A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
Pick and place machines are generally used to manufacture electronic circuit boards. A blank printed circuit board is usually supplied to the pick and place machine, which then picks electronic components from component feeders, and places such components upon the board. The components are held upon the board temporarily by solder paste or adhesive until a subsequent step in which the solder paste is melted, or the adhesive is fully cured.
Pick and place machine operation is challenging. Since machine speed corresponds with throughput, the faster the pick and place machine runs, the less costly the manufactured board. Additionally, placement accuracy is extremely important. Many electrical components, such as chip capacitors and chip resistors are relatively small and must be accurately placed on equally small placement locations. Other components, while larger, have a significant number of leads or conductors that are spaced from one another at a relatively fine pitch. Such components must also be accurately placed to ensure that each lead is placed upon the proper pad. Thus, not only must the machine operate extremely fast, but it must also place components extremely accurately.
In order to enhance the quality of board manufacture, fully or partially populated boards are generally inspected after the placement operation(s), both before and after solder reflow, in order to identify components that are improperly placed or missing or any of a variety of errors that may occur. Automatic systems that perform such operation(s) are highly useful in that they help identify component placement problems prior to solder reflow allowing substantially easier rework or identify defective boards after reflow that are candidates for rework. One example of such a system is sold under the trade designation Model KS Flex available from CyberOptics Corporation of Golden Valley, Minn. This system can be used to identify such problems as alignment and rotation errors; missing and flipped components; billboards, where the part lays improperly on its longer side edge; tombstones, where the part lays improperly on its shorter edge; partial billboards and tombstones, where the part is oriented between its normal orientation and a billboard or tombstone orientation; component defects; incorrect polarity; and wrong components. Identification of errors pre-reflow provides a number of advantages. Rework is easier; closed-loop manufacturing control is facilitated; and less work in-process exists between error generation and remedy. While such systems provide highly useful inspection, they do consume plant floor-space as well as programming time, maintenance efforts and the like.
One relatively recent attempt to provide the benefits of after-placement inspection located within a pick a place machine itself is disclosed in U.S. Pat. No. 6,317,972 to Asai et al. That reference reports a method for mounting electric components where an image of a mounting location is obtained prior to component placement, and compared with an image of the mounting location after component placement to inspect the placement operation at the component level. While the disclosure of Asai et al. marks one attempt to employ in-machine component level inspection to inspect the component placement operation, component orientation errors can also be generated in the process of picking up a component. This process remains a challenge and a major contributor to the quality of the overall operation of the pick and place machine.
The utilization of solder paste to temporarily hold a component upon a circuit board, and later electrically and mechanically couple the component to the circuit board is critical to modern electronics assembly operations. For example, if the solder paste of a single pad of a single component is not placed correctly, it may fail to generate the requisite electrical contact between the component and the circuit board. Further, even a correctly-placed solder pad which is placed simply with far too much solder paste may erroneously generate an electrical connection between two adjacent portions of the circuit board thereby generating an undesirable short circuit. Further still, characteristics of the solder paste and/or pads of the component may affect the ability of the solder paste to temporarily retain the component upon the workpiece until the solder is finally melted to form the permanent connection.
Generally, solder paste for an entire printed circuit board is applied in a screen printing operation. A screen containing a negative image of the circuit board's solder pads is brought into proximity with the circuit board, and solder paste is essentially squeezed through the screen to generate the individual deposits. Sometimes, solder may get stuck, or otherwise lodged within individual apertures of the screen. If this occurs, the solder paste may simply not be present at a certain portion of the circuit board. In order to ensure that solder paste is deposited correctly, solder paste inspection machines are sometimes used in the assembly line. One example of such a solder paste inspection machine is sold under the trade designation SE 300™ Ultra available from CyberOptics Corporation of Golden Valley, Minn. However, even state-of-the-art, advanced, solder paste inspection machines still require a relatively significant capital investment, as well as the occupation of precious floor space in an electronics assembly plant. Providing solder paste inspection functions without requiring the use of the dedicated solder paste inspection machine would significantly benefit the art of electronics assembly.
A pick and place machine includes a placement head configured to releasably grasp a component for placement. A robotic system is coupled to the placement head to generate relative movement between the placement head and a workpiece. An image acquisition system is configured to acquire at least one image of an intended placement location of the component before the component is placed. A controller is operably coupled to the image acquisition system, the controller is configured to process at least one before-placement image to generate a metric relative to solder deposited at the intended placement location.
In accordance with one embodiment of the present invention, a single before-placement image of the placement location is acquired. This single before-placement image can be used to perform two-dimensional image analysis upon the solder paste. Such analysis can be useful in determining whether the solder paste is applied at the correct position, and/or whether the correct amount of solder paste has been applied, to the extent that the amount of solder paste affects the length and width of the solder paste deposit. However, it is also contemplated that a plurality of before-placement images could be acquired each image being from a different point of view. Thus, a plurality of image acquisition systems 350 could be used, with each system 350 observing location 352 from a different point of view. When the plurality of image acquisition systems acquire their respective before-placement images, the two, or more images, can be used to provide depth information in accordance with known stereo vision processing techniques. Examples of the utilization of multiple image acquisition systems for providing depth information related to embedded component inspection in a pick and place machine can be found in U.S. patent application Ser. No. 10/291,074, filed Nov. 8, 2002, entitled PICK AND PLACE MACHINE WITH COMPONENT PLACEMENT INSPECTION.
In the alternative, each and/or both of a plurality of image acquisition systems can include a structured illuminator able to provide structured illumination upon placement location 352. The utilization of structured illumination includes the utilization of laser light, and/or the utilization of light, the intensity of which, varies in accordance with a set pattern, such as a sinusoidal fringe pattern. Providing structured light upon location 352 allows for depth information to be derivable using a single before-placement image from a single image acquisition system. Accordingly, embodiments of the present invention, are able to derive information not only relative to the two-dimensional solder paste length and width, but also relative to the height. In this manner, the total volume of the solder paste deposited can be calculated and compared to a priori information to ensure not only that sufficient solder paste has been deposited, but also to that too much solder paste has not been deposited.
In accordance with another embodiment of the present invention, since the solder paste inspection occurs in real-time slightly before components are placed, the placement of individual components can be varied in response to individual solder paste inspection results. For example, if a pair of solder paste deposits are slightly misplaced, such as skewed in one direction, the placement of the component can similarly be skewed such that the component rests squarely upon the slightly misplaced solder deposits. In this manner, the electrical and mechanical connection to the solder paste deposits themselves is far more robust than if the component were placed in its nominal position upon the slightly misplaced solder paste deposits. It is therefore believed that embodiments of the present invention may actually improve the robustness of electronics devices without requiring significantly more capital investment, or increasing pick and place machine throughput time than current state-of-the-art electronics assembly machines.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
The present application is based on and claims the benefit of U.S. provisional patent application Ser. No. 60/731,848, filed Oct. 31, 2005, which application is herein incorporated by reference in its entirety.
Number | Date | Country | |
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60731848 | Oct 2005 | US |