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Machine vision is a well established method of automating mark inspection on IC packages. Defects such as missing mark, blurred mark, mis-orientation, illegible mark, etc. can be caught accurately and repeatedly by a good mark inspection system. The most common inspection algorithm that is employed is `correlation' (or template matching). Here the mark characters are taught to the system by the operator on one device. These characters are stored as a template set. Subsequent devices are compared with this stored template set and based on the comparison results the mark is classified as good or as reject. One of the setbacks of this traditional method of inspecting mark with a single template set is that it overkills when marking has variation. Variation in the mark can arise from the same or different marking machines and also when device lots with different mark characters are combined. Mark inspection using multiple templates provides a solution to this problem. In this method, instead of one template set, multiple template sets can be taught and subsequent devices are checked with these multiple mark template sets.
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