# pcb fab for low power plane impedance

S

#### [email protected]

Can anyone recommend a pcb fab (not necessarily in the UK) that can
fabricate a board in which the spacing between the power and ground
planes can be around 65um or less? The complete board has 16 layers
and there are two power/ground layer pairs that I would like with as
small a gap as possible. The reason is, I have only recently
understood that the power plane impedance above the last zero of the
decoupling caps (~ 30MHZ) is just given by the bare pcb self-
inductance. The bare pcb self-inductance is proportional to the inter-
plane gap and so I want this as small as possible. The pcb fab that my
employer habitually uses can only fab boards with a gap h=150um with
er=4.2 dielectric which corresponds to a power plane impedance of |Z|
=130mOhms at 100MHz. However, I've heard anecdotally that some pcb
fabs will do 65um.

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

A

#### [email protected]

Can anyone recommend a pcb fab (not necessarily in the UK) that can
fabricate a board in which the spacing between the power and ground
planes can be around 65um or less? The complete board has 16 layers
and there are two power/ground layer pairs that I would like with as
small a gap as possible. The reason is, I have only recently
understood that the power plane impedance above the last zero of the
decoupling caps (~ 30MHZ) is just given by the bare pcb self-
inductance. The bare pcb self-inductance is proportional to the inter-
plane gap and so I want this as small as possible. The pcb fab that my
employer habitually uses can only fab boards with a gap h=150um with
er=4.2 dielectric which corresponds to a power plane impedance of |Z|
=130mOhms at 100MHz. However, I've heard anecdotally that some pcb
fabs will do 65um.

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

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".

J

#### John Larkin

Can anyone recommend a pcb fab (not necessarily in the UK) that can
fabricate a board in which the spacing between the power and ground
planes can be around 65um or less? The complete board has 16 layers
and there are two power/ground layer pairs that I would like with as
small a gap as possible. The reason is, I have only recently
understood that the power plane impedance above the last zero of the
decoupling caps (~ 30MHZ) is just given by the bare pcb self-
inductance. The bare pcb self-inductance is proportional to the inter-
plane gap and so I want this as small as possible. The pcb fab that my
employer habitually uses can only fab boards with a gap h=150um with
er=4.2 dielectric which corresponds to a power plane impedance of |Z|
=130mOhms at 100MHz. However, I've heard anecdotally that some pcb
fabs will do 65um.

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

Hi, Stephen

What are you building? At 16 layers, it must be ambitious.

I've done a bit of TDR testing of actual PCBs, without and then with
bypass caps. It appears to me that with moderate (say, 100-250 um)
plane spacings, there's a generous overlap of frequency ranges in
which the plane is a good bypass, and where the caps are effective. So
there's no need to go to very thin dielectrics.

And decoupling caps are still low impedance past their series
self-resonant points. They don't become useless, they simply look like
a cap in series with their small lead inductance, no surprise.

So go with a reasonable plane spacing and pepper the board with 0805
or 0603 caps; 0.1 or 0.33 uF are good choices and are cheap. There's
really no justification for mixing cap values... the ESL is determined
by body size and vias, so more C is better. The Spice models that
people love to publish, with the bypass SRF dips all superimposed, are
just plain (plane?) silly.

Is your board impedance controlled? That gets nasty at high layer
counts.

John

J

#### Jeff L

John Larkin said:
Hi, Stephen

What are you building? At 16 layers, it must be ambitious.

I've done a bit of TDR testing of actual PCBs, without and then with
bypass caps. It appears to me that with moderate (say, 100-250 um)
plane spacings, there's a generous overlap of frequency ranges in
which the plane is a good bypass, and where the caps are effective. So
there's no need to go to very thin dielectrics.

And decoupling caps are still low impedance past their series
self-resonant points. They don't become useless, they simply look like
a cap in series with their small lead inductance, no surprise.

So go with a reasonable plane spacing and pepper the board with 0805
or 0603 caps; 0.1 or 0.33 uF are good choices and are cheap. There's
really no justification for mixing cap values... the ESL is determined
by body size and vias, so more C is better. The Spice models that
people love to publish, with the bypass SRF dips all superimposed, are
just plain (plane?) silly.

I never understood some of those claims - I still see things like a 0.01 uf
cap in // with a 0.1 in // with a 0.33 uF cap, sometimes even in data
sheets!. (I suppose if someone spaced the caps apart enough, it could form a
nasty LC Pi filter, and the different values could dampen the different
resonances) It might be left from the days of through hole and high
impedance electrolytics // ed with a disc cap. A smaller capacity MLCC cap
is not as good at high frequency as a larger capacity cap in the same
package and construction methods. Cap data sheets which show resonant points
/ impendence graphs show a shift to the left with respect to frequency in
the self resonant points as the cap capacity shrinks, with larger caps
showing a lower impedance then the smaller caps before, at and after the
self resonant point. Series inductance is the limit, not the cap size

For really good bypassing you can try the 0306, 0508 and 0612 capacitors for
much lowered inductance! They seem to work quite well, but it's likely
cheaper to plop down several 0603 caps in // then to buy one "sideways" low
impedance cap. Using more then one via per pad also helps lower the
inductance.

