Page 115 - Building A Succesful Board-Test Strategy

Inspection as Test 101

Figure 3-4 The types of test and inspection that can serve the needs depicted in
Figure 3-3. (Courtesy Teradyne.)

human assemblers have finished their tasks. Post-reflow (it could also be called
"post-wave-solder," but most soldering these days avoids that error-prone step)
takes place after boards have emerged from reflow ovens, when the final solder
joints are available for qualitative and quantitative analysis.

3.2 Post-Paste Inspection
Examining the board immediately after paste deposition enjoys numerous
advantages. Inspection at this stage looks for a number of fault types. A clogged
aperture in the printer stencil, for example, can prevent solder from reaching some
pads, or the solder deposited may be insufficient to create an acceptable joint later
on. This step can also find residual solder where it doesn't belong on the board.
Measuring the area of solder on the pad permits a reasonable estimate of the quan-
tity of solder deposited, a reasonable predictor of final solder-joint integrity. Off-
pad solder, like that on the board in Figure 3-5, can also cause device-connection
problems later on.
One of the primary benefits of post-paste inspection is the ease and low cost
of board repair. Fixing the board at this stage requires merely washing the solder
from the board and returning it to the "back of the line" in the manufacturing
process, ready for another go. This step eliminates many problems before they
escape to downstream test processes. Some manufacturers estimate that up to 80
percent of manufacturing faults on surface-mount boards result from problems at
the solder-paste step.