General Guidelines for Implementation of Statistical Process Control (SPC)
|Publication Date:||1 November 1999|
This guideline describes the general provisions for implementation of SPC and is intended for use by individuals in electronics and other industries involved in the planning, implementation, and evaluation of an SPC system. This guideline outlines SPC philosophy, implementation strategies, tools, and techniques. These may be applied in different sequences depending on the specific company, operation, or variable under consideration. These tools are also used for relating process control and capability to final product requirements.
This document represents the second revision to the IPC Statistical Process Control (SPC) Standard. This revision reflects the principals of SPC represented by the International Organization for Standardization (ISO) Statistical Methods Technical Committee.
NOTE: Text from ISO 11462-1 is included in this document in italics with permission of ISO/TC 69 and has been approved by ANSI.
Subject of this Standard
Statistical process control (SPC) concerns the use of statistical techniques and/or statistical or stochastic control algorithms to achieve one or more objectives:
1. To increase knowledge about the process;
2. To steer a process to behave in the desired way;
3. To reduce variation of final-product parameters, or in other ways improve performance of a process.
These guidelines give elements for implementing an SPC system to achieve the above objectives. The common economic objective of statistical process control is to increase good process outputs produced for a given amount of resource inputs.
Note 1. SPC operates most efficiently by controlling variation of a process parameter or an in-process product parameter that is correlated with a final-product parameter; and/or by increasing the process's robustness against this variation. A supplier's final-product parameter may be a process parameter to the next downstream supplier's process.
Note 2. Although SPC is concerned with manufactured goods, it is also applicable to processes producing services or transactions (for example, those involving data, communications, software, or movement of material).
This International Standard provides elements to guide an organization in planning, developing, executing, and/or evaluating a Statistical Process Control system. By implementing those elements deemed applicable and appropriate by customer and supplier, an organization may satisfy a requirement to adopt a comprehensive and effective SPC system. By also deploying a quality system for assuring that products and services meet customer requirements (such as the system defined by ISO 9001), an organization can install the infrastructure to help hold the gains from its SPC system.
Field of Application
This International Standard specifies SPC system guidelines for use when a supplier's capability to reduce variation in processes associated with design or production needs to be proved, or when a supplier is beginning SPC implementation to achieve such capability.
It is intended that elements in this Standard will be selected based on their applicability and appropriateness to a specific process. Elements' selection, the order in which an organization implements the elements, and the depth of elements' adoption and application by an organization all depend on factors including: customer needs, market being served, nature of product or service, technology, and the nature and speed of production and transaction processes.
It is emphasized that the SPC system guidelines specified in this standard are complementary (not alternative) to technical (product) specified requirements and to quality system requirements. These guidelines specify what elements SPC systems are recommended to encompass. It is not the purpose of these SPC system guidelines to enforce uniformity of Statistical Process Control systems. These guidelines are generic, independent of any specific process, industry, or economic sector. These guidelines are intended to be adopted in their present form by organizations implementing SPC, but on occasions may need to be tailored by adding or deleting certain SPC system elements for specific circumstances. The phrases, "Where appropriate" and "Where applicable" are used to highlight those elements whose particular application is expected to be more process-dependent or more market-sensitive.
These guidelines are not intended for contractual, regulatory, or certification use.
Production Characteristics Covered
This International Standard is applicable in circumstances when:
(a) Variation or deviation from either product requirements or performance to a target value may occur.
(b) Confidence in product conformance can be attained by adequate demonstration of a supplier's capabilities in design, development, production, installation, and/or servicing.
Production characteristics that benefit from SPC implementation include, but are not limited to:
(a) Design and development, production, installation, and/or servicing.
(b) Customized or mass production.
(c) Short runs or long runs.
(d) Small, medium, or large scale production.
(e) Discrete, batch, or continuous processes.
(f) Transactions, as in services, information or communications.
(g) Manual or automated technologies for production, assembly, test, or communications.
(h) First pass or loops for rework, repair, reprocessing, or purging.
References in this Standard to a "product" include service, hardware, processed material, software, or a combination thereof, such as an information or communications transaction.
This guideline also serves to:
• Provide a framework for strategic planning of SPC implementation.
• Outline the general implementation issues that are recommended for an SPC system (see Section 4 and Section 5).
Figure 1-1 establishes a planning framework to use as a guide in applying the elements of SPC for IPC-9191.
Guidelines specific to certain products and/or processes will be specified through the applicable product specifications. Examples of SPC application for specific industries (base materials, printed circuit fabrication, and printed circuit assembly) are shown in IPC-9192, IPC-9193, and IPC-9194. Guidelines for auditing an SPC System are given in IPC-9199. A paragraph cross-reference to IPC-9191's structure is provided in each of these application documents to show relationship to specific sections of IPC-9191.
Techniques for Control and Models of Processes
SPC elements extend to techniques applied on-line within the operation of the process; and off-line either outside the operation of the process, or on the outputs at the end of the process. They are limited neither to traditional control charting techniques, nor to specific models of process data involving specific distributions or specific patterns of correlation. The SPC elements are applicable regardless of the tactics used to control processes; for example: automatic controllers for continuous and batch processes, automated editors for data inputs, control algorithms for timing or spacing of resource inputs, manual maintenance procedures for low volume outputs, and analytical procedures such as control charts. A supplier may use statistical, algorithmic, or model-based methods, or a combination, depending on process data availability, model availability, business needs, and the relative frequency of random, unknown, and assignable causes of variation
"Should" and "may" are used whenever it is necessary to express non-mandatory provisions.
"Will" is used to express a declaration of purpose.
"Applicable" reflects mutual agreements between customer and organizations.
"Documentation" is factual or substantial support for statements made.
"References" are intended to clarify and add information not to be used as auditable criteria. Any reference is noted by the author's last name, followed by the year of publication in parenthesis.
"Examples" provide additional information only and are not to be interpreted as requirements. These examples are shown in outlined sections.
"Where appropriate" indicates the organization is expected to provide rationale for appropriateness.