Presentation - Investigation of the Effects of Ground Vias, Solder ...

Differential Pair Characterization in BGA Packages 

Huihui (Grace) Hu 

Electrical Engineer; 

Package Characterization 

Enabling a 

Microelectronic World

• Introduction 

Outline 

• Full wave simulation results 

– Comparisons 

� Virtual return path vs. Physical grounding 

� Differential driven vs. Single-ended 

– Return current distribution 

• Conclusions 

Ansoft HFSS Workshop 2003, L.A. 

Enabling a Microelectronic World

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 



Tricks of HFSS 

• Engineering judgments are always necessary 

for specific applications 

• The simulation results might be affected by 

– “Grounding” 

– Ineffective absorbing boundary 

– Solving criteria (maximum delta) 

• How to get accurate S parameters for PBGA 

packages? 



Differential 

pairs 

Four-layer PBGA package with four 

differential pairs 

Take a close look at 

the middle section 

Ground plane is split to 

lower the coupling between 

differential pairs 


Mold compound 

Solder ball 

Die 

Ground vias 

Power balls 

Die Attach Wire bond Solder mask 

via Rigid laminate 

Ground plane 

Power plane 


HFSS modeling questions for the specific design 

Q1: What is the proper size of the absorbing 

boundary? 

Q2: Should the power net be treated as signal 

or ground? 

Q3: How to “Ground” the die? 

Q4: Are the simulation results going to be 

changed by removing the ground vias or 

balls? 

Q5: Is there a way to create a return path 

without physical connections? 



Answers for Q1 and Q2 

• It is recommended that the air box should be no less than lamda/4 

away from the strong radiator and no less than lamda/10 from the 

weak radiator. 

• If the solving frequency is set at 20 GHz, the corresponding free 

space 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? 



Edges of power and 

ground planes, mother 

board ground plane 

Start from the simplified structure 

Die ground edges 


• One differential pair is modeled 

Gap source for singleended 

terminal excitation 

Circular excitation 

simulates the real 

measurements 

• All of the ground vias and solder balls are 

removed 

• The edge of the power and ground planes, 

motherboard ground plane and die internal 

ground plane are assigned Perfect-E 

• “Ground” is virtually created 


Single-ended and differential-ended S 

parameters for the simplified structure 

Differentially driven with 

100 ohms impedance 

Single-ended return loss is high at 

low frequency because the planes 

are not physically connected. 

“Bad grounding at DC” 


Single-ended S parameters 

from HFSS 


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 mesh 

size 



Step 1: Extend die ground on the basis of 

simplified structure 

Die ground 


Internal ground plane 

Internal power plane 

Simplified structure Extend die ground to internal ground plane 


Single-ended 

Differential 

Simulation results of extended die-ground 

Return Loss Insertion Loss 

Comparing simplified structure and extended die ground: 

� 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) 



Signal vias 

Signal balls 

Step 2: Add ground vias and solder balls on 

the basis of extended die 

Extended die ground 


Ground ball Power ball 

Ground ball 

Ground via Power via 

Side view 

Top view 

Power ball 


Simulation results of added ground vias and 

solder balls 

Single-ended 

Differential 

Return Loss Insertion Loss 

Comparing extended die ground and added ground vias, solder balls: 


� 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) 



Top view 

Step 3: Add ground (power) wire and trace 

on the basis of added vias and solder balls 


Power wire 

Power trace 

Power via 

� 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) 


Single-ended 

Differential 

Simulation results of added power wire and 

trace 

Return Loss 


Insertion Loss 

Comparing added ground vias, solder balls and added power trace, wire: 


� Single-ended parameters do not change much by adding power wire and trace 


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 lower 

frequencies 



Comparison of simulation time and sources 

Simplified 

structure 

Extended_die 

Ground vias and 

balls added 

Ground wire and 

trace added 

CPU Time 

(Hour:Minute) 

3:14 

1:35 

11:03 

22:33 


RAM Size 

(Gigabytes) 

0.66 

0.34 

1.25 

1.48 

• HFSS version 8.5 was used for all the simulations on a UNIX 

workstation with 2048 megabytes RAM 

• Serial/sequential simulations. 


Return current distribution for differential signals 

Ground ball land 

Current distribution on motherboard ground plane 

Current distribution on 

internal ground plane 


internal power plane 


Power ball land 

Coupling between signal 

ball and ground is weak 

Coupling between 

signal balls is strong 

• The fields are calculated at 

24.2 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 flow 

on the ground plane, right in 

the shadow of the signal traces 

(lowest inductance) 


Return current distribution for single-ended signals 


Ground ball land 

Current distribution on motherboard ground plane 


internal ground plane 


internal power plane 

Power ball land 

Coupling between 

signal balls is weak 

Coupling between signal 

ball and ground is strong 

• The fields are calculated at 

24.2 GHz 

• For single-ended signals, the 

coupling between the signal 

balls is weaker than the 

coupling between each of them 

and ground 

• Most of the return currents flow 

on the ground plane, right in 

the shadow of the signal traces 

(lowest inductance) 


Method 1 

Method 2 

PBGA package with two differential pairs 

Differential pair A 

Differential pair A 

Differential pair B 

Power via and wire 

Differential pair B 


No power/ground vias, balls or wires 

Internal ground plane and power 

plane are virtually connected using 

boundary conditions 

Power ball Ground ball 

Power/ground vias, balls and wires are 

added to physically connect the power 

and ground planes 


Port 1 

Diff pair A 

Eight-port terminal S matrix to four-port 

differential S matrix 

Port 3 

Port 2 

Differential S parameters: 

Diff pair B 

Port 4 

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 

Port 1 

Port 3 


100 ohms 

100 ohms 

1 

2 

3 

4 

.s8p 

100 ohms 

100 ohms 

Eight-port single-ended S parameters were imported 

in ADS to get four-port differential S parameters 

5 

6 

7 

8 

Port 2 

Port 4 


I.S. 

R.T. 

Differential S parameters comparison 

Method 1: Virtual connection 

Method 2: Physical connection 

More solutions needed for 

the interpolate sweeping to 

refine simulation results at 

resonance frequencies 

Insertion Loss S43 


Method 1: Virtual connection 

Method 2: Physical connection 

Return Loss S11 

Pair A Pair B 


Conclusions 

• Virtual grounding is a good method for differential pair 

characterization 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 more 

ground (power) vias, balls, traces or wires

Presentation - Investigation of the Effects of Ground Vias, Solder ...

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