Richard, I'd be more than willing to tell you what I know. As such, it will be fairly brief ... <grin>
Gaps vary greatly as you probably know according to the geometry of the two opposable surfaces under test. It's all a function of Paschen's Law for the breakdown of *** uniform *** gaps. Flat opposable surfaces normal to each other will give differing values than say two pointed opposable geometries. Further, a point opposed to a line will give yet another set of values. Paschen's Law can be stated as V(kV)= 24.2Sh +6.1(Sh)^0.5 where: V is the breakdown voltage in KV S=(293p)/760T h is electrode spacing in cm. p is pressure mm of mercury T is temperature in degrees Kelvin Now, I use from physics the fact that 3MV breaks down 1 meter of air (STP). Using the above equation at STP, we have 30KV/cm, which is a pretty good crosscheck. And thus, 3KV @ 1mm or approx 40 mils. Put in a factor of 2 for safety and you have 3KV for 2 mm or 80 mils. Boards are made out of FR4 which has a Dk of approx 4. Since this shows the strength of the material, we divide the 2 mm by 4 and we get 0.5 mmm for inner layer seperation. Not bad for the 0.4 mm seperation specified. I specified 0.5 mm for the designers. That's my 2.5 cents. Regards, Doug
