Page 20 - Building A Succesful Board-Test Strategy

What Is a Test Strategy? 7

« Service
* Field returns
• The company's "image of quality"

Note that only test, field returns, and service involve testing at all, and field
returns do so only indirectly. Reducing the number of failures that get to the test
process or the number of products that fail after shipment to customers also
simplifies test activities, thereby minimizing costs.
Test-strategy selection goes far beyond merely choosing test techniques.
Design issues, for example, include bare-board construction. An engineer once
described a 50-layer board that was designed in such a way that it could not be easily
repaired. To avoid the very expensive scrapping of bad boards, his colleagues bor-
rowed a technique from designers of random access memory (RAM) components
and large liquid-crystal-display (LCD) panels—they included redundant traces for
most of the board's internal logic paths. Paths were chosen by soft switches driven
by on-board components individually programmed for each board.
Although this solution was expensive, the board's $100,000 price tag made
such an expensive choice viable, especially because it was the only approach that
would work. Without the redundancy, board yields would have been unacceptably
low, and repair was impossible. Unfortunately, the solution created another
problem. The board's components contained specific instructions to select known-
good paths. The bare board defied testing without component-level logic. There-
fore, the engineers created a test fixture that meshed with the sockets on the board
and mimicked its components. In addition to pass or fail information, the test
would identify a successful path, then generate the program with which to burn the
"traffic-cop" devices as part of its output. Including the redundancy as a design
choice mandated a particular extremely complicated test strategy. Sometimes test-
strategy choices reduce to "poor" and "none."
The acceptability of particular test steps depends on whether the strategy is
for a new or existing facility, product, product line, or technology. In an existing
facility, is there adequate floor space for expansion? Is the facility already running
three work shifts, or can a change in strategy involve merely adding a shift?
Test managers must also decide whether to design their own test equipment
or buy it from commercial vendors, whether they should try to "make do" with
existing equipment, and whether new equipment must be the same type or from
the same manufacturer as the installed base.
A test strategy's success also depends on aspects of the overall manufactur-
ing operation. For instance, how does a product move from test station to repair
station or from one test station to the next? Are there conveyors or other auto-
mated handlers, or do people transfer material manually? Concurrent-engineering
principles encourage placing portions of the manufacturing process physically
close to one another, thereby minimizing bottlenecks and in-transit product
damage. This arrangement also encourages employees who perform different parts
of the job to communicate with one another, which tends to increase manufactur-
ing efficiencies and lower costs.