WO2009121051A2

WO2009121051A2 - X-ray inspection systems and methods

Info

Publication number: WO2009121051A2
Application number: PCT/US2009/038748
Authority: WO
Inventors: Steven Ainsworth; Qiang Fu; Dragos Golubovic; Jason Mcgaffey; Benjamin Kingsley Stuart Peecock; Owen Yik-On Sit
Original assignee: Nordson Corporation
Priority date: 2008-03-28
Filing date: 2009-03-30
Publication date: 2009-10-01
Also published as: WO2009121051A3

Abstract

X-ray inspection systems and methods for inspecting a region of interest (15) on a printed circuit board (16). The system includes an X-ray source (12) configured to emit an X-ray beam (24), an X-ray detector (18), an X-Y transport mechanism (14) configured to hold and move the printed circuit board (16) in an inspection plane (21) situated between the X-ray source (12) and the X-ray detector (18), and an X-Y transport mechanism (20) having a carriage (40) coupled with the detector (18) such that the detector (18) faces toward the X-ray source (12), a first stage (42, 44) coupled with the carriage (40), and a second stage (46, 48) also coupled with the carriage (40). The first stage (42, 44) is configured to move the carriage (40) in an X-direction. The second stage (44, 48) is configured to independently move the carriage (40) in a Y-direction orthogonal to the X-direction so that the detector (18) is movable within an imaging plane (19).

Description

X-RAY INSPECTION SYSTEMS AND METHODS

Cross-Reference to Related Applications

[0001] This application claims the benefit of U.S. Provisional Application No. 61/040,244, filed March 28, 2008, the disclosure of which is hereby incorporated by reference herein in its entirety.

Background

[0002] The invention relates generally to X-ray inspection systems and methods and, more specifically, to X-ray inspection systems and methods for acquiring off-axis images of a region of interest on a printed circuit board.

[0003] Printed circuit boards (PCBs) of various constructions pervade modern electronics products. A PCB populated with electronic components, such as integrated circuits, capacitors, resistors, connectors, etc., is known as a printed circuit assembly (PCA). As technology advances, consumers demand increasingly complex functionalities from electronics products and, simultaneously, there is a relentless drive for product miniaturization. These factors cooperate to increase the complexity of PCBs and the difficulty in manufacturing PCBs. One approach for increasing component density is to solder electronic components onto both sides of the printed circuit board, which is known as a double-sided PCB. Various advanced packing techniques, such as Ball Grid Array (BGA) and Quad Flat Pack (QFP), are also used to achieve miniaturization. [0004] Conventional PCBs are manufactured using sophisticated automated machines that perform specific functions in the manufacturing process. To assure a high level of quality, automated inspection machines are used throughout the manufacturing process. Automated X-ray inspection machines are particularly useful for ensuring the quality of printed circuit boards by detecting the presence or absence of know defects in the solder joints that connect the components to the circuit board. In addition to automated X-ray inspection relied upon in a manufacturing environment, three dimensional X-ray imaging instruments may also be used in a higher resolution process control mode to identify, characterize, and classify defects. These instruments, which have a comparatively reduced degree of automation, are staples in many failure analysis (FA) laboratories. [0005] Solders, which may be composed of alloys of zinc and lead or a lead-free material such as a tin-silver-copper alloy, have a high atomic density compared to other materials (i.e., plastic and fiberglass) of which the printed circuit board is composed. An X-ray image represents a density projection of the analyzed specimen. In particular, solder joints of a populated printed circuit board are projected in an X-ray image as dark areas on a light background and different solder thicknesses appeared as various gray levels in the image. These attributes make X-ray imaging ideal for solder joint inspection on single sided printed circuit boards.

[0006] However, solder joints attaching electronics components to double sided printed circuit boards are more difficult to analyze in an X-ray image. On such double-sided printed circuit boards, the electronics components are coupled by soldering to both sides of the printed circuit board. In a traditional two dimensional X-ray image of the printed circuit board, the solder joints on the two sides will overlap with each in the image, which generates interference that degrades the image and may preclude an adequate inspection of the board quality. To overcome this shortcoming, three dimensional X-ray inspection systems have been developed that can target a selected layer (slice) of the printed circuit board. [0007] Conventional three dimensional X-ray imaging systems acquire multiple oblique or off- axis images of a device under test (DUT) and then combine these off- axis images using an algorithm to compute slice images. One conventional technique for analyzing off-axis images is called laminography. Laminography-based X-ray imaging systems feature rather complex hardware in the form of an electronically steerable X-ray source and a detector mounted on a circular transport mechanism. The hardware cooperates to produce off-axis images of the DUT in a continuously manner. Because the relative motion between the X-ray beam and the detector is synchronized, laminography machines can produce X-ray images of a selected Z-height above the DUT. Unfortunately, the hardware required in such laminography-based X-ray imaging systems is sophisticated, comparatively expensive, and requires significant maintenance because of the complexity.

[0008] Another conventional technique for acquiring and analyzing off-axis X-ray images relies upon a cone-shaped beam X-ray source and a large fixed-position flat panel detector to concurrently capture on-axis and off-axis X-ray images of the DUT. The slice images are produced from the off-axis images using tomosynthesis techniques. However, the large flat panel detectors found in these tomosynthesis-based X-ray imaging systems are expensive. Typically, the cost scales dramatically with increasing imaging area. The imaging area of large flat panel detectors may also contain defects from the manufacturing process that produce image defects must be taken into account during image analysis. To obtain an off-axis image of sufficient angle, the flat panel detector must be placed very close to the DUT. As a result, printed circuit boards with tall components cannot be inspected in tomosynthesis-based X-ray imaging systems. [0009] Both types of conventional three-dimensional X-ray imaging systems have a magnification that is fixed at the factory and that is not adjustable by the use. Both types of conventional three dimensional X-ray imaging systems require engineering level personnel to operate and maintain. In additional to being expensive, conventional three dimensional X-ray imaging systems lack flexibility and do not permit selective three dimensional imaging of only selected areas on the printed circuit board.

[0010] What is needed, therefore, is an improved three dimensional X-ray imaging system that overcomes these and other deficiencies of conventional three dimensional X-ray imaging systems.

