Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1999-07-06
2001-06-05
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
C356S237100
Reexamination Certificate
active
06243164
ABSTRACT:
TECHNICAL FIELD
This invention relates to machine vision systems for inspecting integrated circuits and, in particular, to a method and apparatus for inspecting the leads of packaged integrated circuits.
BACKGROUND OF THE INVENTION
Machine vision is the analysis of images by a computing device such that some determination can be made about a scene or object that is viewed by an image sensor or sensors connected directly or indirectly to the computing device.
Electronic integrated circuits (ICs) are generally enclosed in plastic or ceramic packages of a generally rectangular parallelepipedal hexahedral shape. These packages include a plurality of leads about the periphery to provide electrical contact to the circuits within the package, and the leads include a means for attachment to a printed wire circuit board that typically forms part of a larger assembly of similar integrated circuits and other electronic components.
The old standard practice generally deployed the leads to protrude from the top and/or bottom major surfaces of the package such that all the leads were oriented in a direction normal to the plane of the major surfaces of the package. These leads were then inserted through corresponding holes in the printed wire board for mechanical and electrical attachment by soldering. These packages are known in the art as through-hole packages.
In the last 10 to 15 years, conventional practice has transitioned to deploying the leads about the periphery of the IC package in a plane parallel to the package with the distal portions of the leads offset from the plane of the package. The distal portions of the leads are then positioned on corresponding pads on the printed wire boards for soldering. This technique allows closer spacing of the leads, a reduction in size of the overall package, and the placement of IC packages on both sides of the printed wire board. These packages are known in the art as surface-mount-technology (SMT) packages.
FIG. 1
is an isometric view showing a conventional SMT package
10
having a defective lead
12
among a plurality of leads
14
a
-
14
j
(collectively or generically,
14
) that protrude from major side surfaces
16
and/or minor side surfaces
17
of a plastic or ceramic casing
18
. With reference to
FIG. 1
, for each lead
14
of SMT package
10
to be successfully soldered to a substantially planar printed wire board (not shown), leads
14
should all be substantially within the same plane, a property known in the art as lead coplanarity. For example, a defective or noncoplanar lead
12
may exhibit a property known in the art as lead standoff, a defect characterized by a lead
12
that sits too high above a printed wire board for a reliable solder connection to be made between the lead
12
and a corresponding copper pad on the printed wire board.
The methods of determining the coplanarity of leads
14
of SMT packages
10
can be divided for the purposes of discussion into two groups: those that can generate a three-dimensional surface map of an object with one view and those that synthesize a three-dimensional map of the object viewed by combining the information from a plurality of two-dimensional maps.
The first group includes techniques that use a particular type of illumination with a property that renders a two-dimensional image with additional information on the distance of each point in the image from the image sensor. Examples of such techniques include structured lighting techniques, scan-angle modulated techniques, moire techniques, interferometric techniques, holographic techniques, and time-of-flight techniques. For example, Beiman and Michniewicz disclose a moiré interferometry technique for measuring surface contours of an object in U.S. Pat. No. 5,636,025.
The second group uses a plurality of two-dimensional images that are generated by a plurality of two-dimensional image sensors such as conventional video cameras and uses the a priori knowledge of the positions of the image sensors to mathematically construct the three-dimensional nature of the object viewed.
As a convenience, to reduce the size of the coplanarity sensing device, and as an economic measure to reduce the number of image sensors required, two or more of the required views may be combined by mirrors or by prisms, or by a combination of both, onto a single two-dimensional image sensor.
For example, Lebeau and Hopkins disclose a technique employing at least one mirror and a pedestal
20
that serves as a physical reference plane
22
for determining SMT lead coplanarity in U.S. Pat. No. 5,563,703 ('703 patent). Smeyers and Vanderheydt similarly disclose a technique employing a plurality of mirrors, a pedestal
20
that serves as a physical reference plane
22
, and a diffusive top surface
24
upon which a shadow of leads
14
are cast from a plurality of angles to determine SMT lead coplanarity in U.S. Pat. No. 5,440,391 ('391 patent). In FIG.
1
and with reference to the '703 and '391 patents, the top surface
24
of pedestal
20
provides the reference plane
22
, which is also commonly known as the Z-axis reference plane
22
. The height, or Z coordinate, for each lead
14
is then determined with respect to the physical reference plane
22
.
These inspection systems require specialized handling devices, such as a Z-axis actuator, capable of deploying an SMT package
10
along a specified axis or path to position SMT package
10
to be substantially in contact with and coplanar with the surface
24
of pedestal
20
at a predetermined fixed location that is within a view of an image sensor. The view is from above, but not normal to, the physical reference plane
22
.
The typical requirement for motion along the physical reference plane places two burdens on SMT package inspection. One burden is the requirement for a Z-axis actuator and its attendant power and control devices. The other burden is the time required to deploy the SMT package
10
in the Z axis onto pedestal
20
, the pause required during which the images of SMT package
10
on pedestal
20
are acquired, and the time to reverse the deployment of SMT package
10
from pedestal
20
so that SMT package
10
may resume travel along its original path.
SUMMARY OF THE INVENTION
An object of the present invention is, therefore, to provide a method or system for determining the relative coplanarity of leads of an SMT package.
Another object of the invention is to employ machine vision technology to determine the magnitude of deviations from the coplanarity of leads of the SMT package.
An advantage of the invention is that a plurality of views of the SMT package can determine the relative coplanarity of its leads without the use of a pedestal and without the delays associated with deploying the pedestal.
In a preferred embodiment, a camera provides a first view of an SMT package
10
from an axis substantially normal to the plane of SMT package
10
. One or more additional cameras provide one or more additional views of the SMT package
10
from an axis that is substantially collinear with the plane of SMT package
10
, such as collinear views for each side of SMT package
10
that exhibits leads
14
. The views are combined, and three leads
14
of SMT package
10
are arbitrarily chosen to define a virtual plane in a first coordinate system. The remaining leads
14
are then measured with reference to the virtual plane. For convenience, the virtual plane can be defined as Z=0 and may have height coordinates in Z that are negative or positive with respect to the virtual plane. The virtual coordinates are analyzed, and three leads
14
having the lowest Z coordinates are determined to define a reference plane. In an alternative preferred embodiment, the three lowest leads that circumscribe the center of mass of the package (as projected along the Z axis) are selected as the reference plane. For convenience, the reference plane can be defined as Z=0 in a second coordinate system, which may be referred to as the real coordinate system. A mathematical transfo
Baldwin Leo B.
Kelley Michael P.
Electro Scientific Industries
Pham Hoa Q.
Stoel Rives LLP
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