Optics: measuring and testing – Position or displacement
Reexamination Certificate
1999-06-24
2002-06-04
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Position or displacement
C356S638000, C356S625000, C356S613000, C356S400000, C356S138000
Reexamination Certificate
active
06400459
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to optical sensor systems which precisely determine the correct angular orientation and lateral position of a component for precise placement of a component on a work surface by a component placement machine. More specifically, the invention relates to methods and apparatus for more efficiently aligning and placing components using optical component sensor systems.
BACKGROUND OF THE INVENTION
Circuit boards that carry integrated electronic circuits as well as discrete electronic components are well known. To properly place an integrated circuit or other component on a circuit board, the leads of the component must be aligned to corresponding pads on the circuit board within a specified tolerance. The pattern of the pads on the circuit board is determined by the function of the circuit board and is designed on the circuit board prior to assembly.
The separation between centers of any pair of adjacent leads on electronic components is referred to as the pitch. Currently, a commonly manufactured lead separation is 0.025 inches (25 mil) pitch, meaning that the center of the leads are spaced at 25 thousandths of an inch intervals. Advances in component manufacturing technology, however, have produced integrated circuits having 15 and 10 mil pitches and tape automated bonding (TAB) components have been created having several hundred leads spaced with a 4 mil pitch. The bottom ends of the leads form a seating plane that will meet the plane formed by the pads on the circuit board when the component is placed in position.
The dimensions of components placed on circuit boards normally vary between 0.02 inch and 2.0 inches, although larger components may need to be accommodated. For quality manufacturing, component leads must be placed with at least 80% overlap of the lead onto the corresponding pad of the circuit board. For example, a device having a 20 mil pitch generally has 10 mil wide leads. With an 80% overlap, at least 8 mils of the lead width must be on the pad with no more than 2 mils of the lead width off the pad. In general, sensing systems used to align parts for placement must have five to ten times better resolution than the accuracy required. Therefore, 0.2 to 0.4 mil image resolution is required to achieve the maximum placement error of 2 mils specified for quality manufacturing methods for a component with 20 mil pitch. Correspondingly smaller image resolution is required for components with smaller pitch.
To perform this delicate task, precision surface mount component placement machines have been developed. While the particular design of the component placement machine is not relevant, all component placement machines generally pick up a component at one location, properly orient the component and place the component in its proper location on the circuit board. The components are not precisely aligned in the component bins where they are picked up. Therefore, components may be out of position by as much as plus or minus 50 mils and plus or minus 5 degrees angular orientation. To obtain proper placement, the orientation and lateral position of the components from the bins must be determined and then corrected prior to placement.
In a surface mount component placement machine, a component placement head picks up the component from a component bin utilizing a vacuum quill. The vacuum gently picks up the component to be placed and transports it between the component bins and the circuit board. A transport arm moves the placement head with the vacuum quill and the component from the bin to a circuit board located on a work table. Sometime during transport, the angular orientation of the component and the offset of the component from the center of the quill must be determined. The vacuum quill is then precisely lowered to fit the component on the circuit board. In current component placement machines, the transport arm and quill move at approximately one meter per second.
Assuming that the leads of the component have not been damaged, the position of the leads are known from the position and orientation of the body of the component. Therefore, if the lateral position and orientation of the body of the component are determined, the component can be properly placed. Alternatively, systems have been developed that can perform measurements on the leads of the component to adjust for irregularities in the leads during placement and to discover and reject damaged components prior to placement.
Mechanical systems have been commonly used to obtain correct angular orientation and lateral positioning of a component on the end of a quill. The mechanical contacting of the component can cause damage to the components. It is also extremely difficult to achieve the very high degree of accuracy both as to angular orientation and lateral position that is required by the design rules in use in today's technology where lead spacing and widths are often only 10-25 mils wide. Therefore, non-contact, higher accuracy methods are desirable.
Conventional vision systems used in conjunction with component placement machines use solid state television cameras having a resolution of 512×512 picture elements or pixels. When viewing a two inch component, a corresponding two inch field of view with 512 elements produces a basic resolution of 4 mils or 4 thousandths of an inch. This is not sufficient resolution and, in fact, as pointed out above, it is necessary to achieve a resolution which is at least an order of magnitude greater. One solution is to use several cameras, but the use of several cameras is expensive.
Light based systems utilizing one or more focused light sources have been proposed which align a component by making a measurement of a shadow cast by the body of the component. U.S. Pat. No. 4,615,093, entitled Method and an Apparatus for the Positioning of Components With Reference to a Workpiece, describes several possible embodiments of focused light based component alignment systems. One of these embodiments, shown in FIG. 8 of the '093 Patent, uses a row of laser diodes with sharply focused beams and a detector array with a detecting diode corresponding to each laser diode. The component is rotated until the number of diodes detecting laser light is maximized. This aligns the faces of the body of the component parallel to the sides of the frame around the measurement space.
U.K. Patent Number 2,183,820, entitled Electronic Component Placement, describes an alternative optical system for aligning a component based on the shadow that it casts. The system in the '820 Patent uses two perpendicular light sources each with a corresponding array detector. When the machine is in the process of picking up the component, the machine waits until a shadow from the component is detected. If a component is not detected by the system, the system will try another attempt to pick up the component with up to three attempts made. To correctly orient the component, the component is rotated until the shadow cast on the first detector begins to lengthen. Rotation is then reversed until the shadow begins to lengthen again. Rotation is stopped when the component has its narrowest dimension oriented in a specific direction.
U.S. Pat. No. 5,278,634, assigned to one of the assignees of the present invention, entitled A High Precision Component Alignment Sensor System, incorporated herein by reference, discloses a non-contact, laser-based alignment sensor located on a placement head. The sensor is utilized to generate the correct angular orientation of the component for placement. The sensor also determines any offset in the X-Y plane of the center of the component with respect to the vacuum quill, which carries the component to the circuit board, to allow lateral alignment of the component. The high speed laser-based system disclosed in the '634 Patent uses a stripe of laser light which is directed horizontally at the component whose alignment is being sensed.
The shadow cast by the component is detected by a linear array detector whose output
Haugan Carl E.
Peterson Bruce
van de Ven Jan T. A.
Cyberoptics Corp.
Dicke, Billig & Czaja P.A.
Font Frank G.
Lauchman Layla
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