modeling of dynamic behavior of printed circuit...

MODELING OF DYNAMIC BEHAVIOR OF PRINTED CIRCUIT BOARD

(PCB) SUBJECTED TO MECHANICAL SHOCK LOADING

LAU CHIN WEI

A dissertation submitted in partial fulfilment of the

requirements for the degree of Master of Engineering

in Mechanical Engineering

Faculty of Mechanical Engineering

Universiti Teknologi Malaysia

MAY, 2006

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Dedicated to my country,

And

To my beloved father, mother and brothers.

iv

ACKNOWLEDGEMENT

Praises to God for giving me the patience, strength and will to go through and

complete my study. I would like to express my deepest gratitude to my supervisor,

Dr. Nazri Kamsah for his generous guidance, cooperation and advices from time to

time throughout the duration of this dissertation. His valuable knowledge and support

is greatly appreciated.

My sincere appreciation also extends to my laboratory mate, Mr. Liew Yik

Ban and my team mate, Mr. Vijay Anand Ramadass for their patient guidance on

using the ABAQUS finite element software and knowledge sharing. My recognition

also extends to Assoc. Professor Dr. Mohd Nasir Tamin, Professor Dr. Roslan Abdul

Rahman and Assoc. Professor Dr. Abdullah Idris for their kind guidance and sharing

the knowledge on certain technical aspects of this dissertation. Besides, I would like

to express my appreciation to the Vibration Laboratory technician, Mr. Elfendy for

his technical support.

Special thanks go to my friends, Goh Hock Ann, Opong and Lee Yuk Choi

for always giving me their utmost encouragement and moral support.

Last but not least, I would like to thank my family and all my friends for

giving their cheering supports.

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ABSTRACT

The reliability assessments of PCB and PCB related products had been adopted by

industries for years. Among the reliability test, mechanical shock damage is one of

the most predominant modes of failure when PCB experience deformation that

creates excessive strain. For dynamic analysis, it is very expensive, time consuming

and difficult to conduct drop/shock test to detect the damage mechanisms and

identify their behaviors. However, this assessment can be performed through a

computer simulation by Finite Element Method (FEM). FEM is one of such

techniques to achieve solution within reasonable computational time and cost. This

study presents dynamic analysis of PCB and PCB devices by FEM. As a

fundamental study, the natural dynamic properties of a bare PCB and workstation

chassis are obtained by FEM and verified with impact hammer test. Good agreement

of natural frequencies and mode shapes had been shown between FEM and tested

results. From the verified dynamic properties, the shock analysis was carried out

further for PCB with components. Two different ways had been carried out. First, to

model the drop impact behavior, the free-fall of PCB with attached chips was

modeled for seven drop orientation. The bouncing motion was captured and the

dynamic responses on all chips had been analyzed and had good agreement

compared to reported research. The second impact analysis is done by simulate the

PCB with chips that fixed on a drop block. To implement the input-G method, a short

impulse was impose to the PCB fixture surface. The natural dynamic behavior was

obtained for fixed boundary condition followed by detail deflection motion of PCB

due to impact. The simulated result shows that the center of PCB experiences

greatest impact, which is well verified with recent related research.

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ABSTRAK

Pengukuran kebolehtahanan papan litar cetak (PCB) dan produk berkaitan telah lama

diamalkan dalam industri. Antaranya, kerosakan kejutan mekanikal merupakan salah

satu mod pradominan yang menyebabkan kerosakan pada PCB akibat terikan hasil

daripada lenturan. Untuk analisa dinamik, adalah mahal, makan masa dan susah

untuk menjalankan ujikaji bagi mengesan mekanisma kerosakan dan mengenali

kelakuannya. Namun, pengukuran ini dapat dijalankan dengan simulasi komputer

melalui Kaedah Unsur Terhingga (FEM). Ia merupakan salah satu teknik

penyelesaian yang menggunakan perbelanjaan dan masa yang berpatutan. Kajian ini

mempersembahkan analisa dinamik PCB melalui FEM. Sebagai kajian dasar, sifat

getaran semulajadi pada PCB dan rangkanya telah dikaji dan disahkan dengan ujikaji

