Lecture 3 Statistical Process Control Using Control Charts

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Lecture 3

Statistical Process Control

Using Control Charts

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Review

Apakah yang anda ketahui tentang

peta kendali / Shewhart control chart?

Jelaskan pentingnya diagram pareto

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Outline

Pengantar

Penyebab Variasi

Statistik Dasar untuk Control Charts

Pemilihan Subkelompok rasional

Analisis Pola pada Control Chart

Pemeliharaan Control Chart

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Introduction

Proses kontrol statistik (Statistical

process control) adalah kumpulan alat

yang bila digunakan bersama-sama

dapat menghasilkan stabilitas proses

dan pengurangan variabilitas

Control Chart: alat grafisuntuk

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Introduction

Manfaat menggunakan control chart

Ketika untuk mengambil tindakan korektif Jenis tindakan perbaikan yang diperlukan

Ketika meninggalkan proses sendirian

Ketika meninggalkan proses sendirian Kapabilitas proses

Kemungkinan sarana peningkatan kualitas Cara menetapkan spesifikasi produk

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Cause of Variation

Penyebab variasi

Chance Cause

melekat proses Sesuatu (sebagai variasi alami

dalam proses ) dalam proses )

Assignable Cause

Sesuatu yang dapat diidentifikasi ditentukan Contoh: alat yang salah, kesalahan operator

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Cause of Variation

Chance and Assignable Causes of Quality

Variation

Sebuah proses yang hanya disebabkan chance causes ,

proses tersebut dikatakan dalam pengendalian statistik. Alam variabilitas atau kebisingan latar belakang. Fluktuasi

15 October 2010 Materi ke-3

Alam variabilitas atau kebisingan latar belakang. Fluktuasi

Sebuah proses yang beroperasi di hadapan penyebab

dialihkan dikatakan di luar kendali. Misalnya Kesalahan operator, bahan baku yang rusak, pengaturan yang tidak tepat.

Tujuan akhirnya SPC adalah pengurangan atau

penghapusan variabilitas dalam proses identifikasi penyebab dialihkan.

A process that is operating with only chance causes of

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Statistical Basis

for Control Charts

Basic Principles

Assumed to have approximately

normal distribution

Control limits : 99.74 % ( 3

σ

limits )

A control chart : on line process

control

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Statistical Basis

for Control Charts

Selection of Control Limits

Let θ represent a quality characteristic of interest and represent an estimate of θ

( )

ˆ ˆ ˆ ˆ ˆ CL E UCL E k SD LCL E k SD θ θ θ θ θ = = + = − If k = 3 → 0.0026 of a sample statistic falling outside

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Statistical Basis

for Control Charts

Errors in making inference from control chart

Type I : process is out of control when it is actually in

control

Type II : process is in control when it is really out of control

Effect of control limits on errors in making inference

Effect of control limits on errors in making inference Warning limit

Usually 2 standard deviation

Effect of sample size on control limits Influence in standard deviation

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Statistical Basis

for Control Charts

Basic PrinciplesBasic Principles

A typical control chart has control limits set at values such that if the process is in control, nearly all points will lie between the upper nearly all points will lie between the upper

control limit (UCL) and the lower control limit (LCL).

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Statistical Basis

for Control Charts

A control chart contains

A center line

An upper control limit A lower control limit

A point that plots within the control limits indicates the process A point that plots within the control limits indicates the process

is in control

No action is necessary

A point that plots outside the control limits is evidence that the

process is out of control

Investigation and corrective action are required to find and

eliminate assignable cause(s)

There is a close connection between control charts and hypothesis testing

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Photolithography Example

Important quality characteristic in hard bake

is resist flow width

Process is monitored by average flow width

Sample of 5 wafers Sample of 5 wafers

Process mean is 1.5 microns

Process standard deviation is 0.15 microns

Note that all plotted points fall inside the

control limits

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Selection of rational

Subgroups

The premise : chosen is such manner that the

variation within it is considered to due only to chance causes.

Basis : Time order

Two approaches ( Besterfield , 1990 ) Instance of time method

Period of time method

Subgroup Size ( the number of items in each

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Selection of rational

Subgroups

Subgroups or samples should be selected so

that if assignable causes are present, the chance for differences between subgroups will be maximized, while the chance for

will be maximized, while the chance for

differences due to these assignable causes

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Selection of rational

Subgroups

Selection of Rational Subgroups

Two general approaches to constructing rational subgroups.

Select consecutive units of production. Each Select consecutive units of production. Each

sample consists of units that were produced at the same time (or as closely together as possible)

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Select a random sample over the entire

sampling interval.

Often used to make decisions about the

acceptance of all units of product that have been acceptance of all units of product that have been produced since the last sample.

Can be effective at detecting if the mean has

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Analysis of Patterns

in Control Chart

Five Rules for identifying an out-of-control process 1. A single point outside the control limits

2. Two out of three consecutive points fall outside the 2σ warning limits on the same side

3. Four out of five consecutive points fall beyond the 1σ warning limits on the same side

4. Eight or more consecutive points fall to one side

5. A run of eight or more consecutive points –up, down, above or below the CL , or above or bellow the

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Analysis of Patterns

in Control Chart

Nonrandom patterns can indicate out-of-control conditions

Patterns such as cycles, trends, are often of

considerable diagnostic value (more about this in Chapter 5)

Look for “runs” - this is a sequence of Look for “runs” - this is a sequence of

observations of the same type (all above the center line, or all below the center line)

Runs of say 8 observations or more could indicate an out-of-control situation.

Run up: a series of observations are increasing

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An x chart with a nonrandom, up-run, down-run patterns

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Analysis of Patterns

in Control Chart

Interpretation of Plots ( Non random pattern )

Determination of causes associated with out-of-control points

Require a thorough knowledge of the process and the sensitivity

of the output quality characteristic to the process parameters

The pattern and associated causes

Change in the level of the plotted pattern ( a jump ) ( change

quality raw material , change operator , failure component )

Trend in the plotted pattern ( tool wear , change in pressure ) Cyclic behavior in the plotted pattern ( seasonal effects of

quality , operator fatigue )

Concentration of points near the control limits ( two or more

operator plotted on the same chart , different production method )

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Maintenance of Control Chart

Proper placement of the control

charts on the shop floor is important

→

easy to access

The control chart should draw the

attention and curiosity of everyone

involved

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Quiz

1. What are benefits of using control chart ? 2. Explain the different between chance

causes and assignable causes ? give example of each ?

example of each ?

3. How are rational subgroups selected ? explain the importance of this in the total quality systems approach?