Summary

[0011] Embodiments of the invention are directed to apparatus and methods for acquiring off-axis X-ray images of a sample, such as a printed circuit board, and then applying tomosynthesis techniques to produce three dimensional slice images of the sample from the off-axis X-ray images. Additional machine vision techniques, as understood by a person having ordinary skill in the art, may then be applied to the off-axis X-ray images to produce inspection results for the sample in the form of slice images or three dimensional models. [0012] In one embodiment, an X-ray inspection system is provided for inspecting a region of interest on a printed circuit board. The system includes an X-ray source configured to emit an X-ray beam, an X-ray detector, and a support configured to hold the printed circuit board in an inspection plane at a location between the X-ray source and the X-ray detector. The support is configured to be position the printed circuit board so that the X-ray beam is transmitted through the region of interest in transit to the X-ray detector. The X-ray inspection system further includes an X-Y transport mechanism having a carriage coupled with the detector such that the detector faces the X-ray source, a first stage coupled with the carriage, and a second stage also coupled with the carriage. The first stage is configured to move the carriage in an X-direction. The second stage is configured to independently move the carriage in a Y-direction orthogonal to the X-direction so that the detector is movable within an imaging plane.

[0013] In another embodiment, a method is provided for inspecting a region of interest on a printed circuit board with an X-ray beam. The method includes acquiring a first X-ray image of the region of interest with an X-ray detector located within an imaging plane at a first position having a first X-coordinate and a first Y-coordinate, moving the X-ray detector in an X-direction within the imaging plane from the first X-coordinate to a second X- coordinate of a second position, and moving the X-ray detection in a Y-direction within the - A -

imaging plane from the first Y-coordinate to a second Y-coordinate of the second position. While the X-ray detector is located at the second position within the imaging plane, a second X-ray image of the region of interest is acquired.

Brief Description of the Drawings

[0014] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.

[0015] FIG. 1 is a diagrammatic side view of an automated X-ray inspection system in accordance with an embodiment of the invention.

[0016] FIG. 2 is a diagrammatic perspective view of a portion of the automated X-ray inspection system of FIG. 1 with the region of interest shown in four different locations at which an off-axis image is acquired.

[0017] FIG. 2A is a diagrammatic top view illustrating the movement of the detector between two different positions within the imaging plane.

[0018] FIG. 3 is a diagrammatic perspective view similar to FIG. 2 in which the automated X-ray inspection system is configured in a mode to acquire a two-dimensional image.

[0019] FIG. 4 is a perspective view of an enclosure housing an X-ray inspection system in accordance with an embodiment of the invention.

[0020] FIG. 5 is a perspective view of the X-ray inspection system inside the enclosure of FIG. 4.

[0021] FIG. 6 is an exploded view of X-ray inspection system shown in FIG. 5. [0022] FIG. 7 is a cross-sectional view taken generally along line 7-7 in FIG. 5. [0023] FIG. 8 is a perspective view of an X-ray inspection system constructed in accordance with an alternative embodiment of the invention.

[0024] FIG. 9 is a cross-sectional view of the X-ray inspection system shown in FIG. 8. [0025] FIG. 10 is an exploded view of a portion of the X-ray inspection system shown in FIGS. 8 and 9.

Detailed Description

[0026] With reference to FIGS. 1 and 2 and in accordance with an embodiment of the invention, an X-ray inspection system 10 generally includes an X-ray source 12, an X-Y transport mechanism 14, a detector 18 in the representative form of an X-ray imaging panel, and another X-Y transport mechanism 20. The X-Y transport mechanism 14 is configured to support and move an object, such as a printed circuit board 16, that is under inspection, while the X-Y transport mechanism 20 is configured to support and move the detector 18. [0027] The X-ray source 12 of the X-ray inspection system 10 is mounted on a motorized slide 22 that is configured to move the X-ray source 12 over a range of travel upward and downward along a Z-axis, generally indicated by the double-headed arrow 26, that is aligned normal to the confronting surface of the printed circuit board 16. A drive motor 23 is mechanically coupled with the slide 22. Changing the distance between the X-ray source 12 and printed circuit board 16 operates to change the magnification of the images acquired by the detector 18. The magnification is increased by operating the motorized slide 22 to advance the X-ray source 12 along the Z-axis 26 closer to the confronting bottom surface 16a of the printed circuit board 16. Conversely, the magnification is decreased by operating the motorized slide 22 to withdraw the X-ray source 12 along the Z-axis 26 away from the confronting bottom surface 16a of the printed circuit board 16 and the X-Y transport mechanism 14. When operating, the X-ray source 12 emits a beam 24 of X-rays toward the printed circuit board 16.

[0028] Generally, the X-ray source 12 includes a tube that generates the beam 24 of X- rays by accelerating electrons from an electron gun and causing these energetic electrons to collide with a metal target. In one embodiment, the X-ray source 12 may be a stationary or non-steerable type of source, which lacks the capability to move the electron beam to strike the metal target at more than one location. The X-rays contained in the beam 24 are sufficiently energetic to penetrate through the thickness of the printed circuit board 16 and the electronics component(s) in a region of interest 15 so that the attenuated X-rays reach the detector 18. The differential levels of X-ray attenuation by the materials of different density projection produces contrast in the resulting image captured from each region of interest 15 on the printed circuit board 16.

[0029] The X-Y transport mechanism 14 of the X-ray inspection system 10 transports different regions of interest 15 on the printed circuit board 16 into the path between the X-ray source 12 and the detector 18 for exposure to the X-rays and imaging with the detector 18. The X-Y transport mechanism 14 positions the printed circuit board 16 in an inspection plane 21 that is located between the X-ray source 12 and the detector 18. The Z-axis 26 is generally orthogonal or normal to the inspection plane 21. The region of interest 15 occupied an area on the printed circuit board 16 that is less than the entire surface area of the printed circuit board 16. In most instances, the area occupied on the printed circuit board 16 by the region of interest 15 is significantly smaller than the entire surface area of the printed circuit board 16. Each region of interest 15 on the printed circuit board 16 may include a distinct device under testing (DUT) that has solder joints securing the DUT to the printed circuit board 16 and constitute the objects for inspection.

