US20070013772A1

US20070013772A1 - In-circuit test fixture with integral vision inspection system

Info

Publication number: US20070013772A1
Application number: US11/401,240
Authority: US
Inventors: Yew Tham; Wee Yong; Chris Jacobsen
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2005-07-12
Filing date: 2006-04-10
Publication date: 2007-01-18
Also published as: KR100663364B1; US7632748B2; US20070023860A1

Abstract

An in-circuit test fixture performs both electrical tests on a Printed Circuit Assembly (“PCA”) and reads distinguishing features of a feature of interest of the PCA. The in-circuit test fixture physically supports an image sensor array. A light focusing means has a position relative to the distinguishing features and the image sensor such that a focused real image of the distinguishing features is imaged onto the image sensor. The image sensor outputs image information of the distinguishing features. A processor performs image analysis based on the image information of the distinguishing features to determine if defects exist.

Description

FIELD OF THE INVENTION

The invention relates to the field of the automated testing of printed circuit assembly assemblies.

BACKGROUND OF THE INVENTION

A printed circuit assembly (“PCA”) is subject to many different types of defects during the assembly process. Accordingly, various test and inspection techniques are employed to locate these defects. Today, there are three general test methods used to find PCA defects: electrical test, optical (or visual) inspection, and x-ray inspection. Of these, electrical test, and in particular a technique known as “in-circuit test”, is the most mature and most commonly used technique. However, as physical access to nodes on the PCA via bed-of-nails probing decreases, in-circuit test is becoming more difficult.
Some prevalent defects on PCA assemblies are missing, incorrect type or mis-oriented components. Missing components can occur when the components are either never loaded onto the board or they fall off during the assembly process. An incorrect type of component can occur when a component with the wrong electrical value is inadvertently loaded onto the board. Improperly oriented components can occur when a component is loaded onto the board with a reversed polarity. Prior methods for detecting defects at the electrical test stage of the process include in-circuit test, which can include functional tests, and additionally include capacitive measurement test, scan test, automated optical test, and automated x-ray test.
In-circuit test, including unpowered in-circuit analog test (for discrete analog components) and digital in-circuit test for digital components, utilizes an in-circuit tester. The in-circuit tester includes a bed-of-nails test-head having a number of tester interface pins. A fixture having a number of probes is mounted over the bed-of-nails of the tester such that the fixture probes align with and contact the tester interface pins. A PCA under test is mounted in the fixture such that the fixture probes electrically contact various nodes of interest on the PCA under test. Analog in-circuit tests detect manufacturing defects on the PCA for analog parts such as missing components, incorrect components, mis-oriented components, solder opens, and shorts on the PCA under test by probing the appropriate nodes to which the component under test should be attached, and measuring the value, in appropriate units (e.g., resistance, capacitance, etc.), of the component under test. Digital in-circuit tests detect manufacturing defects on the PCA for digital parts such as missing parts, incorrect parts, mis-oriented parts, solder opens, and shorts on the PCA under test by probing the appropriate digital nodes and applying digital values to the input nodes and collecting digital states on the output nodes.
Similarly, in functional test, input and output connections on the edge of the board are made and analog and digital signals are applied that test the large functional blocks of the board.
Capacitive measurement test, such as AGILENT TECHNOLOGIES' TestJet® probe and technique (described in detail in U.S. Pat. No. 5,254,953 to Crook et al.), detects when a device pin is not properly connected to its trace on the PCA. The technique uses an external plate, suspended over the device under test and separated from the lead frame by the plastic or ceramic material of the device housing. The lead frame and external plate form a small capacitor that can be measured by stimulation with an AC source. When the device pin is not electrically connected to the trace, an additional capacitance results in series with the TestJet® capacitor. This additional capacitance exists due to the tiny air gap between the pin and trace. This is a very small capacitance, much smaller than the TestJet® capacitor, so the series combination of the TestJet® and this additional pin capacitor is smaller than either capacitor.
The above techniques each require at least some physical probing of the PCA nodes and are therefore ineffective for PCA assemblies with limited nodal access. To overcome loss of test coverage in non-probed areas of the PCA, alternate test methodologies have emerged. These include automated optical inspection (AOI) and automated x-ray inspection (AXI). Although these methodologies can detect missing devices very effectively, they each suffer from their own limitation and disadvantages. The major disadvantage of these techniques is that they require expensive manufacturing line equipment entirely separate from the in-circuit tester, and therefore also require an entirely new test step to be added to the manufacturing process. The cost of adding such machines to the manufacturing process may be appropriate in some cases, but in other cases the need to do so represents a large disadvantage to these methods.
Additionally, some devices are electrically untestable even with probing. The primary example of this is parallel bypass capacitors. While it is theoretically possible (e.g., on the bench with a single device under test (DUT)) to detect a single missing capacitor, in practice such detection is often not possible. The tolerances and guardbands that must be added to the test limits completely hide small measurement differences due to a single (or even multiple) missing capacitors. As MSI and LSI are replaced by VLSI components, FPGAs and large ASICs, the ratio of bypass capacitors to digital components is increasing, which decreases the number of possible faults that are detectable by even a perfect electrical test.
Coverage for all electrical and imaging test techniques can be quantified. Coverage gaps for each test stage can be identified with a coverage tool. Such coverage tools are described in U.S. Pat. No. 6,792,385 to Parker et al. and owned by AGILENT TECHNOLOGIES, INC., the assignee of the present invention. In this patent, potentially defective properties are enumerated for a board, without regard for how the potentially defective properties might be tested. For each potentially defective property enumerated, a property score is generated. Each property score is indicative of whether a test suite tests for a potentially defective property. Property scores are then combined in accordance with a weighting structure to characterize board test coverage for the test suite. Use of these tools can determine which defects should be tested with electrical methods and which can be tested with imaging methods for optimal defect coverage.
Since most manufacturing lines already use electrical testers (primarily in-circuit testers), it would be beneficial to have the ability to provide extra test coverage during the in-circuit stage of the manufacturing process. Accordingly, it is an object of the invention to detect additional defects on a PCA while the PCA is being electrically tested on an in-circuit tester.

