Grace Hu Amkor

8/4/2019 Grace Hu Amkor

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Enabling aMicroelectronic World

Differential Pair Characterization in BGA PackagesDifferential Pair Characterization in BGA Packages

Huihui (Grace) Hu

Electrical Engineer;

Package Characterization


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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


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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


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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?


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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


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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?


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?



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



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



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



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



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:



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



Simulation results of added ground vias andsolder balls

Single-ended

Differential


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:



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


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Simulation results of added power wire andtrace

Single-ended

Differential


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


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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


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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.


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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


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Return current distribution for single-ended signals

Ground ball land

Power ball land

Current distribution on motherboard ground plane


internal ground plane


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)


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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


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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


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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

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Grace Hu Amkor