Page 136 - Building A Succesful Board-Test Strategy

122 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

a board simulation—and metrology tools to establish pass/fail criteria. Some
systems learn from a known-good board (subject to the usual caveats), and can
automatically locate, inspect, and evaluate each solder connection. This approach
involves training the system to recognize all of the solder-joint geometries that it
must inspect and storing the information in a library with corresponding thresh-
olds and tolerances. The engineer can modify the library criteria to allow for
deviations and customizations.

3.3.5.2 X-Ray Imaging
X-ray inspection falls into two broad categories, covering two-dimensional
and three-dimensional techniques. Figure 3-23 shows two-dimensional, or trans-
mission x-ray, where a stationary x-ray source looks directly through the board,
inspecting both board sides simultaneously. Image intensity indicates depth of the
feature under scrutiny. The approach works best with single-sided boards.
The mechanics of transmission x-ray equipment are considerably simpler
than those of the more complex three-dimensional techniques. It is easier to imple-
ment and the equipment is less expensive. Test time is also faster, although propo-
nents of three-dimensional approaches argue that a slower diagnostic time reduces
the test-time advantage.
On the downside, transmission x-ray cannot easily distinguish features on
double-sided boards because images of the two sides overlap. To compensate, some
industry experts recommend staggering components on the top and bottom of the
board, as Figure 3-24 illustrates.

Figure 3-23 Transmission x-ray sees through the board, presenting it as a two-
dimensional image. (Courtesy Agilent Technologies.)