1. SMT solder paste printing process
quality improvement through
Six Sigma Approach
By Dan Coada

2. Abstract
The solder paste printing process is an important process in the assembly of
Surface Mount Technology (SMT) devices using the reflow soldering technique.
There is a wide agreement in the industry that the paste printing process accounts
for the majority of assembly defects. Experience with this process has shown that
typically over 60% of all soldering defects are due to problems associated with the
screening process.
Operation and parameter setup of the stencil printing process are the key elements
when trying to minimize defects. Parameters such as squeegee pressure, squeegee speed,
stencil separation speed, snap-off and stencil cleaning interval are the most important
factors in the process to achieve a better yield.

3. Introduction
The electronic assembly technology is progressing following the consumer
electronic products demands like light, thin, short, and small goods. Surface-mounted
components are often smaller than their leaded counterparts, and potentially more reliable
products can be designed and manufactured using SMT.
The Surface Mount Technology (SMT) became the mainstream of the modern
electronic assembly industry because reduces the manufacturing cost and it is making
high quality products.
The solder paste stencil printing process is a very critical step in the surface-
mount manufacturing process
In this complex SMT manufacturing environment, many worries appear during
the manufacturing process as a result of the high-density electrical components.
Operation and setup of the stencil printing process are key elements to be
considered when trying to minimize defects.
The main reason for printing solder paste onto the Print Circuit Board (PCB) is to
supply solder alloy for the solder joints. To achieve this objective, the solder paste print
must be aligned correctly, the correct amount of solder paste for each joint must be
present and the print should form an even layer of paste for perfect component
placement. The solder paste on top of the stencil is partly rolled and pressed into the
stencil apertures and onto the PCB solder lands by a moving and angled squeegee. The
squeegee angle must be between 45 to 60 degree (usually not adjustable) and the rolling
solder paste should have a diameter of 15 to 20 millimeter for optimum conditions. Thin
steel and hard as possible squeegees are used for metal stencils. The squeegee printing
edge must be sharp to secure a well-defined print. When using an old worn out squeegee
with rounded printing edge, the squeegee angle is reduced and the solder paste will not
roll as desired
Nowadays, as packages shrink in size and increase in lead count, we see a very
high defect rates (100 to 200 PPM) on 20-mil pitch and below while manufacturing sees
six sigma quality rates on larger pitch components.
The main sources for solder paste stencil printing process defects are stencil,
environment, solder paste, and stencil printing parameter.
This document is using Six-Sigma method DMAIC to improve the process quality
of SMT solder-paste print. We will need to find out the key quality factors and evaluate
the repeatability and reproducibility of the measurement system. After confirming this
capability of the process, we will discover the key factor that affects the process, and then
using “Design of Experiments (DOE)” we will get the ultimate process parameters.
The specification of the solder-paste printing thickness per IPC standards is
150±20µm (6±0.8mils).
As of a result in using DOE we found the best process parameters and the average
of the solder-paste printing thickness was increased from 137.95µm (5.43mils)
to144.98µm (5.71 mils); Standard Deviation was reduced from 2.22 to 1.31 and Cpk was
improved from 1.16 to 3.16.

