Transcript Full Wave Simulation and Validation of a Simple Via Structure

Full Wave Simulation and
Validation of a Simple Via
Structure
Bruce Archambeault,
Samuel Connor, Daniel N. de Araujo, C. Schuster,
A.Ruehli, IBM
[email protected], [email protected],
[email protected], [email protected],
[email protected]
M. R. Has hemi, R. Mistral, Pennsylvania State
University
mmh244 @engr.psu.edu, [email protected]
PCBs and Very High Frequency
Data Rates
• Gaga-bit signal rates in common practice
today
• 10-12 Gb/s plan in near future
• Require at least 5th harmonic for
reasonable signal quality
– 12 Gb/s  30 GHz
• Both measurements and simulations
require extreme care at these frequencies!
Measurements @ 10-50 GHz+
• Calibration of VNA requires special
calibration fixtures
• Special probing techniques
• Extremely expensive equipment
• Requires costly PCBs with various
structures to be studied
• Difficult but possible
Full Wave Electromagnetic
Simulations
• Can save costs associated with test
equipment and test PCBs
• Eliminate calibration concerns
• Probes within models can be ‘perfect’
– No impact on results from the act of
measuring
• Software tools are mature and reasonably
priced (sometimes)
The High Speed PCB Problem
• Many layers
• Entry/exit on different
layers
• Inexpensive dielectrics
have more loss at high
frequencies
• Need to be able to
properly analyze all
effects
• Quasi-static models not
accurate at high
frequencies
Initial Geometry
Single Plane for Initial Models
Source
100 mil
10 mil
10 mil
50 mil
Round
Geometries!
Load
Dielectric Constant = 3.8
Trace Thickness = 1 mil
Via Barrel Diameter = 20 mil
Plane thickness = 1 mil
Via Antipad Diameter = 44 mil
Trace terminated in characteristic impedance = 80 Ohm
Via Pad Diameter = 34 mil
PCB size = 300 mil x 300 mil
Trace width = 7 mil
Metal = perfect electric conductor
Loss tan = 0
Absorbing boundary conditions!
Background DC = 1
Validation of Simulation Results
Extremely Important
• Measurement data not available
• Assumptions must be known
– By user
– Built into the software
– Inherent in simulation technique
• Different techniques have different
assumptions!
• Simulation validation by using multiple
modeling techniques
Initial Models Used r=1  Good Agreement
Via Single Plane Transition (Air Dielectric -- 136 ohm load)
S21 -- Zero Degree (trace enters & exits on same side of Via)
2
0
-2
-4
-6
-8
S21 (dB)
-10
-12
-14
-16
-18
FDTD Modified Square 0 degree
PEEC Square 0 Degree
CST Modified Square Size 0 degree
HFSS Modified Square Hole 0 degrees
cFDTD Zero Degree Modified Square
-20
-22
-24
-26
-28
-30
1.00E+03
1.00E+04
Frequency (MHz)
1.00E+05
How Good is “Good”?
• “I know it when I see it”
• Feature Selective Validation (FSV)
technique
– Allows a numerical comparison that agrees with
‘experts’
– Used in IEEE Standard on Model Validation
– Performs an FFT, separates low frequency data
and high frequency data
• Low frequency data provides indication of overall
amplitude agreement (ADM)
• High frequency data provides indication of rapidly
changing feature agreement (FDM)
FSV Results for ADM
Example
(One Plot for Each Pair of Techniques)
cFDTD vs FIT
PEEC vs FDTD
FSV Grade and Spread
• Easier to quickly compare plots
• Grade  Number of categories required to
get 85% Confidence starting at highest
(excellent agreement)
– Quality of agreement
• Spread  Number of categories required
to get 85% Confidence starting with the
largest category
– Consistency of the agreement
Summary of FSV Grade/Spread for Initial
Agreement Between Techniques
Techniques
Grade
Spread
cFDTD-FIT
2
2
cFDTD-FDTD
4
4
cFDTD-FEM
1
1
cFDTD-PEEC
3
3
FIT-FDTD
4
4
FIT-FEM
2
2
FIT-PEEC
3
3
FTDT-FEM
5
4
FDTD-PEEC
4
4
FEM-PEEC
3
3
3.1 (Excellent-Good)
3.0 (Excellent-Good)
Average
Final Models Used r=3.8  Good Agreement
Via Single Plane Transition (Dielectric=3.8 -- 50 ohm load)
S21 -- Zero Degree (trace enters & exits on same side of Via)
2
0
-2
-4
-6
-8
S21 (dB)
-10
-12
-14
PEEC Modified Square 0 degrees Background Dielectric
-16
CST Modified Square 0 degrees Background Dielectric
-18
FDTD Modified Square 0 Degrees Background dielectric
-20
-22
-24
-26
-28
-30
1.00E+03
1.00E+04
Frequency (MHz)
1.00E+05
FSV Results  Excellent-to-Very Good
FDTD-PEEC
FDTD-FIT
PEEC-FIT
Grade
Spread
FDTD-CST
4
4
FDTD-PEEC
3
3
CST-PEEC
3
3
Traces often Enter/Exit in different
Directions
Will This Make a Difference?
Zero Degrees between
entry/exit traces
90 Degrees between
entry/exit traces
180 Degrees between
entry/exit traces
Direction of Entry/Exit for Traces
Via Single Plane Transition (Background Dielectric=3.8 -- 50 ohm load)
S21 -- Comparison of Various Trace Entry/Exit Configurations
2
0
-2
-4
-6
-8
S21 (dB)
-10
-12
-14
CST Modified Square 0 degrees
-16
CST Modified Square Size 90 degrees
-18
CST Modified Square Size 180 degrees
-20
-22
-24
-26
-28
-30
1000
10000
Frequency (MHz)
100000
Two Plane Geometry
Source
100 mil
10 mil
21 mil
Round
Geometries!
