# [SI-LIST] Re: Power planes

• From: jeff_latourrette@xxxxxxxxxxx
• To: Raymond.Anderson@xxxxxxx, si-list@xxxxxxxxxxxxx
• Date: Thu, 24 Jul 2003 11:43:23 -0600

```Hi Karan, Ray:

I think comments below referenced to Geoff were from my posting of 7/18.  As
Ray points out velocity in microstrip is only dependent on effective dielectric
constant and given you scale line for same impedance when changing the
dielectric thickness, velocity shouldn't change.  So this effect is moot if you
are maintaining line impedance.  Epsilon effective will be slightly larger (&
velocity slightly slower) for low impedance lines, than for high imedance lines.

According to equation previously referenced, Epsilon effective is slightly
dependent on W/h and even less on t/h-in many stack-ups, we can often ignore
thickness.  Although this is equation-based and probably limited in accuracy to
reasonable values of W/h & t/h intuitively, one should be able to imagine the
difference in field distribution between a narrow, high-Z line and that of a
wide low-Z line.  Ray also correctly points out that stripline's velocity is
solely dependent on material dielectric constant and W/h, t/h doesn't matter
because all fields are contained in the dielectric.

OK-so how bad is this effect for microstrip??  I dug and found a very old
design guideline, plotting Epsilon effective vs W/h.

For Alumina (thin-film), it varies from 5.9 (W/h=0.1, Z=130 ohms) to 6.5
(W/h=1.0, Z=50 ohms) to 7.4 (W/h=3.0, Z=26 ohms)

The only reference on the page is "Report No. 03-68-85", probably funded by
(D)ARPA.

Hope this can help,

Jeff LaT.

-----Original Message-----
From: Ray Anderson [mailto:Raymond.Anderson@xxxxxxx]
Sent: Thursday, July 24, 2003 9:28 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Power planes

>
>This is quite a general question.
>
>---------------------------------------------
>
>If I place the GND plane too close to the track wouldn't it affect the
>velocity
of signal on track ?
>
>Possible reason is the capacitance b/w track and GND will increase which will
cause the delay in changing signal(rising edges) as it will need more time to
charge the increased capacitor value.Geoff just told us that placement of GND
affects the velocity of Microstrip and Not Strip Line. How far is it true ?
>
>

The distance between a signal trace and a reference plane in a
stack-up does not effect the velocity of the signal propagating
on the signal trace. It can effect the impedance of the signal
trace. The velocity of propagation is dependent only on the effective
dielectric constant (Er) or electric permittivity of the substrate material.

Yes, there will be more distributed capacitance between the signal
trace and the plane for small spacings, but this does not effect
the velocity of propagation as the PUL inductance of the signal
trace will decrease as well.

some relevant equations:
------------------------

Velocity = C/sqrt(Er)    <= velocity is a function of Er
(where C is speed of light in this equation)

Velocity = 11.8/sqrt(Er) inches/nSec  <= numeric version of above

Velocity = 1/sqrt(L*C)  hence:
L = (delay^2)/C  and C = (delay^2)/L

-Ray Anderson
Sun Microsystems

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