[SI-LIST] Re: [!! SPAM] Re: 6 layers stackup

Curt
Actually, to be technically correct, the impedance of the top layer 
traces are the same whether a ground or a power layer is used directly 
under the surface. It is the transition from package to PCB that can 
cause common modes to occur, with subsequent radiation.  Whether or not 
that happens is dependent, not upon what layer is used underneath the 
traces, but rather, what the signals were referenced to prior to exit 
from the package body, and the loop area between the signal balls/pins 
and reference balls/pins.

Many packages use ground referenced traces, therefore ground referencing 
of the traces on the top layer of a PCB will lower common mode crosstalk 
and subsequent emissions.  However, not all packages utilize ground 
referencing for all signals.  Nor do all packages keep the loop area 
between the signal and return path through the balls/pins small.  In 
those cases, there will be ample common mode noise traveling across the 
nearest plane.


regards,

Scott

Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC



Curt McNamara wrote:
> Steve, every trace on the surface and the IC packages are exposed =
> antennas! Ground under the surface allows lower impedance of traces and =
> crystals along with lower package impedance to GND (lower Vss bounce in =
> your terms).=20
>
> To translate your other point: having traces inside the power<> ground =
> pair traps that energy, which can excite the planes and the cavity. This =
> is another part of the trade-off. So if we have to use S1 G S2 S3 P S4 =
> we route critical traces on the surface layers.
>
>                                                               Curt
>
>
> Curt McNamara, P.E.=A0// principal electrical engineer=20
> Logic Product Development
> 411 Washington Ave. N. Suite 400
> Minneapolis, MN 55401
> T // 612.436.5178
> F // 612.672.9489
> www.logicpd.com=20
> / / / / / / / / / / / / / / / / / / / / / / / / /=20
> This message (including any attachments) contains confidential =
> information intended for a specific individual and purpose, and is =
> protected by law. If you are not the intended recipient, you should =
> delete this message and are hereby notified that any disclosure, =
> copying, or distribution of this message, or the taking of any action =
> based on it, is strictly prohibited.
>
>
>
> -----Original Message-----
> From: steve weir [mailto:weirsi@xxxxxxxxxx]=20
> Sent: Monday, February 25, 2008 11:19 AM
> To: Curt McNamara
> Cc: DAVID CUTHBERT; Fernando Yuitiro Mori; si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: 6 layers stackup
>
> Curt, from the emissions stand-point, Vss bounce matters when that=20
> bounce occurs on an exposed antenna.  Sometimes a die w/o a heatsink=20
> above the planes forms that antenna and reducing the loop height to Vss=20
> is helpful.  In one of life's nothing comes for free categories is the=20
> problem that for CMOS the greater the I/O coupling through the PDN is:=20
> the greater the tendency for both the I/O signals and the PDN to both=20
> radiate, and the greater I/O susceptibility to external disturbances.
>
> Best Regards,
>
>
> Steve.
>
> Curt McNamara wrote:
>   
>> Steve, that is true for the designs we do as well. The trade-off seems =
>>     
> to be trace impedance. With traces inside the power<>ground pair then =
> impedance matching between layers is closer, but at the cost of =
> inter-plane capacitance and all its benefits.
>   
>> Since a main concern of ours is emissions and immunity, reduction of =
>>     
> common mode (your Vss bounce) is more important than Vcc bounce, which =
> can be dealt with other ways.
>   
>>                                                              Curt
>>
>>
>> Curt McNamara, P.E. // principal electrical engineer=20
>> Logic Product Development
>> 411 Washington Ave. N. Suite 400
>> Minneapolis, MN 55401
>> T // 612.436.5178
>> F // 612.672.9489
>> www.logicpd.com=20
>> / / / / / / / / / / / / / / / / / / / / / / / / /=20