[0030] The detector 18 of the X-ray inspection system 10 may have a construction for a digital detector recognized by a person having ordinary skill in the art. Generally, the detector 18 includes an active area, a sensor that converts the incoming X-rays over the active area into another signal type that can be measured or imaged, and an amplifier used to boost the amplitude of the signals. The signals are converted from an analog form to a digital form within the detector 18 and a digital image format is output from the detector 18. An exemplary digital detector is a digital charged coupled device (CCD) camera, such as a complementary metal-oxide-semiconductor (CMOS) flat panel detector that includes a two dimensional pixel array of silicon photodiodes constituting the active area. In one embodiment, the detector 18 is a flat panel detector characterized by a 1.3 megapixel, 50 mm x 50 mm active area. The detector 18 may include an image intensifier, such as a scintillator (e.g., a phosphor screen). The active area of the detector 18 faces toward the printed circuit board 16 and the X-ray source 12 so that the beam 24 of X-rays passing through the printed circuit board 16 intersects the detector 18, when the printed circuit board 16 and detector 18 are properly positioned.

[0031 ] The X-Y transport mechanism 20 is configured to move the detector 18 in an imaging plane 19 that is normal to the line of emanation 27 of the X-ray source 12, which allows off-axis imaging of one or more of the regions of interest 15. The movement varies the field of view of the detector 18 so that the detector 18 can be repositioned within the imaging plane 19 among the different imaging locations. Typically, during imaging, a centerline 33 of the field of view on the active area of the detector 18 is aligned with a centerline 35 of the region of interest 15 on the printed circuit board 16. The region of interest 15 and the detector 18 are each moved to plural locations centered about the Z-axis 26 and spaced from the Z-axis 26 by a radius or distance governed by the particular off-axis oblique angle, θ.

[0032] The beam 24 of X-rays emitted by the X-ray source 12 has a line of emanation 27 that is coincident with the Z-axis 26. In one embodiment, the X-ray beam 24 fans out in a cone, which may be axially symmetric about the line of emanation 27 of the beam 24. The line of emanation 27 of the X-ray beam 24 emitted from the X-ray source 12 is aligned substantially perpendicular to the confronting surface 16a of the printed circuit board 16 and the outer envelope of the beam 24 intersects a section in the inspection plane 21 and a larger section in the imaging plane 19. The intersected sections in the planes 19, 21 may be circular if the outer envelope of the beam 24 is conical and symmetrical about the line of emanation 27.

[0033] Generally, the X-Y transport mechanisms 14, 20 may include motorized stages or a positioning table to which the printed circuit board 16 and detector 18 are respectively mounted and that effect powered movement or motion in respective X-Y planes of the printed circuit board 16 or detector 18. The powered motion of the printed circuit board 16 is contained within one X-Y plane (i.e., the inspection plane 21) that is parallel to another X-Y plane containing the motion of the detector 18 (i.e., the imaging plane 19). The X-Y planes of motion are typically horizontal and are spaced from each other. The X-axis and Y-axis of the imaging plane 19 intersect at an origin 37, which is likewise intersected by the Z-axis 26 to define a reference frame. A similar reference frame is defined at the intersection point of the Z-axis 26 with the inspection plane 21.

[0034] In particular, the X-Y transport mechanism 14 includes a carriage 30 that serves as a mechanical support and mount for the printed circuit board 16, a first drive mechanism or stage including a first drive motor 32 and a first power transmission device 34 that mechanically couples the first drive motor 32 with the carriage 30 for motion in the X- direction, and a second drive mechanism or stage that includes a second drive motor 36 and a second power transmission device 38 that mechanically couples the second drive motor 36 with the carriage 30 for motion in the Y-direction. The first drive motor 32 is used to drive the first power transmission device 34 for providing a powered movement of the carriage 30 in the X-direction. The second drive motor 36 is used to drive the first power transmission device 38 for providing a powered movement of the carriage 30 in the Y-direction. The X- direction is orthogonal to the Y-direction within the X-Y plane of motion and the first and second drive motors 32, 36 are configured to independently move the carriage 30 in the X- direction and in the Y-direction. The carriage 30 of the X-Y transport mechanism 14 may include one or more clamps (not shown), which may be pneumatically actuated, that are used to secure the printed circuit board 16 against movement relative to the carriage 30 when the X-Y transport mechanism 14 moves the printed circuit board 16. Alternatively, the clamps may be omitted from the X-Y transport mechanism 14 such that the printed circuit board 16 is merely resting without mechanical restraint on a tray or plate. [0035] The X-Y transport mechanism 20 includes a carriage 40 that mechanically supports and serves as a mount for the detector 18, a first drive mechanism or stage that includes a first drive motor 42 and a first power transmission device 44 that couples the first drive motor 42 with the carriage 40 for motion in the X-direction, and a second drive mechanism or stage that includes a second drive motor 46 and a second power transmission device 48 that couples the second drive motor 46 with the carriage 40 for motion in the Y- direction. The first drive motor 42 is used to drive the first power transmission device 44 for providing a powered movement of the carriage 40 in the X-direction. The second drive motor 46 is used to drive the first power transmission device 48 for providing a powered movement of the carriage 40 in the Y-direction. The X-direction is orthogonal to the Y- direction within the X-Y plane of motion and the first and second drive motors 42, 46 are configured to independently move the carriage 40 in the X-direction and in the Y-direction. The carriage 40 of the X-Y transport mechanism 20 is spaced from the carriage 30 of the X-Y transport mechanism 14 so that the printed circuit board 16 fits in the open space and can be moved without obstruction to the various positions required for imaging each region of interest 15.

[0036] The drive motors 32, 34, 42, 46 may be any suitable type of electrical motor, such as stepping motors or servo motors. The power transmission devices 34, 38, 44, 48 may be, for example, lead screws that are mechanically connected by couplings with the respective one of the carriages 30, 40. The X-Y transport mechanism 20 is configured to move the carriage 40 and the detector 18 mechanically supported by the carriage 40 with discrete movements in the X-direction and discrete movement in the Y-direction so that the active area of the detector 18 is positioned at the various positions within the imaging plane 19. Similarly, the X-Y transport mechanism 14 is configured to move the carriage 30 and the printed circuit board 16 mechanically supported by the carriage 30 with discrete movements in the X-direction and discrete movement in the Y-direction so that the printed circuit board 16 is positioned at the various positions within the inspection plane 21. These X-Y movements are coordinated such that the printed circuit board 16 and the detector 18 are located at the same oblique angle, θ, and have their respective centerlines 33, 35 aligned. [0037] The X-ray inspection system 10 further includes a motion controller 60 that configured to regulate the indexed movements of the X-Y transport mechanisms 14, 20. The motion controller 60 is electrically coupled with the drive motors 32, 34, 42, 46 and incorporates electrical circuitry that supplies control signals to the drive motors 32, 34, 42, 46 to cause respective motions.

