Probably most of you are well aware with how crucial stackup
information is to a SI engineer. A comprehensive knowledge of design
stackup can greatly contribute to the success and accuracy of a
simulation task. Some fundamental questions regarding design stackup
include:
1. The type, number and order of layers.
2. Do plane layers include any traces? do signal layers contain
ground fills?
3. Are power planes split (i.e. multiple power such as +5.0 V and
3.3 V) , or continuos (for instance single power +5.0 V) ?
4. Are ground or power planes meshed (hatched) or solid?
5. What are the values of substrate thickness and dielectric
constant?
It is of great importance for the SI engineer to have a detailed
understanding of the stackup, so that he or she can input the simulator
program with the most accurate data. Errors or inaccuracies in the
stackup can result in a varying range of simulation problems which
include:
False impedance values
Faulty waveforms and flight times
Incompletely generated topology
Halting of some simulation operations
Stackup is usually provided to the SI staff by the circuit design
engineer or the PCB designer. However, it is beneficial to know how to
extract stackup information from various PCB databases. Allow me to
elucidate this interesting concept with the aid of ALLEGRO and PADS
databases, as well as QUAD's Intermediate Segment File (.isf).
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:
TOP
GND2
INNER3
VDD
GND5
INNER6
GND7
BOTTOM
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
w/cm-
degC!1.2mil!
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
mil!
II. PADS DATABASES
The PADS viewer program can be used to observe a PADS PCB database
(.pcb) or ASCII file (.asc) and hence identify its stackup. Most
PADS-power PCB databses that I have worked with have included up to 30
layers consisting of several signal layers, several power and ground
plane layers, primary and secondary solder paste, primary and secondary
solder resist, primary and secondary assembly, annotation layer, etc.
Frequently, it is the PADS .asc and not the PADS .pcb file which is
inputted to the simulation program for extraction purposes. I usually
review the PADS ASCII file for stackup content. An illustration is
presented below:
Layer5 = Ground
16837 7565 0.000 24 100 7
Layer6 = Inner 2
16839 7432 0.000 24 100 7
Layer 7 = + 5V, STB5V
16839 8732 0.000 24 100 7
III.QUAD'S INTERMEDIATE SEGMENT FILE
Information regarding the stackup of a design is also present in the
Quad's Intermediate Segment File (.isf), as illustrated by the following
example:
N: 960DCLK2R
S: 700700, 742500,695100,742500,0,6
S:703700,743700,700700,742500,0,6
S:69990,747900,703700,743700,11,6
S:686500,745600,686500,759250,0,6
S:686900,742500,686500,745600,0,6
S:695100,742500,689600,742500,0,6
V:703700,743700,3,111111111111
P:TP459,1,1699900,747900,730,000000000001
P:RP4,3,1,686500,759250,732,100000000000
P:U11,14,1,695100,742500,690,110000000000
Above describes a net (N: ), its associated segment (S: ), via (V: )
and pads (P: ), as well as what layers they belong to, which also
allows to examine connectivity.
The main purpose of the Allegro layer.dat, PADS .asc and Quad .isf
examples has been to illustrate that numerous files may contain
important stackup details. In closing, stackup extraction involves
obtaining information regarding design stackup from various files to
check or to supplement the stackup diagrams often furnished by the
design engineer or the PCB designer. The ultimate aim is to enables the
SI engineer to input the simulator with the most accurate stackup,
towards a smooth and reliable simulation.
Like always, your comments are genuinely appreciated.
Respectfully,
Abe Riazi
Anigma, Inc.
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