Re: [SI-LIST] : stackup question

John Fisher (jmfisher@cisco.com)
Tue, 14 Apr 1998 18:24:58 -0700

Roland,

I thought that the standard core thicknesses at the thin end were .002 and
.004 " .

Your fab house must be using a .002 single ply core plus some prepreg.

This thickness is close to what the buried capacitance vendors use.

Are your fab houses comfortable with this?
Any issues using the single ply laminates?

Regards,

John

*********************************************************************
John M. Fisher -- Cisco Systems -- jmfisher@cisco.com
170 W. Tasman Dr., Bldg G, San Jose CA 95134-1706
__ _ _
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Signal Integrity Engineer, Enterprise Line of Business
408-527-9186, Fax 408-526-5504, N6PFN
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At 05:38 PM 4/14/98 -0700, you wrote:
>Paul,
>
>I have a simalar stackup for my 10 layer board. Only difference is the
>location of Power 2 and Ground 2 .
>
>Put the most heavily used, high frequency, etc. power plane pair in the
>middle of the board. Separate this
>pair by 2.5 mils of FR4. The results will be capacitive coupling of
>these two planes (500 to 1000 pf/ sq. inch). This should help very high
>frequency return currents. This doesn't mean you can remove the 0.1uf
>0603 bypass caps that you have scattered around the board. I haven't
>had much success removing bypass caps.
>Make up the board thickness by adding between Signals 2/3 and signals
>4/5.
>
>Be sure BOTH grounds are electrically the same (tied together) (each
>ground via attaches to both).
>
>That leaves a Ground at layer 2 and Power at layer 9. (See Below)
>
>Be sure to make Signal 1, 2, 3, 4, 5, 6 nearly the same impedance. Be
>sure to separate the
>traces on Signal 1 and Signal 2 a little more to reduce crosstalk.
>Remember, crosstalk drops off
>by the square of the distance between the traces, so you don't have to
>separate them too much more
>to gain significant results. Then concentrate your routing
>on the top side to signal 1 and 2, and on the bottom to signal 5 and
>6. This will help make sure the
>return currents go to the same return plane when you switch layers.
>(We are trying to eliminate
>signal 3 and 4 and go for a 8 layer board, keeping the rest of the
>stackup the same.)
>
>I hope this helps.
>
>Roland
>
>
>
>Paul Thompson wrote:
>>
>> I'd like to know what people think about the following stackup for a 10
>> layer board with 2 power planes and 2 ground planes:
>
>
>> Signal 1
>> -4 mil-
>
>Ground
>
>> -5 mil-
>> Signal 2
>> -8 mil-
>> Signal 3
>> -5 mil-
>
>Power 1
>-2.5 mil-
>Ground
>
>> -5 mil-
>> Signal 4
>> -8 mil-
>> Signal 5
>> -5 mil-
>
>Power 2
>
>> -4 mil-
>> Signal 6
>>
>> The idea is first to get the impedance of all layers quite close, and
>> secondly to get as much of the return currents as possible in the ground
>> planes rather than the power planes (this is obviously not the case for
>> signal layers 3 and 4, but I can keep my critical signals off them.
>>
>> Can anyone think of a better stackup, or are my priorities mixed up for a
>> high speed (~100 MHz) digital board? Am I sacrificing high speed
>> decoupling by not putting power and ground planes right next to each other?
>> (One plane is 5V and the other is 3.3V, BTW)
>>
>> Regards,
>>
>> Paul
>>
>> --
>> Paul Thompson crash@apple.com
>> System Integrator, Macintosh Desktop Systems Apple Computer
>
>Signal 1
>-4 mil-
>Ground 1
>-5 mil-
>Signal 2
>-8 mil-
>Signal 3
>-5 mil-
>Power 1
>-6 mil-
>Power 2
>-5 mil-
>Signal 4
>-8 mil-
>Signal 5
>-5 mil-
>Ground 2
>-4 mil-
>Signal 6
>
>The idea is first to get the impedance of all layers quite close, and
>secondly to get as much of the return currents as possible in the ground
>planes rather than the power planes (this is obviously not the case for
>signal layers 3 and 4, but I can keep my critical signals off them.
>
>Can anyone think of a better stackup, or are my priorities mixed up for
>a
>high speed (~100 MHz) digital board? Am I sacrificing high speed
>decoupling by not putting power and ground planes right next to each
>other?
>(One plane is 5V and the other is 3.3V, BTW)
>
>Regards,
>
>Paul
>
>--
>Paul Thompson crash@apple.com
>System Integrator, Macintosh Desktop Systems Apple Computer
>
>