[0038] A system controller 62 is interfaced with the motion controller 60, the detector 18, and the X-ray source 12, and coordinates the operation of the X-ray inspection system 10. System controller 62 may represent practically any computer, computer system, or programmable device recognized by a person having ordinary skill in the art. System controller 62 typically includes at least one processor 64 coupled to a memory 66. Processor 64 may represent one or more processors (e.g., microprocessors), and memory 66 may represent the random access memory (RAM) devices comprising the main storage of system controller 62, as well as any supplemental levels of memory, e.g., cache memories, nonvolatile or backup memories (e.g. programmable or flash memories), read-only memories, etc. In addition, memory 66 may be considered to include memory storage physically located elsewhere in system controller 62, e.g., any cache memory in a processor 64, as well as any storage capacity used as a virtual memory, e.g., as stored on a mass storage device 68 or another computer (not shown) coupled to system controller 62 via a network. [0039] System controller 62 also typically receives a number of inputs and outputs for communicating information externally. For interface with a user or operator, system controller 62 typically includes one or more user input devices (e.g., a keyboard, a mouse, a trackball, a joystick, a touchpad, a keypad, a stylus, and/or a microphone, among others) in the form of a user interface 70. System controller 62 may also include a display 72 (e.g., a CRT monitor, an LCD display panel, and/or a speaker, among others). [0040] System controller 62 operates under the control of an operating system 74, and executes or otherwise relies upon various computer software applications, components, programs, objects, modules, data structures, etc. In general, the routines executed by the system controller 62 to operate the X-ray inspection system 10, whether implemented as part of an operating system or a specific application, component, program, object, module or sequence of instructions will be referred to herein as "computer program code". The computer program code typically comprises one or more instructions that are resident at various times in various memory and storage devices in a computer, and that, when read and executed by one or more processors in a computer, causes that computer to perform the steps necessary to execute steps or elements embodying the various aspects of the invention. [0041] The system controller 62 includes digital and/or analog circuitry that interfaces with the motion controller 60 for supplying control signals to the drive motors 32, 34, 42, 46 and receiving positional information from sensors, such as encoders, relating to the positions of the X-Y transport mechanisms 14, 20. Drive software 75 resident as an application in the memory 66 is executed by the processor 64 in order to issue commands that control the operation of the motion controller 60. The system controller 62 also includes circuitry interfaced with the motion controller 60. The system controller 62 supplies commands to the motion controller 60, which in turn supplies control signals to the drive motor 23 of the slide 22 connected with the X-ray source 12.

[0042] With specific reference to FIG. 2A, the detector 18 or, more specifically, the active area of the detector 18 is depicted being moved from a first position 59 in the imaging plane 19 to a second position 61 in the imaging plane 19. The detector 18 and the region of interest 15 on the printed circuit board 16 are held stationary at each of the first and second positions 59, 60 while an off-axis image is acquired. The first position 59 is characterized by a first set of X-Y coordinates (xi, yi) and the second position 61 is characterized by a different second set of X-Y coordinates (x₂, y₂). The distance from the first position 59 to the origin 37 is equal to the distance from the second position 61 to the origin 37, which reflects that the oblique angle, θ, is held constant while the images are acquired at the first and second positions. In an alternative embodiment, the detector 18 and the region of interest 15 on the printed circuit board 16 may be continuously moved, as opposed to being brought to rest at the positions 59, 61, while acquiring the images at positions 59, 61 on the fly to collect a large number of images over a shorter time.

[0043] The X-Y transport mechanism 20, under instructions from the system controller 62 communicated to the motion controller 60, is operated to move the carriage 40 such that that the detector 18 is serially moved from the first position 59 along a first segment 63 a in the X-direction and then along a second segment 63d in the X-direction to arrive at the second position 61. The number of discrete segments in the X-direction may differ from the representative embodiment in that multiple discrete segments may be conjoined to translate between the different X-coordinates. Similarly, the number of discrete segments in the Y- direction may differ from the representative embodiment in that multiple discrete segments may be used to translate between the different Y-coordinates. Although not shown, the X-Y transport mechanism 14, also under instructions from the system controller 62 communicated to the motion controller 60, is operated to move the carriage 30 so that the region of interest 15 on the printed circuit board 16 is aligned relative to the detector 18 such that a portion of the beam 24 passes through the region of interest 15 in transit from the X-ray source 12 to the detector 18.

[0044] With renewed reference to FIGS. 1 and 2, the system controller 62 also includes digital and/or analog circuitry 76 that interfaces with the detector 18 for receiving the images captured for each region of interest 15 on the printed circuit board 16. In particular, the system controller 62 is configured to execute suitable mathematical algorithms that tomographically reconstruct the off-axis images 80 received from the detector 18 into slice images 82 representing surface areas on the opposite sides of the printed circuit board 16 in each region of interest 15. Alternatively, the mathematical algorithms executed by the system controller 62 may tomographically reconstruct the off-axis images 80 received from the detector 18 into a three-dimensional model 84.

