[PCB_FORUM] Re: Bga pins sharing vias
- From: "Budathoki, Trilok (GE Consumer & Industrial)" <trilok.budathoki@xxxxxx>
- To: <icu-pcb-forum@xxxxxxxxxxxxx>
- Date: Mon, 6 Sep 2004 10:54:32 +0530
Jean,
We have shared power pins in many of our boards in past, didn't face any
problem & the boards were thru in 1st shot. Try to put more than one via for
decoupling caps.Thus will reduce inductance. I assume you have enough bulk caps
around BGA.
Hope it helps
Trilok
-----Original Message-----
From: Jean Bratton [mailto:jean.bratton@xxxxxxxxxxxxxx]
Sent: Saturday, September 04, 2004 4:23 AM
To: icu-pcb-forum@xxxxxxxxxxxxx
Subject: [PCB_FORUM] Bga pins sharing vias
Hi all,
I have a board we're looking at the possibility of sharing a single
through-hole via between two, possible three, power or ground pins. Has anyone
any experience with doing this? Here is my engineer's concern...she's pretty
much determined that she's going to let me do it, but I wanted to know if any
of you have any negative experience doing it. Or, conversely, if you do it all
the time with high speed nets and never have a problem.
Thanks,
Jean
The problem would be that when there is high speed switching, the voltage pins
that are sharing the vias will draw current through the same via at the same
time, resulting in a localized voltage noise around the via. This noise will
be more localized (hence somewhat higher spike) then when there are two vias.
The decoupling caps will try to provide the needed current, and combat the
voltage noise. But due to the inductance associated with the caps, there will
be a time delay before the needed current can be provided to the device voltage
pins, so some noise remains.
Jean Bratton
Sr. Printed Circuit Designer
Freedom CAD Services, Inc.
603-864-1300 x1349
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