Page 106 - Building A Succesful Board-Test Strategy

92 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

A combinational tester can alleviate headaches that many very densely pop-
ulated boards cause. Test programmers subdivide a board's circuitry and create the
most effective test on each section. For example, where bed-of-nails access is pos-
sible at every node or where there is a lot of analog circuitry, an in-circuit test is
generally the best choice. Functional test works better in time-critical areas or
where surface mounting or other mechanical board features prevent convenient
probing,
Combinational testers can find in-circuit and functional failures in the same
process step (although not necessarily in the same pass). Use of one machine
instead of two minimizes factory floor space devoted to test operations and may
reduce the number of people required. Eliminating one complete board-handling
operation (between in-circuit and functional testers) reduces handling-induced fail-
ures, such as those from electrostatic discharge (ESD).
On the other hand, taking advantage of the ability to subdivide a board for
testing necessitates performing the subdivision during test-program development.
This extra analysis step often lengthens programming schedules and increases costs.
Combinational testers can also represent the most expensive test alternative.
As with functional testers, some lower-cost solutions are emerging. Smaller
than their more expensive siblings, the size of these models limits the amount
of test capability that fits inside the box. Speed, accuracy, fault coverage, and
throughput capacity are generally lower than with high-end machines. Also,
low-end systems do not offer multiplexing of test pins. Therefore, each pin driver and
receiver is independent of the others, but the total number of available pins is limited.

2.3.73 Hot-Mockup

The expense and other drawbacks of conventional functional and emulation
testing often prohibit their use. Many manufacturers follow in-circuit test with a
hot-mockup. This approach plugs the board under test into a real system that is
complete except for that board, then runs self-tests or other tests specifically
designed for this situation.
Disk drives, for example, are electromechanical systems with considerable
analog circuitry. Manufacturing occurs in very high volumes, with fast changeover
and short product life. Figure 2-30 shows an appropriate disk-drive test strategy.
During hot-mockup test, an operator attaches the board under test to a PC-
driven hard-disk assembly using clamps and pogo pins. He or she then executes
intense read/modify/write cycles for 5 minutes or more. If the drive fails, the board
is bad. One prominent disk-drive manufacturer employs more than 400 such hot-
mockups in one Singapore factory, with four per operator on a 5-foot workbench.
To change quickly from one board to another, the operator simply pulls two cables.
four thumb screws, and four Allen screws.
One fault that only hot-mockup testing can find relates to the way in which
disk drives store files wherever there is empty space, often scattering many file pieces
across the disk surface. Conventional test scans the disk from the outside in or the