[0045] Imaging software 78 resident as an application in the memory 66 is executed by the processor 64 in order to execute the mathematical algorithms needed for tomographic reconstruction of the off-axis images 80. The off-axis images 80 acquired using the detector 18 are stored in the mass storage device 68 for manipulation by the imaging software 78 to generate slice images 82. The mathematical algorithms used for tomographic reconstruction are understood by a person having ordinary skill in the art, as is the software 78 implemented by the system controller 62 for performing the tomographic reconstruction of the off-axis slice images 80 captured for each individual region of interest 15 on the printed circuit board 16. Generally, tomosynthesis involves digitally shifting and combining the off-axis images 80 for each region of interest 15 to produce a tomosynthetic slice image 82, which represents a horizontal cross-section or image slice through the plane of the region of interest 15. [0046] In use for acquiring off-axis images of regions of interest 15 on the printed circuit board 16, the printed circuit board 16 is placed on the X-Y transport mechanism 14. Fiducial marks on the printed circuit board 16 may be used to align the printed circuit board 16 within the inspection plane 21 and to correct any rotational misalignment. The system controller 62 supplies commands to the motion controller 60 to operate the drive motor 23 for the 22 to move the X-ray source 12 along the z-axis relative to the printed circuit board 16, which selects a magnification at which the images are to be acquired. The X-ray inspection system 10 is initially configured for two dimensional inspection and the X-Y transport mechanism 14 moves the different regions of interest 15 into the field of view of the X-ray source 12 for capturing two-dimensional images. As needed, the X-ray source 12 is operated to emit the beam 24 of X-rays toward the regions of interest 15 on the printed circuit board 16 and the two dimensional images are captured using the detector 18. The voltage and power of the X- ray source 12 may be adjusted to achieve a suitably contrasted image for the operator. [0047] Next, the regions of interest 15 for off- axis imaging are selected by the operator and examined through a series of coordinated, synchronous movements of the printed circuit board 16 and detector 18. For each region of interest 15, the system controller 62 provides instructions to the motion controller 60 to operate the X-Y transport mechanisms 14, 20 to index the printed circuit board 16 and detector 18 simultaneously and synchronously within the respective inspection and imaging planes to a plurality of off-axis locations. In one embodiment, the printed circuit board 16 and detector 18 are indexed by the respective X-Y transport mechanisms 14, 20 under the control of the system controller 62 to either four (as shown in FIG. 2) or eight off-axis locations that may be equally spaced in a rotational sense about the Z-axis 26. At each off-axis location, the oblique angle, θ, of the centerline 35 of the region of interest 15 and the centerline 33 of the detector 18 relative to the Z-axis 26 may be maintained constant so that the off- axis images of each region of interest 15 are acquired at the same off-axis angle, θ. In one embodiment, the maximum off-axis angle, θ, may be in the range of 20° to 35°. Off-axis images are sequentially captured with the detector 18 at each of the plurality of off-axis locations for each region of interest 15 and with the detector located at a constant off- axis angle, θ.

[0048] Tomosynthesis is applied by the system controller 62 to re-construct each imaged region of interest 15 on the printed circuit board 16 based upon the off-axis images. Specifically, a tomosynthesis algorithm in implemented in computer software executing on the system controller 62 to compute slice images 82 of each imaged region of interest 15 on the printed circuit board 16 from the off-axis images captured for that particular area. In one embodiment, two image slices are deduced for each imaged region of interest 15 from the three dimensional re-construction. One of the slices images 82 represents the top side of the imaged region of interest 15 on the printed circuit board 16. The other of the slice images 82 represents the bottom side of the imaged region of interest 15 on the printed circuit board 16. As used herein, the top side of the printed circuit board 16 is the surface that is closest to the detector 18 and the bottom side 16a of the printed circuit board 16 is the surface that is closest to the X-ray source 12.

[0049] Inspections may be performed on these two re-constructed tomosynthetic slice images 82 to determine solder quality and assembly quality of the board assembly in each imaged region of interest 15 on the printed circuit board 16. Specifically, the slice images 82 are analyzed by the system operator for flaws such as voids, shorts, off-position components, and absent components.

[0050] Each successive printed circuit board 16 with nominally identical configurations for its electronics components (i.e., board layout) may be subjected to the same inspection recipe learned from an initial board 16. Of course, the inspection recipe can be altered to adjust to printed circuit boards 16 with different configurations for its electronics components. In addition, the order of image acquisition can be altered so that the three dimensional slice images are captured before the two dimensional images. [0051] In an alternative embodiment, the maximum off-axis angle, θ, may be as large as 60° and the printed circuit board 16 and detector 18 may be indexed by the respective X-Y transport mechanisms 14, 20 under the control of the system controller 62 to a larger number of off-axis locations (e.g., as many as 7,200 or more locations) that may be equally spaced in a rotational sense about the Z-axis 26. Sets of off-axis images for a region of interest 15 may be taken at different off-axis angles, θ. Oblique views are not limited to a fixed angle but are adjustable within the angular limits of the cone of the x-ray beam 24. The system controller 62 applies tomosynthesis to re-construct a three-dimensional model 84 of each imaged region of interest 15 on the printed circuit board 16 based upon the off-axis images 80. [0052] Alternatively, the tomosynthetic slices may be used to identify, characterize, and classify defects in a failure analysis (FA) laboratory. In this instance, the number of discrete locations at which an off-axis image is acquired for each region of interest 15 may be increased to increase the resolution of the reconstructed image slices and for the construction of a three-dimensional model of each region of interest 15. In addition, the image resolution may be further increased by acquiring multiple off-axis images at each of the locations. For example, a full-size image may be acquired of a quarter of the region of interest at each location and then stitched together to provide a complete off-axis image of the region of interest at each location.

[0053] With reference to FIG. 3, a two-dimensional X-ray image of one or more regions of interest 15 on the printed circuit board 16 can be obtained with an appropriate spatial arrangement of the X-ray source 12, the printed circuit board 16, and the detector 18. Specifically, the X-Y transport mechanism 14 is operated under the control of commands communicated from the system controller 62 to the motion controller 60 to move the region of interest 15 on the printed circuit board 16 to a central location near the Z-axis 26. The X- Y transport mechanism 20 is likewise operated under the control of commands communicated from the system controller 62 to the motion controller 60 to move the detector 18 to a central location near the Z-axis 26.

[0054] The configuration of the X-ray inspection system 10 can be switched from a two- dimension imaging mode to a three-dimension imaging mode, and vice versa, merely entails re-positioning the printed circuit board 16 and the detector 18 in their respective X-Y planes using the X-Y transport mechanisms 14, 20. Consequently, two-dimensional and three- dimensional inspection can be readily combined in one inspection program or inspection recipe. The slice images are displayed by the systems controller 62 and may be stored or analyzed for defects. [0055] In comparison with conventional X-ray inspection systems, X-ray inspection system 10 does not rely on acquiring multiple off-axis images simultaneously using a large flat panel detector. Instead, the X-ray inspection system 10 acquires the off-axis images sequentially by moving a smaller detector 18 to a plurality of different imaging locations. By collecting the off-axis images sequentially, a smaller and less expensive X-ray imaging panel may be employed as the detector 18 in comparison with conventional X-ray inspection systems. For example and in one embodiment, the detector 18 may be an imaging panel having an active imaging area of about two inches by two inches. In contrast, conventional X-ray inspection systems may require imaging panels with an active imaging area of six inches by six inches to permit the simultaneous acquisition of images from multiple regions of interest 15.