4. Details of Printing Process Parameters
1. Stencil
Metal stencils can be made of different materials. Besides stainless steel, they can
be made of copper, bronze, or nickel.
There are 3 different metal stencil-manufacturing methods:
Etching,
Electroforming and
Laser cutting.
The apertures in both laser-cut and electroformed stencils have very sharp edges
and are slightly conic. This makes the solder paste slip easily off the aperture edges and
thereby secures a uniform print.
The metal stencils are attached to the printing frame using tensioned mesh or
directly using a special frame with gripping systems, which can easily damage the
stencils and thereby resulting in poor printing quality.
The thickness of the metal stencil is typically 150 microns(6mils) but 100(4mils),
125(5mils) and 200(8mils) microns are also available. The thickness is chosen based on
the job in hand.
For very fine pitch such as 0.3 mm lead pitch, 100 or 125-micron stencils could
be used and for lead pitch down to 0.5 mm, 150-micron stencils can be used.
The stencil thickness together with the aperture size determines the amount of
solder paste present to form each solder joint during reflow soldering.
The minimum stencil aperture width must be at least 3 times (preferable 5 times)
the diameter of the largest solder particle and the stencil aperture width is larger than the
stencil thickness.
Rounded aperture corners reduce clogging of fine pitch apertures and smearing.
The top surface of the metal stencil is slightly roughened to achieve a perfect
solder paste roll during printing.
2. Environment
Dust and dirt from the air that ends up on the PCBs and stencils can cause defects
such as bridging and poor wet-ability in the reflow soldering process.
A small piece of fiber or hair between two fine pitch solder pads can easily cause
bridging.
It is very important that the PCBs are stored in sealed packages and if necessary,
cleaned before use.
Air draught in the production area can speed up evaporation of the solvents in the
solder paste and thereby makes the solder paste dry out. High temperature can also make
the solder paste dry out quickly.
If the room temperature in the production area varies a lot, it will be very difficult
to control the printing process because the viscosity of the solder paste changes with the
temperature and the solder paste print will sometimes be perfected and at other times,
paste will slump and result in bridging.
The temperature window should be between 21 – 25°C.

5. 3. Solder Paste
Several paste characteristics must be carefully controlled to achieve optimum
production results.
These include: percent of metal, viscosity, slump, solder balls, flux activity
working life and shelf life.
4. Stencil Printing Parameters
Stencil Printing Parameters are the most important factors in the solder paste
printing process to achieve better yield.
a. The squeegee pressure.
• It should be as low as possible to scrape the stencil clean of
solder paste particles when printing.
• The amount of pressure is determined by the printing speed and
stencil type. The printing speed is usually recommended by paste
manufacturer typically between 20 and 80 mm per second.
b. Snap off is the distance between the stencil underside and the PCB placed
in print position but without the squeegee touching the stencil.
• For metal stencil printing, the snap off is zero and also called
contact printing.
• A high snap off will result in a thicker layer of solder paste.
c. The speed of separation between stencil and PCB after printing is
important.
• A too rapid separation speed when printing fine pitch will result
in clogging of the stencil apertures. It will also result in tailing
and formation high of edges around the solder paste deposits.
d. Cleaning
• Can be done either manually or automatically.
• The wiper does not clean the stencils underside but simply move
the solder paste particles from around the apertures to the
complete stencils underside.
• Stencil cleaning prior to use is important to prevent dust and dirt
from entering the solder joints.
• If all printing parameters are in control, stencil underside
cleaning is not necessary.

6. 5. Solder Paste Printing Process
The beginning of solder paste printing
Squeegee
Pressure Squeegee/Printing
Speed
Squeegee Stencil
angle 45° Aperture
Snap-off
Solder Paste Printing
Pad
Solder paste printing completed w/ the board off from the stencil
Separation
Speed

7. Experiment Design and Result Analysis
1. Experimental Design
Five factors and their actual low and high levels can be chosen for the screening
experiment that might influence both: the average amount of solder paste applied and the
process variability
a. Squeegee pressure; levels (4 and 7 kg).
b. Squeegee speed; levels (30 and 60 mm/s).
c. Snap off; levels (–0.1 and 0.1 mm).
d. Separation speed; levels (1 and 5 mm/s).
e. Cleaning internal (Stroke) (5 and 10)
There were also another two potential factors identified: temperature and
humidity but because large variation in these factors could potentially adversely affect the
experimental results was decided to hold these factors constant during the experiment.
Even if all the above factors are controlled, we will pay close attention to the
height of deposited solder paste during process.
For this study a product with the highest failure data from ETMS (EPIC Technologies
Manufacturing Systems) was chosen:
• Vocollect1000-5087 Hudson Hydra, lead free, VP reflow

8. This product will use the existing manufacturing machines / inspection
equipment:
1. A “DEK INFINITY” solder printing machine ( model year 2000)
2. EM907 Kester no clean lead free solder paste (EPIC # 1601-0955)
3. A CyberOptics LSM ( Laser Section Microscope) that measures deposited
solder paste thickness
4. An OMEGA i ® -Server MicroServer TM networked device which monitors
the level of temperature and humidity to be constant in the plant.

