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8/4/2019 Grace Hu Amkor
1/24
Enabling aMicroelectronic World
Differential Pair Characterization in BGA PackagesDifferential Pair Characterization in BGA Packages
Huihui (Grace) Hu
Electrical Engineer;
Package Characterization
8/4/2019 Grace Hu Amkor
2/24
Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Outline
Introduction Full wave simulation results
Comparisons
Virtual return path vs. Physical grounding Differential driven vs. Single-ended
Return current distribution
Conclusions
8/4/2019 Grace Hu Amkor
3/24
Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Why full wave simulation?
Challenges of chip packages
High routing density in limited space
High-frequency performance demand
Discontinuities caused by vias and solder balls
8/4/2019 Grace Hu Amkor
4/24
Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Tricks of HFSS
Engineering judgments are always necessaryfor specific applications
The simulation results might be affected by
Grounding
Ineffective absorbing boundary Solving criteria (maximum delta)
How to get accurate S parameters for PBGApackages?
8/4/2019 Grace Hu Amkor
5/24
Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Four-layer PBGA package with fourdifferential pairs
Differential
pairsGround plane is split to
lower the coupling between
differential pairs
Take a close look at
the middle section
Ground vias
Power balls
Mold compound
Die
Wire bondDie Attach Solder mask
Solder ballvia Rigid laminate
Ground planePower plane
8/4/2019 Grace Hu Amkor
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Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
HFSS modeling questions for the specific design
Q1: What is the proper size of the absorbingboundary?
Q2: Should the power net be treated as signalor ground?
Q3: How to Ground the die?Q4: Are the simulation results going to be
changed by removing the ground vias orballs?
Q5: Is there a way to create a return pathwithout physical connections?
8/4/2019 Grace Hu Amkor
7/24Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Answers for Q1 and Q2
It is recommended that the air box should be no less than lamda/4away from the strong radiator and no less than lamda/10 from theweak radiator.
If the solving frequency is set at 20 GHz, the corresponding freespace wavelength is 15 mm and the minimum distance from the air
box to the traces is 1.5 mm. Power net is supposed to carry DC voltage and it is part of the
return path for high-frequency currents. Thus power net should be
treated as part of the Ground system in the HFSS model.
Q1: What is the proper size of the absorbing boundary?
Q2: Should the power net be treated as signal or ground?
8/4/2019 Grace Hu Amkor
8/24Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Start from the simplified structure
Die ground edges
Edges of power and
ground planes, mother
board ground plane
Gap source for single-
ended terminal excitation
Circular excitation
simulates the real
measurements
One differential pair is modeled
All of the ground vias and solder balls are
removed
The edge of the power and ground planes,motherboard ground plane and die internalground plane are assigned Perfect-E
Ground is virtually created
8/4/2019 Grace Hu Amkor
9/24Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Single-ended and differential-ended Sparameters for the simplified structure
Single-ended S parameters
from HFSS
Differentially driven with
100 ohms impedance
Single-ended return loss is high at
low frequency because the planesare not physically connected.
Bad grounding at DC
8/4/2019 Grace Hu Amkor
10/24Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Create better DC grounding
Extend die ground to internal package ground Simulate the ground vias in real packages
Metal box takes much less time to solve
Physically connect power and ground planes
No virtual connections needed
Continuous return path over frequency range
Simulation time increases because of the meshsize
8/4/2019 Grace Hu Amkor
11/24Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Step 1: Extend die ground on the basis ofsimplified structure
Die ground
Internal power plane
Internal ground plane
Simplified structure Extend die ground to internal ground plane
8/4/2019 Grace Hu Amkor
12/24Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Simulation results of extended die-ground
Single-ended
Differential
Return Loss Insertion Loss
No significant difference in differential S parameters over frequency range
Single-ended return loss at DC decreased by 12 dB by extending the die ground
(Better DC Grounding)
No significant difference in single-ended S parameters at high frequency (over 15 GHz)
Comparing simplified structure and extended die ground:
8/4/2019 Grace Hu Amkor
13/24Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Step 2: Add ground vias and solder balls onthe basis of extended die
Signal balls
Signal vias
Extended die ground
Ground ball Power ball
Ground via Power via
Side view
Top view
Ground ball
Power ball
8/4/2019 Grace Hu Amkor
14/24Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Simulation results of added ground vias andsolder balls
Single-ended
Differential
Return Loss Insertion Loss
No significant difference in differential S parameters over frequency range
Single-ended return loss at DC decreased by 16 dB by adding ground via and balls
(Better DC Grounding)
No significant difference in single-ended S parameters at high frequency (over 15 GHz)