[0056] Furthermore, because the image set for each field of view is collected independently, three dimensional imaging can be performed only on selected areas of the printed circuit board 16 that actually require three dimensional inspection for image clarity. For other areas of the printed circuit board 16, the X-ray inspection system 10 can be readily reconfigured to use the relatively faster method of two dimensional X-ray inspection so that two dimensional and three dimensional inspection can be beneficially combined in a single inspection recipe. This attribute of the X-ray inspection system 10 provides a high level of flexibility in comparison with conventional X-ray inspection systems that require the operator to consider a three dimensional image of the entire printed circuit board 16, which is acquired at a fixed magnification, even if only a small area on the printed circuit board 16 actually needs three dimensional inspection.

[0057] The X-ray inspection system 10 does not require a steerable X-ray source, rotating mechanical parts as required for a rotating detector, or a large flat panel detector, which are relatively expensive components in some types of conventional X-ray inspection systems. Instead, the X-ray inspection system 10 solves the problems of high cost because of the implementation of only five axes of movement and a single flat panel detector. Therefore, X- ray inspection system 10 may have a lower initial equipment cost and cost of ownership than these types of conventional X-ray inspection systems.

[0058] The X-ray inspection system 10 can capture high quality images because of the superior control over the image magnification and the exact angle, θ, at which the off-axis images are captured. This promotes the realization of the full potential of the tomosynthesis technology used to analyze the off-axis images. Conventional X-ray inspection systems are limited to a single fixed magnification and a fixed angle for capturing off-axis images. [0059] With reference to FIGS. 4-7 in which like reference numerals refer to like features in FIGS. 1-3 and in accordance with an embodiment of the invention, an X-ray inspection system 90 includes the X-ray source 12, the motorized slide 22 for the X-ray source 12, the X-Y transport mechanism 14 for the printed circuit board 16, the detector 18, the X-Y transport mechanism 20 for the detector 18, and the motion controller 60 and system controller 62, which are all housed inside an enclosure 92 (FIG. 4). The enclosure 92 is in the form of a cabinet with sheet-metal walls lined by a material like lead that blocks the escape of X-rays from the interior of the enclosure 92. The enclosure 92 includes an access opening 94 and a chute door 96 that is configured to be movable to an open position for loading and unloading the printed circuit board 16 through the access opening 94 and to a closed position for inspecting the printed circuit board 16 in which the access opening 94 is blocked by the chute door 96. A support 98 is suspended from the exterior of the enclosure 92 and is used to hold the user interface 70 and display 72 (FIG. 1) for use by the instrument operator. The X-ray source 12, X-Y transport mechanism 14, detector 18, and X-Y transport mechanism 20 are constructed and are arranged inside the enclosure 92 generally as described with regard to FIGS. 1-3.

[0060] The X-Y transport mechanism 14 includes a carriage 100 that acts as a mechanical support and mount for the printed circuit board 16, a first drive motor 102, a first power transmission device 104 that couples the first drive motor 102 with the carriage 100 for motion in the X-direction, a second drive motor 106, and a second power transmission device 108 that couples the second drive motor 106 with the carriage 100 for motion in the Y- direction. The first drive motor 102 is used to drive the first power transmission device 104 for providing a powered movement of the carriage 100 in the X-direction. The second drive motor 106 is used to drive the second power transmission device 108 for providing a powered movement of the carriage 100 in the Y-direction. The X-direction is orthogonal to the Y- direction within the X-Y plane of motion and the first and second drive motors 102, 106 are configured to independently move the carriage 100 in the X-direction and in the Y-direction, respectively. The carriage 100 of the X-Y transport mechanism 14 may include one or more clamps (not shown), which may be pneumatically actuated, that are used to secure the printed circuit board 16 against movement relative to the carriage 100 when the X-Y transport mechanism 14 moves the printed circuit board 16.

[0061] The X-Y transport mechanism 14 also includes X-direction rails 110, 112 aligned in parallel with each other in the X-direction and in a spaced relationship, as well as Y- direction rails 114, 116 aligned in parallel with each other in the Y-direction (orthogonal to the X-direction) and in a spaced relationship. The X-direction rails 110, 112 are located along opposite side edges of the carriage 100, which is movably connected to the X-direction rails 110, 112. Similarly, the Y-direction rails 114, 116 are connected to the other set of opposite sides of the carriage 100. The Y-direction rails 114, 116 are located along opposite side edges of the carriage 100, which is movably connected to the Y-direction rails 114, 116. The carriage 100 is constrained to move in the X-direction along the X-direction rails 110, 112 and in the Y-direction along the Y-direction rails 114, 116.

[0062] The first drive motor 102, first power transmission device 104, and X-direction rails 110, 112 constitute a first drive mechanism or stage of the X-Y transport mechanism 14. The second drive motor 106, second power transmission device 108, and Y-direction rails 110, 112 constitute a second drive mechanism or stage of the X-Y transport mechanism 14. The respective stroke lengths of the first and second stages are selected to provide a range of movement within the imaging plane adequate to move the printed circuit board 16 for imaging the regions of interest 15. An electromagnetic force produces torque in each of the drive motors 102, 106 that is converted to linear motion by the respective power transmission devices 104, 108.

