Automatic Optical Inspection of Printed Circuit Boards

INTRODUCTION The inspection of bare printed circuit boards and photo-tooling is a task that is in urgent need of automation. It is estimated1 that at present some 15% of the labour content of a board is taken up by inspection. Since the work requires a high degree of concentration with little 'job interest', it is difficult to find competent and conscientious inspectors, and the fault detection rate inevitably drops off towards the end of the shift. Since the value of a board increases considerably as it is processed, it is desirable to inspect as early as possible in the processing cycle. In practice, this means inspecting photo-tooling before it is used, and then inspecting the resulting boards at the post-etch stage, before adding extra value in the form of gold plating, solder masks, component identification, etc. The problems of inspection are becoming more severe as reduction in component sizes leads to narrower tracks and spacings, as the increased use of multilayer boards brings a greater need for reliability in every layer, and as end users become accustomed to a higher overall reliability. Various machines have been produced 2,3 for automatic optical bareboard inspection, but the methods used for fault identification have so far limited them to inner layers of multilayer boards, and the cost of such machines has been at least twice that of an electrical bare-board tester. This paper will describe a machine, the Trackscan 2000, which is capable of inspecting single-sided and double-sided boards as well as inner layers and photo-tooling at speeds typically 5 to 10 times those of a human inspector, and which is comparable in cost to an electrical bare-board tester. Compared with an electrical bare-board tester, the Trackscan 2000 is slower, but has the advantages that it requires no special tooling, can detect partial breaks and shorts, and can provide greater information about the location and nature of a fault. It can also be used to inspect photo-tooling, and can provide additional information such as checking hole concentricity, counting holes, and determining the percentage area of copper. Previous machines have used two different approaches to the problem of determining whether a fault exists on the board under inspection— generally known as the 'comparison' and 'feature recognition' techniques. The Trackscan 2000 operates using a different 'analytical' technique which offers the advantages of both methods. The 'comparison' technique uses a board which has already been visually checked or the original photo-tooling as a master. The board under inspection is then compared with this on a point-by-point basis. This requires very precise alignment between the two boards and the two optical systems used to inspect them, and since it will respond to any difference in the boards, however trivial it is to the educated human eye, the false-alarm rate is inevitably high. The 'feature recognition' technique uses a set of circuits each one of which is programmed to detect a specific feature such as a pad or a track, which may occur on the board under inspection. Any feature which is not recognised is regarded as a potential fault. This technique does away with the need for a master board to use as a reference, but it suffers from limitations in that it will detect, and flag as faults, many minor imperfections such as tiny specks of copper in places where they can do no harm, and will fail to detect a fault which conforms to the design rules used to programme the system. For instance, a bridge between two holes of the same width as the nominal track width will not be detected. Moreover, such a system will cope with only a limited range of features,
Automatic Optical Inspeards*
K. G. Doyle
Lloyd Doyle Ltd, Walton-on-Thames, England
ABSTRACT—An automatic optical printed circuit board inspection machine will be described which provides considerable improvements on existing methods of PCB inspection and testing. The machine can detect breaks, nicks, shorts, pinholes, inadequate track clearances, inadequate track widths and holes which are undersized, oversized, missing or not concentric with the pad. It can be used in a non-comparison mode to detect small defects in boards or photographic negatives, and can also be used in a comparison mode to provide a complete functional test of a board similar to that provided by a bed-of-nails tester, but with improved detection of marginal faults. The board is optically scanned and the resulting large amounts of data are reduced by special hardware and software techniques which allow rapid throughput and virtually eliminate 'false alarms' caused by lettering, logos, dust specks etc. which do not affect the functionality of the board. This technique differs considerably from the 'Feature recognition' and the 'Comparison' techniques at present employed in automatic optical PCB inspection. Operation of the machine is very simple as the board does not need to be accurately positioned, and a simple 'good/bad' indication is provided as well as more detailed defect location information.