It is well known within the electronics industry that lead coplanarity is essential to the successful placement of fine-pitch surface mount devices (SMD's). Non-coplanar leads, i.e. those bent up or down with respect to the body of the component, typically result in opens or poor solder joints on the printed circuit board (PCB). It is desirable to reject any components that do not meet coplanarity requirements prior to placement on the printed circuit board. The present industry standards require that all lead tips be coplanar within a 0.004 inch band, forcing inspection equipment to measure coplanarity with .+-.0.002 inch accuracy. Although some non-contact 3-D inspection techniques are capable of providing this degree of accuracy, a number of system constraints prohibit their implementation for this application. These constraints include but are not limited to:
(1) large part sizes,
(2) establishment of a reference (datum) plane,
(3) mechanical space and packaging limitations within the pick and place machine, and
(4) real-time inspection to maintain high pick and place throughput rates.
Other techniques, particularly "implied" techniques based around passive triangulation (i.e. estimation of lead height via measured lead length), have severely limited depth resolution. They may also have other drawbacks such as confusion between lead length error and vertical lead displacement as well as ambiguous polarity information (the inability to differentiate between leads bent up or down with respect to the plane of the part).
It is accordingly an object of the present invention to overcome these constraints while providing the necessary degree of accuracy for proper inspection of lead coplanarity. The optical setup inherent in the disclosed system provides a true, direct measurement of lead height. It produces sufficient depth resolution to measure lead coplanarity to an accuracy of .+-.0.002 inch using conventional sensors and image processing hardware.
An advantage of the disclosed system is its ability to handle a range of SMD components including PQFP, SOIC, PLCC, SOJ, and TapePak with pitch values down to 15 mils. It will accommodate 25-mil pitch parts up to 1.25 inches square in a single field of view. Parts with a larger number of leads or finer leads can also be inspected by breaking up the image into multiple fields of view, typically two or four. If necessary, large fine-pitch devices may be inspected through the use of high resolution image sensors such as line scan or non-RS-170 area cameras, though at present this requires additional complexity for image acquisition.
Other objects and advantages of the present invention will become apparent from the following portion of the specification and from the following drawings which illustrate in accordance with the mandate of the patent statutes a presently preferred embodiment incorporating the principles of the invention.