[SI-LIST] Re: Noise on BGA core voltage rail

        Hi Larry,

        The board has 2 power-ground pairs. The spacing b/n the power and
the ground plane is 4 mils. The power planes carry 2.5 V rail (the noisy
rail) along with other volttages on the board. 

        I have one question.

        In addition to having power-ground pairs yeilding low impedances at
high frequencies above 100Mhz (which seems to be the only solution according
to you), what other precautions should be taken during the design or layout
to have low target impedances from DC to around 200Mhz-600Mhz? 

        Anand.




        From:   Larry Smith <Larry.Smith@xxxxxxx> on 08/09/2002 12:03 AM
        Please respond to Larry Smith <Larry.Smith@xxxxxxx>@SMTP@Exchange
        To:     si-list@xxxxxxxxxxxxx@SMTP@Exchange, ANAND KURIAKOSE/Apex
Data Inc/01@Apex Data Inc
        cc:      

        Subject:        Re: [SI-LIST] Noise on BGA core voltage rail

        Anand - It is difficult to do anything about power distribution
noise
        at 200 MHz and above by using discreet decoupling capacitors.

        Your best ally at high frequency is the PCB power planes.  They are
        effective at high frequency but may have cavity resonances which
depend
        upon the dimensions of the board.

        The best way to make the PCB power planes effective is by defining a
        stackup that has power/Gnd plane pairs next to each other.  The
        dielectric thickness between the planes determines the capacitance
and
        spreading inductance of the planes.  Thinner is better.  4 mils
between
        planes is good, 2 mils is better.

        Several years ago, it was common to see power and ground planes
        separated by 14 mils or so in order to accommodate 2 signal layers
        between the planes.  With that spacing, the performance of the
        decoupling capacitors is limited by the spreading inductance of the
        power planes.  If you have more than a few dozen ceramic capacitors,
        you must use adjacent power planes in the stackup in order to make
them
        effective.  Otherwise, the impedance of the planes dominates over
the
        impedance of the capacitors at high frequency.

        regards,
        Larry Smith
        Sun Microsystems

        > Delivered-To: si-list@xxxxxxxxxxxxx
        > X-Lotus-FromDomain: APEX DATA INC
        > From: Anand.Kuriakose@xxxxxxxxxx
        > To: si-list@xxxxxxxxxxxxx
        > Date: Thu, 8 Aug 2002 22:06:59 +0630
        > Subject: [SI-LIST] Noise on BGA core voltage rail
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        > 
        > 
        > 
        > Hi,
        > 
        > Heres a situation where i have a BGA (chipset with interface to
processor, DDR
        > memory and other high speed proprietry buses) with sufficient
decoupling
        > sprinkled around the BGA. The decoupling on the core voltage rail
(2.5V, which
        > is also the I/O voltage for DDR interface)  basically consists of
2 high value
        > bulk capacitors, six 1uf caps, ten each of 0.1uF and 0.01uF caps.
I am 
        noticing
        > noise around 150mv of noise during activity on the DDR (using
software
        > utilities) and roughly 80-100 mV  during almost no activity across
the chip.
        > This amplitude is significantly more than the noise noticed at
other high 
        speed
        > chips on the board. Using the FFT function on the DSO, i figured
out that the
        > frequencies where it peaks are 200Mhz and integral multiples of
200Mhz.
        > 
        > I tried a couple of things:
        > 
        > 1>   Since i am seeing peaks at 200Mhz and its integral multiples,
i thought
        > that there could be insufficient high frequency decoupling and
hence i replace
        > the  0.1uF caps with 1000pF caps.
        > 2>   Secondly, fearing that there could be some resonance
happening due the
        > different values of caps used, i replaced all the 0.1 uF caps with
0.01uF caps
        > (in  addition to existing 0.01uF caps).
        > 
        > Both the above strategies failed to reduce the noise. There was no
change in 
        the
        > amplitude of the noise and also the frequencies where peaks were
noticed.
        > 
        > Can somebody out there throw some light on what is lacking in the
strategies
        > mentioned above and how to reduce this noise satisfactorily.
        > 
        > Thanx in advance.
        > Anand.
        > 
        > 
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