9. Information of the PCB board
(CAD file, Gerber file: overall size, thickness, fiducial locations)
Solder paste printing
Component placement
Vapor Phase Reflow
OK
Miscellaneous No Good AOI, 5DX
Assembly ICT/Functional
inspection
No Good
Scrapped Packaging
PCB Loader, Solder Paste Printer, Glue Machine Chip Shooters (Component Placement Machines) Reflow Oven
SMT Process Flow Chart

11. Sigma DPMO Sigma DPMO Sigma DPMO
Level Level Level
0.5 841300 3.5 22750 5.0 233
1.0 697700 4.0 6210 5.2 107
1.5 501349 4.2 3467 5.4 48.12
2.0 308770 4.4 1865 5.6 20.67
2.5 158686 4.6 967 5.8 8.55
3.0 66810 4.8 483 6.0 3.4
DMAIC Implementation Steps
Stage Item Content Tools used
a. 5087 solder paste printing process SIPOC ( Suppliers,
b. SIPOC analysis Objectives settings and scope Inputs, Process,
(D) Define c. Target settings study Outputs, Customer)
a. CTQ(Critical to Quality) selection a. Identifying key quality
characteristics characteristics
b. Describing performance standards b. Volume measurements a. 80/20 Rule
c. Volume measurement system c. Improving the b. MSA (Measuring
(M) Measure characterization manufacturing process System Analysis)
a. Process
capability
analysis
a. Solder paste printing process b. Control chart
capability c. Normal machine
b. Y definition of the performance rate plans
indicators a. Data collection and analysis d. Characteristics of
(A) Analysis c. Vary confirmed sources of change b. Identifying the key factors the light map
a. Experimental planning
b. To be due to screening a. Experiment to find the
c. The experimental results and standard
analysis b. Experimental conditions to a. DOE
(I) d. The best combination of confirm the validity of b. ANOVA
Improvement parameters c. Ability to upgrade process c. Response surface
a. Standardization of the best a. Process
parameters capability
b. The implementation of process analysis
(C) Control control Ability to control process b. SPC

12. Input-Output Schematic Diagram
INPUT OUTPUT
Process
(Suppliers) (Inputs) (Process) (Outputs) (Customer)
• Operator in • PCB • SMT solder • Complete Reflowed
the • Stencil paste printed PCB PCBA`s
preceding • Solder paste printing
modules
1. Providers (Suppliers) - refers to the key operational procedures that provide
information regarding to materials and supply people or organizations.
2. Input (Inputs) -provides the process information and materials in the consumption or
conversion.
3. Flow (Process) - refers to the conversion step
4. Outputs (Outputs) - customer’s products or services.
5. Customer (Customer) - refers to the person or organization acceptance of the output
flow.

13. TRUE
The accuracy of the solder paste measuring system in µm
1 µm=0.04 mils
The same operators that used the identical equipment to measure the alike solder
sample height repeatedly resulted in the above poor accuracy true value measurement
graph derived from an average number of measurements data.

14. Process indicators capability control table
(The average standard deviation when adding 1.5*σ “shift”)
Favorable Rate Rate of Adverse
Specification Limits % ppm
±1σ 30.23% 697700
±2σ 69.13% 308700
±3σ 93.32% 66810
±4σ 99.379% 6210
±5σ 99.9767% 233
±6σ 99.99966% 3.4
This soldering print study is for the printed circuit board assembly Vocollect 1000-
5087 Hudson Hydra, lead free:
• 0.062-inch panels
• EPIC recommendations for nominal solder thickness per IPC standards
is 150μm or 5.9 mils (max 170μm or 6.7 mils and min 130μm or 5.2
mils).
Measurement method:
1. solder paste deposit thickness is measured by dividing first the solder
blob on the pad in three sections as is shown in the below picture:
2. the “Mean” or the average solder deposit is calculated using the
formula:
• (Min + Max)/2 where Min is the minimum amount of
measurement points and Max is the maximum amount of the
measuring point
Print Mean Print
Beginning Ending