SUMMARY OF THE INVENTION

The present invention provides broad test coverage of defects on a Printed Circuit Assembly (“PCA”) while the PCA is being electrically tested on an in-circuit tester by adding digital imaging capability to the in-circuit tester.
More particularly, the present invention comprises an in-circuit test fixture which performs both electrical tests on a PCA and images distinguishing features of a feature of the PCA. The in-circuit test fixture physically supports an image sensor array. A light focusing means has a position relative to the distinguishing features and the image sensor such that a focused real image of the distinguishing features is imaged onto the image sensor array. The image sensor array outputs image information of the distinguishing features. A processor performs image analysis, which might include pattern recognition, based on the image information of the distinguishing features to determine if defects exist.
The present invention also includes a method for performing both in-circuit electrical tests on a PCA and for imaging distinguishing features of a feature of the PCA using a single test fixture of an in-circuit tester comprising the steps of: loading the PCA into the test fixture; focusing a digital camera on distinguishing features of a feature of interest of the PCA; capturing an image of the distinguishing features of the feature of interest with the camera; analyzing the image; and outputting results of whether or not the feature of interest of the PCA has a defect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of one embodiment of the in-circuit test fixture with an integral vision inspection system of the present invention.
FIG. 2 shows a electrolytic capacitor loaded onto the PC board 105 with the correct orientation.
FIG. 3 shows the electrolytic capacitor loaded onto the PC board with a reversed orientation.
FIG. 4 shows a more detailed diagrammatic view of an embodiment of the in-circuit test fixture with an integral vision inspection system of the present invention.
FIG. 5 is semi-diagrammatic view of a light image sensor for the acquisition of the digital image signals, or image information, using the image sensor array.
FIG. 6 shows a hardware interface between the sensing elements and the ICT tester or external personal computer.
FIG. 7 a is a representation of a stored template.
FIG. 7 b is a representation of an acquired image.
FIG. 8 shows an embodiment using a single digital camera to image multiple features of interest.
FIG. 9 is an operational flowchart of a method for operating the visual inspection system.
FIGS. 10 a and 10 b, illustrate a test access point structure.

