Where you inspect depends on what you're looking for. X-Ray is a fairly specialized piece of gear and doesn't lend itself for example to paste registration or thickness measurement. A laser system is better suited. An X-Ray system would allow you to check for excess paste shorts under a BGA for example or perhaps identify internal shorts in multi-layer boards or detect counterfeit components. It is not particularly useful for component inspection like an AOI machine. The key is to put it immediately after the process that you suspect is creating defects. For example if you are experiencing shorts under a BGA, finding the problem before they are reflowed greatly reduces repair cost. Rule of thumb, rework cost goes up by a factor of ten for each process step completed before the defect is caught.
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