(1) If the solder paste printing process meets the requirements after solder paste printing, the number of defects discovered by AOI can be greatly reduced. Typical printing defects include the following:
A. Insufficient solder on the pad.
B. Too much solder on the pad.
C. Poor adhesion of the solder to the pad.
D. Solder bridge between pads.
At AOI, the probability of a defect relative to these conditions is directly proportional to the severity of the situation. Slightly less tin leads to fewer defects, and serious conditions such as no tin at all, almost always cause defects in the ICT. Insufficient solder may be a cause of component loss or open solder joints. Nonetheless, deciding where to place an AOI requires recognizing that component loss may have occurred for other reasons that must be placed on the inspection plan. Inspection of this position most directly supports process tracking and characterization. Quantitative process control data at this stage includes print offset and solder volume information, and qualitative information about printed solder can also be generated.
(2) The pre-reflow inspection is performed after the components are placed in the solder paste on the board and before the PCB is fed into the reflow furnace. This is a place where the inspection machine is typically placed, as most defects from solder paste printing and machine placement can be found here. Quantitative process control information generated at this location provides information on the calibration of high-speed film machines and fine-pitch component placement equipment. This information can be used to modify component placement or to indicate that the placement machine needs calibration. The check of this position satisfies the goal of process tracking.
(3) Check the final step of the SMT process after reflow. This is the most popular choice for AOI at this time, because it can find all the assembly errors. The post-reflow inspection provides a high level of safety because it identifies errors caused by solder paste printing, component placement and reflow processes.