DETAILED DESCRIPTION

FIG. 1 shows one embodiment of the in-circuit test fixture with an integral vision inspection system 101 of the present invention. A test fixture 103 of an in-circuit tester 115 is shown positioned around a printed circuit assembly (“PCA”) 105. Only a small portion of the in-circuit tester 115 is shown in FIG. 1.
Mounted on the test fixture 103 are one or more digital cameras 107, 109. Each of the digital cameras has one or more light focusing means 117 which can be a lens or, in the case of pinhole cameras, can be a pinhole. The light focusing means 117 is used to image a feature of interest from the PCA 105. In this example, the features of interest are through- holes 121, 123. Examples of other features of interest are electrolytic capacitors, capacitors used as bypass capacitors, bead probes or, for example, any feature that is excluded from coverage during a previous ICT stage or which requires double-checking from a previous ICT stage. Each of the digital cameras 107, 109 can include a separate image sensor array 107′, which can be a semiconductor image sensor that records light electronically.
Looking at the camera 109, the light focusing means 117 is mounted at a position between the image sensor array 107′ and the feature of interest 121 so that a focused image of the feature of interest 121 is imaged onto the image sensor array 107′. The distances between the feature of interest 121, light focusing means 117, and image sensor array 107′ can be adjustable to aid in focusing the image onto the image sensor array 107′. This feature allows the camera 109 to be focused on features above, below or co-planar with the surface of the PCA 105. In the case of a pinhole camera, focusing can mean assuring that the size of the image projected onto the image sensor array 107′ corresponds to a desired region and area size of the image sensor array 107′. In some embodiments the distance between the image sensor, lens or lenses, and feature of interest, is fixed once a focused image of a feature of interest is obtained. This is especially true when the image sensor array 107′ is used to acquire an image of a single feature of interest.
The image sensor array 107′ and light focusing means 117 can be enclosed in a common digital camera housing to be part of the digital camera 109. Alternatively, they can be separate. The light focusing means 117 and image sensor array 107′ can also be separate components mounted to the test fixture. In other embodiments, the distances are left adjustable so that the camera can be focused on different features of interest during testing.
Also mounted on the test fixture 103 can be a light source 113 for providing general illumination to the PCA 105 to allow for the acquisition of brighter and clearer images by the image sensor array. The light source 113 can be a lamp, for example. In addition or alternatively to using the light source 113, the invention can make use of ring lights 119 to illuminate features of interest. A ring light provides diffused illumination over a small area. With the light focusing means 117 axis through the center opening of the ring light assembly, the ring light illuminates the area directly in front of the camera. Extending from the ring light 119 is a communications line which can connect to communications paths 125 connecting the lights and cameras to a vision engine and controller 111, thereby providing control signals and power.
Rather than mounting the image sensor array 107′ or digital cameras 107, 109 on the test fixture 103, they can be mounted directly on the in-circuit tester 115 or at another location so long has they can acquire an image of a feature of interest on the PCA 105. For example, the image sensor array 107′ can be located quite some distance apart from the test fixture 103 and apart from the in-circuit tester 115 if fiber optics or another light-transmission method is used to bring the image from the PCA 105 to the image sensor array 107′.
Some in-circuit test systems are used in an automated environment, where an automatic conveyor moves the PCA into position on the test fixture. In such a system the cameras 107, 109 and light sources 113, 119 can be mounted in the conveying system rather than on the test fixture 103.
The image sensor array 107′ can be a complementary metal oxide semiconductor (CMOS) or charge coupled device (CCD), for example. The image sensor is made up of many photosites or pixels, each acquiring a portion of the image. Both CCD and CMOS image sensors convert light into electrons. A processor then breaks this electronic information down into digital data. The image sensor can use filtering to create a full color image.