Comparing extended die ground and added ground vias, solder balls:
8/4/2019 Grace Hu Amkor
15/24Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Step 3: Add ground (power) wire and traceon the basis of added vias and solder balls
Top view
Power wire and trace are added right adjacent to the signal
One end of the power wire is extended to die internal ground (part of the return path)
Power ball bottom is touching motherboard ground plane (part of the return path)
Power trace
Power via
Power wire
8/4/2019 Grace Hu Amkor
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Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Simulation results of added power wire andtrace
Single-ended
Differential
Return Loss Insertion Loss
Comparing added ground vias, solder balls and added power trace, wire: No significant difference in differential S parameters over frequency range
Single-ended parameters do not change much by adding power wire and trace
8/4/2019 Grace Hu Amkor
17/24
Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Review the comparisons
Different Grounding approaches have been tried
Perfect-E edges
Extended die internal ground
Ground (power) balls and vias added
Ground (power) wire and trace added Differential S parameters do not show significant
differences over whole frequency range
Single-ended S parameters can be affected by the
return path connections, especially at lowerfrequencies
8/4/2019 Grace Hu Amkor
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Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Comparison of simulation time and sources
1.4822:33Ground wire andtrace added
1.2511:03Ground vias andballs added
0.341:35Extended_die
0.663:14Simplifiedstructure
RAM Size
(Gigabytes)
CPU Time
(Hour:Minute)
HFSS version 8.5 was used for all the simulations on a UNIXworkstation with 2048 megabytes RAM
Serial/sequential simulations.
8/4/2019 Grace Hu Amkor
19/24
Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Return current distribution for differential signals
Ground ball landPower ball land
The fields are calculated at24.2 GHz
For differentially-driven signals,the coupling between the signal
balls is stronger than the
coupling between each of them
and ground
Most of the return currents flowon the ground plane, right in
the shadow of the signal traces
(lowest inductance)
Current distribution on motherboard ground plane
Current distribution on
internal ground planeCurrent distribution on
internal power plane
Coupling between
signal balls is strong
Coupling between signal
ball and ground is weak
8/4/2019 Grace Hu Amkor
20/24
Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Return current distribution for single-ended signals
Ground ball land
Power ball land
Current distribution on motherboard ground plane
Current distribution on
internal ground plane
Current distribution on
internal power plane
Coupling between signal
ball and ground is strong
Coupling between
signal balls is weak
The fields are calculated at24.2 GHz
For single-ended signals, thecoupling between the signal
balls is weaker than the
coupling between each of them
and ground
Most of the return currents flowon the ground plane, right in
the shadow of the signal traces
(lowest inductance)
8/4/2019 Grace Hu Amkor
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Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
PBGA package with two differential pairs
Differential pair A
Differential pair B
No power/ground vias, balls or wires
Internal ground plane and power
plane are virtually connected using
boundary conditions
Differential pair ADifferential pair B
Power via and wire
Power ball Ground ball
Power/ground vias, balls and wires are
added to physically connect the power
and ground planes
Method 1
Method 2
Eight port terminal S matrix to four port
8/4/2019 Grace Hu Amkor
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Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Eight-port terminal S matrix to four-portdifferential S matrix
Port 3Port 1
Port 2
Port 4
Eight-port single-ended S parameters were imported
in ADS to get four-port differential S parameters
100 ohms
100 ohms100 ohms
100 ohms1
2
5
3
4
6
7
8
.s8p
Port 1
Port 4Port 3
Port 2
Diff pair A
Diff pair B
Differential S parameters:
S11, S22 ---- return loss for pair A
S33, S44 ---- return loss for pair B
S12, S21 ---- insertion loss for pair A
S34, S43 ---- insertion loss for pair B
8/4/2019 Grace Hu Amkor
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Enabling a Microelectronic WorldAnsoft HFSS Workshop 2003, L.A.
Differential S parameters comparisonMethod 1: Virtual connection
Method 2: Physical connection
Method 1: Virtual connection
Method 2: Physical connection
Insertion Loss S43 Return Loss S11
More solutions needed for
the interpolate sweeping to
refine simulation results at
resonance frequencies
I.S.
R.T.
Pair A Pair B
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Enabling a Microelectronic WorldA f HFSS W k h 2003 L A
Conclusions
Virtual grounding is a good method for differential paircharacterization in PBGA package
Without losing accuracy over frequency range
Shorten simulation time by more than 90%
Accurate single-ended characterization requires physical
connection for return path Extend die ground to simulate ground vias in real package
Connect power and ground planes by vias and solder balls.
No significant changes in the simulation results by adding moreground (power) vias, balls, traces or wires