[SI-LIST] Re: Regarding the air gap between via hole and copper

Hi Ken,

Thanks for the instructions. As you mentioned, via hole impedance will 
change with the gap size. Just for this case, I think the 5mil gap might 
have little impact on the signals of my board. I will try to find out how 
much the influence will be by simulation.

Thanks,
Peter

----- Original Message ----- 
From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
To: <dagmara.avanindra@xxxxxxxxx>; <pangning2000@xxxxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>
Sent: Saturday, July 16, 2005 3:50 AM
Subject: [SI-LIST] Re: Regarding the air gap between via hole and copper


Peter,
It depends on the risetime of your signal.  The greater the (L/C)^1/2 ratio,
the higher the Zo of the via.  In a general sense you can look at the pad as
the L and the antipad as the C.  Keeping the pad size constant, the smaller
the gap, the higher the C, and the lower Zo goes.  You need a 3-D solver to
analyze the geometry with any accuracy.
Ken

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Dagmara Avanindra
Sent: Friday, July 15, 2005 11:30 AM
To: pangning2000@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Regarding the air gap between via hole and copper


   Peter,=20

   Sounds like you are talking about the size of antipad, and the size
of your antipad is 10 mils larger than pad, correct? (5 mils all
around).

  Depending on who you use to manufature this board, there should be
no problem with this antipad size.

I wouldn't worry much about the xtalk either, unless you are talking
about some super-sensetive analog planelet, in which you have lots of
extra vias for noisy signals.

 Dagmara

On 7/15/05, Pang Ning <pangning2000@xxxxxxxxx> wrote:
> Dear Experts,
>=20
> I have a simple question about the gap between the via hole pad and
> surrounding copper. In VCC layer, I used positive plane. Because the boar=
d
> size is small, in order to give wider spacing for power between via holes=
, I
> used 5mil air gap between via pad and the copper. I don't know whether th=
is
> spacing(5mil) will cause some problems such as crosstalk from via holes, =
low
> yield issue... Is 5mil too small for the gap? Could you give me some
> suggestions if you think there is anything wrong?
>=20
> Thanks,
> Peter
>=20
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