hentaman tukul. Keputusan antara FEM dan ujikaji didapati bersetuju. Daripada

keputusan tersebut, analisa kejutan dibuat dengan PCB berkomponen. Dua cara telah

diperkenalkan. Pertamanya, untuk mengkaji kejutan jatuh, PCB berkomponen telah

dimodel jatuh bebas pada tujuh arah yang berlainan. Pergerakan lonjakan dan tindak

laku dinamik telah diperoleh dan setuju dengan perbandingan kajian lain. Cara kedua

disimulasi dengan PCB berkomponen yang ditempatkan pada blok jatuh. Untuk

mengimplikasi cara kemasukan-G, kejutan pantas telah dikenakan pada permukaan

ikatan PCB. Sifat getaran semulajadi telah diperolehi bagi keadaan terikat dan

seterusnya pergerakan lenturan PCB akibat daripada kejutan. Keputusan simulasi

menunjukkan bahawa bahagian tengah PCB mengalami lenturan yang paling ketara,

di mana pendapatan ini adalah bersetuju dengan kajian-kajian baru yang berkaitan.

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TABLE OF CONTENTS

CHAPTER TOPIC

PAGE

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

GENERAL NOTATIONS xiv

CHAPTER 1 INTRODUCTION 1

1.1 Introduction 1

1.2 Prior Research On Mechanical Vibration and Impact 4

1.3 Objectives of the Project 7

1.4 Scopes of the Project 7

CHAPTER 2 LITERATURE REVIEW 8

2.1 Material Science of Printed Circuit Board 8

2.1.1 PCB Construction 9

2.1.2 Base Material 10

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2.1.3 Desktop Motherboard 12

2.2 Mechanics of Composite Materials 13

2.3 Plate Theory 16

2.3.1 Thin Plate Theory 17

2.3.2 Boundary Condition Governing Equations 20

2.4 Vibration Test 22

2.4.1 Excitation Technique 22

2.4.2 System Assumptions 23

2.4.3 Excitation Assumptions 24

2.5 Boundary Condition 25

2.5.1 Free Boundary Condition 26

2.5.2 Grounded Boundary Condition 27

2.5.3 Comparison of varies boundary condition 27

2.6 Drop/Impact Test Method 28

2.6.1 General Free Fall Test Method 29

CHAPTER 3 METHODOLOGY 34

3.1 Vibration Testing 36

3.1.1 PCB Hammer Impact Test 36 3.1.2 Chassis Hammer Impact Test 37

3.2 Computational Modeling and Simulation 40

3.2.1 ABAQUS 41

3.2.2 Pre-processing Procedure 42

3.2.3 Modeling of the Geometry 43

3.2.4 Specification of Material Property 43

3.2.5 Analysis Step 45

3.2.6 Specification of Boundary and Loading Condition 46

3.2.7 Meshing 46

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3.3 Chassis vibration simulation 40

