[PCB_FORUM] Re: Via inside SMD discrete..

There is another issue which hasn't been discussed and that is the direction
of the signal entry. If you are decoupling the power pins of an IC, you
should actually treat the bypass cap as a four terminal device. Decoupling
is ALWAYS necessary at the source of the HF interference. If you only
decouple at the power pins of your IC's you will actually make things worse
as you will attract CVN HF noise to the low-impedance of the caps.
Decoupling at the IC itself is done to deal with external sources over which
you have little control (and over-the-air coupling by sources in your
system). What I mean is:
 
Netlist:
VCC -> Cap (pin1); GND -> Cap (pin2)
 
Better:
IC_VCC -> Cap (pin1) -> VCC (plane/pour); IC_GND -> Cap (pin2) -> GND
 
It is better to route the signal across the cap than to route the cap and IC
to the planes independently. When routing the cap send the trace from the IC
into the cap pad from the opposite side as the trace exits the cap pad to go
to the via. If possible, performance will be much improved by creating a V
shape with the entry and exit traces as this will turn the trace parasitic
inductances and resistances in your favor.
 
I'm a research EE who specializes in this field. If you need more info on
this, please feel free to contact me (jf@xxxxxxxxxxx).
 
Warm regards to all and happy holidays,
-Jonathan
Jonathan Friedman, GSR
Networked Embedded Systems Laboratory (NESL)
University of California, Los Angeles (UCLA) 

 


  _____  

From: icu-pcb-forum-bounce@xxxxxxxxxxxxx
[mailto:icu-pcb-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Naren Thesia
Sent: Friday, December 09, 2005 9:05 AM
To: icu-pcb-forum@xxxxxxxxxxxxx
Subject: [PCB_FORUM] Via inside SMD discrete..




Hi All,

 

My question is related to general PCB discussion.

Recently I saw via between the component pad for SMD decap (As shown in
attached screen shot). I got clarification that it is to improve the
performance of decap in High speed design! 

 

I unable to understand that clearly. Any one can share his or her
experience?

 

Thanks in advance

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