WO2001036929A2

WO2001036929A2 - Via inspection with an optical inspection system

Info

Publication number: WO2001036929A2
Application number: PCT/US2000/041911
Authority: WO
Inventors: Gary E. Frame
Original assignee: Midas Vision Sysems, Inc.
Priority date: 1999-11-05
Filing date: 2000-11-06
Publication date: 2001-05-25
Also published as: AU2751101A; WO2001036929A3; TW466337B

Abstract

An optical inspection apparatus includes a platform (14) for holding a device under inspection (32) and a light source (16) disposed to illuminate a backside portion of the device. The inspection apparatus also includes a camera (12) disposed over a front side portion of the device to receive optical energy that penetrates through the device and a diffuser (36) disposed over a front side portion of the device. The illumination source provides an image on a bottom surface of the diffuser, for processing by the camera.

Description

VIA INSPECTION IN OPTICAL INSPECTION SYSTEM

BACKGROUND This invention relates to optical inspection systems .

Optical inspection systems are used to inspect objects such as printed circuit boards. Such systems generally include a camera and a light source to illuminate the printed circuit board. The camera is used to produce a video signal that is processed to determine whether the printed circuit board passes or fails an inspection. Such systems may include a telecentric lens. The telecentric lens is disposed between the camera and the assembly, and is as wide as the light source. Several inspection algorithms are known including so-called template matching, pixel counting, and design rule inspection processing.

SUMMARY Problems can occur when examining vias (holes through a printed circuit board) with optical inspection systems. When systems use cameras with larger fields of view, vias on the periphery can appear to be missing or only partially drilled through because of the way light is captured by the camera. This system provides a technique to inspect vias while enabling the use of larger fields of views for the camera to make the inspection process faster. The inspection process is faster because there is less motion i.e., scanning required of the camera. The system can process the image without having to adjust the processing to take into consideration the distortion. According to an aspect of the invention, an optical inspection apparatus includes a platform for holding a device under inspection and a light source disposed to illuminate a backside portion of the device. The inspection apparatus also includes a camera disposed over a front side portion of the device to receive optical energy that penetrates through the device and a diffuser disposed over a front side portion of the device, said diffuser producing an image on a bottom surface of said diffuser, for processing by the camera.

The apparatus is arranged such that the image is of features of the device produced from light that penetrates the device. The apparatus can be used to inspect vias in printed circuit boards, ceramic substrates or substrates having an array of vias with the vias having a high aspect ratio. The apparatus can use a collimated light source. The diffuser can be a ground glass plate or a sheet of plastic having an etched surface. The apparatus can also include a processor to process a video signal from the camera to determine whether the device passes an inspection.

A method for optically inspecting a substrate having a via includes arranging a diffuser between a platform for holding a substrate under inspection, illuminating a backside portion of the substrate with a light source and producing an image on a bottom surface of said diffuser, for processing by a camera disposed over the front side of the substrate in response to received optical energy that penetrates through the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a machine inspection system including a diffuser placed over a device under inspection. FIG. 2 is a diagrammatical view of the system of

FIG. 1.

FIG. 3 is a plan view of a printed circuit board having vias that are inspected by the system of FIG. 1. FIG. 3A is a cross-sectional view taken along lines 3A-3A of FIG. 3. FIG. 4 is a plan view of a diffuser disposed over the PC board of FIG. 3.

FIG. 4A is a cross-sectional view taken along lines 4A-4A of FIG. 4. FIG. 5 is a blown up view of a portion of FIG.

4.

FIG. 5A is a blown up view of a portion of FIG. 4A.

FIG. 6 is a flow chart depicting a process to examine vias in the system of FIG. 1.

DETAILED DESCRIPTION Referring now to FIG. 1, a machine inspection system 10 is shown. The machine inspection system 10 includes a camera 12 that is disposed above a platform 14. The platform 14 is supported by a stand 15. The platform 14 holds an assembly 30 including a device to undergo an inspection such as a printed circuit board 32 (PCB) and a diffuser 36 disposed on a surface of the printed circuit board 32. The diffuser 36 is further described in conjunction with FIG. 2. The system 10 also includes a light source 16 disposed under the platform 14 so that the light is incident on a surface of the assembly 30 that is opposite to the surface of the assembly 30 having the diffuser 36. In addition, to printed circuit boards, the inspection system 10 can be used to inspect ceramic substrates, or any other material. It is especially useful on boards that contain an array of vias where the vias have a high aspect ratio, i.e., depth to width of e.g., 3-5 or more.

