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