I was also wondering what effect do holes have in the ground plane,
I know that slots are a big problem, but circular holes (such as pad or vias
clearances) I'm not so sure.
Could someone clear this up for me.
Thanks

Patrick Jabbaz
Halliburton Energy Services
Circuit Board Designer
(281) 596-6697
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-----Original Message-----
From: Per Viklund <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Tuesday, January 13, 1998 9:09 AM
Subject: Re: [DC] Trace Impedance over a gridded ground plane ...


>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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