Sorry for the late response Dennis,

I'm not an experienced board designer but ...
I've seen this discussed in _High Speed Digital Design_
by Howard Johnson and Martin Graham.  They use a thing
called "Rent's Rule".  It's an equation that gives you
average trace spacing by putting in a few parameters
and it goes like this ...

#1)  ave. trace pitch = (2.7*M*[X*Y]^0.5)/N

 M = number of routing layers
 N = number of connections
 X = board width in inches
 Y = board length in inches

You could of course turn this around and
solve for M (layers)

 M = N*(ave. trace pitch)/(2.7*[X*Y]^0.5)

You'll have to round up to get a whole number.

I came up with a slightly different equation that
is a little more conservative.

#2)  ave. trace pitch = (2*M*[X*Y]^0.5)/(N+2)

 Variables mean the same as above.

Turning this around, you could solve for M as

 M = (N+2)*(ave. trace pitch)/(2*[X*Y]^0.5)

You'll have to also round up to get a whole number.

Both work equally well in an Excel spreadsheet.
Keep in mind this number gives you *routing layers*
so power planes and ground planes don't count.
You'll have to add them into your final layer count.

And also, these equations asume an awful lot of stuff
that might not be very realistic, such as everything
is perfectly evenly distributed all over the board.

Given your 16.5 x 11.2 inches with 10 or 12 layers yields

Using Rent's rule
                         Qty    Units
Number of Connections    1000
Board Length             16.5   inches
Board Width              11.2   inches
Number of Routing Layers 10
-> Average Trace Pitch = 0.367  inches

Double the number of connections and the
average trace pitch goes down by a half,
so for 2000 connections, ave pitch = 0.183,
for 5000 connections, ave.pitch = 0.073
That's why I used 1,000 connections.

Increasing the number of layers increases
the trace pitch.

As a side note -

Using Doug's Rule
                         Qty    Units
Number of Connections    1000
Board Length           16.5   inches
Board Width              11.2   inches
Number of Routing Layers 10
-> Average Trace Pitch = 0.271  inches


Hope that's not too lengthy.  You can run
the same type of comparison for finding the
number of *routing* layers needed.

Regards,  Doug


At 09:20 AM 2/18/99 -0500, Dennis Ward wrote:
>Good morning,
>
>I was hoping some of you experienced PCB designer would
>share a few pearls of wisdom with me.
>
>Our design team is ramping up for our next design and
>there are some real concerns if everything will fit on the
>board.
>
>When a design is first initiated how do you determine the
>proper amount of space versus layer count??
>Right now we are using a 16.5 X 11.2 board, populated
>both sides. Right now we're hoving at 10 -12 layers per
>board. Is there some formula, methodology, metric, etc,etc
>that actually works??
>
>Right now would would like to keep the 16.5 X 11.2 size,
>but if the board becomes so packed that it becomes a
>nightmare to manufacture there is no gain in keeping
>our current size.
>
>thanks for the help
>Dennis

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