[SI-LIST] Re: copper-to-copper separation
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 2 Aug 2002 17:21:48 -0400
> According to my notes from Lee Ritchey's class, the dielectric
breakdown
> voltage, DBV, of standard FR-4 is 1100 Volts/mil.
I'm not an expert; but I believe FR4 is like a sponge and is easily
affected by humidity, considerably reducing its breakdown potential
especially on outer layers.
One reference I have, suggests 600 V/mil for inner layers, but only 30
V/mil for outer layers. These numbers supposedly have a 2:1 safety
factor built-in.
Even altitude plays a factor.
Somewhere long ago I saw a table of breakdown voltages which didn't make
much sense to me, because among other things the data was very, very
nonlinear, without explanation.
Keep in mind that the copper on most boards has a smooth side and a
rough side (for better adherence to the dielectric layer), and the rough
side has these little dendrite-like fingers that may put adjacent pieces
of copper closer together than the nominal spacing would have them. I
don't know if this is enough to make a difference. Since sharp points
tend to concentrate charge, they could be the most likely places for
breakdown to occur. But like I say, I'm not an expert on this.
Regards,
Andy
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