Page 114 - Building A Succesful Board-Test Strategy

100 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

Smaller parts
More complex parts
Denser boards
Ever-shrinking device-pin spacings
Increasing use of BGAs, flip-chips, and other technologies with hidden nodes
Increased use of expensive, low-volume boards for many products
Figure 3-2 These factors both encourage and discourage inspection as part of a "test'

Figure 3-3 The various stages of the manufacturing process, and the test or inspection
requirements at each stage. (Courtesy Teradyne.)

Historically, "inspection" evoked images of armies of human beings exam-
ining tiny board features looking for problems. In fact, human visual inspection
(HVI) remains a huge part of the industry's inspection arsenal. A few years ago,
Stan Runyon at Electronic Engineering Times speculated that 40,000 human beings
still performed this function, despite its declining effectiveness in the face of
advances in board technology.
In fact, inspection covers much more ground than the human beings armed
with magnifying glasses or microscopes looking for anomalies on the board
surface. Figure 3-3 shows the various stages of the manufacturing process, and the
test or inspection requirements at each stage. Figure 3-4 shows the types of test
and inspection that can serve those needs.
As you can see, inspection can take place at any or all of three process
locations. Post-paste inspection examines the board after the paste-printer
has deposited solder, before assembly operations have added components. Post-
placement inspection occurs after pick-and-place machines, chip shooters, and