15. The CyberOptics LSM (Laser Section Microscope) thickness measurements
(Unit: μm; 1μm=0.04 mils)
Printing Samples
Operator Number 1 2 3 4 5 6 7 8 9 10
1 134.67 135.46 133.75 133.6 134.44 134.29 133.07 136.61 133.02 132.92
2 134.68 135.48 133.62 133.29 134.47 134.19 133.04 136.47 132.87 133.06
A 3 134.67 135.53 133.73 133.31 134.29 134.21 133.02 136.36 133.05 132.94
1 134.69 135.46 133.70 133.58 134.34 134.11 132.99 136.58 133.03 132.94
2 134.77 135.58 133.69 133.53 134.39 134.31 133.02 136.48 133.05 132.97
B 3 134.85 135.54 133.49 133.49 134.42 134.16 133.00 136.40 133.07 132.99
1 134.68 135.55 133.75 133.45 134.40 134.20 133.02 136.22 132.89 132.84
2 134.91 135.44 133.56 133.76 134.48 134.11 133.13 136.32 132.95 132.92
C 3 134.56 135.40 133.65 133.73 134.33 134.28 133.08 136.29 132.91 132.97
Summary Output – Regression Statistics
It is assumed that main factor A has a levels (and A = a-1 df), main factor B has b
levels (and B = b-1 df), n is the sample size of each printing, and N = abn is the total
sample size. The overall degrees of freedom df is one less than the total sample size.
Source DegOfFreedom SumOfSq MeanSq F
Main Effect A given A, SS / df MS(A) / MS(W)
a-1
Main Effect B given B, SS / df MS(B) / MS(W)
b-1
Interaction Effect given A*B, SS / df MS(A*B) / MS(W)
(a-1)(b-1)
Within (the error) given N - ab, SS / df
ab(n-1)
Total sum of others N - 1,
abn - 1

16. The following results are calculated using the Quattro Pro spreadsheet. It provides
the p-value and the critical values are for alpha = 0.05
Summary:
Two-Way ANOVA Table With Interaction P value> 0.05
Alpha=0.05
Source DF SS MS F P
Part Numbers 9 107.833 11.9814 903.821 0
Operators 2 0.012 0.006 0.45 0.644
P/N *Operator 18 0.239 0.0133 1.748 0.055
Repeatability 60 0.455 0.0076
TOTAL 89 108.539
We use the Gage R & R analysis to determine the uncertainty of our measurement
system before we can compare, control or optimize our printing processes.
a. REPEATABILITY is the variation obtained from LSM and one operator
when measuring the same solder height several times.
b. REPRODUCIBILITY is the difference in the average measurements
made by different operators using the same LSM when measuring the
same solder height.
c. As it can be seen below the solder print height measurement change is
8.41 %< 10% which indicates a good measurement with the LSM
% Contribution
Source VarComp (of VarComp)
Total Gage R&R 0.00948 0.71
Repeatability 0.00759 0.57
Reproducibility 0.00189 0.14
Operators 0 0
Op*P/N 0.00189 0.14
Part-To-Part 1.3298 99.29
Total Variation 1.33927 100
Study Var %Study Var
Source StdDev (SD) (6 * SD) (%SV)
Total Gage R&R 0.09734 0.58406 8.41
Repeatability 0.0871 0.52257 7.53
Reproducibility 0.04348 0.26086 3.76
Operators 0 0 0
Op*P/N 0.04348 0.26086 3.76
Part-To-Part 1.15317 6.91901 99.65
Total Variation 1.15727 6.94362 100
Number of Distinct Categories = 16

