrees and not just the standard numbers, but actual
>calculations/tests
>
>I've searched a lot on the web and found next to nothing on this
>subject. I have ordered the www.ipc.org IPC-D-330, but will take days
>to arrive.....
>
>So does anyone have a clue about this?
>
If breakdown is across the surface, then the material strength is
irrelevant, and cleanliness/pollution/environment is critical. If you
are providing 2.8mm as a standard layout guideline, this should meet
most production requirements, providing that the boards meet the CTR
of the safety documents, are cleaned and nobody leaves their
fingerprints behind.
Check with your board vendor about the CTR for the process that you
are buying - there is no guarantee that it meets the standards'
assumptions, unless specifically stated by the vendor.
In dirty environments, slots will assist in avoiding accumulation of
crud, but it's only a matter of time.
RL
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