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

• From: Scott McMorrow <scott@xxxxxxxxxxxxx>
• To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
• Date: Tue, 26 Feb 2008 09:55:25 -0500

```Lee
I apologize for misinterpreting your calculation of capacitor
inductance.  However, your numbers still do not hold.  Lets take a board
where the planes are 40 mils down with a via inductance of 17.5 pH/mil
per via pair for 0402 capacitors and 5 pH/mil per 6 via mount for X2Y
capacitor.

40 mil case
Via attach inductance = 17.5 pH/mil x 40 = 700 pH
0402 capacitor inductance = 450 pH
total inductance 0402 capacitor = 1.15 nH

X2Y capacitor via attach inductance = 5 pH/mil x 40 = 200 pH
X2Y capacitor inductance = 120 pH
total inductance X2Y capacitance = 320 pH

Inductance ratio  0402 vs. X2Y = 1150/320 = 3.59:1 (planes 40 mils down)

If we perform the same calculations with the planes 20 mils down, we get:

Total inductance 0402 capacitor = 800 pH
Total inductance X2Y capacitor = 220 pH
Inductance ratio 0402 vs. X2Y = 3.64:1

By virtue of the fact that low inductance capacitors utilize multiple
vias that take advantage of mutual inductance coupling, there are
inherent inductance advantages in the body of the capacitor itself and
within the coupled via system that penetrates the planes.  For example,
if you look at an X2Y capacitor via pattern you will see that although
there are 6 vias used, there are 4 well-coupled via pairs.  This
provides lower overall inductance than what would be provided by 3 sets
of spatially isolated via pairs, as is the case with 0402 capacitor
mounting patterns.

No matter how we cut it, simulate it, or measure it, a well-mounted X2Y
capacitor has better than 3:1 performance advantage over an 0402
capacitor.   It's not magic, it's science.

Regards,

Scott

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

http://www.teraspeed.com

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

Lee Ritchey wrote:
> I disagree that ultralow inductance capacitors perform significantly better
> that standard two leaded capacitors such as 0402 and 0603 due to the added
> inductance associated with the mounting pads and mounting vias.  This has
> been supported many times by physical measurements on real PCBs and is
> documented in my book, "Right the First Time, A Practical Handbook on High
> Speed PCB and Systetm Design" as well as in several other sources.
>
> I am mystified at how it can be claimed that this is not so when the
> inductance of the vias connecting capacitors to the associated planes
> averages something like 35 pH per mil of length- no matter what  kind of
> magical capacitor they are connected to.  It is especially true when one is
> speaking of a six layer PCB that has planes 20-40 mils below the surface.
> That is 0.7 nH to 1.4 nH per lead.  Even if the capacitor has 0.1 nH
> inductance as an IDC capacitor is said to have, the total inductance is
> driven up near 1 nH.  How can any capacitor make this go away?
>
> It is time to stop pretending that there are magic capacitors out there.
>
> Lee Ritchey
> Speeding Edge.
>
>
>
>> [Original Message]
>> From: steve weir <weirsi@xxxxxxxxxx>
>> To: QU Perry <Perry.Qu@xxxxxxxxxxxxxxxxxx>
>> Cc: <si-list@xxxxxxxxxxxxx>
>> Date: 2/25/2008 11:52:30 AM
>> Subject: [SI-LIST] Re: 6 layers stackup
>>
>> Perry, the discussion has been limited to 4/6 layer boards with a single
>> symmetric power cavity.  The Z axis inductance for an IC or bypass cap
>> to the cavity depends on the distance to the center of the cavity.  With
>> a single, symmetric cavity, that is going to be half the board
>> thickness.  This is not to be confused with higher layer count boards
>> where we can place at least one modestly thin cavity near the IC /
>> capacitor mounting surface.  In all cases, the cavity spreading
>> inductance becomes a limiting factor in PDN impedance.  Cavity impedance
>> varies directly with cavity height.
>>
>> In the case of any almost thickness PCB, the via attachment structure is
>> a significant, if not dominant contributor to the mounted capacitor
>> inductance, that is the inductance as seen at that attachment point at
>> the planes.  Capacitors like X2Y(r)'s, and IDC(r)'s when properly
>> attached yield much lower inductance than discrete caps.  This is
>> readily modeled in any number of tools, and confirmed by properly
>> constructed experiments.  So low inductance caps still work, whether or
>> not you have a thin cavity, and whether or not the cavity is adjacent to
>> the caps.
>>
>> We are talking about two different resonances.  The resonance that you
>> are referring to are the half-wave modes.  I am talking about the
>> parallel resonance between the bypass network and the power cavity.
