A useful guideline is to look to the various standards for conductor spacing
to yeild a safe voltage standoff. A reasonable minimum of 8mm for 2500V 
standoff between conductors on the same side of the board will give you
a good chance to avoid "tracking" or arcing along the surface due to
normal contamination. Moisture absorption has a significant effect, too;
even properly cured FR4 will absorb .5% water, which can elevate leakage
currents and thus reduce both puncture and tracking voltage.

Often slots are added to the board between conductors to reduce tracking 
effects, for instance between A and K of a diode multiplier string, or to
isolate the input and output pins of Optoisolators. 

I've seen many failures due to tracking, but none due to puncture.

Brian

At 02:48 PM 3/18/02 -0800, you wrote:
>Sean,
>       the standard answer from IPC for general purpose (Epoxy E-glass) FR4
>(is there such a thing today?)is 39,400 Volts/mm (approx. 62,500V at
>0.062"). This is derived from a test using 0.125mm and they warn that it
>should not be considered linear for high voltage applications with minimal
>dielectric spacing, i.e. less then 0.09mm (0.0035"). Don't think that counts
>for most general designs, that would be 3.5mil laminate.
>
>       Now a word of warning. This would be through the dielectric, not
>around the edge of the material, through air, nor over the surface(s).
>Considering most applications and the means for contamination of these
>surfaces, I would be checking the surface or around the edge requirements as
>well. Especially is there is copper near the board edges.
>
>Sincerely,
>Brad Velander.
>
>Lead PCB Designer
>Norsat International Inc.
>Microwave Products
>Tel   (604) 292-9089 (direct line)
>Fax  (604) 292-9010
>email: [EMAIL PROTECTED]
>http://www.norsat.com
>
>See us at Booth S8155 at NAB 2002 in Las Vegas April 8 - 11.
>
>
>-----Original Message-----
>From: Sean James [mailto:[EMAIL PROTECTED]]
>Sent: Monday, March 18, 2002 2:15 PM
>To: Protel EDA Forum
>Subject: [PEDA] Voltage standoff of FR4
>
>
>What is the voltage standoff (front to back) for .062" FR4?
>
>Sean James
>PCB Designer
>Telecast Fiber Systems
>102 Grove Street
>Worcester, MA 01603
>TEL 508-754-4858 x33
>FAX 413-541-6170
> 

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