Page 81 - Building A Succesful Board-Test Strategy

Tesf Methods 67

an MDA or some form of inspection to find manufacturing problems and then
proceeding directly to functional test.
One recent development is actually making in-circuit testing easier. As
designers integrate more and more functionality onto single devices, real-estate
density on some boards is declining.
For example, shrinking boards for desktop computers, telephone switch-
ing systems, and similar products realizes few advantages. Even notebook
computers cannot shrink beyond a convenient size for keyboard and display,
A few years ago, IBM experimented briefly with a smaller-than-normal form
factor notebook computer. When the user opened the lid, the keyboard unfolded
to conventional size. Unfortunately, reliability problems with the keyboard's
mechanical function forced the company to abandon the technology. (Although
some palmtop computers feature a foldable keyboard design, their keyboards
are much smaller than those of a conventional PC, making the mechanical
operation of the folding mechanism considerably simpler.) Since that time, mini-
mum x-y dimensions for notebook computers have remained relatively constant,
although manufacturers still try to minimize the z-dimension (profile). In fact,
some new models have grown larger than their predecessors to accommodate large
displays.
Two recent disk-drive generations from one major manufacturer provide
another case in point. The earlier-generation board contained hundreds of tiny
surface-mounted components on both sides. The later design, which achieved a
fourfold capacity increase, featured half a dozen large devices, a few resistor packs,
bypass capacitors, and other simple parts, all on one side. Boards destined for such
products may include sufficient real estate to accommodate through-hole compo-
nents, test nodes, and other conveniences.
In-circuit testing of individual devices, however, has become more difficult,
especially in applications such as disk-drive test that contain both analog and
digital circuitry. At least one in-circuit tester manufacturer permits triggering
analog measurements "on-the-fly" during a digital burst, then reading results after
the burst is complete. This "analog functional-test module" includes a sampling
DC voltmeter, AC voltmeter, AC voltage source, frequency- and time-measurement
instrument, and high-frequency multiplexer.
Consider, for example, testing a hard-disk-drive spindle-motor controller
such as the TA14674 three-phase motor driver with brake shown in Figure 2-11.
This device offers three distinct output levels—a HIGH at 10V, LOW at 1.3V, and
OFF at 6V. Durickas (1992) suggests that although a CMOS logic-level test will
verify basic functionality, accurately measuring the voltages themselves increases
test comprehensiveness and overall product quality.
To conduct such a test, Durickas requires prior testing of certain passive
components that surround the controller, then using those components to set
important operating parameters for the primary device test. Therefore, his ap-
proach necessitates bypassing the controller test if any of the passive compo-
nents fails.