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UNIVERSITI TEKNOLOGI 1\1ALA YSIA
BORANG PENGESAI-IAN STATUS TESIS·
.JUIJUL: V ARIABILlTY REDUCTION IN STENCIL PRINTING OF SOLDER
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::
I :lJ",r,lIl I'r<'id. '-.;arialla (.lud:1 ("'-.;\ I J
VARIABILITY REDUCTION IN STENCIL PRINTING OF SOLDER PASTE
FOR SURFACE MOUNT TECHNOLOGY
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
November, 199H
BAHAGIAN A - Pellgesahan Kerjasama*
Adalah disahkan bahawa projek penyelidikan tesis ini telah dilaksanakan melalui
keljasama antara ___________ dengan __________ _
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 : ....................... ~~ ........................................... . Nama
ii
" 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
Date .. ~.~ .. -:-.l.~.-:-. .~.l ....................... .
DEDICATION
Ulltlik ibll dall isteri lersayallg.
!-IeU'ah Halillwh Ele. Mohammed dall Widimt'ati @ Wahidah Etc Saia 'all,
pellgorballall mil hallya Allah saja yallg dapal l11el11/Jalasllya.
III
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
directly or indirectly tn the success or this project.
1\
v
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
the thesis.
vi
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
lebih lanjut di dalall1 tesis ini.
vii
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
2.4 Solder Paste Application Background 10
3.
4.
2.5 The Need for Research in Solder Paste Printin!,!
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
1 i
14
14
I"
I"
IG
1('
17
1~
1 ~
1~
2()
21
21 ,~
-.' ,~
0- _,
25
2~
3()
3()
31
~ ~ , ,
1111
5.
4.3 Solder Paste Deposition Consistency
4.3.1 Consistency Experiment
4.4 Viscosity
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
'}.7
' -~"I /
-w ~ 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
" -" '
1\
6.
5.8
5.7.2 Average Effects of Squeegee Speed
5.7.3 Average Effects of Separation Speed and
Print Gap
Conclusions
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
84
85
87
89
89
90
90
91
92
References 94
Appendix A - Surface Mount Technology Terms and Definitions 99
Appendix B - Taguchi Design of Experiment 118
XI
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
5.6.2.2 O.4mm tvlain Effects 68
XII
5.6.2.3 O.4mm ANOV A with Pooled Factors 68
5.6.2.4 O.4mm Optimum Condition and Performance 69
5.6.3.1 0.3mm Experimental Result 71
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)
with Pooled Factors 77
xiii
LIST OF FIGURES
FIGURE ITEM PAGE
2.2 Through-Hole 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
5.5.1 Response Graph 57
XIV
5.6.1.1 Interaction between Squeegee Pressure and
Squeegee Speed 64
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
5.7.4 A verage Effects of Print Gap 87
CHAPTER 1
INTRODUCTION
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
the assembly process at the first stop. The result is a more robust process that can
tolerate additional variation in the fine-pitch lead coplanarity, PCB variations, and
sl ight component misplacement.
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
investigating the various printing forces.
2
3
1. Hwang (1989) identified several forces that contribute to the paste transfer during
printing.
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
used in this study are :
I) Chemically etched
~ Laser cut
e Electroformed
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
viscosity so it does effects the printing process.
4
o Machine programmable parameter setting optimization
To investigate the effect of equipment variables (squeegee speed and
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.
5
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
the potential areas for further work.
1.5 THE INDUSTRIAL COLLAHORA TORS
Three industrial paJiners wcre involved in the project reported in this thesis.
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
1 mil 0.025 111m
6