18. Fifty samples were collected from the solder printing process two every half hour
The amount of solder paste thickness measurement results are as follows:
(Unit: μm; 1μm=0.039mils)
Solder Solder Solder Solder Solder
paste paste paste paste paste
Thickness Thickness Thickness Thickness Thickness
No. No. No. No. No.
1 135.8 11 137.2 21 137.9 31 136.7 41 133.4
2 142.3 12 136.4 22 134.6 32 135.8 42 138.4
3 136.9 13 138.5 23 137.6 33 140.7 43 141.4
4 138.5 14 137.4 24 138.6 34 139.9 44 137.3
5 136.4 15 140.2 25 135.9 35 139.9 45 136.5
6 140.8 16 140.5 26 134.7 36 140.6 46 139.2
7 138.4 17 138.3 27 138.4 37 140.8 47 137.8
8 135.2 18 135.7 28 137.6 38 138.9 48 137.5
9 140.1 19 136.3 29 134.9 39 138.3 49 135.5
10 136.4 20 137.1 30 144.2 40 140.4 50 135.7
Xbar-R Chart of Solder Thickness
Initial Printing Process to be improved

19. Probability Plot of Solder Paste Thickness
Probability Capability of Solder Paste Thickness

20. The target of solder paste thickness specification process is 150 ± 20μm
From the above charts we found that the thickness of solder paste Cpk = 1.16,
which standard deviation does not meet the objective of a Cpk=1.5 standard
Process Improvement
(1μm=0.039mils)
Average Value Std Deviation Cpk
Solder Paste
Thickness 137.95µm 2.22 µm 1.16
The potential variables that are impacting the printing process would be:
a. Blades Material
b. Blade angles
c. Blade speed
d. Squeegee pressure
The SMT solder paste printing process takes place in a fully controlled
environment: 23 ± 1°C, 50 ± 5% RH,
Cause-and-Effect Diagram
Measurements Material
Precise Temperature PCB Material
Solder Paste Type
Precise Humidity
Viscosity Factors that affect solder
Precise Defect Measurement paste thickness
Stencil Gap Blades Material
High/Low Temperature
Squeegee Pressure
Level printer platform
High/Low Humidity Blade Angles
Printer hardware wearing down
Blade Speed
Environment Methods Machines

21. We are going to change the above variables characteristics and study the impact that it
has on the solder paste printing.
Squeegee material (A: electroform or B: steel)
Squeegee blade angles (45°, 60°, 75°),
Squeegee speed (5, 17, 30 mm/s).
Squeegee pressure; levels (4, 7 and 11 kg).
Snap off (–0.1 mm)- constant
Separation speed ( 1 mm/s)- constant
Cleaning internal (5 Internal) – constant
Temperature and Humidity 23 ± 1°C, 50 ± 5% RH -constant in the plant
The DOE steps are defined as identifying the response or output, identifying factors
for the study, determination of factor levels and range of factor setting, choice of
appropriate experimental design, run the experiment, collect and analyze the data,
draw conclusion and act on the results. These steps can be summarized and
used as a strategy in designing, performing and analyzing experiment.
General Linear Model: solder paste thickness versus blade material, squeegee angles, squeegee speed, squeegee pressure
Factor Type Levels Values
Squeegee Pressure Fixed 2 -1 , 1
Squeegee Material Fixed 2 B,A
Squeegee Blade Angles Fixed 2 -1 , 1
Squeegee Speed Fixed 2 -1 , 1
Analysis of Variance for Solder Paste Thickness, using Adjusted SS for Tests
Source DF Seq SS Adj SS Adj MS F P
Squeegee Pressure 1 66.038 8.801 8.801 30.88 0
Squeegee Material 1 95.024 95.024 95.024 333.42 0
Squeegee Blade Angles 1 83.104 83.104 83.104 291.59 0
Squeegee Speed 1 95.346 95.344 334.54 334.54 0
Error 11 3.138 3.138 0.285
Total 15 342.65
S=0.543367 R - Sq=98.96% R - Sq(adj)=98.74%

22. Multi-Vari Chart for Solder Paste Thickness by Scraper Type-Blade Type
4 kg 7 kg
30mm/sec, 45° 30mm/sec, 60°
Blade Type
Solder
Paste
Thickness
60mm/sec, 45° 60mm/sec, 60°
4 kg 7 kg
Squeegee Pressure
Squeegee variables: blade speed, blade angles
We are going to use for our experiment A. E-FORM squeegee blades
Experimental solder paste thickness table
High Level (+) Standard (*) Low Level (-)
1. Squeegee Pressure 11Kg 7kg 4kg
2. Squeegee Blade Angles 75° 60° 45°
3. Squeegee Blade Speed 30mm/sec 17mm/sec 5mm/sec

