Re: [SI-LIST] : Q: Plane-jumping return currents

Christian Schuster (schuster@iis.ee.ethz.ch)
Wed, 22 Sep 1999 09:33:09 +0200

Hi Eric,

I have been using the finite-difference time-domain (FDTD)
method for electromagnetic field simulation in
similar stackup and via configuration probelms. If you
are not familiar with FDTD these comments might give you
another view of the problem:

> Stackup 2
>
> plane =========== | | ==========
> trace A ------------| |
> trace B | |-----------
> plane =========== | | ==========
>
>
> Stackup 3
> | |----------- trace B
> plane =========== | | ==========
> plane =========== | | ==========
> trace A ------------| |
>
> In stackups 2 and 3, the return current for trace A moving to trace B has
> to jump planes. The only place that can occur is via a capacitance. Some
> capacitance is provided by the interplane capacitance which works better in
> stackup 3 than it does in stackup 2. Otherwise, a nearby bypass cap must
> be found. The farther away the cap is, the larger the inductance (and
> impedance) of the return current path.
>
> My system is running pretty fast (> 1 Gbps).
>
> My questions:
>
> 1. Is what I've described generally true?
>
> 2. How could one analyze how far away a "nearby" cap can be and not degrade
> the signal too much?
>
> 3. How does the value of the cap affect this? Clearly we want a low
> inductance package. Do I just go for the largest capacitance that fits in
> a low-inductance package?
>
> 4. How could one analyze if the interplane capacitance is sufficient for
> this purpose?
>

When simulating with FDTD you see that in stackup 2 and 3 you
will generate spurious parallel plane modes at the via
location that will propagate radially away. These modes lead to
losses in your signal energy and electric potential fluctuations
between your planes (sometimes this is called "ground bounce").