>> This message (including any attachments) contains confidential =
>>     
> information intended for a specific individual and purpose, and is =
> protected by law. If you are not the intended recipient, you should =
> delete this message and are hereby notified that any disclosure, =
> copying, or distribution of this message, or the taking of any action =
> based on it, is strictly prohibited.
>   
>> -----Original Message-----
>> From: steve weir [mailto:weirsi@xxxxxxxxxx]=20
>> Sent: Monday, February 25, 2008 10:46 AM
>> To: Curt McNamara
>> Cc: DAVID CUTHBERT; Fernando Yuitiro Mori; si-list@xxxxxxxxxxxxx
>> Subject: Re: [SI-LIST] Re: 6 layers stackup
>>
>> Curt, true Vss bounce improves when Vss is closest to the IC, just the =
>>     
>
>   
>> same as Vcc bounce improves when it is closest.  But for most 4/6 =
>>     
> layer=20
>   
>> PCB circumstances I find I lose more by making Vss the near plane than =
>>     
> I=20
>   
>> gain.
>>
>> Best Regards,
>>
>>
>> Steve.
>> Curt McNamara wrote:
>>  =20
>>     
>>> One other point about stack-ups -- putting ground near the top layer =
>>>       
> provides lower impedance to ground for any IC packages located there.
>   
>>>                                                     Curt
>>>
>>>
>>> Curt McNamara, P.E. // principal electrical engineer=20
>>> Logic Product Development
>>> 411 Washington Ave. N. Suite 400
>>> Minneapolis, MN 55401
>>> T // 612.436.5178
>>> F // 612.672.9489
>>> www.logicpd.com=20
>>> / / / / / / / / / / / / / / / / / / / / / / / / /=20
>>> This message (including any attachments) contains confidential =
>>>       
> information intended for a specific individual and purpose, and is =
> protected by law. If you are not the intended recipient, you should =
> delete this message and are hereby notified that any disclosure, =
> copying, or distribution of this message, or the taking of any action =
> based on it, is strictly prohibited.
>   
>>>
>>> -----Original Message-----
>>> From: si-list-bounce@xxxxxxxxxxxxx =
>>>       
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of steve weir
>   
>>> Sent: Saturday, February 23, 2008 5:44 PM
>>> To: DAVID CUTHBERT
>>> Cc: Fernando Yuitiro Mori; si-list@xxxxxxxxxxxxx
>>> Subject: [SI-LIST] Re: 6 layers stackup
>>>
>>> Dave, Fernando my $0.02 on 4/6 layer stack-ups with a single =
>>>       
> symmetric=20
>   
>>> power cavity:
>>>
>>> 1) The Z-axis inductance seen at the IC solder pads to the power =
>>>       
> cavity=20
>   
>>> is pretty much fixed by:
>>>
>>> a. The total thickness of the PCB.
>>> b. The pin-out of the IC.
>>> c. The via drill diameter.
>>>
>>> 2) Similarly the Z-axis inductance seen between the bypass caps and =
>>>       
> the=20
>   
>>> power cavity is fixed by:
>>>
>>> a. The total thickness of the PCB.
>>> b. The type of bypass capacitors used.
>>> c. The via pattern used w/ the bypass caps.
>>> d. The via drill diameter.
>>> e. The areal density of the bypass caps used.
>>>
>>> b/c/d Determine the mounted inductance of each cap.  X2Y(r)'s and=20
>>> IDC(r)'s yield the best results.  In all cases the via pattern used=20
>>> makes a big difference in the number of caps used and the behavior at =
>>>       
>
>   
>>> parallel resonance.  In my mind it is a lot better to floor plan =
>>>       
> bypass=20
>   
>>> caps w/ optimal via patterns up front, than to have the PCB designer =
>>>       
> try=20
>   
>>> to fit them in later.
>>>
>>> 3) As the power cavity is made thinner, six notable things happen:
>>>
>>> a. The horizontal spreading inductance of the planes falls.  The=20
>>> extremes for six layer 0.062" stack-ups can be almost 10:1 going from =
>>>       
> a=20
>   
>>> 4 mil to a 38 mil power core.
>>> b. The high frequency impedance of the power system comes down.  On =
>>>       
> the=20
>   
>>> bad side one will be in PCB wave effects at lower frequencies.  =
>>>       
> Detuning=20
>   
>>> w/ discretes takes about the same number of parts independent of the=20
>>> cavity thickness.  Tolerances are more forgiving for the thinner =
