Variability reduction in stencil printing of solder paste for surface mount technology

24  Download (0)

Full text

(1)
(2)
(3)

PASTE FOR SURFACE MOUNT TECHNOLOGY

SESI PENGA.JIAN: 1998/99

Saya __ MUSTAFFA BIN HAJlIBRAHIM

(IIURIiF IlESAR)

rncngaku Illclllbcnarkan tcsis *S,nj.llld ldddaiSarjana iDl~It'<c)1' rfll~tlrnh ini disirnran eli Pcrrllstakaan Institut Tcknologi TUIl I IlIsscin Onn dcngan syarat-syarat kcgllnaan scpcrti bcrikllt:

I. Tcsis adalah hakrnilik Univcrsiti Tcknologi Malaysia.

2. Pcrpllstakaan Univcrsiti Tcknologi Malaysia elibcnarkan IllCmbllat salinan lIntllk tujllan pcngajian sahaja.

3. Pcrpustakaan dibcnarkan Illclllbllat salinan tcsis ini scbagai bahan pcrtllkaran eli antara institusi pcngajian tinggi.

4. t*Sila tanelakan (./) SULIT

(

TERIIAD

Mcnganelllngi Illakllllllat yang bcrelarjah kcsclal11atan atau kcpcntingan 1vJalaysia scpcrti yang tcrrnaktllh eli dalam AKTA RAIISIA RAS1vl! 1972)

(ivlcnganJlIngi l11aklumat TERI lAD yang tclall ditcntllkan alch arganisasiibaelan eli mana pcnyclielikan elijalankanl

o

TIDAK TERHAD

Alalllat Tctap: NO 28 I, JALAN 1\ll:LAJ'lT ~

KG. BENTONG Di\Li\M, 86000 KLUANG

--~---, ---~---_ ..

-.101 lOR Di\RUL TAKZ!I",l

Nama Pcnyclia Tarikh:

1.1 - \

L - .,

"1

C:\l/\Ti\N

+

l'otong yang tidak krLcnaall,

Jika tcsis ini SLILIT atau TERII:\ D. sib larnl'irkan sural darirada pili:l!: bakuasa10rganisasi l'crkcllaall dcngall IllCnyalal.all >cbli IL'rnr"il Ic,i, illi i','rll' dikclasbn scbag:li Sl ;LlT alau TElU 1..\1)

Tcsis dimaksudkan scba~ai {csi, bar:1 li:l/.d-, j)nl:tllr Fal':d',"1 J::11 ',rid",

pl'll:clidik:-Hl. JUt! di~2rtJsi t'agi PL'!l!;:'.ii:,f] ~_·\.·,!r,: kl'ri,! ~ Of,,1:,-d'\1] r~"I''''':' I::

(4)

MUSTAFFA BIN HAJI IBRAHIM

A thesis suhmittcd in fulfillment of thc I"cquit-cments for thc award of the degree

of Mastel" of Engineering (Mcchanical)

Institllt Tclmologi Tun Hussein Onn

Univcrsiti Teknologi Malaysia

(5)

Disahkan oleh: Tandatangan Nama Jawatan (Cop rasmi) Tarikh:

*

.film pellyediaall tesis/projek lIlelibatkall keljasallla.

BAHAGIAN B - Untuk Kegunaan Pejabat Pusat Pengajian Siswazah

Tesis ini telah diperiksa dan diakui oleh:

Nama dan Alamat Pemeriksa Luar

Nama dan Alamat Pemeriksa Dalam I:

Pemeriksa Dalam II:

Nama Penyelia Lain Uika ada)

Dr. Ismarani bt. Ismail SIRIM Berhad

1, Persiaran Dato' Menteri Seksyen 2, P. O. Box 7035 40911 Shah Alam

Selangor

Prof. Dr. Awaluddin bin Mohamed Shaharoun FakuIti Kejuruteraall Mekanikal

UTM, Skudai

Prof. Madya Dr. Hashim bin Saim Institut Teknologi Tun Hussein Onn Bahl Pahat

Johor

Disahkan oleh peno\:~~~ftar di PPS:

Tandatangan : ...

