In order to fulfill the requirements of printed circuit board inspection, a variety of inspection equipment has been produced. Automated Optical Inspection (AOI) systems are often used for inner layer testing before layering; after layering, X-ray systems monitor alignment accuracy and fine defects. Scanning laser systems provide inspection of pad layers prior to the reflow method. These systems, combined with visual inspection technology on the production line and component integrity inspection for automated component placement, help ensure the reliability of final assembled and soldered boards.
However, even with these efforts to minimize defects, final inspection of the assembled printed circuit board is still required, which is perhaps the most important because it is the final unit of product and overall process evaluation.
Final inspection of assembled printed circuit boards may be done by automated methods or by automated systems, and is often done using a combination of both methods. “Manual” means that an operator visually inspects the board using optical instruments and makes correct judgments about defects. Automated systems use computer-aided graphic analysis to determine defects, and many consider automated systems to include all inspection methods except manual light inspection.
X-ray techniques provide a means of assessing solder thickness, distribution, internal voids, cracks, desoldering and the presence of solder balls (Markstein, 1993). Ultrasonics will detect voids, cracks, and unsealed interfaces. Automated optical inspection evaluates external features such as bridging, solder flux and shape. Laser inspection provides three-dimensional images of external features. Infrared detection detects internal solder joint failures by comparing the thermal signature of the solder joint with a known good solder joint.
It is worth noting that all the defects that cannot be found by the limited inspection of assembled printed circuit boards by these automated inspection techniques have been found. Therefore, manual visual inspection methods must be used in conjunction with automatic inspection methods, especially for those few applications. A combination of X-ray inspection and manual optical inspection is the best way to detect defects in assembled boards.
Assembled and soldered printed circuit boards are prone to the following defects:
1) Missing components
2) Component failure
3) The components have installation errors and misalignment
4) Component failure
5) Poor tin dip
7) Insufficient solder
8) Too much solder forms tin balls
9) Form welding pinholes (air bubbles)
10) There are pollutants
11) Inappropriate pads
12) Wrong polarity
13) The pin floats
14) The pin sticks out too long
15) Cold welding spots
16) Too much solder
17) Solder voids
18) There are blow holes
19) The inner circle fillet structure of the printed line is poor
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