Re: [SI-LIST] : 18 layer stackup question

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From: sweir (weirsp@a.crl.com)
Date: Wed Apr 12 2000 - 12:52:20 PDT


Ellis,

1 oz is very heavy for inner layers on this dense a stack-up. 1 oz may
help on the DC distribution layers if your current draw is very high. 1/2
oz is more typical, and before vias closely approximates 1 milliohm inch /
inch. I assume at 2 mils you are using the Hadco material, as 3 mils is
generally about it for FR4, unless you pay for special handling. The
spacing on signal layers should be set based on a target line width and
impedance. If you don't have a field solver, your board house should.

Regards,

Steve.
At 02:36 AM 4/12/00 -0700, you wrote:
> > I am currently in layout with an 18 layer board stacked up as follows:
>
>1 - TOP
> 3 mil
>2 - GND
> 5 mil
>3 - IN1
> 4 mil
>4 - IN2
> 5 mil
>5 - GND
> 2 mil (buried cap)
>6 - 3.3V
> 5 mil
>7 - IN3
> 4 mil
>8 - IN4
> 5 mil
>9 - GND
> 2 mil (buried cap)
>10 - 5V
> 5 mil
>11 - IN5
> 4 mil
>12 - IN6
> 5 mil
>13 - GND
> 2 mil (buried cap)
>14 - 1.8V
> 5 mil
>15 - IN7
> 4 mil
>16 - IN8
> 5 mil
>17 - GND
> 3 mil
>18 - BOTTOM
>
>All layers use 1 oz Cu except TOP and BOTTOM use 0.5 oz. I have specified
>minimum of 20 mil center-to-center distance between signal layers to
>minimize cross talk. This give a D/H ratio of 4, per
>Howard Johnson's book. Is this overkill? Most signals are running
>between 66-100 MHz, with rise times of 1-3 ns. I believe Dr. Johnson
>provides an example where he set the D/H ratio to 5 minimum.
>
>Does anyone have any comments on the stackup? Is there anything I should
>watch out for?
>
>All inner signal layers are closer to each other (4 mil) than to their
>respective reference planes (5 mil). Will this cause any problems?
>
>
>
>Thanks in advance.
>Ellis
>
>
>
>
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