Design for eXcellence

Unit 4: Design for test

Section 1: Background to test and inspection

Introduction

The hidden costs of poor quality are very significant, so inspection and test are not isolated processes which occur after manufacture has finished, but vital elements integrated into production, whose role is to ensure quality rather than just identify defects. What we are trying to do in the test and inspection process is to

Test processes, and the adequacy of test coverage, are becoming increasingly important in electronics manufacturing. This is because higher-density circuits and advanced component packaging with smaller pitches are used to meet marketplace demands for smaller products with more functions. Unfortunately, these are proving more difficult to test.

Along with Design for Assembly and Design for Fabrication, Design for Test (DfT) is a ‘Design for’ that a PCA designer will be considering in an organisation’s DfM or DfX process. DfT is a methodical approach to design that incorporates the needs of the chosen test process and develops the test programmes and structures alongside the design rather than as an afterthought. Those needs are communicated to the designer with sets of constraints, rules and guidelines.

Most of the activity of PCB layout engineers relates to their responsibility for integrating testability requirements into a layout. In addition, they may be involved in test process decision-making (although the test engineers will carry out DfT and provide recommendations for redesign), so need to understand terminology such as Boundary Scan and the reasons behind any design constraints for a specific application.

‘Inspection’ and ‘test’ are terms which are sometimes used interchangeably. The distinction drawn in this module is that:

In the first part of this unit, which is taken from the Materials and processes for EDR module, we will be looking first at inspection, and then at test, describing the test sequence, and looking at bare board and assembly test procedures. In the parts that follow, there is an explanation of common design recommendations made after a DfT review or suggested on a DfT review checklist . In the last section we provide some information on the type of decisions made when conducting a DfT analysis and deriving the test process for a product.

Types of inspection system
Types fo test system
Practical testing
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