SMT solder paste printing process quality improvement through

Six Sigma Approach

By Dan Coada

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.

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.

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.

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.

5. Solder Paste Printing Process

The beginning of solder paste printing

Squeegee Pressure Squeegee/Printing

Speed Squeegee angle 45°

Stencil Aperture

Snap-off

Solder Paste Printing

Pad

Solder paste printing completed w/ the board off from the stencil

Separation Speed

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

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.

Information of the PCB board (CAD file, Gerber file: overall size, thickness, fiducial locations)

Solder paste printing

Component placement Vapor Phase Reflow

PCB Loader, Solder Paste Printer, Glue Machine Chip Shooters (Component Placement Machines) Reflow Oven

SMT Process Flow Chart

AOI, 5DX ICT/Functional

inspection

Miscellaneous Assembly

Packaging Scrapped

OK

No Good

No Good

Sigma

Level DPMO Sigma

Level DPMO Sigma

Level DPMO

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

Stage Item Content Tools used

(D) Define

a. 5087 solder paste printing process b. SIPOC analysis c. Target settings

Objectives settings and scope study

SIPOC ( Suppliers, Inputs, Process, Outputs, Customer)

(M) Measure

a. CTQ(Critical to Quality) selection characteristics

b. Describing performance standards c. Volume measurement system

characterization

a. Identifying key quality characteristics

b. Volume measurements c. Improving the

manufacturing process

a. 80/20 Rule b. MSA (Measuring

System Analysis)

(A) Analysis

a. Solder paste printing process capability

b. Y definition of the performance indicators

c. Vary confirmed sources of change a. Data collection and analysis b. Identifying the key factors

a. Process capability analysis

b. Control chart c. Normal machine

rate plans d. Characteristics of

the light map

(I) Improvement

a. Experimental planning b. To be due to screening c. The experimental results and

analysis d. The best combination of

parameters

a. Experiment to find the standard

b. Experimental conditions to confirm the validity of

c. Ability to upgrade process

a. DOE b. ANOVA c. Response surface

(C) Control

a. Standardization of the best parameters

b. The implementation of process control Ability to control process

a. Process capability analysis

b. SPC

DMAIC Implementation Steps

INPUT OUTPUT

Process

Input-Output Schematic Diagram

• Operator in the preceding modules

• SMT solder paste printing

• PCB • Stencil • Solder paste

• Complete printed PCB

Reflowed PCBA`s

(Suppliers) (Inputs) (Process) (Outputs) (Customer)

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.

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.

Process indicators capability control table (The average standard deviation when adding 1.5*σ “shift”)

Specification Limits Favorable Rate

% Rate of Adverse

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 Ending

Print Mean Beginning

(Unit: μm; 1μm=0.04 mils)

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.

Samples Operator

Printing 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

Source DegOfFreedom SumOfSq MeanSq F

Main Effect A given A, a-1

SS / df MS(A) / MS(W)

Main Effect B given B, b-1

SS / df MS(B) / MS(W)

Interaction Effect given A*B, (a-1)(b-1)

SS / df MS(A*B) / MS(W)

Within (the error) given N - ab, ab(n-1)

SS / df

Total sum of others N - 1, abn - 1

The CyberOptics LSM (Laser Section Microscope) thickness measurements

Summary Output – Regression Statistics

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

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

P value> 0.05 Alpha=0.05

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)

No.

Solder paste

Thickness No.

Solder paste

Thickness No.

Solder paste

Thickness No.

Solder paste

Thickness No.

Solder paste

Thickness

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

Probability Plot of Solder Paste Thickness

Probability Capability of Solder Paste Thickness

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,

Environment

Factors that affect solder paste thickness

High/Low Temperature Stencil Gap

Squeegee Pressure

Blade Angles

Blade Speed

Blades Material

Level printer platform

Printer hardware wearing down High/Low Humidity

Precise Defect Measurement

PCB Material

Viscosity

Solder Paste Type Precise Humidity

Precise Temperature

MachinesMethods

MaterialMeasurements

Cause-and-Effect Diagram

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.

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%

General Linear Model: solder paste thickness versus blade material, squeegee angles, squeegee speed, squeegee pressure

Multi-Vari Chart for Solder Paste Thickness by Scraper Type-Blade Type

Squeegee Pressure

Solder Paste

Thickness

Squeegee variables: blade speed, blade angles

Blade Type

4 kg 7 kg

4 kg 7 kg

60mm/sec, 45° 60mm/sec, 60°

30mm/sec, 60° 30mm/sec, 45°

We are going to use for our experiment A. E-FORM squeegee blades

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

Experimental solder paste thickness table

Analysis of variable number of abnormal forms (pre-screening)

Run Order

Squeegee Pressure

(Kg) Squeegee

Angles

Squeegee Speed

(mm/sec)

Solder Paste

Thickness (µm)

Run Order

Squeegee Pressure

(Kg) Squeegee

Angles

Squeegee Speed

(mm/sec)

Solder Paste

Thickness(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

Solder paste thickness variance analysis table

Estimated Effects and Coefficients for Solder Paste Thickness

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.

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 0.495Error 27 20.555 20.555 0.761 Total 53 552.943

S = 0.872523 R-Sq = 96.28% R - Sq(adj) = 92.70%

Squeegee Pressure

Blade Angles

Blade Speed

Pareto Chart of the Standardized Effects (Response is Solder Paste Thickness, Alpha = .05)

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

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.

Squeegee Pressure Blade Speed

Blade Angles

Main Effects Plot (data means) for Solder Paste Thickness

4 Kg 7 Kg 11 Kg 5 mm/sec 17 mm/sec 30 mm/sec

45° 60° 75°

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.

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.

No.

Solder Paste

Thickness No.

Solder Paste

Thickness No.

Solder Paste

Thickness No.

Solder Paste

Thickness No.

Solder Paste

Thickness1 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

Figure 4.17 after the solder paste process to improve the thickness normal machine rate plans

Probability Plot of Solder Paste Thickness

Process Capability of Solder Paste Thickness

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