The cameras are controlled by the vision engine and controller 111 which sends signals to the cameras causing them to capture the images, acquires the digital image signals, or image information, from the cameras and which can also control the focusing of the cameras, all through the communications paths 125. The vision engine and controller 111 can also perform image processing and analysis of the acquired digital images or alternatively can send the digital data to another external computer for processing and analysis. The light sources 113, 119 can also be controlled by the vision engine and controller 111 or an external computer. The vision engine and controller can be part of or separate from the in-circuit tester 115.
An example of a digital camera that can be used in the present invention is an A4 Tech Pk35N camera.
Referring to the diagrammatic block diagram of FIG. 5, a more detailed description is provided of a light image sensor 500 for the acquisition of the digital image signals, or image information, using the image sensor array 107′. The elements in the image sensor 500 may be implemented as discrete components or may be components integrated in a semiconductor package. The image sensor 500 includes the image sensor array 107′ for capturing light signals. A representative CMOS image array is integrated into the ADCS-2021 manufactured by AGILENT TECHNOLOGIES. The image sensor array 107′ can include a plurality of conventional sensing elements 510. The sensing elements 510 can be charge coupled device (“CCD”) sensors, or alternatively can be complementary metal oxide semiconductor (CMOS) sensors, which are generally much less expensive than CCD sensors, but may be more susceptible to noise. Other types of sensors may be used in the image sensor array 107′. The size of the image sensor array 107′ can be 640×480 to permit VGA resolution. In response to a detected image signal 535 from one of the features of interest, for example the through- holes 121, 123 on the PCA 105 (see FIG. 1), the image sensor array 107′ will generate an analog electrical signal 537 that has a coded value indicating the color and intensity of the detected light signal 535.
One or more programmable amplifiers 515 are coupled to the image array 107′. The amplifiers 515 provide a suitable gain to improve signal to noise ratio of the electrical signals 537 generated by the image array 107′.
An analog-to-digital converter (AID converter) 520 converts the analog signals 537 from the image array 107′ into digital signals 525, for example 8/10 digital output. An A/D converter function is integrated into the AGILENT ADCS-2021 CMOS image sensor device from AGILENT TEXHNOLOGIES, Palo Alto, Calif.
The sensor 500 further includes a timing controller 530 for providing proper synchronization between the analog electrical signal 537 output of the image sensor array 107′ and the digital signals 525 output from the AID converter 520. Typically, the AID converter 520 has a 10-bit parallel output. The timing controller function is integrated into the AGILENT ADCS-2021 CMOS image sensor device, for example.
The timing controller 530 can be part of the vision engine and controller 111 of FIG. 1.
A conventional clock source (not shown in FIG. 5) provides clock signals 536 to the timing controller 530.
The AGILENT ADCS-2021 CMOS image sensor provides an 12C serial bus interface 540 to facilitate external read and write of the ADCS-2021 internal registers. The bus interface 540 is a summation of an output bus (DRDY, nFRAME_nSYNC, nROW, nIRQ_nCC).
The conventional ICT unit 115 receives the digital signals 525 for analysis. One suitable ICT unit 115 is the 3070 ICT from AGILENT TECHNOLOGIES.
In another embodiment the digital signals 525 are received by an external personal computer for processing and in yet another embodiment the vision engine and controller 111 is also implemented using the personal computer.
In one embodiment, the sensor 500 includes a sampling stage 560 that reads the analog output 537 of the image array 107′. The sampling stage 560 can include a Bayer filter pattern and an alternating pixel pattern of red, green, and blue. The Bayer filter pattern is typically used in the majority of today's consumer digital cameras. The Bayer filter pattern alternates a row of red and green filters with a row of blue and green filters to create an image that the human eye will perceive as a true color. As the image sensor array 107′ in the sensor 500 records the light image 535, each pixel is translated into an electronic signal that can be ported via the analog-to-digital converter (ADC) 520 to the ICT unit 115 or external PC. This electronic signal (converted by ADC 520 to a digital signal 525) is analyzed by the In-Circuit Tester unit 115 or processed by the PC which communicates with the sensor 500. Conventional software tools are typically used by the ICT unit 115 or personal computer to analyze the signal 525 so as to match patterns and/or colors of the features of interest to expected values to detect defects in the PCA. Alternatively, any appropriate image analysis method known to those skilled in the art can be used to analyze the signal 525 representation of features of interest.
