[PCB_FORUM] Re: Bga pins sharing vias
- From: Jean Bratton <jean.bratton@xxxxxxxxxxxxxx>
- To: icu-pcb-forum@xxxxxxxxxxxxx
- Date: Mon, 6 Sep 2004 06:13:59 -0700
If you put a size in the pad flash, but then put a flash name like "BLANK"
in, then you'll see the pad in Allegro when you're working, but it won't be
there in your output data, and Allegro will check to the specified pad size
for maintaining clearance.
_____
From: Budathoki, Trilok (GE Consumer & Industrial)
[mailto:trilok.budathoki@xxxxxx]
Sent: Monday, September 06, 2004 1:25 AM
To: icu-pcb-forum@xxxxxxxxxxxxx
Subject: [PCB_FORUM] Re: Bga pins sharing vias
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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