Hah?
The wavelength of 800Mhz is 1.2 feet. So y/20 = .72 inches. Even the
higher harmonics are over 100 mil.
The length of a via to an internal plane is about 20 mils? The is 1/5 of
the higher harmonics. Give me a break.
You must be joking if you think placing a "Swiss Cheese" plane on the PCB
top for saving a 10mil via distance is going to affect signal impendence.
That "Swiss Cheese" plane is probably a greater Z than the vias. Still
compared to a wavelength of 1.2 feet it is not going to be measureable.
Bob Kondner
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dwight Mattix
Sent: Wednesday, September 16, 2009 11:27 AM
To: [log in to unmask]
Subject: Re: [TN] Ground Pour Under BGA
At 07:36 AM 9/16/2009, Lee Hill wrote:
>Ok Bob I will try to find it.
>Yes, the problem assumes "high frequency". F>100 MHz would be high enough.
>And for today's technology that is pretty "low" :-)
Hah. These days DC is about 800 MHz. /heh
>Traditional lumped-element noise theory models capacitive coupling as
>"small" value of capacitance. In other words, it is "accidental", "stray",
>or "parasitic". If it were a big value it would be obvious to the designer
>and he/she would avoid it at all costs. The Z of the stray capacitance at
>the noise frequency determines the amount of noise current injected. The
>impedance of the victim circuit determines the noise voltage that is
>developed. You mention the impedance of vias, I'm not sure what they are
>but I don't think they are relevant to the E-field coupling from the body
of
>the noisy IC to say, a nearby enclosure cover, or a wire attached to the
PCB
>a few cm away, or to data transceiver IC a few cm away.
>
>Tiny amounts of mutual inductance (picoH) and capacitance (pF) do cause
many
>noise problems. The case I was referring to above is not "electrostatic
>coupling", "static" implies stationary and/or "DC".
>
>Best Regards
>
>Lee
>
>-----Original Message-----
>From: Robert Kondner [mailto:[log in to unmask]]
>Sent: Wednesday, September 16, 2009 10:21 AM
>To: 'TechNet E-Mail Forum'; 'Lee Hill'
>Subject: RE: [TN] Ground Pour Under BGA
>
>Lee,
>
> If you still have any links to that research I would love to read it.
>
> Ok, I can see a electrostatic (Near Field) effect is possible, I recall
my
>Hog Wash label from that effect. But as far as the chip body to board
>capacitance it would have to be a pretty high frequency to be less than the
>Z of a couple 10 mil long vias. At least I think.
>
> Thanks again, it is definitely something to think about.
>
>Bob Kondner
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee Hill
>Sent: Wednesday, September 16, 2009 10:10 AM
>To: [log in to unmask]
>Subject: Re: [TN] Ground Pour Under BGA
>
>There was good EMC research done about 5-7 years ago at the University of
>Missouri-Rolla (UMR - now Missouri University of Science and Technology)
>that investigated this design technique and explained how, why, and when it
>might be helpful. I know it is tempting sometimes to dismiss seemingly
>weird or useless PCB EMC design techniques out-of-hand, but there are a lot
>of smart people around the world spending a lot of time doing mathematical,
>computational, and experimental research work to analyze, understand, and
>prove the usefulness or uselessness of many different aspects of PCB
design.
>
>For this specific example, the basic idea is to encourage capacitive
>coupling from the top of the IC back down to the "low side of the source",
>which in general is the "ground plane" of the PCB. For it to be perceived
>as effective, many conditions must be met 1) the IC must be troublesome
>source of E field coupling, 2a) there must be a "victim" of the E field
>coupling, and 2b) the victim is sensitive enough or the regulatory
emissions
>limit low enough that the noise coupling is troublesome. "plane impedance"
>is a little vague, it is not clear whether this refers to power bus
>impedance (then there must be a second power conductor somewhere), or just
>the impedance of the ground plane(s) alone, which is not defined without
>first defining a complete signal or noise loop geometry. If we have a PCB
>where there already is a "ground" plane at layer 2, then there probably is
>not as much benefit to the technique for noise control. I think the
research
>was spurred by the use of 4 or 6 layer boards, where the IC might not
>"normally" have a "ground" plane directly beneath it.
>
>Best Regards
>
>Lee UMR '92
>
>Lee Hill
>
>SILENT
>10 Northern Boulevard, Suite 1
>Amherst, NH 03031
>USA
>+1 (603) 578-1842 (v)
>+1 (603) 578-1843 (f)
>+1 (508) 341-3947 (m)
>[log in to unmask]
>
>Electromagnetic Compatibility and RF Design, Troubleshooting and Training
>
>
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
>Sent: Wednesday, September 16, 2009 8:42 AM
>To: [log in to unmask]
>Subject: Re: [TN] Ground Pour Under BGA
>
>Hi,
>
> What would be the goal of placing atop side GND plane under the BGA?