The sampling stage 560 advantageously permits programmability for window size, panning, and gain. Thus, to select the window size, or to change panning and/or gain, the sampling stage 560 will sample particular subsets of the sensing elements 510 in the image array 107′.
It is further noted that the sampling stage 560 can be implemented in or integrated in the image array 107′.
FIGS. 2 and 3 show sample images taken of an electrolytic capacitor 203 which might, for example serve as a bypass capacitor, using the present invention 101 of FIG. 1. Electrical ICT tests can not be used to determine the proper polarity of mounted electrolytic capacitors, therefore the present invention is particularly useful in this case. Of course the present invention can be used to analyze many different components and features of interest and is not limited to electrolytic capacitors. In this example the digital camera 107 is mounted over the capacitor 203. The camera 107 is focused on a top surface of the capacitor 203 so that distinguishing features within rectangular target areas or imaging windows 205, 207 and 209 are in focus. In this case one distinguishing feature is a polarity marker 210 along the outer edge of the top surface of the capacitor 203. The polarity marker 210 is within the target area 209 in FIG. 2 and within the target area 205 in FIG. 3. Also, in FIGS. 1 and 2 within the target area 207 are grooves 211 passing through the center of the top surface of the capacitor 203. Image processing and analysis is performed on the markings within these target areas. The image in FIG. 2 shows the capacitor 203 loaded onto the PC board 105 with the correct orientation while FIG. 3 shows the capacitor 203 loaded onto the PC board 105 with a reversed orientation.
The vision inspection system 101 analyzes the images within the imaging windows 205, 207 and 209 by matching the images to a stored template. Again, it should be emphasized that any appropriate image processing method known to those skilled in the art can be used to analyze the capacitor 203 or other features of interest. These techniques include grey level or intensity measurements and comparisons, edge measurements, contrast measurements and comparisons, color component measurements and optical character recognition (OCR) among many others.
FIG. 7 a is a representation of a stored template 701. The stored template is actually a table of digital values stored in the memory of, for example, the vision engine and controller 111. If FIG. 7 b represents an acquired image 703, then a processor of the vision engine and controller 111 can compare the digital values of the acquired image 703 with those of the stored template 701 and in this particular case would recognize that they are substantially different. The processor searches for distinguishing features 703 a, 703 b, 703 c within the image and compares these distinguishing features to distinguishing features 701 a, 701 b, 701 c within the stored template 701. For example, the distinguishing features can be one or a combination of a crack, a barcode, a design, alpha numeric characters, a color, or a hot-spot. Hot-spots are detected when the sensor array 107′ is sensitive to and used to detect infrared radiation.
In the image of FIG. 2, the images within the imaging windows 205, 207 and 209 all match the stored template within a threshold value. However, in the image of FIG. 3, the capacitor 203 is loaded onto the PCA 105 with an incorrect reversed orientation and so the images within the imaging windows 205, 207 and 209 do not match the stored template. Therefore, in the case of FIG. 3, the vision inspection system 101 will indicate the orientation of the capacitor 203 is incorrect. This information is transferred to the ICT unit 115, or external PC, and the machine operator so that appropriate action can be taken.
The visual inspection system 101 can also be used to detect indications of many other kinds of defects. These defects and indications include, but are not limited to: a) missing, mis-oriented or incorrect devices for which no efficient electrical test exists; b) solder defects which might also have no efficient electrical test; c) the presence of properly formed or improperly formed bead probes; or d) incorrect infrared signatures of devices drawing power.
The visual inspection system 101 is also particularly advantageous for detecting defects in PCA's which include LEDs (light emitting diodes). These PCAs can be parts of displays such as dashboards, status panels, etc. The visual inspection system can determine if LEDs are missing or can determine if they are or are not emitting the proper colors.