Platform 14 engages the assembly 30 on its side, as shown. Other arrangements can be used as long as it does not obstruct light from entering the vias from underneath the PC board. The light source 16 is disposed under the platform 14 such that light rays 16a are incident on the bottom surface of the board 32, and shine through the vias in board 32 and the diffuser as light rays 16b that are intercepted by camera 12. The camera 12 produces a video signal that is fed to processor 18. The video signal is used in the processor 18 to process the video signal into an image signal. The image can be displayed on display device 20 and information from the video signal or image can be used to inspect the assembly by various techniques such as design rule, pixel counting or template matching as disclosed, for example in U.S. Patent 4,648,053, incorporated herein by reference. The exact details of the optical inspection process are generally conventional and need not be further discussed here. Referring to FIG. 2, the camera 12 has a field of view that is larger than the diameter of the lens. The camera includes lens 12a that is part of an optical system and the detector 12b. A large field of view is provided by using a detector 12b with a large number of detector elements e.g., 2,048 elements, 4,096 elements or 8,192 elements. Each element may represent a predefined area e.g., .001 inches or .002 inches. Thus, a 8,192 element detector would have a 8.192 or 16.384 inch field of view. Such a detector 12b provides a larger field of view to make the inspection process faster. The process is faster because there is less motion i.e., scans of the camera 12 across the device to be inspected, as illustrated by the three phantom positions shown for the camera 12 in FIG. 1. While a larger field of view provided by a camera having a larger number of elements is desirable to make inspections faster, there are some problems associated with this arrangement, in particular, when examining vias. Large fields of view progressively exacerbate these problems when compared to smaller fields of view. As cameras with higher fields of view are used, vias on the periphery of the field of view can appear to be deformed because of the way that light is captured by the camera.

This field of view problem becomes especially important with high aspect ratio vias, e.g., those vias that have a depth substantially greater than their width (e.g., 5-20 or more aspect ratios) . In these arrangements, the only light that comes through the vias travels in a straight line parallel to the via axis. The light coming from vias at the periphery of the board will not hit the lens 12a in the camera since it is relatively small. For example, with a 6 inch field of view, the lens 12a of the camera is typically 1 inch in diameter. The light from vias at the edges of the board will not hit the lens and thus via will not be seen by the camera.

In addition to having vias with high aspect ratios, some printed circuit boards may also have vias that are large. For the larger vias, light can come in at a wider range of angles. That makes the intensity of the light appear brighter at the center of the large vias because more light gets into the wider via since the light comes in from a wider range of angles.

Referring now to FIGS. 3, 3A, printed circuit board 32 includes a plurality of vias, e.g., three vias 33a-33c, drilled or otherwise disposed through the board 32. As shown in FIG. 3A, via 33a is disposed towards the periphery of the board 32; whereas, via 33b is disposed towards the center of board 32. Via 33c is not further referenced since it is shown in the subsequent cross- sectional views of the remaining figures. When camera 12, as shown in FIG. 1, has a large field of view and a relatively high magnification, the light that enters the camera through via 33a disposed at the periphery of the field can be distorted making the via 33a appear as an oval when in fact it is completely round. This would not be a concern with the vias that are at the center of the field of view. Via 33a may appear partially plugged because of the way that light is captured by the camera from the via 33a. The via 33a may have a high aspect ratio i.e., the board may be ten or more times as thick as the width of the via 33a. The smaller the via 33a, the less likely that light through the via 33a will hit the lens and the more likely it is that the via 33a can be mistaken as a malformed or plugged via by the inspection process in processor 18.

A second problem occurs with simple backlighting of the image. The use of non-collimated light produces a hot spot in the center of the image. Thus, as light travels straight up into the camera lens, it generates a nonuniform light intensity pattern which is exacerbated with smaller holes and holes that are disposed about the periphery of the image. This nonuniform light intensity over the surface of the holes may also cause the machine inspection processing 18 to interpret that occurrence as a malformed via.