V

#### vasile

Can anyone recommend a pcb fab (not necessarily in the UK) that can
fabricate a board in which the spacing between the power and ground
planes can be around 65um or less? The complete board has 16 layers
and there are two power/ground layer pairs that I would like with as
small a gap as possible. The reason is, I have only recently
understood that the power plane impedance above the last zero of the
decoupling caps (~ 30MHZ) is just given by the bare pcb self-
inductance. The bare pcb self-inductance is proportional to the inter-
plane gap and so I want this as small as possible. The pcb fab that my
employer habitually uses can only fab boards with a gap h=150um with
er=4.2 dielectric which corresponds to a power plane impedance of |Z|
=130mOhms at 100MHz. However, I've heard anecdotally that some pcb
fabs will do 65um.

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

The biggest problem for manufacturer is not using the ZBC1 or ZBC2
materials but:
- registration problems using very low thickness material
(any lamination need a central core with a largest thickness than the
others layers of the lamination)
- simetry problems in laminated stack (the laminations must be
simetricaly)
- keeping the number of laminations as low as possible (most
manufacturesr will say 3)
- blind and buried vias (including laser drilled vias) can't be
manufactured on less than
0.003" or 0.0025" tickness by most manufacturers

Even the gap between power and ground is less than 65um, some vias
connected with power or ground
will require copper plating, which will increase the copper layer
thickness.

So, you need a deep talk with your manufacturer before sending the
gerbers. The problem with those talks
are the fact that often you need to redesing entire stuff once or
twice before they gave you the ok for manufaturing.

I will focus to tawanese manufactures if the price is an issue.

A

#### [email protected]

For really good bypassing you can try the 0306, 0508 and 0612 capacitors for
much lowered inductance! They seem to work quite well, but it's likely
cheaper to plop down several 0603 caps in // then to buy one "sideways" low
impedance cap. Using more then one via per pad also helps lower the
inductance.

I'd like to see sources for either claim. We are in the process of
transferring large chunks of our startup capital into the pockets of
consultants which are making us place the classic "octave of caps"
like 18pf, 22pf, 33pf,47pf, 100pf on sensitive analog nets. We are
running at 6GBps+ and looking at mV of noise on reference signals.

I understand that the superposition of bode plots for each cap is
meaningless since the point we are decoupling doesn't "see" the caps,
it sees what it sees. The caps are "parasitic L" further away than the
point that needs the charge.

My personal take on it is to reduce the parasitic L by using 0201 caps
with two vias per pad, and use the largest C value possible in the
package. A point of view not shared by others.

J

#### John Larkin

I'd like to see sources for either claim.

There's no shortage of sources for claims here. There are a lot of
silly claims.
We are in the process of
transferring large chunks of our startup capital into the pockets of
consultants which are making us place the classic "octave of caps"
like 18pf, 22pf, 33pf,47pf, 100pf on sensitive analog nets. We are
running at 6GBps+ and looking at mV of noise on reference signals.

Not Howard Johnson, I hope! All those caps are ludicrous.
I understand that the superposition of bode plots for each cap is
meaningless since the point we are decoupling doesn't "see" the caps,
it sees what it sees. The caps are "parasitic L" further away than the
point that needs the charge.

My personal take on it is to reduce the parasitic L by using 0201 caps
with two vias per pad, and use the largest C value possible in the
package. A point of view not shared by others.

Overkill. I do tiny signals with picosecond timing that would be
trashed by millivolt noise, on the same board with uPs and bus
interfaces and switching regulators. Most of our jitter problems can
be found with a 1 MHz oscilloscope, looking at supply ripple and such.

As I noted, the ground planes themselves are an excellent
low-impedance structure, with a broad overlap with a scattering of
caps. The dual-via thing doesn't help enough to matter.

Fire those consultants before they bleed you dry, and get your product
working. Where are you located?

John

A

#### [email protected]

There's no shortage of sources for claims here. There are a lot of
silly claims.

Not Howard Johnson, I hope! All those caps are ludicrous.

Overkill. I do tiny signals with picosecond timing that would be
trashed by millivolt noise, on the same board with uPs and bus
interfaces and switching regulators. Most of our jitter problems can
be found with a 1 MHz oscilloscope, looking at supply ripple and such.

As I noted, the ground planes themselves are an excellent
low-impedance structure, with a broad overlap with a scattering of
caps. The dual-via thing doesn't help enough to matter.

Fire those consultants before they bleed you dry, and get your product
working. Where are you located?

John

Hi John,
I need 0201 caps so I can squeeze all the decoupling, coupling and
terminating networks close enough to the MLF16 packages. It would be a
strange sight to see MLF16 with something as big as 0603 or 0805s
around it. 0402 is already hampering me enough as it stands.

The problem I think we have is that our board is so physically small,
that we can't get any significant charge stored in PCB. The planes are
eaten up by vias.

I can understand using board capacitance if you're building VME sized
cards or 19 inch rack mount stuff. My stuff is the size of a CC.

What's wrong with Johnson? I thought he was the one backing the
"biggest cap you can find" theory?

S

#### [email protected]

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

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