Gold Contact Inspection Using a Scanning Laser Beam

A laser scanner inspection machine was built to detect defects on printed wiring board gold contact fingers. The scanner sweeps a focused laser beam in raster fashion across the contacts at 60 scans/sec and the scattered beam is detected and electronically analyzed to yield the defects. Marking is accomplished with a relay-driven felt tip pen which marks the back of the board under the defective contact. The inspection of a board about four inches on a side can be done in a few seconds. A range of defects can be detected with this machine, including pinholes, gradual indentations, scratches, discolored gold, etc. The detection levels can be adjusted to pass imperfections which are not considered serious. This advantage makes the machine programmable in an objective manner, easily accommodating revisions in specifications. In this report, the operation of the machine is described including a detailed account of alignment procedures and electronic processing. Results of a preliminary test of the system effectiveness are discussed and it is shown that the level of detection must be empirically determined.