3.3.1 Part 47

3.3.2 Material 49

3.4 Impact analysis simulation 51

3.4.1 PCB drop 51

3.4.2 PCB impact 52

CHAPTER 4 VIBRATION ANALYSIS

4.1 PCB vibration 54

4.2 Chassis vibration 60

CHAPTER 5 DROP AND IMPACT ANALYSIS

5.1 PCB drop (Case 1-7) 68

5.2 PCB impact 99

5.2.1 Vibration with fixed boundary condition 99

5.2.2 Impact at 4 pinned points 103

CHAPTER 6 CONCLUSIONS AND SUGGESTIONS

6.1 Conclusions 107

6.2 Suggestions 108

REFERENCES 109-112

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LIST OF TABLES

TABLE TITLE PAGE

2.1 Laminate designations and materials 11

3.1 Material properties 44

4.1 First 10 natural frequencies comparison with 56

experimental results

4.2 Natural frequency result for finite element analysis 57

4.3 Natural frequencies comparison with experimental results 62

(a) First 20 natural frequencies comparison for FEM

Case 1 and Case 2 (b) Natural frequencies comparison at

selected position for experimental and FEM method

5.1 Natural frequencies comparison between simulated 100

bare PCB with free boundary condition and PCB with

pinned boundary condition with attached chips

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LIST OF FIGURES

FIGURE TITLE PAGE

1.1 Critical solder ball and failure interface 2

1.2 PCB warpage distributions during maximum bending 2

1.3 PCB impact failure and related further studies 3

2.1 FCBGA Package 9

2.2 Nominal Board Stack Up 10

2.3 An ATX motherboard 12

2.4 Stress reference coordinates 14

2.5 Plate Geometry 17

2.6 CPT cross-sectional assumption 17

2.7 Resultant Forces in Plate 19

2.8 Plates with fully clamped edges and fully free edges 20

2.9 Hammer excitation 22

2.10 Shaker excitation 23

2.11 Setup for board-level tester 30

2.12 Setup for board-level tester with a special designed jig 30

2.13 Schematic diagrams showing different orientations for both 31

methods

3.1 Flow chart of overall problem solving method 36

3.2 Vibration testing set up for Impact/Hammer Test 37

3.3 Basic Step needed for FEM simulation 41

3.4 ABAQUS analysis step 42

3.5 Simplified motherboard model 43

3.6 Material orientation 45

3.7 Specified analysis step 46

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3.8 Plate meshing 47

3.9 Chassis as a whole part 48

3.10 Assembly parts of chassis 48

3.11 ‘Tie’ connections 49

3.12 Chassis in assembled condition 49

3.13 Iron base material 50

3.14 Zink coated material 50

3.15 Chips layout and PCB position for free-fall 51

3.16 Fixed PCB with applied transient force 52

3.17 Applied transient force in trapezium shape 53

4.1 Bare PCB acceleration responses in frequency domain 55

4.2 Comparison between experimental and FEM results 56

4.3 Mode shapes of bare PCB free vibration by 58

ABAQUS simulation

4.4 Mode shapes of free vibration 59

4.5 Chassis acceleration responses in frequency domain 60

(a) Resonance at side plate (b) Resonance at bottom plate

4.6 Selected FEM results for chassis natural frequencies 61

comparison (a)Resonance at bottom plate (b)Resonance at

side plate

4.7 Comparison between FEM Case 1 and FEM Case 2 results 63

4.8 The first 20 mode shapes of the chassis. 67

5.1 Case 1 PCB drop orientation with 5 concentrated masses 69

5.2 First 6 motions captured after free fall (a) to (f) 70

5.3 Energy results versus time 71

5.4 Acceleration histories of five chips on PCB 72

5.5 Velocity histories of five chips on PCB 72







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5.22 First 8 motions captured after free fall (a) to (h) 86





