Page 97 - Building A Succesful Board-Test Strategy

Test Methods 83

A functional-test bed-of-nails cannot inject signals. It provides observation
points only. Reducing functional-test logic depth, however, simplifies test-program
generation considerably.
Functional beds-of-nails remain unpopular with most board manufacturers
because of fixture costs, scheduling pressures, and nail capacitances. These con-
cerns have spurred the growth of boundary-scan designs (see Chapter 5) as an alter-
native for internal-logic access.
An MDA or an in-circuit test measures the success of the manufacturing
process. Functional testing verifies board performance, mimicking its behavior in the
target system. Because this test tactic addresses the circuit's overall function, it can
apply equally well to testing large circuit modules or hybrids and to system testing.
Functional test can occur at full speed, thereby uncovering racing and other
signal-contention problems that escape static or slower-speed tests. This test also
verifies the design itself, as well as how well the design has translated to the real
world. Test times for passing boards are the fastest of any available technique, and
failure analysis can indicate the correct fault, almost regardless of board real-estate
population density.
Functional test is traditionally the most expensive technique. Also, automatic
functional testing is still basically a digital phenomenon. Programming is difficult
and expensive and traditionally involves a complex cycle of automatic and manual
steps. Analog automatic program generation is nearly nonexistent.
Most functional testers work best at determining whether a board is good or
bad. Pinpointing the cause of a failure can take much longer than the test itself
Diagnostic times of hours are not unheard of. Many companies have "bone piles" of
boards that have failed functional test, but where the cause remains unknown.
A bed-of-nails tester can identify all failures from a particular category
(shorts, analog, digital) in one test pass. For a functional test, any failure requires
repair before the test can proceed. Therefore, a test strategy that eliminates bed-of-
nails techniques must achieve a very high first-pass yield with rarely more than one
fault per board to avoid multiple functional-test cycles.
Solutions that address these issues are emerging. Some new benchtop func-
tional testers are much less expensive than their larger siblings. A traditional func-
tional tester can cost hundreds of thousands of dollars, whereas benchtop prices
begin at less than $100,000. Larger testers still perform better in applications
requiring high throughput and very high yields, but for many manufacturers, small
functional testers can offer a cost-effective alternative.

2.3.10 Functional Tester Architectures
Digital functional testing comes in several forms. Differences involve one or
more of certain test parameters:
« Test patterns: Logical sequences used to test the board
* Timing: Determines when the tester should drive, when it should receive,
and how much time should elapse between those two events