Page 110 - Building A Succesful Board-Test Strategy

96 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

2.3.15 Other Options

Manual testing consists of training technicians to analyze board performance
using an array of individual instruments and some kind of written procedure. The
approach is most appropriate in small companies and other low-volume applica-
tions. It can also provide a good way to analyze very early production runs for a
new product, where information from manual tests helps programmers develop
automatic tests. Startup costs are relatively low—the cost of people, training, and
instruments. The method is quite flexible if technicians move easily from one board
type to another. Of course, the technique is too slow for many applications. It
requires highly skilled and well-trained people, and results may be inconsistent,
from technician to technician or from day to day.
Inspection techniques are also gaining popularity to complement or supple-
ment traditional test. Chapter 3 will examine this option.
The most interesting alternative board-test strategy is not to test at all, as with
the Japanese television manufacturer referred to in Chapter 1. Test engineering is
perhaps the only occupation whose ultimate goal is its own demise. Ensuring vendor
material quality and monitoring the production process at every step will, at some
point, produce 100 percent good boards and systems. Test professionals can rest
assured, however, that we will not likely achieve that goal anytime soon.

Finding a fault at any production stage can cost 10 times what it costs to find
that same fault at the preceding stage. Monitoring processes to prevent problems
from occurring at all represents the least expensive option. Failing that approach,
manufacturers generally try to remove as many failures as possible at board level,
before assembling systems.
Many test techniques are available to accomplish this task. Bed-of-nails
methods, such as shorts-and-opens, manufacturing-defects analysis (MDA), and
in-circuit testing, operate on individual components, board traces, or functional
clusters to ensure correct assembly. Most test failures have one and only one cause,
minimizing diagnostic efforts.
Functional-test techniques, which control boards through a few nodes at the
edge connector or a test connector, examine overall board behavior and verify it
against designer intentions. Because of the logic depth between a failing node and
the access point, fault diagnosis can be lengthy and may involve guided probing or
other additional steps.
Emulation testers perform a functional-type test on microprocessor-based
logic. Hardware pods instead of software models mimic complex circuitry, allow-
ing testers to examine logic states and assess board performance.
Ultimately, the goal is to have processes that are sufficiently in control to
eliminate test completely. Fortunately, it will be some time before that goal comes
within reach.