[SI-LIST] Re: Current Return Vias

  • From: "Perry Qu" <perry.qu@xxxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 26 Jul 2006 12:58:52 -0400

Interesting topic. We recently had a design with a SDRAM interface consists
of 5 SDRAMs, 2 pairs clamshelled and one standalone. It's running on 155 MHz
clock single data rate so not a fast interface by today's standard. For the
clamshelled SDRAMs, we measured the address signals and the waveform on the
top and bottom sometimes are quite different, specifically, one of the SDRAM
(could be either top or bottom depending on which address pin we probe)
shows very visible crosstalk and worse shelf on edge.  The result is less
margin for timing. 

This can't be caused by topology as we length-matched the trace from SDRAM
pin to the branch via in a mixture of star/far-end cluster topology. The
only thing I can think of is return current as most of the signals vias are
under the middle of the SDRAM packaging with no ground/power via really
closeby. The SDRAM packaging is not well done in my opinion as they have
most of the power/ground pins on one half of the chip, and very few
reference pins on the other half where all address pins locate. To make it
worse,  the routing layer is all over the stackup and it can jump from a
layer close to top to a layer close to bottom and then jump back, with no
return vias close to signal vias, although they all reference to GND (we
have multiple GND planes throughout the stackup). The worse waveform seems
always occurs at the SDRAM with bigger jump in terms of reference planes
thus larger return loop area, which is the cause for crosstalk and higher
inductance on return and shelf on edge. I decide to add some ground vias
close to the signal vias for our next design which uses same chips. 

Regards

Perry


======================================= 

Perry Qu 

Design & Qualification, Alcatel Canada 

600 March Road, Ottawa ON, K2K 2E6 

DID: 613-7846720  Fax: 613-5993642 

Email: perry.qu@xxxxxxxxxxx 

======================================= 


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