From: Larry Smith ([email protected])
Date: Wed Jun 14 2000 - 10:39:01 PDT
I am glad that we completely agree on the necessity of thin dielectric
between power planes!
But, let's further consider the role of the dielectric constant. I
don't want to discourage all the folks out there who are developing
high K materials for pcb power planes. It is true that there are some
time of flight issues as we raise the dielectric constant, but I don't
think they are major.
We have just about reached the "thinness" limit with FR4 woven glass
and resin with the 2 mil Hadco BC technology. I agree that we can
connect pairs of power planes in parallel to achieve higher
capacitance, lower inductance and lower impedance power distribution,
but there is a better way. It will require a new, thinner material
between the power planes.
You have asked for someone to develop materials with higher loss to
deal with plane resonances. I believe the best material for that is a
thin dielectric. Several suppliers have demonstrated the ability to
build 1 mil thick dielectric cores with copper planes attached to each
side. At least one suplier has a road map showing dielectric thickness
at a fraction of a mil. They are working hard to make these
technologies consistent with PCB manufacture. The cores would simply
be inserted into the stackup like an FR4 core.
So, how does thin dielectric increase the loss? For low loss transmission
lines, the lossy propagation constant alpha = R/2*Z0 + G*Z0/2. R is
the conductor resistance, G is the dielectric loss
(tan(delta)*omega*C), Z0 is the impedance of the transmission line.
The power planes are well represented an xy array of transmission
lines. At the frequencies that we are concerned with (1 MHz to 5 GHz),
R and G are dominated by skin loss and dielectric loss. R goes as
sqrt(freq). G is proportional frequency and C. As we reduce
the dielectric thickness, the skin losses remain the same but the
impedance of the transmission line and goes down, thus increasing the
first term in alpha. Z0 is in the numerator of the second term so it
might appear that alpha is going to increase as we reduce thickness.
But G goes as C and Z0 goes as 1/sqrt(C), so the second term of alpha
also increases with decreasing dielectric thickness. Thin dielectric
power planes increase the loss of the power planes, a very fortunate
Attached is a (hopefully small) .pdf file that shows the simulation of
power plane impedance vs frequency for several dielectric thicknesses.
One AC amp is forced along the edge of our familiar 6x6 square inch
pair of power planes. You can see the Capacitive down slope (-20 dB
per decade) at low frequencies and can see where the plane resonances
begin above 100 MHz. Skin effect and dielectric loss has been
included in these simulations. Notice how the plane resonances damp
out nicely as we simulate 1, 0.5, 0.25 and 0.1 mil thick dielectrics.
A second panel is in the simulation that shows the effect of raising
the dielectric constant between planes from 4 to 16. Notice how the
impedance of every curve has dropped. Also notice how the damping has
been increased for each dielectric thickness. This is because the
impedance of the planes go as sqrt(L/C) and the dielectric constant has
increased C. A reduction in plane impedance has increased the alpha of
the propagation constant and reduced the magnitude of the cavity
The first priority is to reduce the dielectric thickness because that
reduces L as well as increases C. But if we happen to get a high
dielectric constant in that thin material, I will definitely take it!
> X-Sender: [email protected]
> Date: Tue, 13 Jun 2000 15:34:26 -0700
> I advocate thin dielectric layers, using ordinary FR-4
> dielectric materials. I don't like using high-dielectric
> materials for power-and-ground plane separation.
> As we go to higher frequencies, in order to attain ever-lower values
> of spreading inductance, we will need thinner and thinner power-ground
> spacing. If we cannot make our power-ground
> dielectric layers sufficiently thin, we may have to switch to
> a layer-cake approach with several pairs of power-ground layers.
> To deal with our resonance problems, I'm hoping that someone
> will develop some materials with somewhat higher loss
> properties that FR-4 and copper. Either a more lossy dielectric,
> or perhaps a copper layer with a surface treatment that
> increases its skin-effect loss (like plating with tin or
> something less conductive that copper) could begin to help
> in this area.
> Best regards,
> Dr. Howard Johnson
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