~~

... .

(6)

" I declared that this thesis entitled" Variability Reduction In Stencil Printing Of

Solder Paste For Surface Mount Technology" is the result of my own research

except as cited in references. The thesis has not been accepted for any degree and is

not concurrently submitted in candidature of any degree".

Signature

Name Of Candidate MUST AFF A BIN HAJI lBRAHlM

(7)

DEDICATION

Ulltlik ibll dall isteri lersayallg.

!-IeU'ah Halillwh Ele. Mohammed dall Widimt'ati

@

Wahidah Etc Saia 'all,

(8)

The \\Titer woulcllike tn cnJl\'ey his sincerL' apprL'ciatilln tll his supen ispl.

Tn. I

Ii.

Khalid B. Ilasnan ror his consistent encnuragemcnt. alhice and ill\;t!\1;lhk

guidance thr{lughnut the course or this research pr()icct.·\ spL'ci;ll th;lIlb \I) tk

:-;apura Uectronics Industries Sdn nhd. allli Jlr(lmosol o·\sia Pal'ilie Stlno I\hd

r'lI

their permission and C(l-opl'r<ltinn gi\"l'n througlwut the research aeti\'itiL'S in thl'

l~lCtory ami lab environment. ;\Iso Illy appreciatioll to the (lj'jicers from Siril11 Ikrhad ror their contributioll. Finally I would als(1 thank those who kl\°l' c()ntrihutc

(9)

ABSTRACT

Competition in stencil printing to produce excellence in the finished product

is intense. Faults in the printing process are a major source of board failure. Studies

have shown that over 63% of defects identified after reDow originated from the

solder paste printing ( A. Lotfi ,1998 ) . However. understanding these failures are a

challenging problem as the printing process has a large number of non linearly

dependent variables such as factors relating to paste (formulation. viscosity), the

environment (temperature, humidity) and machine parameter (alignment, pressure

and speed of squeegee, blade hardness etc). The process engineer is challenged to

widen the process window so that future modifications to the process, such as the

addition of a new component, can be achieved with little. if any, change in materials

or process parameters. This thesis reports the effect of temperature and humidity

variation from the manufacturing environment on the solder paste consistency and

optimization of the essential parameters of squeegee pressure, squeegee speed.

separation speed and print gap. The outcome of variation in temperature and

humidity to the solder paste viscosity were analyzed and tests were done to

determine the characteristic of the solder paste. The tests results indicate that the

temperature and humidity has an impact on the solder paste printability. thus some

attempts must be taken to control these variables. For parameter optimization. the

analysis was carried out using statistical optimization. The main aim was to combine

these parameters with three main pitch categories to produce the acceptable print

formation. The results showed that. the ideal print result requires optimum statistical

combinations of four parameters essentially related to a particular pitch. It is also

shown that there is a diversity and contrasts of the combination of the parameters for

each category of pitch. Detailed explanations as to the phenomenon are outlined in

(10)

AbstJ'ak

Persaingan dalam meneetak pekatan pateri ke arah menghasilkan produk

yang bcrmutu tclah menjadi semakin sengit. Kerosakan yang disebabkan oleh proses

meneetak adalah punea utama kepacla kegagalan papan litar. Kajian yang lalu

ll1enunjukkan bahawa \ebih 63% kerosakan yang clikesan datang nya clari proses

meneetak pekatan pateri. Walau bagaimanapun. memahami kerosakan ini aclalah

masalah yang meneabar kcrana proses ini clipengaruhi oleh banyak faktor yang ticlak

bertinclak seeara terus clan saling bcrtinclak antara satu sama lain. Faktor-faktor yang

berkaitan c!cngan pekatan patcri ( formula, kelikatan ), keadaan sekeliling ( suhu,

