What is the value of capacitance between a two layer PCB?

[dkace]

As you have allready notice I am new in PCB design ( coming from IC design). I found out that it is possible to build a capacitance - small due to the thickness of the inside layer- between two lines, or copper fillings on top and bottom layer. Does it work? How big this capacitance can be? Is it worth the try in commercial applications? I mean does anyone uses it?
If this capacitance is dealed as strain capacitance what problems might cause?
Thanks,
D.

 

[WA]

Re: What is the value of capacitance between a two layer PCB

Capacitence depends on the dielectric and plate size, for your problem your plate sizes will be the pcb's L x W - removed foil. dielectric is the material and space between both foils. Most basic circuit analysis books have the basic formulas to determine capacitence.

To answer other question, it's effects? I think it is best to answer this way.
1. At low frequencies - there shouldn't be too much problem however...
2. At high frequencies - there could be significant problems.
in the earlier days, RF designers would space the foil a certain distance next to another to transfer the signal from one foil to another with coils. the capacitence was built into the board.

if strain capacitence (stray) this is usually not good, the idea the capacitor blocks one thing and passes another (usually block DC and pass AC).
the best way to explain is to think of a tube(valve) the distance between the plate and grids were called inter-electrode capacitence. this capacitence effected the tube at higher frequiencies. - some problems, parasitic oscillations, passing frequiences out of expected range etc...

I was taught to keep these capacitences to a complete minimum.

finally, I think the values you asked are around the pico values(very small).
the valuse range to the RF and higher frequiences.

I hope this helps, this is good question and I'm sure many good members will add more to your knowledge on the subject.
wa

 

[cancel]

with FR4 1.5mm between plates with 10x10mm dimensions C=2.8pF .. try RFSIM99 for your specific conditions

 

[atmaca]

Re: What is the value of capacitance between a two layer PCB

what is the dielectric constant of the PCB in interest

 

[Moss]

Re: What is the value of capacitance between a two layer PCB

Dielectric constant vary with diferent manifaturers, but in general is about 3,7-4,7 for FR-4 material. There is special materials with higher ε for special RF proposses. They based on same ceramics matiral and tipicly ε is in region of 9- 10 at the working frequencies. This "higher" dieledtric constant, helps to keep phisical demensions smaler.