>> This is the beast that eats most people's lunch.  It is a matter of
>> capacitance per unit area of the cavity which depends directly on
>> height, and the area / unit inductance of the bypass network.
>>
>> Best Regards,
>>
>>
>> Steve.
>>
>> QU Perry wrote:
>>
>>> Steve:
>>>
>>> My understanding on the impact of thinner power cavity is mainly the
>>> reduction of spread inductance, such that any added benefit of IDC/X2Y
>>> placed at the peripheral of BGA will not be compromised by the planes..
>>> In most applications however, we rely heavily on the decoupling caps
>>> (0402) directly placed underneath BGAs, and in those cases, I would
>>> think thickness of power cavity is not important as the total inductance
>>> looking into PCB from BGA pads to the planes and to the decoupling caps
>>> don't change. Your thoughts ?
>>>
>>> I'm also not clear when you say parallel resonance frequency is driven
>>> by thickness. Comparing Z dimension vs. X/Y for a normal power plane/PCB
>>> thickness, I would say the resonance frequency is mainly determined by
>>> how big the plane is not how thick the cavity ?
>>>
>>> Thanks
>>>
>>> Perry
>>>
>>> =======================================
>>>
>>> Perry Qu
>>>
>>> Design & Qualification, Alcatel-Lucent Canada Inc.
>>>
>>>
>>> DID: 613-7846720  Fax: 613-5993642
>>>
>>> Email: perry.qu@xxxxxxxxxxxxxxxxxx
>>>
>>> =======================================
>>>
>>>
>>>
>>>
>>>
>>>> -----Original Message-----
>>>> From: si-list-bounce@xxxxxxxxxxxxx
>>>> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of steve weir
>>>> Sent: Saturday, February 23, 2008 6:44 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 power cavity:
>>>>
>>>> 1) The Z-axis inductance seen at the IC solder pads to the
>>>> power cavity 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 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 IDC(r)'s yield the best results.  In all cases the via
>>>> pattern used 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 caps w/ optimal via
>>>> patterns up front, than to have the PCB designer try 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 extremes for six layer 0.062" stack-ups can be almost
>>>> 10:1 going from a
>>>> 4 mil to a 38 mil power core.
>>>> b. The high frequency impedance of the power system comes
>>>> down.  On the bad side one will be in PCB wave effects at
>>>> lower frequencies.  Detuning w/ discretes takes about the
>>>> same number of parts independent of the cavity thickness.
>>>> Tolerances are more forgiving for the thinner cavity.
>>>> c. The parallel resonant frequency of the power system comes
>>>> down as the square root of the power cavity thickness.
>>>> Typical resonant frequencies typically vary over a 300MHz to
>>>> 1.5GHz range depending on bypass scheme over the 4mil to
>>>> 38mil cavity thicknesses.
>>>> d. The Q of the parallel resonance goes up.  On the good
>>>> side, higher Qs
>>>> are generally easier to detune.   The bad side is that the natural
>>>> magnitude of Zpeak is fairly independent of the cavity
>>>> thickness, now it is much more likely to be where there is
>>>> more signal energy.  The moral 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 stack-up become more pronounced and routing density
>>>> can suffer severely.  Maintaining 50Ohms and/or acceptable
>>>> cross talk values on 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 can be broken out and routed completely on S1.
>>>> Otherwise S1 P S2 S3 G
>>>> S4 is usually better breaking out high speed signals on layer
>>>> S4 first and layer S3 second, minimizing via stubs.  In
>>>> either case prioritizing the traces with the most high speed
>>>> energy to the routing layer(s) adjacent an image plane
>>>> connected to the dominant coupling rail in the IC will help
>>>> reduce demands on the PDN.  That rail is usually ground.
>>>>
>>>> Best Regards,
>>>>
>>>> Steve.
>>>>
>>>>
>>>> DAVID CUTHBERT wrote:
>>>>
>>>>
>>>>> 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:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>> 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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>>>> --
>>>> Steve Weir
>>>> Teraspeed Consulting Group LLC
>>>> 121 North River Drive
>>>> Narragansett, RI 02882
>>>>
>>>> California office
>>>> (707) 780-1951 Fax
>>>>
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>>>> (401) 284-1840 Fax
>>>>
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>>>
>>>
>> --
>> Steve Weir
>> Teraspeed Consulting Group LLC
>> 121 North River Drive
>> Narragansett, RI 02882
>>
>> California office
>> (707) 780-1951 Fax
>>
>> Main office
>> (401) 284-1840 Fax
>>
>> Oregon office
>> (503) 430-1285 Fax
>>
>> http://www.teraspeed.com
>> This e-mail contains proprietary and confidential intellectual property
>>
> of Teraspeed Consulting Group LLC
>
> ----------------------------------------------------------------------------
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