23. Analysis of variable number of abnormal forms (pre-screening)
Solder Solder
Squeegee Squeegee Paste Squeegee Squeegee Paste
Run Pressure Squeegee Speed Thickness Run Pressure Squeegee Speed Thickness
Order (Kg) Angles (mm/sec) (µm) Order (Kg) Angles (mm/sec) (µm)
1 7 45° 30 132.9 28 4 75° 17 138.4
2 11 60° 17 140.2 29 7 75° 30 138.2
3 11 60° 30 141.6 30 7 75° 5 142
4 4 45° 30 130.4 31 4 60° 17 135.7
5 11 60° 17 139.6 32 7 75° 17 140.4
6 7 75° 30 138.7 33 7 75° 5 142
7 11 45° 5 138.6 34 11 45° 30 134.6
8 7 75° 17 140.7 35 4 75° 5 140.7
9 4 75° 30 138.6 36 11 45° 5 138.2
10 11 75° 5 144.5 37 4 60° 17 135.1
11 7 60° 30 135.3 38 4 75° 5 140
12 4 75° 17 138.7 39 7 45° 5 136.3
13 7 45° 17 134.9 40 7 60° 17 137.9
14 11 75° 17 139.6 41 4 45° 17 132.4
15 7 60° 30 139.5 42 4 45° 5 134.8
16 7 60° 5 139.7 43 11 60° 30 137.9
17 11 75° 5 144.8 44 11 75° 30 140.6
18 4 60° 5 137.5 45 11 60° 5 141.5
19 7 45° 17 134.6 46 11 60° 30 137.2
20 7 60° 30 135.2 47 4 45° 5 134.6
21 4 60° 30 133.7 48 7 60° 5 138.3
22 4 60° 30 133.6 49 4 45° 30 130.0
23 4 45° 17 135.2 50 7 45° 30 132.4
24 11 45° 17 136.7 51 11 75° 30 140.5
25 4 60° 5 137.9 52 11 45° 17 136.5
26 4 75° 30 136.5 53 7 45° 5 136.0
27 7 60° 17 137.5 54 11 45° 30 134.2

24. Solder paste thickness variance analysis table
Estimated Effects and Coefficients for Solder Paste Thickness
Source DF Seq SS Adj SS Adj MS F P
Squeegee Pressure 2 104.513 108.077 54.039 70.98 0
Squeegee Blade Angles 2 294.608 279.215 139.607 183.38 0
Squeegee Blade Speed 2 114.829 115.23 57.615 75.68 0
Squeegee Pressure * Squeegee Blade Angles 4 4.41 4.853 1.213 1.59 0.205
Squeegee Pressure * Squeegee Blade Speed 4 3.203 4.229 1.057 1.39 0.264
Squeegee Blade Angles * Squeegee Blade Speed 4 5.049 6.142 1.536 2.02 0.12
Squeegee Pressure * Blade Angles * Blade Speed 8 5.776 5.776 0.722 0.95 0.495
Error 27 20.555 20.555 0.761
Total 53 552.943
S = 0.872523 R-Sq = 96.28% R - Sq(adj) = 92.70%
The results indicate that Squeegee Pressure, Squeegee angles, Squeegee speed have a
result of the P value <0.05 therefore because the interaction is not significant, we will
delete one the factors and keep the effect of the other three factors.
Pareto Chart of the Standardized Effects
(Response is Solder Paste Thickness, Alpha = .05)
Squeegee Pressure
Blade Angles
Blade Speed

