Page 117 - Building A Succesful Board-Test Strategy

Inspection as Test 103

stencil loading and positioning, paste-pressure adjustment, and proper paste dis-
pensing. They inspect the first few boards and make any necessary process modi-
fications, then turn off the camera to avoid slowing the process during routine
production.
In addition to solder-paste area and position, three-dimensional inline tech-
niques profile solder height to precisely calculate its volume. Proponents contend
that irregularities in the shape of the solder on the board make solder-volume cal-
culations from only pad-area coverage inaccurate. Some 3-D systems inspect the
entire surface of every board through the process. Slower and less-expensive
systems examine critical board areas or production samples.
The more common approach to three-dimensional post-paste inspection
scans a laser across each solder pad, starting with a line across the pad edge
and moving perpendicularly until the laser has covered the entire pad. The
equipment then measures solder height by a technique called reflective interference,
from which it calculates the volume. Some such systems create a complete volume
map of solder paste. To attain adequate speeds for full production, other systems
may calculate volume by measuring height from line-scans at one or a few
locations.
Another approach projects a diffraction fringe pattern on the board, then cal-
culates height and volume by examining the pattern with a stroboscopic white light.
This variation avoids the need to scan the board, thereby increasing inspection
speeds. Supporters of this variation contend that it provides as much and as accu-
rate information about the solder joint as the laser scan in less time.
Programming for three-dimensional inspection inline begins with sites that
will accommodate fine-pitch components, BGAs, flip-chips, and other chip-scale
packages. If time permits, the step can look for clogged stencil apertures and similar
problems.
Manufacturers can use this type of inspection as a "real-time alarm"—stop-
ping the line if measurements exceed established tolerances—or merely record the
information and move on.
The primary limitation in all three-dimensional inspection techniques is that
obtaining a useful reflection for analysis depends on the angle of incidence between
the light source and the board under test. Irregularities or reflection variations in
the board surface, as well as the reflectivity of the solder itself may affect results.
The inspection step must also locate the board surface to determine how much of
the measured height is solder. Examining both sides of a double-sided board
requires two separate inspections.

3.3 Post-Placement/Post-Reflow

Once components have been assembled onto the board, inspection looks for
a number of additional manufacturing conditions: the presence or absence of com-
ponents, component height, some incorrect components (if the incorrect part is
significantly different in size or appearance), as well as the accuracy of pick-and-
place machines and chip shooters.