Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1998-09-10
2001-07-17
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
Reexamination Certificate
active
06262803
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to systems and methods for three-dimensional inspection of articles and more particularly, to a system and method for three-dimensional inspection of electronic packages, such as ball grid array (BGA) devices, using a projected pattern of light.
BACKGROUND OF THE INVENTION
Projecting patterned or structured light onto an article is a well known technique for obtaining three-dimensional information pertaining to the article. As shown in
FIG. 1
, a projector
1
is used to project a pattern of light, such as a series of parallel lines
2
, onto surfaces
4
,
6
. The axis
3
of the projector
1
is oriented at an angle with respect to these surfaces
4
,
6
. When the lines
2
are projected onto a surface
4
that is raised with respect to another surface
6
, the lines
2
appear to shift laterally between these surfaces
4
,
6
when viewed from above, for example, using camera
8
and monitor
9
. The magnitude of the lateral shift between the lines
2
on surfaces
4
,
6
yields information about the distance between the surface
4
and the surface
6
. For example, the lateral shift between the lines
2
and the angle of projection can be used to calculate the height of the surface
4
with respect to the surface
6
using triangulation.
Existing systems and methods for three-dimensional inspection using projected light patterns, however, do not adequately provide an accurate inspection of electronic packages having specular surfaces, such as Ball Grid Array (BGA) devices. Accurate inspection of electronic packages and other such articles requires high resolution measurements of the lateral shift in the lines or pattern projected onto the article. If the projected pattern or image is not properly focused or is distorted, measurements of the lateral shift in the lines of the projected pattern may not be accurate. In the existing systems having an angled projector
1
, the projected image may not be in focus if the Scheimpflug condition is not satisfied, as will be discussed in greater detail below. Blurring of the lines in the projected pattern also typically occurs as the lines move away from the focus of the projector
1
. As a result, the width of the lines projected onto the article may not be consistent over the entire range of the article being inspected. The width and spacing of the projected lines can also vary as a result of an effect commonly referred to as keystoning, as will be described in greater detail below.
Existing patterned light projectors also encounter problems as a result of specular surfaces, such as the solder balls on BGA devices. The reflection of light from specular surfaces often causes a saturation of pixels in the camera and necessitates the use of cameras with high dynamic ranges or logarithmic responses. Also, if a series of lines or a similar pattern is projected with a spacing equal to the spacing of the solder balls on a BGA device, light will reflect between neighboring solder balls. This type of reflection will adversely affect the image detected by the camera and thus will result in an inaccurate measurement of the shift in the lines. Furthermore, when the article being inspected has a surface and surface objects with different reflectivities, such as the solder balls and the substrate of a BGA device, it is difficult to view both surfaces with a single exposure without losing information on one of the surfaces by either saturating one of the lines or causing one to be in the noise.
Accordingly, a need exists for a system and method for three-dimensional inspection that projects patterned light in a manner that reduces unwanted reflection from specular surfaces, provides a projected pattern that is in focus, and allows high resolution measurements of the reflected light pattern to accurately determine three-dimensional information. In particular, a need exists for a system and method for three-dimensional inspection of BGA devices or similar articles having rounded specular surfaces and surfaces of different reflectivities.
SUMMARY OF THE INVENTION
The present invention features a system for three-dimensional inspection of an article having at least one three-dimensional object protruding from or on the surface of an article to be inspected and which article is supported generally in a plane. A patterned light projector having an optical axis is disposed at an oblique angle with respect to the plane of the article. The patterned light projector includes a light source for generating light and directing the light toward the article along the optical axis. A light patterning member is disposed between the light source and the article at an oblique angle with respect to the optical axis, for creating the light pattern as the light passes through the light patterning member.
The system also comprises a light pattern detector, for detecting the light pattern reflected from the surface of the article. In one example, the light pattern detector includes an image detector, such as a camera, disposed generally above the article, for detecting the image of the light pattern reflecting from the article. The image of the light pattern is preferably in a plane parallel to the plane of the article such that the light pattern projected onto the article and the article itself are simultaneously in focus. The system also comprises an image processor, for receiving the image detected by the image detector and for processing the image to determine three-dimensional information pertaining to the article.
The present invention also features a patterned light projector for use in the system for three-dimensional inspection of an article. The preferred embodiment of the patterned light projector includes an extended light source for generating light and projecting the light toward the article generally along the optical axis disposed at an oblique angle with respect to the plane of the article. The light patterning member preferably includes a pattern of lines and creates lines of light projected onto the article.
The patterned light projector also includes one or more projection lenses disposed between the light patterning device and the article, for projecting the light pattern on the surface of the article. A condenser lens is also preferably disposed between the extended light source and the light patterning member, for providing a substantially uniform illumination of the light patterning member.
Examples of the extended light source include a fiber optic bundle, a light line, or an array of light emitting diodes (LEDs). In one example, the light patterning member includes a transparent slide with a mask forming the pattern of lines. Alternatively, the light patterning member includes a programmable mask.
The pattern of lines on the light patterning member preferably have a varying, non-uniform spacing and thickness such that the lines of light projected onto the article at the oblique angle have a substantially equal spacing and thickness on and along the surface of the article. The spacing of the lines of light is preferably greater than the spacing of the specular elements or objects on the article being inspected, for example, the solder balls on the BGA device.
The patterned light projector also preferably includes a depth of focus modifier, for providing a lower f-number (i.e. less depth of focus) in a direction along a length of the lines of light projected on the surface of the article, and a higher f-number (i.e. greater depth of focus) in a direction along a width of the lines. This provides very crisp, clear edges on the projected lines. In one example, first and second projection lenses are used with an elongated aperture disposed between the first and second projection lenses and oriented lengthwise with respect to the lines.
According to one embodiment, the projector shifts the lines of light projected onto the article. The shifting can be accomplished by a mechanism for moving the projector, a rotatable transparent light shifting plate disposed between the patterned light projector
Hallerman Gregory R.
Ludlow Jonathan E.
Stern Howard K.
Acuity Imaging, LLC
Bourque & Associates P.A.
Rosenberger Richard A.
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