Mechanical Vision System Using Selective Wavelength and Brightness Illumination

1. Technical Field

This invention generally relates to mechanical visions systems and more particularly, this invention relates to using selective wavelengths and brightness' of illumination to optimize pattern recognition of components such as chips and semiconductor dies.

2. Background

Mechanical vision systems are used in semiconductor manufacture to aid in placement of dies on leadframes, wire bonding of the dies to the leadframe, placement and bonding of chips on multichip module substrates, inspection and quality control. These systems employ a video camera to produce a highly magnified image of a component which can be viewed by an operator or analyzed by a pattern recognition algorithm to aid in the positioning of tooling such as a wire bonding head.

U.S. Pat. Nos. 4,441,205, 4,853,968 and 5,119,435 teach one such system. This particular system creates a digital image of the subject by assembling an array of pixels which are each obtained by converting an analog signal to a four bit word whose value corresponds to the magnitude of the intensity of the reflection of light received from a portion of the subject.

The light sources which are known to the inventors for illuminating the subjects are monochromatic and consist of one or more red light emitting diodes (LEDs). Some surface features tend to obscure using a single monochromatic light source. Additionally, because different surfaces reflect light differently because of their texture and color some components simply do not show up well under monochromatic illumination. Increasing the intensity of monochromatic illumination can bring out the obscured features however this can cause prominent features to wash out as the light sensors in the video camera become saturated.

What is needed is tunable illumination source which uses limited multiple wavelength sources which each can be adjusted to provide an optimum combination of colors and intensities of illumination for a mechanical vision system.

This object and others is accomplished by an illumination subsystem which is used in conjunction with a mechanical vision system and which has a plurality of intensity adjustable monochromatic light sources, at least two of which are of different wavelength illumination.

The invention provides for both oblique and normal incidence of multiple wavelength illumination. An oblique illumination assembly is positioned off to the side of the video camera and is pointed at the subject to be viewed. A direct illumination assembly is positioned within the light path of the lens of the camera to facilitate direct illumination of the subject. At least one of the illumination assemblies, preferably both, contains at least two different wavelength light emitters, such as two differently colored LEDs. Each of the emitters is preferably separately controllable to adjust its intensity. However, the emitters may be controllable by one or more group controls, where for instance the emitters may be grouped by color, so that the intensity of all the members of a group can be adjusted at the same time.

Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentality's and combinations particularly pointed out in the appended claims.

FIG. 1 is a schematic representation of a mechanical vision system using a multiple wavelength illumination assembly according to the invention;

FIG. 2 is a bottom plan view of the oblique illumination subassembly;

FIG. 3 is a bottom plan view of the direct illumination subassembly; and

FIG. 4 is a simplified circuit schematic of an LED driver for an LED illumination source.

Referring now to the figures, a multiple monochromatic wavelength illumination assembly is generally designated as 10 and is shown attached to a video camera 2 typical of those used in mechanical vision systems. The illumination is monochromatic multiple wavelength in the sense that it comprises two or more separate narrow band wavelengths of light.

In this preferred embodiment, multiple monochromatic wavelength assembly 10, hereinafter simply illumination assembly 10, has an oblique illumination subassembly 12 attached adjacent to the aperture of camera 2 by a bracket 16 which is attached to the main body of camera 2 and a direct illumination subassembly 14 fixed in the light path of camera 2.

Oblique illumination subassembly 12 here has a semi-circular body to which a plurality of light emitters are attached. In this embodiment, the light emitters are two different colored LEDs, yellow LEDs D1-D4 and orange LEDs D7 and D8. Subassembly 12 is positioned the direct light toward subject 4 such that the general angle of reflection is away from the light path of camera 2. In this sense, subassembly 12 provides a diffused light source for camera 2.

Direct illumination subassembly 14 is attached to camera 2 and positioned to emit multiple monochromatic wavelengths of light into the light path of camera 2. This illumination provides the normally incident illumination for subject 4. The light which is reflected into the lens of camera 2 to render the video image is a combination of light emitted from subassembly 12 and subassembly 14. The inventors have found that the combination of yellow and orange light in a 2:1 ratio provides excellent resolution for a wide range of semiconductor manufacturing applications. However, depending upon the geometry, colors and surface qualities of the subject components other color combinations might be more beneficial.

The LEDs are driven by one or more emitter source driver circuits. One such circuit is illustrated in FIG. 4. Here, each LED D1-D9 is driven by a series resistor network having a current limiting resistor R2, R4, . . . R18 and a variable resistor R1, R3 . . . R17 for brightness control. This particular configuration provides only individual control of each LED. However, it may be desirable to control the LEDs as a single group or as multiple groups. For instance, grouping yellow LEDs D1-D6 as one group and orange LEDs D7-D9 as a second group. A single actuator, either mechanical or electrical, can be used to control the variable resistors of all the members of the group at once. Alternatively, a second set of variable resistors under single group control could be added to enable both individual and group control.

In use, the operator adjusts the intensity of the various emitters as well as their wavelengths, i.e. colors, to optimize the visual image captured by camera 2. This is accomplished empirically by the operator viewing a video monitor, not shown in the figures, which is connected to camera 2 for viewing the image it produces. It can also be determined analytically by bracketing a series of exposures using different combinations of intensities and wavelengths and quantifying the recognition accuracy for each configuration.

While there is shown and described the preferred embodiment of the invention, it is to be distinctly understood that this invention is not limited thereto but may be variously embodied to practice within the scope of the following claims.