kelembapan uclara ) clan teknologi mesin pcneetak ( tekanan 'squeegee', kelajuan

·squeegee'. kekerasan bahan 'squeegee'), dan lain-lain faktor. Jurutera proses

c!ieabar untuk ll1emper luaskan lagi tingkap proses pembuatan agar pacla ll1asa

haclapan. ll10clifikasi terhadap tingkap proses pembuatan seperti penambahan

komponen baru, boleh clilakukan clengan seclikit, jika perlu, perubahan bahan atau

perubahan proses pembuatan. Tesis ini ll1elaporkan kesan variasi suhu dan

kelembapan udara seke1iling kilang pembuatan clan proses mengoptimumkan

parameter-parameter asas sepelii tekanan 'squeegee', kelajuan 'squeegee', kelajuan

pembebasan 'squeegee' clan kelegaan peneetakan. Kesan clari variasi suhu dan

kelembapan udara terhadap kelikatan pekatan pateri telah dianalisis dan ujian-ujian

dijalankan untuk menentukan eiri-eiri pakatan pateri. Bagi mengoptimumkan

parameter-parameter. anal isis dijalankan dengan kaedah statistik. Tujuan nya adalah

untuk menggabungkan parameter-parameter ini dengan tiga kategori pic untuk

menghasilkan eetakan yang unggul. Keputusan nya menunjukkan bahawa terdapat

pereanggahan clan kelainan dalam kombinasi parameter-parameter untuk setiap

kategori pic. Penjelasan yang lebih jelas terhadap fenomena ini diterangkan dengan

(11)

CONTENT

DEDICATION III

ACKNOWLEDGEMENT IV

ABSTRACT V

ABSTRAK VII

LIST OF TABLES XI

LIST OF FIGURES Xlll

CHAPTER ITEM PAGE

1. INTRODUCTION

1.1

Overview and Problem Description

1.2

Previous Studies

2

1.3

Research Methodology 4

1.4

Structure of the Thesis 5

1.5 The Industrial Collaborators 6

1.6 Units 6

2.

Surface Mount Technology 7

2.1

Introduction 7

2.2

What is Surface Mount Technology 7

2.3

SMT Processes 9

(12)

3.

4.

The Stencil Printing Process

3.1 Introduction

3.2 Objective of the Printing Process

3.3 Preparations Before Printing

3.3.1 Solder Volume

3.3.1.1 Stencil Thicknes~ 3.3.1.2 Component Land~ 3.3.1.3 }\perture Dimensions

3.3.2 Choice

or

Solder Paste

3.3.2.1 Paste Features to Consider

3.3.2.2 Storage

3.3.3 Stencil Selection

3.3.3.1 Stencil Type IVlanul~lcturing i'v1cthod

3.3.4 Squeegee Selection

3.3.4.1 Material

3.3.5 Machine Parameters

3.4 Conclusion

rvlanufi.lcturing Environment Efrect on Solder Paste

4.1

4.2

I ntrocluction

The Equipment

4.2.1 The Printer

4.2.2 Vision System

4.2.3 Inspection Systcm

4.2.3.1 System Calibrati()n

4.2.4 IIumiditylTL'mperature l\ 1c:lsurement Equipmcnt

4.2.5 Viscometer

4.2.5.1 S'stcm Calihratil1n

4.2.() Ilumidit:- Cham her

14 14 I" I" IG 1(' 17 1~ 1 ~ 1~ 2() 21 21 ,~

-.'

,~

0 -_ ,

(13)

5.

