> I. ALLEGRO DATABASES
> An ALLEGRO PCB database (i.e. a .brd file) can be viewed by an
>Allegro viewer program, freely available from the Cadence Web Site,
>which identifies and displays the stackup of the design. For example
>stackup of an eight layer board (consisting of four signal layers, three
>ground, and one power layer) using Allegro viewer, is identitfied as:
> When extracting an Allegro database, I also examine the Layer.dat
>file ( an ASCII data file which results from the .brd file after
>execution of the Allegro EXTRACT utility batch program) for stackup
>details. For example, the following section of a Layer.dat file includes
>information regarding the thickness and dielectric constant of the Top
>layer, ground layer, an inner signal layer and substrate:
> S!1!TOP!POSTIVE!!YES!!595900mho/cm!COPPER!NO!3.98 w/cm-degC!1.2mil!
> S!2!!!NO!4.500000000000e+000!0 mho/cm!FR-4!!0.012 w/cm-degC! 8mil!
> S!3!GND2!NEGATIVE!EMBEDED_PLANE!YES!!595900 mho/cm!COPPER!YES!3.98
> S!4!!!!NO!4.500000000000e+000!0 mho/cm!FR-4!!0.012 w/cm-degC!8 mil!
> S!5!INNER3!POSITIVE!!YES!!595900 mho/cm!COPPER!NO!3.98 w/cm-degC!1.2
As you point out, the Allegro database "cross sectional characteristics"
contain data for layer thickness, dielectric constant and conductivity.
That information, combined wi the trace information, allows post-layout
circuit equivalents to be accurately extracted.
Allegro database information such as cross sectional characteristics,
device CLASS, PINUSE, etc. is often not set up correctly in a given
database. Layers have certain default properties unless overridden with
actual values by the user. Since the electrical information is not
actually needed to manufacture the board, some people never set this up in
Bottom line, you need to make sure the electrical characteristics specified
in the Allegro database are correct for the board under consideration, or
your SI analyses are for naught. Some SI tools that rely on data extracted
from .dat files (as you mention) allow the board's cross sectional
characterisitics to be specified in a separate text file - but this is
really a poor second to putting the correct data in the Allegro database to
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