Re: [SI-LIST] : different 4-layer board Stack up (S-P-G-S) ?

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From: Shawn X. Arnold (sarnold@cpcibackplanes.com)
Date: Tue Mar 07 2000 - 08:39:09 PST


Take a look at reversing the power and gnd planes in the stackup. The noisiest thing on the PCB is the power plane, even with good decoupling. Using a standard 4 layer construction, i.e. a .038 core in the middle, the power plane, layer 2 in your stackup, is 3X closer to the SMD pads than it is to the gnd plane. A lot of the digital switching noise will couple to the SMD pads and could lead to radiated emissions problems. Also, change the core thickness to get a more desirable impedance. Don't just let the fab house run it with their "standard" core thickness. Once you have the impedance that you want across most of your PCB, you can modify the trace width on things like the SCSI bus to push the impedance back to 100 ohms.

Shawn Arnold
International Product Design Inc.
    -----Original Message-----
    From: Alex Li <alexl@ati.com>
    To: 'si-list@silab.eng.sun.com' <si-list@silab.eng.sun.com>
    Date: Monday, March 06, 2000 6:53 PM
    Subject: [SI-LIST] : different 4-layer board Stack up (S-P-G-S) ?
    
    
    Recently I saw a 4-layer mother board with 100 Mhz 128-bit memory bus. This board has unusual signal-power-ground-signal stack up. I talked to one of their engineer for this kind of arrangement. They said since most PC motherboard has several power plane split and on the top level there are a lot of components with pads. they think if they route all the 128-bit memory bus on the back and put it close to ground plane, they have much routing area and this will help to keep the signals clean.
     
        This is kind of new idea to me, does anyone see any drawback by this arrangement ? Will this decrease the decoupling caps performance ?
     
                                                    Alex
     
        

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