As PCBs become smaller and more densely populated, they become harder to inspect for quality without automating the inspection process. In general, printed circuit boards have many very small components that need to be inspected for correct component placement, skew, rotation, part number, orientation and even solder joint analysis. All of these inspection criteria can now be accomplished with an Automated Optical Inspection system (AOI). AOI machines are programmed to look at all of the components and solder joints and determine if they are placed and soldered correctly. This data is captured for future reference and traceability by bar coded PCB part number. AOI machines have evolved from the early days of a basic photographic comparator to more data-driven software that looks for where and what component should be in each location from the PCB board assembly data. AOI machines are for any PCB’s that have standard surface mounted components.
However, many PCBs now have bottom-terminated components (BTC). These components have all their solder connections under the body of the component and these terminations can only be seen using x-ray inspection. X-ray inspection is done in a lead-lined chamber using x-ray tubes of varying power depending on the thickness of the PCB and component density. Typical x-ray machines for PCB applications utilize x-ray tubes between 80 and 130Kv. They also use software to determine solder joint quality for these components, specifically looking for solder bridging (too much solder that connects solder joints that should not be connected), insufficient solder (poor solder connection between component and PCB) and excessive solder voiding (too many air-pockets in the individual solder joints causing poor solder connection strength). As the individual solder connections under the bottom terminated connections are very small, highly magnified images and software detection tools are required to identify defects. A typical BGA or BTC can have more than 1,000 individual connections that all must be inspected by x-ray.