Re: [SI-LIST] : RE: Power planes

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From: Alan Hilton-Nickel (ahilton@transmeta.com)
Date: Wed May 02 2001 - 19:16:29 PDT


Hi Doug,

I have a couple of comments and some questions, the answers to which
will probably generate some more comments.

I would always place a ground plane next to a power plane. High
frequency transients exist in the power distribution network, not just
the signal lines. You need a low-impedance path from your power supply.
So layer 15 needs an associated ground plane. the two grounds on either
side of power layer 3 are overkill.

If, as Matt suggests, you use a 3-4 mil separation between the power and
ground pair, then splits will not bother you too much as the interplane
capacitance will provide a low-inductance path for return signals. You
could combine the 3.3V and 2.5V power on a split plane as well,
depending on how the placement segragates the power pins.

You don't mention much about the use of the boards and planes. I'd like
to know what your edge rates (and clock frequencies) are. Are all layers
meant to be high-speed layers? Are the outer layers meant only to
breakout signals from BGAs and pads, for immediate routing to the
high-speed layers, or are they also for high-speed routing? Are you
doing any differential signals?

8 routing layers out of 16 seems like an inefficient stackup to me, but
the purpose of the board will determine what is really necessary... :-)

Alan Hilton-Nickel
Transmeta Corp.

Doug Hopperstad wrote:
>
> I am currently looking at the following stackup for a design and would like
> some feedback:
>
> 1. ---- ---- Top layer (Very little routing) (2 oz)
> 2. --------- Ground (1 oz)
> 3. --------- Split Power (1 oz)
> 4. --------- Ground (0.5 oz)
> 5. ---- ---- Routing (0.5 oz)
> 6. ---- ---- Routing (0.5 oz)
> 7. --------- Ground (0.5 oz)
> 8. ---- ---- Routing (0.5 oz)
> 9. ---- ---- Routing (0.5 oz)
> 10. --------- Ground (0.5 oz)
> 11. ---- ---- Routing (0.5 oz)
> 12. ---- ---- Routing (0.5 oz)
> 13. --------- Ground (0.5 oz)
> 14. --------- Power - 3.3v (1 oz)
> 15. --------- Power - 2.5v (1 oz)
> 16. ---- ---- Bottom layer (Very little routing) (2 oz)
>
> The dual-striplines are separated by grounds and the power-to-grounds are
> adjacent to each other for high frequency noise coupling. The board will be
> made from Getek with Er = 3.9 with a total thickness of 125 mils.
>
> Dielectric spacing between layers:
> 1:2, 15:16 = 14 mils
> 2:3, 14:15 = 6 mils
> 3:4, 4:5, 6:7, 9:10, 10:11, 12:13, 13:14 = 4 mils
> 5:6, 8:9, 11:12 = 12 mils
>
> Layers 14 and 15 are going to be dedicated power planes, each for a single
> voltage source (2.5v and 3.3v). The split plane will have 1.35v, 1.8v and
> 5v.
>
> 1. Is there any concerns with making the outer layers 1 or 2 ounce compared
> to the inner layers set at 0.5 ounce?
> 2. Put the split power plane between two grounds planes?
> 3. How much of a dielectric spacing would be best between two adjacent power
> planes?
>
> I am open to using layer 2 as a power plane instead of a ground. I look
> forward to any comments on this issue, thanks.
>
> Doug Hopperstad
>
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