[SI-LIST] Re: remove non-functional pads: measuring via capacitance with TDR
- From: <lin-wl_wang@xxxxxxxxxxx>
- To: <zasio@xxxxxxxxxxxxxxxxxxx>
- Date: Wed, 7 Apr 2004 11:42:26 +0800
Hi John,
Thanks for your sharing! I think this is a very good comparison.
I have some questions on your experiment. What rise time did you use in =
TDR measurement? If the rise time is fast enough, the small parasitic C =
of via can be identified from TDR waveform. I happen to see an article =
in following site:
http://cp.literature.agilent.com/litweb/pdf/5988-6505EN.pdf
with title: Measure Parasitic Capacitance and Inductance using TDR
So I think the C of via can also be estimated using your TDR result. =
I'll prefer this method than using a C meter, the reasons are on the =
first page of this article.=20
Another thing I'm thinking is that with faster TDR step, you'll see a =
deeper dip, so a bigger capacitance value. Does this mean there's no =
fixed value for capacitance of via? It's always changing with TDR rise =
time or bandwidth?
How do you think about this?
Thanks!
-WL
-----Original Message-----
From: John Zasio [mailto:zasio@xxxxxxxxxxxxxxxxxxx]
Sent: Tuesday, April 06, 2004 2:57 AM
To: WANG,LIN-WL (A-Singapore,ex1)
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: remove non-functional pads: any improvement
be seen
WL,
I choose to remove non-functional pads from all high speed vias. If =
space
permits I also design the power plane anti-pad so that the via looks =
like
a 50-Ohm coaxial transmission line.
Example: OC48 line card (2.448 Gb/s)
26-layer FR406 PC Board=09
12 PWR/GND planes
12 Signal planes
2 Surface pad layers
Agilent OC48 SFF Transceiver
Via characteristics:
Standard 50-Ohm
Board Thickness 102-mil 102-mil
Drilled hole dia. 13-mil 13-mil
Signal pad dia. 25-mil 25-mil
Pwr anti-pad dia. 35-mil 64-mil
Capacitance
With 12 signal pads 1.304 pF 0.756 pF
With 1 signal pad 0.829 pF 0.501 pF
The 2.448 Gb/s path from the transceiver to the SerDes has all the vias
on a 100-mil pitch. These can accommodate the 64-mil antipad. I place =
the
signal wires on the signal plane near the bottom of the PCB. This =
creates
a path from the component (top side of PCB) to the signal wire that =
looks
like 50-Ohms. The via is so clean the it does not show up on a TDR
measurement.
For large BGAS with vias on a smaller pitch, I use the standard via but
without non-functional pads.
Every one of my PC Boards has a built in test coupon with examples of =
these
vias. I use coax probes into the vias and measure the capacitance at 1 =
MHz
with a Boonton capacitance meter designed for measuring semiconductor =
devices.=20
It will measure 1.0pF with a 0.01pF accuracy.
Regards,
John Zasio
=09
=09
lin-wl_wang@xxxxxxxxxxx wrote:
> Thanks to all who responded!
>=20
> I checked with PCB fabricator again. They can easily delete all unused =
=3D
> pads, but it would be time-consuming to selectively delete some of =
them.
>=20
> Just curious, do you always choose to remove non-functional pads for =
GHz =3D
> signal lines? And How much improvement have you seen?
>=20
> Thanks!
> -WL
>=20
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris McGrath
> Sent: Friday, April 02, 2004 10:58 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: remove non-functional pads
>=20
>=20
>=20
> Our board house uses Boardstation and it is my understanding that
> turning unused pads off (removing them) is a configuration option. =
I'm
> not sure if you can selectively do it, but I know that this is not
> difficult to do if you want all pads removed.
>=20
> -Chris
>=20
>=20
>>-----Original Message-----
>>From: lin-wl_wang@xxxxxxxxxxx [mailto:lin-wl_wang@xxxxxxxxxxx]=3D3D20
>>Sent: Thursday, April 01, 2004 8:48 PM
>>To: si-list@xxxxxxxxxxxxx
>>Cc: lin-wl_wang@xxxxxxxxxxx
>>Subject: [SI-LIST] remove non-functional pads
>>=3D3D20
>>=3D3D20
>>Hello,
>>=3D3D20
>>I would like to know how to remove those non-functional pads=3D3D20
>>in order to reduce via capacitance? Manually or automatically=3D3D20
>>by layout software?
>>=3D3D20
>>Currently I'm thinking of two methods.
>>1.In layout design, define via in the way that:
>>(1) on all inner layers, pads defined as diameter=3D3D3D0 or =
12,=3D3D20
>>drilling hole=3D3D3D12mil(for example)
>>(2) on an inner layer where trace is connected, pads defined=3D3D20
>>as diameter=3D3D3D20mil, drilling hole=3D3D3D12mil
>>=3D3D20
>>To what I understand, the unnecessary pads will be removed,=3D3D20
>>and the pads used to route out the traces will remain. But=3D3D20
>>there're many errors when generating the gerber due to this=3D3D20
>>removal. Don't feel comfortable with the errors. We're now=3D3D20
>>using PADS PowerPCB, do not know if the software has a=3D3D20
>>function to remove these pads automatically. Hasn't got reply=3D3D20
>>from vendor yet.
>>=3D3D20
>>2. or manually remove the pads in gerber file, but there're=3D3D20
>>quite a lot.=3D3D20
>>How do you think about the two method? Is there any better=3D3D20
>>way to remove?
>>=3D3D20
>>Thanks!
>>-Wang Lin
>>------------------------------------------------------------------
>>To unsubscribe from si-list:
>>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>=3D3D20
>>or to administer your membership from a web page, go to:=3D3D20
>>//www.freelists.org/webpage/si-list
>>=3D3D20
>>For help:
>>=3D3D20
>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>=3D3D20
>>List FAQ wiki page is located at:
>> http://si-list.org/wiki/wiki.pl
>>=3D3D20
>>List technical documents are available at:
>> http://www.si-list.org
>>=3D3D20
>>List archives are viewable at: =3D3D20
>> //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
>> =3D3D20
>>=3D3D20
>>=3D3D20
>=20
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>=20
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>=20
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>=20
> List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl
>=20
> List technical documents are available at:
> http://www.si-list.org
>=20
> List archives are viewable at: =3D20
> //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
> =3D20
>=20
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>=20
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>=20
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>=20
> List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl
>=20
> List technical documents are available at:
> http://www.si-list.org
>=20
> 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
>=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 FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl
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:
