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Conformability Analysis in Support of Design for Quality
Abstract
Around 80 per cent of the costs and problems of quality are created in product development. The paper overviews a knowledge-based design technique for the prediction of potential variability risks in component manufacture and assembly, and presents a method for linking the risk metrics to notions of design acceptability and potential costs of quality in production and service. The paper considers issues related to application of the work to industrial products and describes its coupling with failure modes and effects analysis.
Just look at any engineering product design drawing revision column and it will become clear that most designs are not right the first time. Inevitably the main problems result from poor tolerance capability. The paper outlines the statistical concepts behind the process capability index Cpk and explains its importance to the engineering designer. The paper goes on to introduce the notion of Process Capable Tolerancing (PCT) and describes a methodology for creating components and products that are robust to process variation. Software that facilitates the application of the methodology is described and its application is illustrated through an industrial case study.
This paper outlines progress made in the development of a knowledge-based technique to support capable design. The analysis method employs detailed knowledge on the capability of a wide range of manufacturing processes, including the effects of material and design geometry, to enable realistic tolerance distributions to be estimated for key characteristics on designs. The method takes into account failure severity through linkage with 'failure mode and effects analysis' for the determination of potential failure costs. The application is illustrated through two industrial case studies and, finally, the contribution of the paper is discussed and a number of areas are identified where further work is being directed.
Successful electronic products rely on a combination of good design, appropriate manufacturing processes and effective test and inspection. In order to achieve all of these within a highly cost constrained environment requires that sufficient, but not excessive, resources are applied to each area. One problem which makes it difficult to balance the allocation of resources is that different measures of performance are used in each context. In the design domain, the objectives are typically design centring, robust design, part count reduction or component cost reduction. Objectives in manufacturing processes are typically yield maximisation, process capability, defect reduction etc. In the test arena, the aim is typically to maximise test coverage and minimising test/inspection times. Given this plethora of different measures of performance, it is difficult to assess trade-offs between different domains and decide appropriate resource allocations.In order to address this problem we have proposed a unified measure of capability across the design/manufacture/test spectrum. This is based on the process capability measure Cpk which has been adapted to measure functional capability: the ability of a design to meet its performance specification in the presence of component parameter variations, and to test capability: the ability of test/inspection processes to correctly identify defects without erroneously indicating defects which are not present. These capability measures may be related to the costs arising from defects through scrap, rework and warranty returns through a failure modes and effects analysis (FMEA) and a quality cost mapping process. This makes it possible to quantify and compare the cost consequences of design decisions. This unifying analysis methodology is termed electronic conformability analysis (eCA). The paper introduces the eCA methodology, explains the functional, manufacture and test capability measures and shows how these may be related to qualit- - y cost. The application of the methodology to a simple example circuit is presented.
In this paper, recent advances in the analysis of products for assessing the consequences of design decisions on quality of conformance are presented. The approach, called conformability analysis, uses estimates of likely process capability levels and likely failure severity to approximate the costs of failure in production and service, associated with non-conforming design characteristics. Improvements in estimating process capability are discussed and the correlation between the estimated values and shop-floor capability levels is reported. A case study is employed to illustrate how the analysis can be used in new product development and the benefits that may accrue from its applications.
Argues that the costs and problems of quality are largely predetermined in product development and, therefore, there are limits to what can be achieved thereafter by the application of best-practice manufacture and quality control. Presents knowledge-based design-for-quality methods which enable a business to identify product characteristics which represent potential failures and their associated costs. Discusses application modes and includes a product case study.
Offers, as a contribution to the debate on probabilistic engineering design, the suggestion that quality problems in development and manufacture can be anticipated and resolved before they occur by applying variance synthesis to the basic design. Only after a satisfactory design has been established should engineering tolerances be applied to control the manufacturing process. Identifies dominant sources of variability variance synthesis in addition to providing a statistical basis for calculating a safety factor. Gives two illustrative examples.
The variations in component parameter values embodied in an electronic circuit can result in it failing to conform to its specification. Such failures can cause the manufacturer to incur significant quality costs, arising from factors including warranty returns and legal liabilities. It can be difficult to predict both the probability that a circuit will fail to meet its specification and the consequent quality costs. It can also be difficult to determine which component parameter variations are most significant in causing faults. In this paper we introduce the concept of functional capability, which is a variation on the familiar process capability, as a measure of the performance variation found in a circuit relative to the specified range of acceptable performance. Based on this capability measure, along with a failure modes and effects analysis and a cost model, we are able to relate functional capability to quality costs. In addition, we introduce a capability breakdown which allows the effect of different parameter variations to be readily visualized so that the designer may focus effort where it will be most effective in reducing quality costs. The resulting technique is primarily intended to aid the design of printed circuit board level analogue and mixed signal circuits, particularly in safety critical applications. The paper illustrates the technique through analysis of a simple circuit and the analysis of a commercial mixed signal circuit. It also shows how the technique may be integrated within a capable design, manufacture and test analysis process. Copyright © 2005 John Wiley & Sons, Ltd.