Referring now to FIGS. 4, 4A, the assembly 30 includes the diffuser disposed over the PC board 32. Several embodiments can be provided with this diffuser. Examples of diffuser 36 include a ground glass plate and a sheet of plastic film such as Mylar which is etched on one surface and is commercially available at drafting supply stores. Other examples are possible. The diffuser 36 serves to diffuse light coming through the vias 33a-33c and project the light as an image at the bottom of the diffuser e.g., glass plate or Mylar film. The plate can have any reasonable thickness, e.g., 0.1 to 0.5 inches or other thickness and the sheet of plastic can be one to several e.g., 10 or so mils thick.

As shown in FIGS. 5, 5A, the diffuser 36 produces an image 37 on the bottom surface thereof that can be seen from the top surface. The image 37 is provided without the concomitant distortion that is seen by the camera 12 when light simply travels through the via towards the camera 12. Ground glass produces the image 37 on one surface so that it can be viewed from the other surface. The Mylar embodiment has a very small amount of surface roughness on one side that gives the Mylar a hazy appearance. This has the same characteristics as ground glass. Mylar is a thinner and less expensive and is available in larger quantities.

The diffuser 36 provides an imaging plane above the board. All of the light that passes through the vias is incident on the imaging plane and actually produces the image 37 on the diffusing surface that is free of distortion. For ground glass or Mylar the image 37 is produce on the bottom of the diffuser 36 while the camera captures the image 37 through the glass or Mylar at the top surface. The image 37 captured in this manner is free of the restrictions caused by the angle of the light.

Therefore, the field of view restriction is not present in the projected image 37 on the image plane.

Referring again to FIG. 2, in order to further reduce the distortion seen by the camera 12, the backlighting source 16 is preferably a collimated source 16'. Thus, source 16 could be a non-collimated light source that has a collimating lens 17 disposed thereover or other arrangements. The collimated source 16' produces light rays that are parallel to each other, such that all light enters the vias at the same angle and is collected on the ground glass or etched surface at the same intensity for any size hole.

Referring now to FIG. 6, a process 60 for inspecting vias using the system of FIG. 1 is shown. The process 60 includes disposing 62 a diffuser 36 over a top surface of the device under inspection such as the printed circuit board having vias. Light, preferably from a collimated light source, is directed towards the backside of the device and passes through the vias and through the diffuser 36 disposed on the device under inspection. The light that travels through the vias forms an image on the bottom surface of the diffuser. The image is captured by the camera 12. The camera 12 receives this image from the bottom portion of the diffuser and processes the image using conventional techniques such that the image can be compared to a template or other vision recognition process to determine whether the vias which represent the image correspond to properly drilled holes.

Other Embodiments

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.

What is claimed is:

Claims

1. An optical inspection apparatus comprises: a platform for holding a device under inspection; a light source disposed to illuminate a backside portion of the device; a camera disposed over a front side portion of the device to receive optical energy that penetrates through the device; and a diffuser disposed over a front side portion of the device, said diffuser producing an image on a bottom surface of said diffuser, for processing by the camera.

2. The apparatus of claim 1 wherein the image is of features of the device produced from light that penetrates the device.

3. The apparatus of claim 1 wherein the apparatus inspects vias in printed circuit boards.

4. The apparatus of claim 1 wherein the apparatus inspects vias in printed ceramic substrates.

5. The apparatus of claim 1 wherein the apparatus inspects vias in substrates having an array of vias with the vias having a high aspect ratio.

6. The apparatus of claim 1 wherein the light source is a collimated light source.

7. The apparatus of claim 1 wherein the diffuser is a ground glass plate.

8. The apparatus of claim 1 wherein the diffuser is a sheet of plastic have an etch surface that is disposed on the device .

9. The apparatus of claim 1 further comprising: a processor to process a video signal from the camera to determine whether the device passes an inspection.

10. A method for optically inspecting a substrate having a via comprises: arranging a diffuser between a platform for holding a substrate under inspection; illuminating a backside portion of the substrate with a light source; producing an image on a bottom surface of said diffuser, for processing by a camera disposed over the front side of the substrate in response to received optical energy that penetrates through the substrate.

11. The method of claim 10 wherein the light source is a collimated light source.

12. The method of claim 10 wherein the diffuser is a ground glass plate.

13. The method of claim 10 wherein the diffuser is a sheet of plastic have an etch surface that is disposed on the device.

14. The method of claim 10 further comprising: processing a video signal from the camera to determine whether the device passes an inspection.