5.27 First 12 motions captured after free fall (a) to (l) 91





5.32 First 12 motions captured after free fall (a) to (l) 96




5.36 Natural frequencies comparison between simulated 100

bare PCB with free boundary condition and PCB with

pinned boundary condition with attached chips

5.37 First 10 natural mode shapes of PCB for pinned 102

boundary condition and with attached chips

5.38 First 10 motions captured during and after impact (a) to (j) 105



5.41 Deflection histories of five chips on PCB 106

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GENERAL NOTATIONS

English Symbols

a - Length

b - Width

E - Young’s modulus

f - Force

G - Shear modulus

G(t) - Acceleration at time t

Gm - Peak acceleration or maximum shock level

g - Gravitational acceleration

H - Drop height

h - Thickness

k - Spring constant

L - Length

M - Moment

m - Mass

M - Bending Moments

S - Compliance Components

T - Impact duration

t - Time

x - Displacement

x& - Velocity

x&& - Acceleration

xv

Greek Symbols

υ - Poisson ratio

ρ - Density

ε - Strain

σ - Stress

λ - Wavelength

ω - Frequency

η - = y/b, dimensionless plate spatial coordinate

ξ - Damping ratio, or = x/a, dimensionless plate spatial coordinate

Abbreviations

CPU - Central Processing Unit

DOF - Degree of Freedom

FCBGA - Flip Chip Ball Grid Array

FE - Finite Element

FEM - Finite Element Method

FR - Flame Retardant

FRF - Frequency Response Function

FFT - Fast Fourier Transform

PBGA - Plastic Ball Grid Array

PCB - Printed Circuit Board

PCBA - Printed Circuit Board Assembly

SDOF - Single Degree of Freedom

SMT - Surface Mounting Technology

CHAPTER 1

INTRODUCTION

1.1 Introduction and Problem Definition

PCB is one of the major mechanical elements in most modern electronic systems.

Transportation and handling of PCBs and PCB related products may generate shock and

vibration on it. As a result, the PCB will experience deformation that creates excessive

strain, which is eventually leaded to failure.

Although portable devices such as PDA or mobile phone are the products which

were easily damage due to drop impact, this mechanical destruction is also the main

damage factor for non-portable products such as motherboard package of desktop

computer. Mishandling during transportation or customer usage may terribly damage the

package.

In the microscopic of view, the PCB experiences deflection during drop impact.

The PCB bending and mechanical shock will create extreme peeling stress (Figure 1.1,

1.2) that cracks the solder joints of electronic components that mounted on the PCB.

Both Tee et al., (2004) and Yang et al., (2000) had shown the results that the critical

stress always occur at the outer row or corner solder balls. The disconnection of

electronic circuit leads to malfunction of the product. Due to environmental protection

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concerns, the usage of lead-free solders instead of lead-containing solders had results

more critical phenomenon because the higher moduli have more brittle mechanical

responses (Yeh and Lai, 2000). Although, conventionally, board-level reliability usually

refers to solder joint fatigue strength during the thermal cycling test (Tee et al., 2004) .

Figure 1.1 Critical solder ball and failure interface (Tee et al., (2004))

Figure 1.2 PCB warpage distributions during maximum bending (Tee et al., (2004))

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Generally, PCB impact failure can be dividing into two fields: thermal impact and

dynamic impact as Figure 1.3. The dynamic impact is part of structural dynamic field

which concerns dynamic parameters such as deflection, velocity and acceleration of the

whole structure. The study then can be furthered by stress, strain analysis under fracture

mechanic analysis. Come to the last, the main reason of the study is to predict life of the

product followed by product improvement which is the main concern for manufacturers

in cost reduction and to benefit end users.

Figure 1.3 PCB impact failure and related further studies

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1.2 Prior Research On Mechanical Vibration and Impact

It has always been a challenge to predict dynamic behavior of mechanical impact

for PCB and related devices. Since 1980, with the blow up of PCB related products, the

manufacturers are suffering from reliability of those products that may damage during

handling and transportation. On the other hand, the end users are demanding for more

robust products.

It was a difficult study to predict the final impact response on the product, even

more difficult is to catch what actually happened inside the product during shock or drop

that occur just within few milliseconds. In recent years, various impact predictions have

been proposed to capture the dynamic behavior of the products.

Most researchers agree that board-level test is much easier to conduct and

evaluate compare to the product-level test due to the complexity. Although there are

numbers of studies have been done on board-level evaluation, only a few have done for

product-level by simulation. However, it was a pre-requirement to study the board-level

structural dynamic before furthering in product-level analysis if the result are frequency

related data.

The investigations of vibration are presented as follows:

Wu et al., (2002) had presents vibration analysis of PCB by FEA model and

verified with the tested result. The simulation result had a close prediction to the tested

data within 7% error range. Yang et al., (2000) investigated dynamic properties of

PBGA assemblies using experimental modal testing and FE analysis. Bare PCB and

PCB assembly with PBGA modules mounted were tested and analyzed separately. The

dynamic investigation was limited to free and force vibration. Geng et al., (2003)

evaluated solder joint shock reliability for Intel’s desktop motherboard. Modal analysis

was introduced to evaluate and calibrate the test board fundamental frequency. An

external mass was mounted to the PCB to adjust the bare PCB behavior to the desktop

system level. The FEM analysis validation had shown good agreement with the tested