[0063] The inspection system 90 may include input and output conveyors (not shown) for shuttling successive printed circuit boards 16 to and from the system 90, as well as transfer mechanisms to shift the printed circuit boards 16 from the input conveyor to a clamped position on the carriage of the X-Y transport mechanism 14 and from the carriage to the output conveyor. This setup makes the inspection system 90 well-suited for use in a production environment that is monitoring for defects in a production line process. [0064] The X-Y transport mechanism 20 includes a carriage 120 that mechanically supports and provide a mount for the detector 18, a first drive motor 122, a first power transmission device 124 that couples the first drive motor 122 with the carriage 120 for powered motion in the X-direction, a second drive motor 126, and a second power transmission device 128 that couples the second drive motor 126 with the carriage 120 for powered motion in the Y-direction. The first drive motor 122 is used to drive the first power transmission device 124 to transfer power to the carriage 120 for providing a powered movement in the X-direction. The second drive motor 126 is used to drive the first power transmission device 128 to transfer power to the carriage 120 for providing a powered movement in the Y-direction. The power transmission devices 124, 128 are coupled by a guide 125 that aligns them for orthogonal powered movement relative to each other. The X- direction is orthogonal to the Y-direction within the X-Y plane of motion and the first and second drive motors 122, 126 are configured to independently move the carriage 120 in the X-direction and in the Y-direction. The carriage 120 of the X-Y transport mechanism 20 suspends the detector 18 in a spaced relationship with the carriage 100 of the X-Y transport mechanism 14 so that the printed circuit board 16 fits in the open space and can be moved without obstruction to the various positions required for imaging each of the regions of interest 15.

[0065] The first drive motor 122 and first power transmission device 124 constitute a first drive mechanism or stage of the X-Y transport mechanism 20. The second drive motor 122 and second power transmission device 124 constitute a second drive mechanism or stage of the X-Y transport mechanism 20. The respective stroke lengths of the first and second stages are selected to provide a range of movement within the imaging plane adequate to move the detector 18 for imaging the regions of interest 15.

[0066] With reference to FIGS. 8-10 in which like reference numerals refer to like features in FIGS. 1-3 and in accordance with an embodiment of the invention, an X-ray inspection system 130 includes X-Y transport mechanisms 14, 20 for the printed circuit board 16 and detector 18, respectively, that are constructed differently than in the X-ray inspection system 90 depicted in FIGS. 4-7.

[0067] The X-Y transport mechanism 14 o f X-ray inspection system 130 includes a carriage 132, a sample tray 134 supported by the carriage and that serves as a mechanical support for the printed circuit board 16, and a pair of linear motors 136, 138 each having a movable platen coupled with the carriage 132. The linear motors 136, 138 are aligned in parallel with each other in the X-direction along opposite side edges of the carriage 132 and are configured to provide powered movement of the carriage 132 in the X-direction. The stationary track of each of the linear motors 136, 138 is coupled with, and supported from, another carriage 140. Carriage 132 is disposed inside, and moves in the X-direction within, a central opening defined in carriage 140. Linear motors 142, 144 each have a movable platen coupled with the carriage 140. The linear motors 142, 144, which are aligned in parallel with each other in the Y-direction along opposite side edges of the carriage 140, are configured to simultaneously provide powered movement of both carriages 132, 140 in the Y-direction. The X-direction is orthogonal to the Y-direction within the X-Y plane of motion (i.e., the inspection plane 19) for the printed circuit board 16. Linear motors 136, 138 are configured to independently move the carriage 132 in the X-direction independent of the movement of both carriages 132, 140 in the Y-direction by linear motors 142, 144. The linear motors 136, 138 constitute a first drive mechanism or stage of the X-Y transport mechanism 14 and the linear motors 142, 144 constitute a second drive mechanism or stage of the X-Y transport mechanism 14.

[0068] The sample tray 134, which supports the printed circuit board 16 to be inspected without active clamping, is thin to minimize X-ray attenuation. The system magnification is only limited by the thickness of the sample tray 134 and not by any tall components on the printed circuit board 16. Maximum magnification of the region of interest 15 is achieved when the sample tray 134 is resting directly on a top surface of the tube of the X-ray source 12.

[0069] The X-Y transport mechanism 20 of X-ray inspection system 130 includes a carriage 150 that acts as a mechanical support and mount for the detector 18, and a first drive mechanism or stage that includes a drive motor 152, a power transmission device 154 that couples the drive motor 152 with the carriage 150 for motion in the X-direction, and X- direction rails 154, 156 aligned in parallel with each other in the X-direction and in a spaced relationship. The X-direction rails 154, 156 are located along opposite side edges of the carriage 150, which is movably connected to the X-direction rails 154, 156. The carriage 150 is constrained to move in the X-direction along the X-direction rails 154, 156. [0070] The X-Y transport mechanism 20 of X-ray inspection system 130 further includes a second drive mechanism or stage that includes a pair of drive motors 158, 160, a pair of power transmission devices 162, 164, a beam 166 mechanically coupled at opposite ends by the power transmission devices 162, 164 with the drive motors 158, 160, and a pair of Y- direction rails 163, 165 that constrain powered movement of the beam 166 in the Y-direction. [0071] The first stage and carriage 150 are suspended from the underside of the beam 166 such that, as the beam 166 is moved by the drive motors 158, 160, the carriage 150 is likewise moved in the Y-direction. The detector 18 is oriented on the carriage 150 so that the active area is oriented to face the X-ray source 12. Drive motor 152 is used to drive the power transmission device 154 for providing a powered movement of the carriage 150 in the X-direction. The drive motors 158, 160 are driven in unison to drive the power transmission devices 162, 164 for providing a powered movement of the carriage 150 in the Y-direction. The X-direction is orthogonal to the Y-direction within the X-Y plane of motion and the drive motor 152 is configured to move the carriage 150 in the X-direction independently of the movement of the carriage 150 in the Y-direction powered by drive motors 158, 160. [0072] The X-Y transport mechanisms 14, 20 are coupled by support members 170, 172 with a machine base 174. The machine base 174 is in turn supported upon a plurality of anti- vibration feet 176. The machine base 174, which may be made from a composite of granite chips and a binding resin, and the anti-vibration feet 176 operate to isolate the X-Y transport mechanisms 14, 20 from extraneous vibration and thermal expansion. The X-ray tube 12 is likewise mounted to the machine base 174 by one or more mounting members (not shown). [0073] In order to generate a three-dimensional reconstruction, the detector 18 is moved to a programmable oblique angle of up to 60° and the carriage 150 is moved to maintain the relative position between the source 12 and detector 18. The detector 18 is moved using discrete movements along the X-axis and the Y-axis in circular path relative to the Z-axis 26, but without rotation about the axis 26. The printed circuit board 16 tracks the movement of the detector in a circular path of smaller radius, also without rotating about Z-axis 26. The off-angle or inclined images are collected and reconstructed by the system controller 62. [0074] The maximum sample size is limited by the size of the X-Y transport mechanism 20 that the printed circuit board 18 sits on. Because the motions are rectangular, there is no wasted space in the corners. An approximate sample size may be 2.25m² and a sample area of 0.2m . Because both the detector 18 and the printed circuit board 16 always maintain the same angular orientation both on the display 72 and within the X-ray inspection system 90, the operator is not disorientated even at high magnifications. By having a fixed distance between the imaging and inspection planes 19, 21, the X-Y transport mechanisms 14, 20 can be mounted on a common framework, which enables high accuracy parallel and linear alignment over longer travel lengths.