4.4.1 Test Conditions

4.4.2 Solder Paste Viscosity Vs TempcratuI\.' Chan~c

4.4.3 Solder Paste Viscosity Vs Humidity Clwnt;l'

4.4.4 Shear Test

4.4.5 Stencil Life Test

4.5 Solder Paste Failure 1\'lodes

4.5.1 Powder Failure

4.5.2 Rheologic Failure

4.5.3 Reaction Product Build-Up

4.5.4 Solvent Separation

4.5.5 Moisture Adsorption

4.5.6 Loss of Powder Suspension

4.6 Conclusions

Optimizing the Printing Process

5.1

5.2

5.3

5.4

Introduction

The Equipment's Used During the Fxpcriments

5.2.1 The Printer

Analysis of Process Variability

Taguchi Design or Experiment

~ 1

~1 ~~ ~.., ~4

-15

45

4(1

~(l

47

47

~7 -I~ 5()

51

51

-

, :>.'

5.4.1 Software For Taguchi Design or Expcrimellt

54

5.5

5.7

Experimental Results

5.5.1 Trial Results

DOE using Qualitek 4

5.(1.1

Pitch Size O.5mm

5.(1.2 Pitch Size ().41l11l1

5.b.'" Pitch Size (Ull1ll1

5.b.4 COll1binatillll Pitch Sill'

Results and Discussillll

h()

"71

"

(14)

6. Conclusions and Future Work

6.1 Summary

6.2 Detailed Conclusion

6.2.1 Manufacturing Environment Effect

6.2.2 Optimization of Parameter Setting

6.3 Future Work

89

89

90

90

91

92

References 94

Appendix A - Surface Mount Technology Terms and Definitions 99

(15)

LIST OF TABLES

TABLE ITEM PAGE

2.4 Variables Affecting Solder Paste Printing 12

3.3.2.1.1 Relationship between Powder Type, Mesh

and Particle Size 18

3.3.2.1. Relationship between Pitch and Recommended

Powder Type 19

4.2.5 Mineral Oil Viscosity standarcl Fluids 35

4.3.1 Taguchi Orthogonal Array 38

4.3.2 Factors ancl Levels Description 38

4.3.3 The Main Effects 38

4.3.4 Analysis of Variance ANOV A 39

4.3.5 Optimum Condition ancl Performance 39

4.3.6 The Main Effects 39

4.3.7 Analysis of Variance ANOVA 39

4.3.8 Optimum Condition and Performance 40

5.5.1 0.5mm Experimantal Results 55

5.5.2 Response Table 57

5.6.1.1 Factors ancl Levels Description 60

5.6.1.2 Taguchi L8 Orthogonal Array 61

5.6.1.3 Results of Trials 61

5.6.1.4 Results Ivlain Effects 62

5.6.1.5 ANOVA with Pooled Factors 63

5.6.1.6 Optimum Condition and Performance 63

5.6.2.1 0.4111111 Experimental Result 67

(16)

5.6.3.2 0.3mm Main Effects 72

5.6.3.3 0.3mm ANOV A with Pooled Factors 72

5.6.3.4 0.3mm Optimum Condition

and Performance 73

5.6.4. I Combination Experimental Result 76

5.6.4.2 Combination Main Effects 76

5.6.4.3 Combination ANOV A with Pooled Factors 77

5.6.4.4 Combination Optimum Condition

and Performance 77

5.7.1 Individual Pitch Size Trials Result 81

5.7.2 A verage Effects of Factors and Interactions 81

5.7.3 Anova (Percent Contribution)

with Pooled Factors 82

5.7.3 Anova (Optimum Condition)

(17)

LIST OF FIGURES

FIGURE ITEM PAGE

2.2 ThroughHole Assembly

-Hybrid Technology - Surface Mount

8

2.3.1 Basic of SMT Processes 9

2.3.2 Main Components of a Typical

SMT Electronic Assembly 9

2.4 Basic Steps in Stencil Printing Operation II

3.3.1 Preparations Before Printing 16

3.3.5 Support on Squeegee 27

4.2.1 Fuji GSP II Printer 30

4.2.2 lVC Vision System 31

4.2.3 LSM CyberOptics 32

4.2.3.1 Calibration Tool 33

4.2.3.2 System Calibration 33

4.2.4 Humidity and Temperature

Measurement Equipment 33

4.2.5 System Calibration 34

4.2.6 ESPEC Humidity Cabinet 36

4.4.2 Viscosity at Extreme Temperature 41

4.4.3 Viscosity at Extreme Humidity 42

4.4.4 Shear Test 43

4.4.5 Stencil Life Test 44

5.2.1 DEK 260 Semi Auto Printer 51

(18)

