Page 67 - Building A Succesful Board-Test Strategy

CHAPTER 2

Test Methods

Chapter I discussed many of the decision parameters that constitute an effective
board-test strategy. This chapter describes various test techniques, examining their
capabilities, advantages, and disadvantages.

2.1 The Order-of-Magnitude Rule

An accepted test-industry aphorism states that finding a fault at any stage in
the manufacturing process costs 10 times what it costs to rind that same fault at
the preceding stage, as Figure 2-1 illustrates. Although some may dispute the factor
of 10 (a recent study by Agilent Technologies puts the multiplier closer to 6) and
the economics of testing at the component level, few would debate the principle
involved. In fact, for errors that survive to the field, 10 times the cost may be
optimistic. Field failures of large systems, for example, require the attention of
field engineers, incurring time and travel expenses, and may compromise customer
goodwill.
The single biggest cost contributor at every test level is troubleshooting time.
Uncovering design problems and untestable circuit areas before production begins
can prevent many errors altogether. Similarly, analyzing failures that do occur and
feeding the resulting information back into the process can minimize or eliminate
future occurrence of those failures.
A prescreen-type tester such as a manufacturing-defects analyzer (MDA) can
find shorts or other simple faults much more easily than a functional tester can. In
addition, because the functional level generally requires the most expensive and
time-consuming test-program development, eliminating a fault class at the earlier
stage may obviate the need to create that portion of the functional test program
altogether. Equipment and overhead for people and facilities at each stage also tend
to be more expensive than at the preceding stage.
As an example, consider an automated assembly-board-manufacturing
process that includes a complex soldered part costing $10. A board test reveals
that the part is bad. Repair consists of removing the part, manually inserting
its replacement, soldering, and cleaning, perhaps 30 minutes' work. At a burdened

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