[SI-LIST] Re: question about the bypass cap
- From: "Gil Gafni" <gil.gafni@xxxxxxxx>
- To: <zhao.dave@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sun, 20 Mar 2005 09:43:52 -0800
David
Both Istvan and Steve have given you sound advice. I would like to offer my
2 cents -
In most of my designs I have been using different Vias for Power and
different for signals.
For instance "regular" via's drill will be 10 to 12.5 mil, power via's drill
will be 40 mils. Having a bigger diameter via reduces its inductance. You
can use one or couple of those Power vias to connect your decoupling to the
planes.
Although you did not mention it, I would imagine that your dilemma relates
to BGAs. Under BGAs you can have a "smart" breakout patterns -
A. one that will allow you to have cooper pours for GND and for the various
voltages, those cooper pours share few vias to the plane. To my humble
opinion cooper pour with few vias, will offer lease inductance than traces.
if you will leave a via free area for placement (mostly I shape it like a
cross) you can stretch the cooper pours to reach your bypass.
B. playing with the breakout to allow placement of 0201 or even 0402
capacitors. Takes more time, and you have hard time to find your signals.
Needs more creativity.
As a rule of thumb I will not try that hard on bypass capacitors with
capacitance higher than 10uF, I will just place them around the perimeter
and use the Power Vias.
Gil Gafni
----- Original Message -----
From: "david" <zhao.dave@xxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Sunday, March 20, 2005 3:03 AM
Subject: [SI-LIST] question about the bypass cap
> hi sir, i try to connect the bypass capacitor to the power pins of
> chips directly to reduce the induction of the circuit.Is it right?
> Generally, the bypass caps are connected to the power planes by a pair
> of vias and so as the power pin ,but i want to make the bypass cap
> and the power pin share the same via to cancel another pair. Is it
> feasible?
>
>
> --
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