Hi,

There is no doubt that the impedance do increase if the ground plane
(or the power plane -or both depending on which is the reference plane)
is broken up in some way. Just as Dough McKean so rightly points out,
the impedance is the square root of per unit length inductance divided
by per unit length capacitance.
Again, as Dough McKean points out, the capacitance between the plane and
the conductor will be lower than if the plane was solid.
-This is also the case of a heavily perforated solid plane.

Then one might wonder how much the impedance really do increase.
Several studies has been done on this topic but I don't remember any
good references. Of cource, the effect is dependant of how mych copper
you leave in.

There are some comercial fieldsolvers around that can solve these kinds
of issues. On the other hand, the practice is to always use solid planes
if you need to control the impedance strictly.
One way is to have a grided plane in general but to fill it solid under
the traces you need to control the impedance to about 3 to 5 trace
widths on either side of the trace.

A hint could be to look for impedance formulas on the MCM foundries
www pages -MCMs of certain types are often done with gridded planes.

Regards

Per Viklund

Steve Horvath wrote:
>
> We've always used solid ground plane for controlled impedance work here
> at Motorola. To answer your question though, I think the impedance will
> increase. To be sure, ask your resident sparkey what he or she thinks.
>
> Steve
>
> *******************************************************
>
> On Mon, 12 Jan 1998, Doug McKean wrote:
>
> > Does anyone know if gridding a ground plane
> > under a trace of controlled impedance increases
> > or decreases by having to actually work with this?
> >
> > I can guess and theorize all day long...
> > My guess is that since there's more distributed
> > capacitance with a solid ground plane, that
> > increasing the grid pattern of the ground will
> > lower the distributed capacitance thus INCREASING
> > the controlled impedance.
snip.

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