[SI-LIST] Re: bypass-caps: trade-off between capacitance and inductance
- From: steve weir <weirsi@xxxxxxxxxx>
- To: sarah@xxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Sat, 23 Apr 2005 00:31:08 -0700
Sarah, you can work out the mutual coupling between the vias for your two
cases. A 3D solver would provide the most accurate answer. For a manual
math exercise you can apply Biot-Savart to this structure. "Physics" by
Halliday and Resnick is a classic reference for determining inductance by
the method of Biot-Savart. Dr. Johnson has demonstrated some derivations
for signal and power via arrangements on his web-site www.sigcon.com.
I believe that you are underestimating your via inductance. For an 0.062"
board, 10 mil drills, and 40mils on centers I come up with 2.6nH for a via
pair in isolation ( 1.3nH for each via ). To get down to 1.8nH for a pair,
I show 20mil via drills as necessary. When you place a bunch of these next
to each other, the inductance / via about doubles over a single via pair in
isolation. When you place a pair that is reversed, at these dimensions,
the inductance falls a little bit ( about 20% ) from the isolated case.
Steve.
At 06:54 PM 4/22/2005 -0700, Sarah Bates wrote:
>Si-folk,
>I am pinning-out a chip that will be used on a board that
>does not use via-in-pad technology. In order to place
>bypass caps in the 1mm ball field, I'll need to place 2 power
>balls in a row and two ground balls adjacent to the power balls,
>and drop an 0402 cap in between them. In order to maximize the number
>of caps, I could then keep adding a pair of power and gnd balls,
>and a cap. However, this will place all powers in one row and
>all grounds in aonther row and so the mutual inductance of the
>caps/vias in the power-delivery path will be increased. If I
>try to alternate power and ground, I will not be able to place
>as many caps, so how do I go about quantifying the trade-off between
>the two scenarios. (Scenario 1 is having more caps, but a topology
>in-which the mutual-inductance works against me. Scenario 2 is
>having fewer caps, but alternating occasionally so that the mutual-
>inductance works for me.)
>For this analysis, I'm estimating a 10 layer board and via inductance
>on the order of 0.9nH. I think this is reasonable. I would use 0.1uF
>0402 caps.
>
>Here are pictures to clarify the question:
>P represents a power ball
>G represents a ground ball
>|| represents an 0402 capacitor
>
>Scenario 1:
>P P P P P P P P P P P P
> || || || || || || || || || || ||
>G G G G G G G G G G G G
>
>
>Scenario 2:
>P P P G G G P P P G G G
> || || || || || || || ||
>G G G P P P G G G P P P
>
>Please point me to any relevant books/papers if you know of any.
>
>Thanks,
>Sarah
>
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
