6S黑带培训教材(1)

Process Capability Analysis
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Process Variation
Process Variation is the inevitable differences among individual measurements or units produced by a process.
Process Limits (LPL and UPL)
• measures the variation of a process • the natural 6 limits of the measured characteristic
Control Limits (LCL and UCL)
• measures the variation of a sample statistic (mean, variance, proportion, etc)
Sources of Variation
• within unit • between units • between lots • between lines • across time • measurement error
(positional variation) (unit-unit variation) (lot-lot variation) (line-line variation) (time-time variation) (repeatability & reproducibility)
Process Capability Analysis
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Three Types of Limits
Distribution of Individual Values
Distribution of Sample Averages
Process Capability Analysis
Cpu

USL 3
Cpl

LSL 3
NSL
Cpk Minimum Cpu , Cpl
3
Process Capability Analysis
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Process Performance
Cpk
Reject Rate
1.0
0.13 – 0.27 %
1.1
0.05 – 0.10 %
1.2
0.02 – 0.03 %
1.3
48.1 – 96.2 ppm
1.4
13.4 – 26.7 ppm
1.5
3.4 – 6.8 ppm
1.6
794 – 1589 ppb
1.7
170 – 340 ppb
1.8
33 – 67 ppb
• Short-Term vs Long-Term Process Capability • Process Capability for Non-Normal Data
– Cycle-Time (Exponential Distribution) – Reject Rate (Binomial Distribution) – Defect Ratocess is highly capable (Cpk>1.5) b) Process is capable (Cpk=1 to 1.5) c) Process is not capable (Cpk<1)
Process is centered
Process is not centered
Process Capability Analysis
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Process Performance
The Cpk index relates the scaled distance between the process mean and the nearest specification limit.
said to be “in statistical control”
Process Capability Analysis
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Types of Variation
Special or Assignable Variation
• May be due to a) improperly adjusted machine b) operator error c) defective raw material
Process Capability Analysis
(Measure Phase)
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Scope of Module
• Process Variation • Process Capability
– Specification, Process and Control Limits – Process Potential vs Process Performance
Process Capability Analysis
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Process Potential
The Cp index assesses whether the natural tolerance (6) of a process is within the specification limits.
1.9
6 – 12 ppb
2.0
1 – 2 ppb
Process Capability Analysis
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Process Performance
a)
Cp = 2
Cpk = 2
b)
Cp = 2 Cpk = 1
c)
Cp = 2
Cpk < 1
• A process operating in the presence of assignable causes of variation is said to be “out-of-control”
Process Capability Analysis
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Process Capability studies can • indicate the consistency of the process output • indicate the degree to which the output meets specifications • be used for comparison with another process or competitor
Capability is often thought of in terms of the proportion of output that will be within product specification tolerances. The frequency of defectives produced may be measured in
Process Capability Analysis
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Process Capability vs Specification Limits
a)
b)
c)
a) Process is highly capable b) Process is marginally capable c) Process is not capable
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Process Capability Indices
Two measures of process capability • Process Potential
– Cp • Process Performance
– Cpu – Cpl – Cpk
Process Capability Analysis
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Types of Variation
Inherent or Natural Variation
• Due to the cumulative effect of many small unavoidable causes • A process operating with only chance causes of variation present is
a) percentage (%) b) parts per million (ppm) c) parts per billion (ppb)
Process Capability Analysis
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Process Capability
Process Capability
Process Capability is the inherent reproducibility of a process’s output. It measures how well the process is currently behaving with respect to the output specifications. It refers to the uniformity of the process.
Cp

Engineering Tolerance Natural Tolerance
USL LSL 6
Process Capability Analysis
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Process Potential
A Cp of 1.0 indicates that a process is judged to be “capable”, i.e. if the process is centered within its engineering tolerance, 0.27% of parts produced will be beyond specification limits.
Cp
Reject Rate
1.00
0.270 %
1.33
0.007 %
1.50
6.8 ppm
2.00
2.0 ppb
Process Capability Analysis
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Process Potential
a)
b)
c) a) Process is highly capable (Cp>2) b) Process is capable (Cp=1 to 2) c) Process is not capable (Cp<1)
Process Capability Analysis
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Three Types of Limits
Specification Limits (LSL and USL)
• created by design engineering in response to customer requirements to specify the tolerance for a product’s characteristic
Process Capability Analysis
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Process Potential
The Cp index compares the allowable spread (USL-LSL) against the process spread (6). It fails to take into account if the process is not centered between the specification limits.