Page 17 - Building A Succesful Board-Test Strategy

4 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

Similarly, design for testability reduces the incidence of some faults and
permits finding others more easily. Feeding failure information back into the
process allows adjustments to improve future yields. These steps also belong as part
of the larger concept of "test."
Embracing those steps in addition to the conventional definition of "test"
allows test people to determine more easily the best point in the process to iden-
tify a particular fault or fault class. Pushing detection of certain faults further
upstream reduces the cost of finding and repairing them. In addition, not looking
for those same faults again downstream simplifies fixture and test-program gener-
ation, shortens manufacturing cycles, and reduces costs.
Test strategies have traditionally attempted to find every fault possible at each
step. Adopting that approach ensures that several steps will try to identify at least
some of the same faults. A more cost-effective alternative would push detection of
all faults as far up in the process as possible, then avoid looking for any fault
covered in an earlier step later on.
Self-test, too, forms part of this strategic approach. Many products include
self-test, usually some kind of power-on test to assure the user that the system is
functioning normally. Such tests often detect more than a third of possible fault
mechanisms, sometimes much more. Which suggests the following "rules" for test-
strategy development:
• Inspect everything you can, test only what you must.
• Avoid looking for any problem more than once.
• Gather and analyze data from the product to give you useful
information that allows you to improve the process.

1.3 Strategies and Tactics

Test strategies differ significantly from test tactics. In-circuit test, for example,
is a tactic. Removing manufacturing defects represents the corresponding strategy.
Other tactics for that strategy include manual and automated inspection,
manufacturing-defects analysis (a subset of in-circuit test—see Chapter 2), and
process improvement.
A strategy outlines the types of quality problems you will likely experience,
then describes which of those problems you choose to fight through the design
process during design verification, which you assign to test, and which you leave
for "Let's wait until the product is in the field and the customer finds it,"
The difference between strategies and tactics boils down to issues of term and
focus. A test strategy lasts from a product's conception until the last unit in the
field dies. During that time, the manufacturer may resort to many tactics. Also,
a tactic addresses a particular place and time in the overall product life cycle. A
strategy generally focuses on the whole picture.
In building a test strategy, we are always looking for "digital" answers to
"analog" problems. That is, we must decide whether the product is good or bad. But
how good is good? How bad does "bad" have to be before the circuit will not function?