25. Estimated Effects and Coefficients for Solder Paste Thickness
Source DF Seq SS Adj SS Adj MS F P
Squeegee Pressure 2 104.513 128.095 64.048 77.2 0
Squeegee Blade Angles 2 294.608 304.735 152.367 183.65 0
Squeegee Blade Speed 2 114.829 114.829 57.415 69.2 0
Error 47 38.994 38.994 0.83
Total 53 552.943
S = 0.910851 R-Sq = 92.95% R-Sq(adj.) = 92.05%
In the normal map P test value is greater than 0.05, so the configuration model is in line
with the assumptions.
Residual Plots for Solder Paste Thickness

26. Probability Plot of Solder Paste Thickness
In order to find the optimum conditions we draw the main factor effect diagram and
interaction effect.
In this experiment that results in three main factor maps (Factorial plots), we can observe:
1. If the squeegee pressure is lower, the solder paste thickness is minimum
2. Greater the squeegee blade angles, higher the solder paste thickness.
3. The use of a lower speed for the squeegee blade allows higher solder paste
thickness.
Main Effects Plot (data means) for Solder Paste Thickness
Squeegee Pressure Blade Speed
4 Kg 7 Kg 11 Kg 5 mm/sec 17 mm/sec 30 mm/sec
Blade Angles
45° 60° 75°

27. Based on experimental results, we have decided that the best factor level settings are:
Squeegee material – electroform ( E-form)
Squeegee blade angles - 75°
Squeegee speed - 5 mm/s
Squeegee pressure - 11 kg
Snap off (–0.1 mm)- constant
Separation speed ( 1 mm/s)- constant
Cleaning internal (5 Internal) – constant
Temperature and Humidity 23 ± 1°C, 50 ± 5% RH -constant in the plant
In the same time the use of MINITAB statistical software analysis of the surface
regression model estimated that the regression model for the solder paste thickness is:
y = 6.32153 +1565.97 X1 +0.1297 X2-0.0112879X3,
where X1 is squeegee pressure; X2 squeegee angle and X3 is the squeegee blade speed
The contour map drawing verifies that the above settings are the best solder paste printing
parameters.

28. 2. Verification and Validity
Control phase
Two solder paste printed samples were taken every half an hour. The settings for the
screen printing are the ones shown above.
As it can be seen the P value> 0.05 is in line with the normal distribution of data.
The data resulted from the process is stable and in line with the assumption of normal
distribution:
• The average paste thickness 144.98μm
• Standard deviation is 1.31μm,
• The process ability indicators Cpk = 3.16.
Solder Solder Solder Solder Solder
Paste Paste Paste Paste Paste
No. Thickness No. Thickness No. Thickness No. Thickness No. Thickness
1 144.9 11 146.8 21 145.1 31 145.4 41 143.9
2 145.6 12 145.7 22 145.6 32 143.4 42 147.6
3 143.6 13 149.9 23 143.4 33 145.7 43 144.2
4 144.7 14 144.1 24 145.6 34 144.2 44 143.9
5 145.6 15 143.1 25 144.6 35 145.2 45 145.6
6 144.6 16 146.2 26 143.5 36 144.8 46 144.5
7 142.8 17 144.9 27 145.6 37 143.7 47 143.6
8 146.6 18 145.6 28 144.9 38 146.7 48 144.7
9 144.7 19 142.8 29 143.5 39 145.9 49 145.8
10 145.6 20 145.8 30 146.2 40 144.2 50 144.5
Xbar-R Chart of Solder Paste Thickness

29. Probability Plot of Solder Paste Thickness
Figure 4.17 after the solder paste process to improve the thickness normal machine rate
plans
Process Capability of Solder Paste Thickness

30. Monitoring phase
In order to maintain the process stability and the process performance, we need to
monitor the process by using the process control chart and sample measure daily the
solder paste thickness deposits for the product we chose for our experiment.
The results of the experimental process compared with the initial status quo shows that
the average solder paste deposit thickness increased significantly reducing the standard
deviation.
Comparison of the process improvement before and after
Solder Paste Thickness
(1μm=0.039mils)
Average Value Std Deviation Cpk
Process to Improve 137.95µm 2.22 µm 1.16
Process Improvement 144.98µm 1.31µm 3.16
Xbar-R Chart of Solder Paste Thickness