45 mil
overall
10 mil
Load
Dielectric Constant = 3.8
Trace Thickness = 1 mil
Via Barrel Diameter = 20 mil
Plane thickness = 1 mil
Via Antipad Diameter = 44 mil
Trace terminated in characteristic impedance = 80 Ohm
Via Pad Diameter = 34 mil
PCB size = 300 mil x 300 mil
Trace width = 7 mil
Metal = perfect electric conductor
Loss tan = 0
Absorbing boundary conditions!
Background DC = 1
Validation Using Different Techniques
Transmission Loss for Two Planes with no GND Return Via
Zero Degrees Between Enter/Exit Signal Via
Grade
Spread
FDTD-MWS
3
3
FDTD-HFSS
3
3
MWS-HFSS
3
3
0
-2
-4
S21 (dB)
-6
-8
-10
-12
-14
FDTD No Return Via (gated 0.25ns)
FIT- No Return Via
HFSS gnd via spacing=90mil
-16
-18
-20
0
10
20
30
40
50
60
Frequency (Hz)
70
80
90
100
Direction of Entry/Exit for Traces
Comparison of S21 for Various Via Configurations
Two Plane Example
0
-5
S21 (dB)
-10
-15
-20
FIT- Zero Degrees (No Return Via)
FIT- 90 Degrees (No Return Via)
-25
FIT- 180 Degrees (No Return Via)
-30
0
10
20
30
40
50
60
Frequency (GHz)
70
80
90
100
Ground-Return Via between Planes
Two Plane Example Using FEM Technique
Distance to Ground-Return Via Varied
0
-2
-4
-6
S21 (dB)
-8
10 mil
50 mil
90 mil
-10
-12
-14
-16
-18
-20
0
10
20
30
40
50
Frequency (GHz)
60
70
80
90
100
Real-World PCBs have many
Layers/Planes
6 Plane Example
10 Plane Example
Comparison of 2/6/10 Planes
Two Techniques for Validation
Comparison Transmission Through Via
90 Degrees Between Entry/Exit Trace (No Gnd-Return Via)
0
-10
S21 (dB)
-20
-30
FIT- 2 Plane
FIT 6-plane
FIT 10-plane
FEM 2 plane
FEM 6 plane
FEM 10 plane
-40
-50
-60
0
10
20
30
40
50
Frequency (GHz)
60
70
80
90
100
Model Detail Subtleties
• Assumptions may be hidden
– Simulation techniques
– Software simulation tool
– User
Model Detail Subtleties
Examples
• Dielectric slab vs background dielectric
– Volume based techniques can handle
dielectric slabs without significant increase in
computer resources
• FDTD, FIT, FEM
– Surface based techniques require significant
increase in computer resources for dielectric
slab
• Background dielectric easy to simulate
• MoM, PEEC
• Does it matter?
Dielectric Model
Using Background vs Slab Dielectric
Via Single Plane Transition (Dielectric=3.8 -- 50 ohm load)
S21 -- Zero Degree (trace enters & exits on same side of Via)
5
0
-5
S21 (dB)
-10
-15
-20
-25
-30
FDTD Modified Square 0 Degrees Slab dielectric only
FDTD Modified Square 0 Degrees Background dielectric
-35
10
20
30
40
50
60
Frequency (GHz)
70
80
90
100
Model Detail Subtleties
Examples
• Round vs Square objects
– Via, Via pad, Via antipad
• Some techniques have non-rectangular grids to
make closer approximation to round objects
– FEM
• Some software tools handle round objects with
special internal calculations
• ‘Regular’ rectangular grids require stairstepping
for round objects
• Does it matter?
Square vs. Stairstep Via Structure
Example
FDTD Via Single Plane Transition (AIR Dielectric -- 136 ohm load)
S21 -- Zero Degree (trace enters & exits on same side of Via)
2
0
-2
-4
S21 (dB)
-6
-8
FDTD-All-Round 0 degree
-10
FDTD Modified Square 0 degree
-12
-14
-16
-18
-20
1
10
Frequency (GHz)
100
Models Can Provide Information
that is Impossible to Measure
• Examples
– Current through ground return via
– Displacement return current mapping
Ground-Return Via Current from FDTD
Simulation (Time Domain)
Example of Current in Via for Different Seperation Distances
Two Plane Example
0.006
signal via current
Ground-Return via @ 10 mils
Ground-Return via @ 50 mils
0.004
Current (amps)
0.002
0
-0.002
-0.004
-0.006
-0.008
50
60
70
80
Time (seconds)
90
100
Ground-Return Via Current from FDTD
Simulation (Frequency Domain)
Current Through Ground Return Via
Two-Plane Example
-20
-25
Current (dB)
-30
-35
-40
10 mil distance
50 mil distance
-45
-50
-55
-60
-65
0
20
40
60
Frequency (GHz)
80
100
View of Displacement Current Through
Dielectric
(Two Ground-Return Vias)
Summary
• Model validation is IMPORTANT
– Do NOT simply believe simulation results because a
technique or tool was accurate on a different model!!!
• FSV technique allows user to compare overall
quality of agreement
– Similar to expert opinion
• Modeling allows the user to view
voltage/currents/fields in ways not possible in
the laboratory
– Helps understand underlying physics
– Helps with validation
• Via configuration changes transmission at high
frequencies significantly
FSV Follow up
• FSV Web site
– http://www.eng.dmu.ac.uk/~apd/FSV/FSV%20web/
• On-line Survey
– http://orlandi.ing.univaq.it/test/default.asp
• FSV Program free download
– http://ing.univaq.it/uaqemc/public_html/