Page 51 - Building A Succesful Board-Test Strategy

What Is a Test Strategy? 37

production and not looking again for those same faults downstream can simplify
program generation and fixture construction considerably.
For a number of years, boards and components have been shrinking, while
the number of components per board has soared. Surface mounting, small-outline
ICs, and other space-saving solutions have permitted cramming huge amounts of
circuitry into ever-smaller areas. With ball-grid arrays (EGAs), flip-chips, and other
technologies, nodes reside underneath the components. Where conventional
wisdom once demanded restricting components to one board side and ensuring
that all nodes feed through to the other side for testing, new products often cannot
afford such luxuries. For these products, bed-of-nails access is becoming more dif-
ficult. Therefore, in-circuit test can verify only component clusters rather than
single parts. Some manufacturers are returning to functional test as a primary tool
because bed-of-nails access is nearly impossible. Lack of access has also fueled
renewed interest in inspection as a viable tactic in a "test" strategy.
At the same time, some products are proceeding in the opposite direction.
Disk drives, for example, have shrunk to the point where laptop computers weigh-
ing barely 5 pounds can have multigigabyte hard-disk capacities, with no end in
sight. To increase reliability and simplify both manufacturing and testing, design-
ers are integrating more functions onto ASICs, reducing the total number of com-
ponents on the board as well as the density of associated traces. CD-ROM and
DVD drives and writers—because of the huge amounts of data they hold and their
high data-transfer rates—are exhibiting these same design characteristics.
This development has pushed much quality assurance back to the compo-
nent level, while providing real estate for the test nodes and through-holes that
designers have been resisting. Assuming that even complex devices worked before
board assembly, many test engineers are returning to beds-of-nails rather than
edge-connector techniques to verify that the board was built correctly, further
reducing test-generation costs and schedules.

1,6.6 The How of Testing

What types of equipment does a test strategy include? Some manufacturers
choose rack-and-stack test instruments controlled from a host PC or workstation.
Others may design custom testers or may select from vendor offerings. Still others
may farm out all test responsibilities to third parties, even if they do the manu-
facturing themselves, although with the expansion of contract manufacturing this
practice is far less common than it was just a few years ago.
Test tactics may involve in-circuit-type or functional-type tests. An emula-
tion test allows a board to behave as in the target system, stopping at convenient
times to monitor register contents and other specific internal conditions.
Inspection may supplement or replace test steps that look for manufacturing
defects. Inspection steps can be manual or automatic, and can occur post-paste,
post-placement, or post-reflow. Each inspection method and location provides
different information about the process and about board quality.