>
> Low Z Gnd return? Electrostatic shield?
>
> Having a plane under the BGA vs another layer is not going to make a
>difference in plane Z.
>
> Sounds like a good chance of creating shorts if the solder mask chips or
is
>damaged during rework.
>
>Bob Kondner
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
>Sent: Wednesday, September 16, 2009 7:43 AM
>To: [log in to unmask]
>Subject: Re: [TN] Ground Pour Under BGA
>
>IPC 7095, BGA Technology by Lau.
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Toby Carrier
>Sent: Tuesday, September 15, 2009 8:45 PM
>To: [log in to unmask]
>Subject: [TN] Ground Pour Under BGA
>
>Hello all,
>
>Is it a good idea to create a top layer ground flood under a BGA package? I
>am
>guessing that the fanout will not allow much area for the ground pour, am I
>correct in thinking so? Does anyone have any good reference info on this
>topic?
>
>Also, if you don't have a ground pour under the BGA, how will that affect
>the
>impedance control of the traces going to the BGA ball?
>
>Thanks for the help,
>
>Toby
>
>---------------------------------------------------
>Technet Mail List provided as a service by IPC using LISTSERV 15.0
>To unsubscribe, send a message to [log in to unmask] with following text in
>the BODY (NOT the subject field): SIGNOFF Technet
>To temporarily halt or (re-start) delivery of Technet send e-mail to
>[log in to unmask]: SET Technet NOMAIL or (MAIL)
>To receive ONE mailing per day of all the posts: send e-mail to
>[log in to unmask]: SET Technet Digest
>Search the archives of previous posts at: http://listserv.ipc.org/archives
>Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
>for additional information, or contact Keach Sasamori at [log in to unmask] or
>847-615-7100 ext.2815
>-----------------------------------------------------
>
>---------------------------------------------------
>Technet Mail List provided as a service by IPC using LISTSERV 15.0
>To unsubscribe, send a message to [log in to unmask] with following text in
>the BODY (NOT the subject field): SIGNOFF Technet
>To temporarily halt or (re-start) delivery of Technet send e-mail to
>[log in to unmask]: SET Technet NOMAIL or (MAIL)
>To receive ONE mailing per day of all the posts: send e-mail to
>[log in to unmask]: SET Technet Digest
>Search the archives of previous posts at: http://listserv.ipc.org/archives
>Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
>for additional information, or contact Keach Sasamori at [log in to unmask] or
>847-615-7100 ext.2815
>-----------------------------------------------------
>
>---------------------------------------------------
>Technet Mail List provided as a service by IPC using LISTSERV 15.0
>To unsubscribe, send a message to [log in to unmask] with following text in
>the BODY (NOT the subject field): SIGNOFF Technet
>To temporarily halt or (re-start) delivery of Technet send e-mail to
>[log in to unmask]: SET Technet NOMAIL or (MAIL)
>To receive ONE mailing per day of all the posts: send e-mail to
>[log in to unmask]: SET Technet Digest
>Search the archives of previous posts at: http://listserv.ipc.org/archives
>Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
>for additional information, or contact Keach Sasamori at [log in to unmask] or
>847-615-7100 ext.2815
>-----------------------------------------------------
>
>---------------------------------------------------
>Technet Mail List provided as a service by IPC using LISTSERV 15.0
>To unsubscribe, send a message to [log in to unmask] with following text in
>the BODY (NOT the subject field): SIGNOFF Technet
>To temporarily halt or (re-start) delivery of Technet send e-mail to
>[log in to unmask]: SET Technet NOMAIL or (MAIL)
>To receive ONE mailing per day of all the posts: send e-mail to
>[log in to unmask]: SET Technet Digest
>Search the archives of previous posts at: http://listserv.ipc.org/archives
>Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
>for additional information, or contact Keach Sasamori at [log in to unmask] or
>847-615-7100 ext.2815
>-----------------------------------------------------
>
>---------------------------------------------------
>Technet Mail List provided as a service by IPC using LISTSERV 15.0
>To unsubscribe, send a message to [log in to unmask] with following text in
>the BODY (NOT the subject field): SIGNOFF Technet
>To temporarily halt or (re-start) delivery of Technet send e-mail to
>[log in to unmask]: SET Technet NOMAIL or (MAIL)
>To receive ONE mailing per day of all the posts: send e-mail to
>[log in to unmask]: SET Technet Digest
>Search the archives of previous posts at: http://listserv.ipc.org/archives
>Please visit IPC web site
>http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
>information, or contact Keach Sasamori at [log in to unmask] or
>847-615-7100 ext.2815
>-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
for additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
