Perhaps the major barrier to perfecting quality in a manufacturing environment is variability. Variability is inherent in every product—no two products are ever identical.
For example, the dimensions of two thin films used for interconnect will vary according to the precise conditions and equipment used to deposit and pattern the films. Small variations might have negligible impact on the final product, but large variations can lead to final products that are unacceptable. Quality improvement may be defined as the reduction of such variability in processes and products.
The variation in the quality of product in any manufacturing process is broadly classified as:
(a)Chance causes
(b)Assignable causes.
a-CHANCE CAUSES
The chance causes are those causes which are inherit in manufacturing process by virtue of operational and constructional features of the equipments involved in a manufacturing process.
This is because of:
1.Machine vibrations
2.Voltage variations
3.Composition variation of material, etc.
They are difficult to trace and difficult to control, even under best condition of production. Even though, it is possible to trace out, it is not economical to eliminate.
The chance causes results in only a minute amount of variation in process.
Variation in chance causes is due to internal factors only the general pattern of variation under chance causes will follow a stable statistical distribution (normal distribution).
Variation within the control limits means only random causes are present.
b-ASSIGNABLE CAUSES
These are the causes which creates ordinary variation in the production quality.
Assignable cause’s variation can always be traced to a specific quality.
They occur due to:
1.Lack of skill in operation
2.Wrong maintenance practice
3.New vendors
4.Error in setting jigs and fixtures
5.Raw material defects
Variation due to these causes can be controlled before the defective items are produced.
Any one assignable cause can result in a large amount of variation in process.
If the assignable causes are present, the system will not follow a stable statistical distribution. When the actual variation exceeds the control limits, it is a signal that assignable causes extend the process and process should be investigated.
For example, the dimensions of two thin films used for interconnect will vary according to the precise conditions and equipment used to deposit and pattern the films. Small variations might have negligible impact on the final product, but large variations can lead to final products that are unacceptable. Quality improvement may be defined as the reduction of such variability in processes and products.
The variation in the quality of product in any manufacturing process is broadly classified as:
(a)Chance causes
(b)Assignable causes.
a-CHANCE CAUSES
The chance causes are those causes which are inherit in manufacturing process by virtue of operational and constructional features of the equipments involved in a manufacturing process.
This is because of:
1.Machine vibrations
2.Voltage variations
3.Composition variation of material, etc.
They are difficult to trace and difficult to control, even under best condition of production. Even though, it is possible to trace out, it is not economical to eliminate.
The chance causes results in only a minute amount of variation in process.
Variation in chance causes is due to internal factors only the general pattern of variation under chance causes will follow a stable statistical distribution (normal distribution).
Variation within the control limits means only random causes are present.
b-ASSIGNABLE CAUSES
These are the causes which creates ordinary variation in the production quality.
Assignable cause’s variation can always be traced to a specific quality.
They occur due to:
1.Lack of skill in operation
2.Wrong maintenance practice
3.New vendors
4.Error in setting jigs and fixtures
5.Raw material defects
Variation due to these causes can be controlled before the defective items are produced.
Any one assignable cause can result in a large amount of variation in process.
If the assignable causes are present, the system will not follow a stable statistical distribution. When the actual variation exceeds the control limits, it is a signal that assignable causes extend the process and process should be investigated.
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