Page 125 - Building A Succesful Board-Test Strategy

Inspection as Test 111

BGA inspection can also look for excess solder-ball oxidation by gray-level
analysis. This condition usually manifests on whole lots of BGAs, rather than on
individual components, so identifying it permits repair or returning parts to
vendors. Although excess oxidation has never proved to represent a real defect,
most manufacturers will reject such BGAs, anticipating possible poor bump bonds
and possible poor wetting during underfill. Identifying this situation on BGAs
before assembly allows correction at much lower cost than scrapping BGAs or
whole boards later.

3.3.3 Design for Inspection
The success of an AOI step depends on the ease with which the camera can
distinguish the features that it inspects and the degree to which the board under
test conforms to the good-board standard. Several simple steps can make inspec-
tion more efficient and more successful.
Maintain consistent size specifications for particular components. For
example, 603 discrete component specifications list a size of 60 x 30 mils. Actual
devices, however, can vary from 50 x 22 mils to 65 x 34 mils, depending on vendor.
If you permit such a wide variation, an AOI system cannot determine whether a
device on the board is correct. Therefore, select a single component vendor or a
group of vendors who provide visually similar products.
Place components on the board with consistent orientation. This step will
make inspection-system programming easier, and will facilitate repair and rework
operations later. For the same reasons, it is preferable to select parts with the same
pin-1 designator (cut corner, colored dot, stripe, dimple).
Some manufacturers advocate specific techniques to facilitate determining
the component's exact position on the board. Placing fiducials on the component,
as in Figure 3-13, or painting the component site with a contrasting color, as in
Figure 3-14, makes position measurements more precise and reduces the number
of false failures. Fiducials on the component site, as in Figure 3-15, help the AOI
system make a presence/absence decision.
AOI is becoming increasingly common, experiencing a year-on-year growth
rate exceeding 20 percent. Current trends in board technology leave manufacturers
with few viable alternatives. Higher speeds, better spatial resolution, and more
accurate fault detection also combine to increase its effectiveness, and therefore its
popularity.

3.3.4 Infrared Inspection—A New Look at
an Old Alternative

Many kinds of board faults exhibit a higher inherent resistance than their
faultless counterparts do. When powered, however briefly, these areas heat up,
becoming detectable by infrared inspection. In the same way, infrared techniques
can also reveal marginal components, traces, and solder joints that often surface
as early field failures.