Page 56 - Building A Succesful Board-Test Strategy

42 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

productive. Increased productivity also generally translates into greater job satis-
faction, thereby lowering turnover and reducing costs still further.
Every piece of test and test-related equipment—from PCs or workstation
hosts to individual instruments, to benchtop testers, to freestanding "monolithic"
testers, to fixtures and other interface hardware—requires maintenance. Mainte-
nance includes instrument calibration and other scheduled activities, pin replace-
ment on bed-of-nails fixtures, and unscheduled repairs of equipment failures,
Choice of specific equipment and classes of equipment can significantly affect
these costs.
For example, most companies can quickly locate a PC to replace a failed PC
host in a tester or rack-and-stack system that conducts measurement and test func-
tions over an IEEE-488 interface. Similarly, a big tester that can pinpoint a faulty
switching card after a failure permits quick board swapping and a minimum of
manufacturing downtime.
On the other hand, more time-consuming equipment repairs increase costs
not only for the repairs themselves but also for lost production while the line is
shut down. In-house-designed test equipment often falls into this category because
of inadequate or outdated documentation or because tester designers have moved
on to other projects and are no longer available for consultation.
Board-design and test-strategy characteristics directly affect the cost of diag-
nosing faulty boards to identify exact causes of failure and then repairing them,
Many strategies will fail some good boards, creating false failures, which increase
testing and dlagnosis-and-repair costs. More forgiving test-strategy elements may
pass bad boards to the next test stage, where finding failures is more expensive
("escapes"). Failing boards may defy diagnosis, leading them to pass through the
test process several times, only to end up on some bone pile.
Board repair costs include the cost of concomitant failures—secondary
failures that result from repair activities or that occur when powering up a board
containing a specific fault causes another device to fail as well.
Test-strategy flexibility and design stability can influence costs related to
product upgrades. Test devices that connect to a board via the edge connector or
test connector or through a microprocessor socket are less affected by many
changes, because these board characteristics are likely to remain relatively con-
stant. Board layout, on the other hand, frequently changes as designers replace
off-the-shelf components with ASICs or with more capable or more reliable
alternatives. A strategy that includes a bed-of-nails works better with a more
stable product, because altering the fixture to accommodate design changes can be
quite expensive.
Other test-operation costs include indirect labor costs, including supervisors"
and managers' salaries. Facilities costs cover floor space, security, utilities, and
other fixed expenses.
Note that reducing the amount of floor space devoted to one product does
not necessarily improve the financial burden on the company as a whole. Factories
are not elastic. Abandoned space does not disappear. Therefore, unless another