It's not such a big deal. It is a patented process and you will have

to find a shop that is ZBC approved. It's ridiculous but there you

have it. At least that's how it works out in North America. And where

did you get that 30MHz figure from? Are you using 0805 caps or

something? You have to use 0201caps with two vias per pad.

Anyways, you have to pay a fee to use this PCB technology. You can

dance around the issue if you can convince an unlicensed PCB shop that

you are not using the PCB as a capacitor, but to "control impedance".

Thanks for hint about ZBC cores, I had not heard of this core

material, their

website says that a core called BC12 has er=4.2 and the gap between

the power and ground plane is 12um.

And where did you get that 30MHz figure from?

I wrote a little program to numerically (using R.F.Harrington's method

of

moments) solve the scalar Helmholtz equation for the electric field in

the

gap between the power and ground plane. I assumed that the E-field was

only

in the z-direction (normal to the planes) and that it did not depend

on z

so it was just Ez(x,y) and that the effect of the decoupling caps

could

be modelled as line sources of displacement current extending in the

z-direction between the planes. I looked at the results of this

program in

order to get a feel for what is going on in terms of a circuit model

for power

plane impedance.

At low frequency the impedance of a bare pcb (without any decoupling

caps)

is the impedance the planar pcb capacitance Cpcb. At higher

frequencies

there is a zero in the impedance when the planar capacitance cancels

the

bare pcb inductance Lpcb. Values for a 100mm x 100mm area with er=4.2

and

the gap h=150um are Lpcb=114pH and Cpcb=2.5nF with the zero at about

300MHz.

The decoupling caps are connected in parallel and attached to the

node

joining Lpcb and Cpcb. The Spice network would be,

Lpcb port n001 114pH

Cpcb n001 0 2.5nF

* One species of cap, 20 caps each with Lesl=1.5nH and C=100nF

Leff n001 n002 75pH

Ceff n002 0 2uF

where Leff and Ceff are the effective series inductance and

capacitance

of a string of decoupling caps of a single species all in parallel.

If there are enough decoupling caps so that the effective inductance

of the decoupling caps is less than the pcb inductance (Leff<Lpcb) and

Ceff>Cpcb then the zero of the decoupling caps at 1/sqrt(Leff*Ceff)

does

not appear and there is a zero at 1/sqrt(Lpcb*Ceff) which is below

the

resonance of the caps alone. Above this frequency the pcb looks

inductive.

That is how I got the 30MHz figure in my original post; I found that

if I

used many small value caps (say 1nF), the expected hole in the

impedance

at the (relatively high frequency) resonance of the small caps did

not

appear and the resonance was always at the lower frequency of

1/sqrt(Lpcb*Ceff). For the typical values in the example, this zero is

just

above 10MHz.

Above this zero the power-plane impedance looks like the impedance

of the bare pcb as Z=s*Lpcb. Then, there is a closely spaced pole/zero

pair

that marks the point at which the power plane impedance looks exactly

like

a bare pcb. This pole/zero pair is at 1/sqrt(Leff*Cpcb). For the

example

this is at 370MHz.

The bare pcb inductance Lpcb is proportional to the interplane gap

and

since the power plane impedance is Z=s*Lpcb above the zero at

1/sqrt(Lpcb*Ceff), it makes sense to have a small gap as possible and

hence

the ZBC cores look interesting. However, the sanmina-sci.com website

says that

the use of these cores can reduce the number of decoupling caps, but

the

power-plane impedance only goes as s*Lpcb provided that there are

enough

decoupling caps that Leff<Lpcb. If the number of caps is reduced so

that

Leff>Lpcb then the power plane impedance is dominated by the

inductance of the caps and Z=s*Leff and you've lost the effect of the

small

pcb inductance until the frequency is above the pole/zero pair at

1/sqrt(Leff*Cpcb) at which the pcb looks exactly like a bare pcb.

Stephen

http://www.stebla.pwp.blueyonder.co.uk