5.6.1.2 Interaction between Squeegee Pressure and

Separation Speed 65

5.6.1.3 Transformation of Performance at Optimum

Condition 66

5.6.1.4 Performance Distri bution

(Current and Improved) 66

5.6.2.1 0.4 mm Transformation of Performance at

Optimum Condition 70

5.6.2.2 0.4 mm Performance Distribution

(Current and Improved) 70

5.6.3.1 Interaction between Squeegee Pressure and

Separation Speed 73

5.6.3.2 0.3mm Transformation of Performance

at Optimum Condition 74

5.6.3.3 The Performance Distribution

(Current and Improved) 75

5.6.4.1 I nteraction bet ween Squeegee Pressure and

Separation Speed 78

5.6.4.2 Combination Transformation of Performance

at Optimum Condition 79

5.6.4.3 The Performance Distribution

(Current and Improved) 80

5.7.1 A verage Effects of Squeegee Pressure 83

5.7.2 A verage Effects of Squeegee Speed 84

5.7.3 A verage Effects of Separation Speed 86

(19)

1.1 OVERVIEW AND PROBLEM DESCRIPTION

iVlanufacturing electronic boards requires an extensive knowledge of the

processes involved. The main processes are paste application. component placement

and renow. Each process has many machine and material parameters that affect the

final product. All of these parameters are important but only a limited number are

considered 'critical'. The process window for each process in an SMT manufacturing

line decreases in size as the pitch of the component decreases. Smaller operating

windows have forced manufacturers to study processes in order to ensure that

operating windows are maintained during full-scale production. Each process has a

number of parameters that critically affect the product as it passes through that

process.

The printing of solder paste is the foundation of the SMT assembly process.

Maintaining a consistent print is the key to avoiding solder joint defects that result

in lower than optimal yields, increased rework costs, and customer dissatisfaction

due to increasedlcvels of field failures. Preventing these defects by controlling the

printing process provides significant cost acivantages by eliminating rework,

improving product yield, and reciucing the number of field failures caused by

marginal solder joints that pass in-circuit and functional testing. By controlling the

process to a relatively stringent tolerance, additional margin can be introduced in

(20)

1.2 PREVIOUS STUDIES

Stencil printing evolves from screen printing, a process originally developed

from the textile industry. Screen printing was embraced by the electronics industry

for printing thick film inks. adhesives and solder pastes. Two approaches have been

adopted in the modeling of printing in Thick Film technology. The first is based

exclusivcly on the rheological behavior of ink. whilst the second considers the forces

generated during the screen printing processes.

1. Trease and Dietz (1972) showed a hypothetical relationship of viscosity as a

function of time during screen printing and the corresponding shear stress rates

associated with major printing steps.

2. Miller (1974) investigated the effect of rheology of polymer solutions on their

screening performance.

3. Evans and Beddow (1987) calTied out morphological characterizations of solder

paste particles and studied the rheological behavior of solder paste.

Most of these studies relate to thick film ink. and the data on solder paste printing

performance is still meager, so efforts in obtaining the data are warranted. The

viscosity of solder paste is much higher than that of inks. In ink printing, it is the

pressure difference above and below the screen that causes ejection of ink during the

snapping off the screen, whereas in solder paste printing it is the adhesion of the

paste to the pad that is the dominant effect.

More recent attempts at modeling the paste printing process are based on

(21)

2. Riemer (1988a, 1988b, and 1989) has investigated the hydrodynamics of screen

printing of ink and derived the relationship between hydraulic pressure and some

printing parameters based on adhesion theory.

