[SI-LIST] Re: copper-to-copper separation
- From: jeff_latourrette@xxxxxxxxxxx
- To: Andrew.Ingraham@xxxxxx, si-list@xxxxxxxxxxxxx
- Date: Fri, 2 Aug 2002 17:02:26 -0600
Ibrahim:
Reference Data for Radio Engineers (ITT Corp.), beginning of chapter 41 (5th
edition) has some breakdown numbers for air gaps, which would affect breakdown
on the top and bottom of your board. It includes tables of temperature and
pressure (altitude). Humidity and dust also factor in, but they don't describe
their impact. I believe I remember that humidity helps improve breakdown
(really dry air conducts the best). I'd assume dust particles work in the
opposite direction, decreasing breakdown.
The base number for dry, dust-free air at room temp. and sea level is 75 peak
kV/inch or 30 peak kV/cm. They also throw in the limitation of <= 300 MHz. So
sounds like Andy's 30 V/mil (or 30 kV/inch) guideline for outer layers is
de-rated by 2.5X from nominal. Keep in mind the pressure and temp effects on
air breakdown are substantial and could eat up that de-rating, depending on
your application.
I know this started out about FR-4 breakdown, but many times your air gaps will
breakdown long before even thin board layers.
Regards.
-----Original Message-----
From: Ingraham, Andrew [mailto:Andrew.Ingraham@xxxxxx]
Sent: Friday, August 02, 2002 2:22 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: copper-to-copper separation
> 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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