Page 116 - Building A Succesful Board-Test Strategy

102 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

Figure 3-5 Inspecting immediately after solder-paste printing will find problems such
as off-pad solder. (Photo courtesy Teradyne.)

Post-paste inspection can take several forms. Some companies still use human
inspectors, but the number and size of solder pads and the difficulty of conclu-
sively determining solder-pad registration limits their effectiveness.
The least expensive equipment-supported technique takes advantage of the
capabilities of the paste printer itself. Paste printers include their own cameras to
detect fiducials on the boards, as well as to facilitate process setup and calibration,
These cameras can double as inspectors. They can look down at the board to detect
solder anomalies and look up to find clogged apertures and other irregularities in
the stencils. This technique is inexpensive and effective within limits (and subject
to the ever-present tradeoff between cycle time and inspection resolution),
However, it provides only two-dimensional images, limiting the accuracy of solder-
volume calculations and, therefore, joint-quality predictions. On the other hand,
some manufacturers contend that the area of solder alone correlates well with
volume, even without height measurements.
The printer camera cannot look for fiducials and examine solder pads at the
same time. The printer deposits the solder, then the camera inspects the results. In
high-volume operations, the time required for this two-cycle process generally
proves prohibitive. On the other hand, manufacturing engineers in low-volume
environments need not concern themselves with the time constraint.
To cope with throughput limitations, some companies use the camera
to examine only certain board areas that they consider either typical or critical.
Also, some engineers use the camera during paste-print setup to ensure correct