3. I-Iuner (1987, 1988) evaluated Riemer's model and proposed an upper bound

alternative based on the blade coation analogy.

Both models, however do not address the problem associated with the rheological

behavior of solder paste during printing, particularly the relatively coarser size of

particles used in solder paste and the paste's vehicle and flux system.

A cooperative study was undertaken by a team of engineers from six companies in

the United State ( Dodly, 1985): Austin American Technology, IBM, Delco, MCC,

Motorola and TI ; to determine the limits of fine pitch solder paste printing

capability. Their investigation involved varying the stencil aperture size. squeegee

blade hardness, squeegee speed, snap off distance. solder pastes and squeegee angle

of attack in order to find the best printing conditions. As this study was undertaken

commercially only very brief details of the results were reported in the literature, and

commercial confidentiality prevented the release of the component pitch sizes tested.

Recently more studies have been carried out on improving the solder paste printing.

1. Li et al (1995) presented a statistical- neural network modeling to optimize the

fine pitch stencil process. The aim of the study was to determine the optimum

setting of the design parameters that would result in minimum solder paste

variation for 20 mil, 25 mil and 50 mil component pitch.

2. Clouthier (1995) studied the performance of three stencils. The three stencils

(22)

3. Myklak and Coleman (1995) also studied the performance of a laser cut and a

chemically etched stencil in which the apertures were electropolished on hybrid

stencil.

Currently there are many universities. companies anci research centers that have

contributed to a broad understanding of the stencil printing process. However, the

information and knowledge gathered is not shared publicly due to the

confidentiality policy of the company or institute.

1.3 RESEARCH METHODOLOGY

The solder paste deposition process is effected by an abundant number of

interactive variables including the solder paste (formulation and viscosity), the

manufacturing environment (temperature. humiciity). the stencil (type, aperture

sizes). the squeegee (material), and the printer setup (squeegee alignment, pressure,

speed, etc.). The goal of this study is to understand the interaction of some ofthese

variables to the manufacturing environment. identify the significant process

variables, and assess the current process capability. The Taguchi design of

experiment (DOE) methodology will be presented (Appendix B). The information

gained from the experimental analysis will be used to determine the capability of the

stencil printing process. The work accomplished can be divided into two main parts:

G The Manufacturing Environment Effect on Solder Paste.

To determine whether solder paste viscosity is a variable with respect to

temperature and humidity. As temperature and humidity has an impact on

(23)

pressure, print gap, squeegee separation speed) and determination of the process

window for defect free printing.

1.4 STRUCTURE OF THE THESIS

There are six chapters in this thesis. Chapter two introduces surface mount

technology assembly and its benefits.

It

also gives fUliher background on the stencil

printing process. The final section of this chapter examines the need for further

research on solder paste printing.

Chapter three describes on the control of the printing process parameters. These

parameters are related to the printer, the solder paste, the squeegee system and the

environmental influence.

Chapter four reviews the experiments and tests done to investigate the influence of

temperature and humidity to the solder paste viscosity and its printability settings.

Chapter five presents the investigation done to determine the optimum settings of the

design parameters (squeegee speed and pressure, print gap [snap off distance] and

squeegee separation speed) that would result in minimum solder paste height

variation for the board with 12 miL 16 mil and 20 mil pitch pad patterns.

Chapter six presents the conclusion of the research. It reviews the results of the

experiments and summarizes the contribution of this research work.

It

also identifies

(24)

These industrial partners are:

o SAPURA Electronics Industries Sdn Bhd

e PROMOSOL Asia Pacific Sdn. Bhd.

e DEK Printing Machines Ltd.

1.6 UNITS

The units used in industry for specifying stencil apertures is the miL which is

equivalent to one thousandth of an inch or conversion S.l units can be achieved

usmg:

1 mil 25.4 micron

1 mil 0.001 inch

Figure

Updating...

References