Page 111 - Building A Succesful Board-Test Strategy

CHAPTER
Inspection as Test

Living with ever-increasing component complexity and board density, along
with decreased nodal access, test engineers must face the reduced efficiency of
traditional test strategies. Yet the need to ship good products has become more
critical than ever. Customers expect that their electronic products will work the
first time without difficulty, and that they will continue to work with a minimum
of fuss.
To cope, manufacturers are turning to inspection as complement or supple-
ment to traditional test. Inspection, when it works, offers numerous advantages
over test. It requires neither bed-of-nails nor edge-connector fixtures. Good-board
criteria against which you compare the board under test may come from a known-
good board (or a number of them to establish appropriate tolerances) or from a
simulation. Also, most inspection is noninvasive. That is, it does not exercise or
otherwise disturb the circuit.
On the other hand, inspection can determine that the board looks correct.
but it cannot verify that the board works. Only a true test can establish function-
ality. Test and inspection represent a tradeoff, as Figure 3-1 illustrates. Taking
maximum advantage of inspection can simplify requirements for subsequent test,
which leads to a recommendation that I hope will become an industry mantra:
"Inspect everything you can, test only what you must."
The inspection equivalent of a test program consists primarily of a repre-
sentation of a good board's physical-layout specifications and a collection of rules
and heuristics to decide whether the board under scrutiny conforms sufficiently.
Perhaps the greatest caveat that accompanies most inspection techniques is that
unless the heuristics allow for sufficient variation in judging what constitutes a good
board, the step will produce excessive numbers of false failures.
Test, on the other hand, relies on input signals and output measurements,
which, at least to some degree, require exercising the circuit to determine its quality.
An improperly designed test on a board containing a catastrophic fault can aggra-
vate existing problems. Powering up a board containing a short, for example,
accomplishes little beyond frying working circuitry and verifying the performance
of the facility's smoke detectors.

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