[0075] While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Thus, the invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants' general inventive concept.

Claims

What is claimed is:

1. An X-ray inspection system for inspecting a region of interest on a printed circuit board, the system comprising: an X-ray source configured to emit an X-ray beam; an X-ray detector; a support configured to hold the printed circuit board in an inspection plane at a location between the X-ray source and the X-ray detector, the support configured to position the printed circuit board so that the X-ray beam is transmitted through the region of interest in transit to the X-ray detector; and a first X-Y transport mechanism having a first carriage coupled with the detector such that the detector faces the X-ray source, a first stage coupled with the first carriage, and a second stage coupled with the first carriage, and the first stage configured to move the first carriage in an X-direction and the second stage configured to independently move the first carriage in a Y-direction orthogonal to the X-direction so that the detector is movable within an imaging plane.

2. The X-ray inspection system of claim 1 wherein the support is a second carriage, and further comprising: a second X-Y transport mechanism connected with the second carriage, the second X-

Y transport mechanism configured to move the second carriage so that the printed circuit board is moved within an inspection plane that is spaced from and substantially parallel to the imaging plane.

3. The X-ray inspection system of claim 2 wherein the X-ray source is configured to emit the X-ray beam centered about an axis that intersects an origin of an X-Y reference frame within the imaging plane, and further comprising: a motion controller coupled with the first and second X-Y transport mechanisms, the motion controller configured to supply command signals to cause the first X-Y transport mechanism to move the first carriage to position the detector at a plurality of positions within the imaging plane relative to the axis and to supply command signals to cause the second X-

Y transport mechanism to move the second carriage to position the printed circuit board within the inspection plane such that the X-ray beam is transmitted through the region of interest in transit from the X-ray source to the detector at each of the plurality of positions.

4. The X-ray inspection system of claim 3 wherein the first and second X-Y transport mechanisms are configured to respectively move the first carriage and the second carriage such that the plurality of positions for the detector are each spaced within the imaging plane at a radius from the axis to define an off-axis angle.

5. The X-ray inspection system of claim 3 wherein the detector is configured to capture a portion of the X-ray beam passing through the region of interest at each of the plurality of positions to generate an image, and further comprising: a system controller coupled with the detector, the system controller configured to process the portion of the X-ray beam at each of the positions to generate one or more slice images of the region of interest or a three-dimensional model of the region of interest.

6. The X-ray inspection system of claim 1 wherein the X-ray source is configured to emit the X-ray beam centered about an axis that intersects the imaging plane, and further comprising: a motion controller coupled with the first X-Y transport mechanism, the motion controller configured to supply command signals to cause the first X-Y transport mechanism to move the first carriage with movements in the X-direction and with movements in the Y- direction to position the detector at a plurality of positions within the imaging plane.

7. The X-ray inspection system of claim 1 wherein the detector is configured to capture a portion of the X-ray beam passing through the region of interest at each of the plurality of positions, and further comprising: a system controller coupled with the detector, the system controller configured to process the portion of the X-ray beam captured by the detector at each of the positions to generate one or more slice images of the region of interest or a three-dimensional model of the region of interest.

8. A method for inspecting a region of interest on a printed circuit board with an X-ray beam, the method comprising: acquiring a first X-ray image of the region of interest with an X-ray detector located within an imaging plane at a first position having a first X-coordinate and a first Y- coordinate; moving the X-ray detector in an X-direction within the imaging plane from the first X-coordinate to a second X-coordinate of a second position; moving the X-ray detection in a Y-direction within the imaging plane from the first Y-coordinate to a second Y-coordinate of the second position; while the X-ray detector is located at the second position within the imaging plane, acquiring a second X-ray image of the region of interest.

9. The method of claim 8 further comprising: computing at least one slice image or a three dimensional model from at least the first and second images of the region of interest.

10. The method of claim 8 wherein the X-ray detector is supported by a movable carriage, and moving the X-ray detector in the X-direction within the imaging plane comprises: operating a first drive motor to cause a powered movement of the carriage so that the X-ray detector is moved from the first X-coordinate of the first position along an X-axis of an X-Y coordinate frame to the second X-coordinate of the second position.

11. The method of claim 10 wherein the X-ray detector is supported by a movable carriage, and moving the X-ray detector in the Y-direction within the imaging plane comprises: operating a second drive motor to cause a powered movement of the carriage so that the X-ray detector is moved from the first Y-coordinate of the first position along a Y-axis of the X-Y coordinate frame to the second Y-coordinate of the second position.

12. The method of claim 8 wherein the X-ray source is configured to emit the X-ray beam with an axis that intersects an origin of an X-Y coordinate frame in the imaging plane, and the first and second positions are equidistant from the origin so that an oblique angle between a centerline of the region of interest and the axis is equivalent at the first and second positions.

13. The method of claim 8 wherein the X-ray detector is mounted to a carriage, and moving the X-ray detector in the X-direction within the imaging plane comprises: constraining the carriage to move in the X-direction of the imaging plane.

14. The method of claim 13 wherein moving the X-ray detector in the Y-direction within the imaging plane comprises: constraining the carriage to move in the Y-direction of the imaging plane.

15. The method of claim 8 wherein acquiring the second image of the region of interest comprises: acquiring a plurality of sub-images each covering a portion of the region of interest; and stitching the sub-images together to form the second image.

16. The method of claim 8 further comprising: while the X-ray detector is located at the second position within the imaging plane, directing the X-ray beam directed through the region of interest to the X-ray detector.