[SI-LIST] Re: [!! SPAM] Re: 6 layers stackup
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Tom Biggs <tbiggs@xxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 26 Feb 2008 20:08:23 -0800
Tom, I am full agreement with Lee that data sheet "capacitor only"
inductance values are not useful for bypass applications. For power
bypass, designers need to know what the inductance is of the combined
capacitor and attachment structure to the plane cavity: wherever that
may be. The "bulldozer" here is not the length of the vias, but the
combination of lightly loaded cavities and test fixtures that are not
deembedded. Lee and John's good intention was to measure actual
attached capacitor performance. For the reasons stated in my earlier
message, the test fixture is not suitable to doing that with any good
accuracy. Further, John's choice of an absolutely ridiculous via and
surface connection pattern for the X2Y(r) cap severely skewed their
results.
The bone of contention is that Lee professes the belief that low
inductance caps get hobbled by vias to the extent that their replacement
ratio over regular caps doesn't hold in a conventional PCB. We have
proven that belief false for any well-designed via attachment pattern.
More importantly, we have proven that the impact of via length: that is
the total inductance added per unit length multiplied by the number of
vias used is substantially better with X2Y(r) caps than it is with
conventional 0402s. What that means is that to get to a given
inductance target, the replacement ratio of X2Y(r) type capacitors to
conventional 0402s does not significantly degrade with distance from the
capacitor mounting surface to the attached cavity. At any inductance
target, and for any distance from the capacitor surface to the power
cavity, X2Y(r) capacitors require fewer total vias than conventional
0402 or 0603 capacitors. Charts for attached inductance versus cavity
depth for various types of capacitors including X2Y(r)s can be found in
application notes on the X2Y(r) web site: www.x2y.com/bypass.htm.
In order to fix Lee's fixture, he needs to do a couple of things:
1) Fix the areal density of capacitors to be representative of intended
design practice. This will fix the impedance and PRF.
2) Device a way to deembed his fixture from the what he is measuring.
I have no doubt that once that is done, Lee will obtain the same or very
similar results to those that we report.
Best Regards,
Steve.
Tom Biggs wrote:
> You can take the bulldozer analogy two ways. It depends on whether you
> care about the weight of the bulldozer with passengers, or just the
> weight of the passengers.
>
> Steve's test fixture was geared toward 'weighing the passengers'. Lee's
> was toward 'weighing the bulldozer'. So they each served their purpose.
> His whole point was that the vias going down 50 mils on a board are
> going to swamp out the advantages of low inductance caps. His test
> fixture, by design, had vias that go down 50mils. I'm sure he'd agree
> that this would be a bad fixture for measuring the cap itself, which was
> not his goal.
>
> The appropriate land pattern to use for the low-inductance caps is a
> separate issue. I'd be curious to see Lee's board with Steve's land
> pattern.
>
> -tom
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Muranyi, Arpad
> Sent: Tuesday, February 26, 2008 1:31 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: [!! SPAM] Re: 6 layers stackup
>
> Can't resist to illustrate this with an example:
>
> If you want to compare the weight of an ant and a cricket and you put
> them on top of the same bulldozer, you will not see much difference in
> their weight...
>
> Arpad
> ------------------------------------------------------------------------
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Lee Ritchey
> Sent: Tuesday, February 26, 2008 11:16 AM
> To: Charles Grasso; Scott McMorrow
> Cc: Steve Weir; QU Perry; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: [!! SPAM] Re: 6 layers stackup
>
> What does the test vehicle have to do with it? Both capacitors are
> seeing
> the same stackup. It's apples and apples. Why 26 layers? Lots of
> PCBs
> have 26 layers, pretty much all of them in terabit routers. This PCB
> was used to test may things besides these two capacitors.
>
> What is being presented is the difference between the two capacitors
> under the same set of test conditions and it is not much.
>
> There are two sets of tests. One with the capacitors connected to the
> first two planes inside the PCB, which is the lowest added inductance
> and the other is with the capacitors attached to two planes further down
> in the PCB.
> ------------------------------------------------------------------
> 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 technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at: =20
> //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
> =20
>
> ------------------------------------------------------------------
> 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 technical documents are available at:
> http://www.si-list.net
>
> 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
>
>
>
>
--
Steve Weir
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
California office
(408) 884-3985 Business
(707) 780-1951 Fax
Main office
(401) 284-1827 Business
(401) 284-1840 Fax
Oregon office
(503) 430-1065 Business
(503) 430-1285 Fax
http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of
Teraspeed Consulting Group LLC
------------------------------------------------------------------------------------------------------
Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC
------------------------------------------------------------------
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 technical documents are available at:
http://www.si-list.net
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:
