Automatic parameter optimization in inspection systems

Automatic inspection systems for IC mark, package and lead inspection are being widely used as in-process controls and check points. Here their primary function is not only to inspect and sort out defective parts but also to provide feedback on how well a process such as marking or trim and form is performing. Inspection results or every part inspected are often accumulated in a statistical process control (SPC) program that can monitor drifts in the process. Not all drifts are caused by problems in the process itself, For example the mark contrast on a package may be reduced not only because of some problem with the marking process but also because of changes in the mold compound af the package or changes in the light intensity of the inspection system. In latter case a statistical tool such as the SPC program may alert the user of a process drift and he will have to retune, recalibrate or change the parameters of the inspection system. Often the change in parameter is done by trial-and-error, A change too much or too little can result in excess overkill or even escapes. Alternatively the statistical data itself can be used to suggest the user what changes should be made to the inspection parameters. This method of automatic parameter optimization is discussed in detail in this paper. A mark inspection system is chosen as a specific example an how to apply this method.