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Breakdown on PCB versus air

  • Thread starter Klaus Kragelund
  • Start date
K

Klaus Kragelund

Jan 1, 1970
0
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?

Thank

Klaus
 
R

Robert Baer

Jan 1, 1970
0
Klaus said:
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?

Thank

Klaus

Look at the following PCB material:
http://oil4lessllc.com/HTPCB.pdf
 
L

legg

Jan 1, 1970
0
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
 
L

lemonjuice

Jan 1, 1970
0
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?

Thank

Klaus
From what I know dielectric breakdown in solids unlike in liquids
usually brings about a permanent conduction channel within the bulk
of the PCB. So you might have a permanent problem. If its a surface
problem apart from cleaning well the surface try using a water
displacement solution then add silicon greases to reduce the surface
tracking . Breakdown voltages are considerably lower for dc then ac
voltages. So if you can do anything about that try it out.

Cutting out a slot may bring in new problems like defects on the board
which make it more vulnerable to breakdown coltages. The pointed areas
from cutting also favor intense electrical fields.
Thermal instability also contributes. Can you cool the board ?
 
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