5

data. However, the shock testing and simulation were not included in this work. It

provides only as a ground work for shock test evaluation. Chen and Lue, (2005) had

study the free vibration characteristic of cross-ply laminated plates with a semi-

analytical method. The result had been verified by comparing the present data with the

exact elasticity solution for a simply supported plate. Allemang, (1983) had a detail

discussion on the experimental modal analysis method, into four categories: forced

normal mode excitation method, frequency response function method, damped complex

exponential function method and mathematical input-output model method.

While the investigations of shock/impact are presented as follows:

Tee et al., (2004) predict impact life by studying PCB bending and mechanical

shock. The drop test had been carried out and compare with explicit solver simulation.

Detail study such as effect of component position, drop orientation and boundary

condition of PCB had been evaluated. The quantitative approach provides both accurate

relative and absolute impact life prediction. Wang et al., (2004) carried out drop test

FEM simulation, especially the methodology for analyzing the reliability of electronic

devices under drop and impact. The structure with high complexity such as TV set

packaging and Aluminum casing had been simulate with same method. Yeh and Lai,

(2000) used the support excitation scheme in stead of the conventional drop test, in order

to omit drop table modeling and contact methodologies. For experimental work, JEDEC

drop test had been carried out. Both FEM and numerical studies had been done. The

comparison for different drop orientation had been done in detail. Wang et al., (2006)

modeled and simulated the drop-impact analysis of PCB. The experimental study was

carried out by actual drop test, where the images of specimen movements were capture

with a high-speed camera. Detail study had been done using FEA simulation for four

different models: isotropic, orthotropic, multi-layer uni-directional fiber and multi-layer

fill-warp PCB models. Allen and Bogy, (1996) studied the effects of shock on hard disk

drive by comparing drop test result from high speed camera with FEM modeling. The

comparison of both method shown discrepancy due to the linear and non-linear

conditions of simulation and testing, respectively. Ong et al., (2003) had studied drop

impact for both product-level and board-level of the Nokia Model 3210 mobile phone

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and its PCB. The mechanical responses were obtained by a load cell, accelerometers and

strain gauges. The method was purely experimental, and several drop direction had been

done to study the relationship between mechanical responds and drop direction. Suhir,

(2002) had studied whether the shock tests could adequately replace drop tests. The

analytical result shows that in order to mimic the actual drop test, the shock test loading

should be as close as possible to an instantaneous impulse, and the duration of the shock

load should be established base on the lowest (fundamental) natural frequency of

vibrations.

1.3 Objectives Of The Project

The project is concerned with modeling of dynamic behavior of PCB subjected

to mechanical shock loading. The objectives are as follows:

1) To understand current experimental method of mechanical shock tested on PCB

(board-level) and PCB with chassis (product-level).

2) To simplify actual condition of product-level PCB and perform simulation

modeling to obtain PCB dynamic responses due to free-fall and input-G.

1.4 Scopes of The Project

The scopes of project are as follows:

1) Undertake the FEM modeling of a PCB at simplified board-level condition.

2) Carry out further modeling study with isotropic and orthotropic model.

3) Conduct parametric study with the best model to investigate PCB response under

effect of other features.

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4) For verification, mechanical shock test of simplified product-level PCB should

be performed either stimulating external impact on a static PCB or free-fall of

PCB itself.

The remainder of the report is organized as follows. In Chapter 2, the theory of

PCB structure, structural dynamic background, vibration test and conventional drop test

is elaborated for better understanding of the solution method. In Chapter 3, the method

of solution is introduced for vibration and shock/impact evaluation, for both

experimental and FEM simulation method. Chapter 4 shows the results of PCB and

chassis vibration analysis. Chapter 5 discussed the free-fall and input-G results for PCB

with chips. In Chapter 6, conclusions of the research are presented with summary on

major findings in the study. Future works for refining the research are recommended.

modeling of dynamic behavior of printed circuit...

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