It is widely recognised that there is a need for enhanced computer support during the early stages of product development since most functions of computer-aided-design and manufacturing tools are aimed at the later stages at which time the majority of product cost is already committed. Product models are an essential element in CAD tools and therefore, it is important that product models are available which are suited to use in the early design stages. This paper reports on an aggregate product model schema which can be used throughout the design of a product. The concept of relations between features is used to allow the extension of the model from conceptual to detailed design stages. Feature relations are an integrated representation system for dimensioning, tolerancing and connectivity modelling. This unified approach enhances the development of design assessment tools, in particular the product quality assessment functionality of the CAPABLE system.
Test and inspection are an increasingly costly element of electronic system design and manufacture and so it is critical that
the cost effectiveness of test and inspection are well understood. This paper presents techniques which may be used to assess
test capability and hence the implied test quality costs of PCB level electronic circuits. The techniques presented are based
on the electronic Conformability Analysis (eCA) approach which combines process capability indices and Failure Modes and Effects
Analysis with a cost mapping procedure. It introduces a new measure of test capability based on the widely used process capability
measure C
pk. This analysis allows the quality costs associated with design and manufacture induced faults to be estimated and the effectiveness
of test strategies in reducing these costs to be determined. It allows the trade-off between quality costs and the component,
manufacturing process and test costs to be explored. The technique has been applied to analogue & mixed signal, safety critical
circuits from automotive systems.
An important measure of design quality is the extent to which a circuit design is able to meets its specification in the presence of component parameter variations. Of equal importance is knowing how to improve this capability most effectively. This paper presents an analysis technique and visualization method which give the designer information about design quality and possible routes to improved quality. The paper introduces two new measures of functional capability, based on the process capability measure Cpk, and shows how these may be linked to the statistical variations in individual component parameters. It then shows how this information may be used to assess the effectiveness of altering the nominal value and variability in each of the component parameters in order to improve circuit performance capability. The analysis required is straightforward and the information is presented in a clear manner which can readily be interpreted by the designer. The effectiveness of the technique has been evaluated through analysis of a number of printed circuit board level analogue and mixed signal circuits and the results of a case study are shown. Copyright © 2007 John Wiley & Sons, Ltd.
This paper describes the adaptation of the conformability analysis technique to the assessment of functional, manufacturing and test capability of PCB level electronic circuits. It combines process capability indices and failure modes and effects analysis (FMEA) with cost mapping to allow the quality costs associated with design and manufacture induced faults to be estimated and the effectiveness of test strategies in reducing these costs to be determined. It allows the trade-off between these costs and the component, manufacturing process and test costs to be explored. The technique is particularly applicable to the relatively low complexity analogue and mixed signal safety critical circuits typically found in automotive and aircraft electronic systems.
Conformability Analysis is a structured analysis to project the
process capability of each component and assembly in a product design,
and estimates a cost of failure for the proposed product. So far,
Conformability Analysis has been applied only to mechanical designs, but
the need to extend the method to electronics design has been recognised.
This paper outlines the developments in capable design aimed at
determining the level of variability associated with printed
circuit-board manufacture and assembly, and the application of worst
case design early on in the design process. Use of the approach in
supplier development is also discussed
A method, called conformability analysis (CA), has been introduced through which the potential quality loss associated with a product design can be predicted for given manufacturing and assembly process routes. In this way, the projection of profitability of a product introduction project can be better informed. CA also provides a way of opening discussions with suppliers in terms of process capability and a means of specifying process capability values (Cpk) on drawings to show special design characteristics important to the customer.
The design of a product largely predetermines its cost and quality, and there are therefore limits to the benefits that can be obtained by the application of best practice in manufacturing and quality control. The paper introduces a general model of design for quality and describes a systematic quality evaluation methodology to aid the development of quality competitive products. The application and performance of the methodology are described and its integration with techniques in design for manufacture and assembly is discussed.
Quality is commonly considered as a down-line ‘measurement’term or goal and yet experts believe that practically all of a product's cost and quality are committed by decisions taken during the design and planning stages. The paper introduces a design for quality model and associated techniques to support the introduction of quality-competitive products. The relationship between these techniques and other quality tools, and their connections with techniques in design for manufacture are also discussed.
Design for quality is not a standalone subject. With ISO 9000 procedures being seriously adopted throughout industry in Europe, the author links design for quality with design methodology and the procedures thereby for handling design quality issues.
Enhanced quality function deployment
- D Clausing
- S Pugh