[SI-LIST] Re: To remove reference planes under the gold finger areas
- From: "Sogo Hsu" <sogo.hsu@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 15 Jun 2005 05:39:46 -0000
Dear Cortex,
I am not the author of design guide and designer of PCI-E add-in
card. I try to answer your question. The purpose of the guide for
removing reference plane under edge finger pads is for better
impedance control. In the gold finger area, the spacing is tipically
spreading. In my opinion, that is why the design guide recommended
to remove the reference plane. Accordingly, the thickness is
increased and achieve better impedance control. However, if you can
keep good impedance control, I don't think you have to follow the
design.
Totally speaking, design guide is just a "guide". Hope it is helpful.
Sogo Hsu, Ph. D
Foxconn
--- In si-list@xxxxxxxxxxxxxxx, "Cortex Chen" <Cortex.Chen@xxxx>
wrote:
> Dear SI experts,
> I'm working to design a FB-DIMM add-in card. To recall previous
experiences for PCI-E, I'm confused why it's recommended to remove
reference planes under the gold finger areas of an add-in card. In
detail, refer to section 2.3.2.6 (page 28 ~ 29) of PCI Express
Design Guide (rev. no, 0.5, 12978). Who can tell me what the main
reason is?
>
> By the way, refer to section 6.11.10 (page 417 ~ 418) from Howard
Johnson's new book, I like to quote some sentences for your
reference as below:
> "......In a differential-pcb pair, most of the returning current
from each trace still flows on the solid plane, not the other trace,
because each differential trace couples much more strongly to the
big, solid nearby plane than it does to its little, skinny
differential buddy....".
>
> If it's true, then it will cause some effects like "Differential U-
Turn"?
> Please give me your comments.
>
> Thanks & Sincerely
>
> Cortex Chen =B3=AF=A5=C3=AAN
>
>
>
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