Assembly and inspection of microelectronic systems

An evaluation of blink inspection

It has been suggested that performance in an industrial inspection task can be improved by rapidly alternating images of the item to be inspected and a perfect item onto the visual field of the inspector. Such a blink inspection system was evaluated for a complex item (a printed circuit board which could contain three flaws) and a simple item (a blank field which could contain one flaw) using eight student subjects for each of five inspection conditions. Three groups, one using blink inspection and two control groups, inspected circuit boards; two groups (one blink and one control) inspected blank fields. Search time was improved by blink inspection for the circuit boards and with different defects showing different degrees of improvement. Only one type of defect showed both reduced search times and reduced errors. For blank fields, performance was worse under blink inspection. Subjects showed a consistent tendency to terminate their search for defects in a logical manner, with stopping time related to search difficulty. Measures of heart rate mean and variability were taken as a pilot study to measure task stress. There was little evidence from the heart rate measures that the task was stressful.

Factors affecting visual inspection accuracy

A four-fold classification of factors which might influence the extent of inspector accuracy is provided. It is emphasized that the influence of these factors has been mainly evaluated in the context of laboratory tasks which simulate real inspection tasks to a varying degree. For each of the four classes of factors one has been selected to illustrate its contribution to reducing inspector error. These are the inspector's visual acuity, the work-place lighting conditions, the time available for inspection and the provision of feedback or knowledge of results to the inspector. There is now sufficient evidence to confirm the advantages that can follow from careful consideration being given to these factors when designing or modifying industrial inspection tasks.

Studies of Visual Inspection

This paper describes the results of four experiments in a series aimed at understanding and improving visual inspection in general and of small integrated circuits (i.e. 'chips') in particular. Stimuli consisted of chips that, although electrically sound, contained visual anomalies. The first experiment found that the modal duration of eye fixations of trained inspectors was about 200 msec. The most accurate inspectors made the fewest eye fixations and were the fastest. The second experiment evaluated the performance of inspectors at one of the many sequential stages of chip inspection and found that 23% of the chips containing anomalies were accepted, whereas only 2% of the chips without anomalies were rejected. When the same chip was judged more than once by an individual inspector the consistency of her judgment was very high whereas the consistency between inspectors was somewhat less. The third experiment showed that variation by a factor of six in inspection speed led to variation of less than a factor of two in inspection accuracy. The fourth experiment showed that inspection via a ground glass screen is only a little worse than the usual method of looking through a binocular microscope. This was true even though the inspectors had no previous experience with the screen.