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availability is still quite limited, however, and the jury is still out on the board types
and factory configurations that will benefit most. Still, by offering a new applica-
tion of an established approach and a way to identify problems that other solu-
tions miss, this technique can provide a viable choice in creating a successful test
strategy.

3.3.5 The New Jerusalem?—X-Ray Inspection
Currently the fastest-growing inspection technique, x-ray inspection can
detect defects that defy the line-of-sight constraint of optical systems. It can
examine hidden components and solder joints on both surfaces of double-sided
boards, and can even inspect inner layers of multilayer boards. X-ray is the only
available technique that can quantitatively analyze solder joints. Results can indi-
cate problems with the solder and assembly processes, measuring such parameters
as solder-joint accuracy, fillet height, solder volume (detecting both insufficient and
excess solder), component existence and position, and polarized-capacitor orien-
tation (by the position of the "slug").
Figure 3-17 shows the geometry of a typical solder joint. The x-ray system
can measure each of the noted features and compare them with a good-joint stan-
dard to permit a pass-fail decision. The gull-wing solder joint in Figure 3-18 will
pass both electrical and x-ray inspection. In Figure 3-19, the joint shows an insuf-
ficient heel. This joint will pass electrical test because a connection exists. Never-
theless, it represents a potential reliability problem. Finding marginal faults such
as this is one of x-ray inspection's strengths.
The x-ray system examines the joint, producing the image shown in Figure
3-20. Combining that information with data that calibrate the darkness of points

Figure 3-17 The geometry of a typical solder joint. (Courtesy Agilent Technologies.)