Page 22 - Building A Succesful Board-Test Strategy

What Is a Test Strategy? 9

percent of a job requires 80 percent of the effort. In testing, the last 10 percent
taking 90 percent of the effort might be more accurate.
This analysis is not meant to advocate shoddy product quality. Every
company must establish a minimum acceptable level of quality below which it will
not ship product. Again, that level depends on the nature of the product and its
target customers.
Companies whose operations span the globe must also consider the enor-
mous distances between design and manufacturing facilities. Language, time zones,
and cultural differences become barriers to communication. In these situations,
manufacturing must be fairly independent of product development. Design-to-
manufacture, design-to-test, and similar practices become even more critical than
for more centralized organizations.
Therefore, selecting an efficient, cost-effective test strategy is a mix of engi-
neering, management, and economic principles, sprinkled with a modicum of
common sense. This book tries to create a successful salad from those ingredients.

1.4 The Design and Test Process
There are only three ways in which a board can fail.
Poor-quality raw materials result from inadequacies in the vendor's process
or design. This category includes bad components; that is, components (including
bare boards) that are nonfunctional or out of tolerance when they arrive from the
vendor, rather than, for example, delicate CMOS components that blow up from
static discharge during handling for board assembly.
If the board design is incorrect, it will not function properly in its target appli-
cation, even if it passes quality-control or test procedures. An example would be
a bare board containing traces that are too close together. Very fast signals may
generate crosstalk or other kinds of noise. Impedance mismatches could cause
reflections and ringing, producing errors in edge-sensitive devices ranging from
microprocessors to simple flip-flops and counters. In addition, if the traces are too
close together, loading components onto the board reliably while avoiding solder
shorts and other problems may be impossible, so that even if the bare board tech-
nically contains no faults, the loaded board will not function.
A board can also fail through process variation. In this case, the board design
is correct but may not be built correctly. Faults can result from production vari-
ability, which can include the compounding of tolerances from components that
individually lie within the nominal design specifications or from inconsistent accu-
racy in board assembly. Sometimes substituting one vendor's component for an
allegedly equivalent component from another vendor will cause an otherwise func-
tioning board to fail. Also in this group are design specifications, such as requir-
ing components on both board sides, that increase the likelihood that the process
will produce faulty boards. In addition, even a correct and efficient process can get
out of control. Bent or broken device legs and off-pad solder or surface-mount
parts fall into this category. The culprit might be an incorrectly adjusted pick-and-