>>>       
> cavity.
>   
>>> c. The parallel resonant frequency of the power system comes down as =
>>>       
> the=20
>   
>>> square root of the power cavity thickness.  Typical resonant =
>>>       
> frequencies=20
>   
>>> typically vary over a 300MHz to 1.5GHz range depending on bypass =
>>>       
> scheme=20
>   
>>> over the 4mil to 38mil cavity thicknesses.
>>> d. The Q of the parallel resonance goes up.  On the good side, higher =
>>>       
> Qs=20
>   
>>> are generally easier to detune.   The bad side is that the natural=20
>>> magnitude of Zpeak is fairly independent of the cavity thickness, now =
>>>       
> it=20
>   
>>> is much more likely to be where there is more signal energy.  The =
>>>       
> moral=20
>   
>>> here is:  detune the resonance.
>>> e. Above and below the resonant frequency noise attenuation improves.
>>> f. The asymmetry between outer and inner routing layers in a 6 layer=20
>>> stack-up become more pronounced and routing density can suffer=20
>>> severely.  Maintaining 50Ohms and/or acceptable cross talk values on=20
>>> outer layers more than about 10 mils from an image plane demands some =
>>>       
>
>   
>>> rather wide traces and routing pitches.
>>>
>>> 4) An S1 G S2 S3 P S4 stack-up works best when the highest speed =
>>>       
> signals=20
>   
>>> can be broken out and routed completely on S1.  Otherwise S1 P S2 S3 =
>>>       
> G=20
>   
>>> S4 is usually better breaking out high speed signals on layer S4 =
>>>       
> first=20
>   
>>> and layer S3 second, minimizing via stubs.  In either case =
>>>       
> prioritizing=20
>   
>>> the traces with the most high speed energy to the routing layer(s)=20
>>> adjacent an image plane connected to the dominant coupling rail in =
>>>       
> the=20
>   
>>> IC will help reduce demands on the PDN.  That rail is usually ground.
>>>
>>> Best Regards,
>>>
>>> Steve.
>>>
>>>
>>> DAVID CUTHBERT wrote:
>>>  =20
>>>    =20
>>>       
>>>> Fernando,
>>>> The S1 S2 G P S3 S4 stackup can provide excellent power plane =
>>>>         
> performance at
>   
>>>> the expense of S1 and S4. Routing S1 and S4 mostly at right angles =
>>>>         
> to S2 and
>   
>>>> S3 can greatly reduce the crosstalk. And using narrow traces to =
>>>>         
> maintain the
>   
>>>> Z0 of S1 and S4 will take care of the Z0.
>>>>
>>>> I often use S1 G S2 -  S3 P S4 for 6-layer boards. The signal traces =
>>>>         
> are
>   
>>>> nicely isolated with a 62 mil board having spacing like so:
>>>> 10 mils, 5 mils, 22 mils, 5 mils, 10 mils. The tradeoff is that the =
>>>>         
> power
>   
>>>> plane Z0 is about 2X that of a board having 10 mils between each =
>>>>         
> layer. The
>   
>>>> power plane Z0 is still quite low with an inductance of about 200 pH =
>>>>         
> per
>   
>>>> square. Contrast this to an S1-G via inductance of about 300 pH and =
>>>>         
> the
>   
>>>> plane Z does not dominate things.
>>>>
>>>>      Dave Cuthbert
>>>>      NARTE Certified EMC Engineer
>>>>      Consulting, SI, EMC, power electronics, analog of all kinds
>>>>
>>>>
>>>> On Wed, Feb 20, 2008 at 2:17 PM, Fernando Yuitiro Mori =
>>>>         
> <mori@xxxxxxxxxxxxx>
>   
>>>> wrote:
>>>>
>>>>  =20
>>>>    =20
>>>>      =20
>>>>         
>>>>> Hi,
>>>>> I normally use S1 S2 G P S3 S4 for the 6 layers stackup. I need the =
>>>>>           
> 4
>   
>>>>> layer with 60 ohms, so there are some problem if I use S1 G S2 S3 P =
>>>>>           
> S4?
>   
>>>>> Regards,
>>>>>
>>>>> Fernando Mori
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>>>>>    =20
>>>>>      =20
>>>>>        =20
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>>>>         
>>>  =20
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>>>       
>>  =20
>>     
>
>
> --=20
> Steve Weir
> Teraspeed Consulting Group LLC=20
> 121 North River Drive=20
> Narragansett, RI 02882=20
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>
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