The general method of one embodiment of the visual inspection system 101 is described by the operational flowchart of FIG. 9. The steps include:
STEP 901: Load the PCA 105 into the test fixture 103 of the in-circuit tester 115.
STEP 903: Focus the camera 107 on distinguishing features of a feature of interest of the PCA 105, such as the top surface of the capacitor 203, so that distinguishing features are in focus within target areas.
STEP 905: Capture an image 701 of the feature of interest having distinguishing features with the camera 107.
STEP 907: Analyze the images by comparing the captured image to a stored template 703.
STEP 909: Determine the similarity of the distinguishing features in the captured image 701 and stored template 703.
STEP 911: Output results of the visual inspection of whether or not the feature of interest of the PCA 105 has a defect.
It will be appreciated that Steps 907 and 909 can be replaced by or combined with any other image processing methods known in the art.
Some PCAs require functional testing. Therefore, rather than using the method of the present invention only with an ICT test fixtures, it can be used with functional test fixtures. Also, the present invention can be used in fixtures used for a combination of ICT and functional test.
In FIG. 6, the following signals embody a hardware interface between the sensing elements 510 (see FIG. 5) and the ICT tester 115 or external PC. Signals D0-D9 are the digital data bits output from a CMOS image sensor. DRDY is a handshaking bit that alerts the ICT tester that data is ready. NRst_nSTBY is a signal input from the ICT tester to the CMOS image sensor to initiate a reset or to place the device in standby mode. nROW (END of Row) and nFRAME_nSYNC (END of FRAME) signal end of row and end of frame respectively to the ICT tester. The clock signal, pin 17, is an I2C, 100 khz, SCLK that acts as a transfer sequencer of the data, SDATA_TxD which is pin 18 in FIG. 6.
Turning now to FIG. 4, an embodiment of the present invention is shown with one particular ICT fixturing configuration. In addition to the illustrated configuration, there are many other ICT fixturing configurations and the present invention will work with any of them. The in-circuit test fixture with integral vision inspection system 101 is shown within a portion of an ICT tester 115 employing several digital cameras 20 a, 20 b, 20 c implemented in accordance with the invention. In addition to the ICT tester 115, also illustrated is the test fixture 103, and the PCA under test 105. Due to the close spacing of the tester interface pins, nodes of the PCA under test, and small size of the components under test, only a small edge portion of the tester is shown for ease of illustration.
The ICT tester 115 includes a plurality of tester interface pins 31 arranged in an array (or “bed-of-nails”) along the top side of the tester 115. The ICT tester 115 includes tester hardware 35 which operates under the control of a controller 36. The controller 36 may be controlled by tester software 37, which may execute within the ICT tester 115 itself, or remotely via a standard communication interface. The controller can be the vision engine and controller 111 of FIG. 1 which can be within the tester 115 or within an external PC. One function of the controller 36 is to configure the hardware 35 to make or not make electrical connections between measurement circuits within the tester and each of the test interface pins 31. To this end, each test interface pin 31 is connectable to or isolated from the tester hardware by a relay 34. Electrical contact between the test resources and a respective test interface pin 31 may be made by closing its corresponding relay 34; conversely, the pin 31 may be isolated from the test hardware by opening its corresponding relay 34.
Mounted on top of the ICT tester 115 and over the bed-of-nails test interface pins 31 is the test fixture 103. The test fixture 103 may directly interface the test interface pins 31 to fixture probes 48, or as shown, may indirectly interface the test interface pins 31 to fixture probes 48 through a test adapter 50. The test fixture 103 is mounted over the tester interface pins 31 of the ICT tester 115 such that the bottom tips of its double-ended spring probes 48 make electrical contact with the top tips of corresponding test interface pins 31 of the ICT tester 115, either directly, or through a test adapter 50 as shown. The top tips of the double-ended spring probes 48 align with and make electrical contact with conductive pads of interest 3 a, 3 b, 3 c, 3 d, 3 e on the bottom side of a PCA under test 105. The test fixture 103, via the fixture probes 48 or the combination of fixture probes 48 and test adapter 50, provides electrical continuity between tester interface pins 31 of the ICT tester 115 and conductive pads of interest 3 a, 3 b, 3 c, 3 d, 3 e of the PCA under test 105, thereby providing the ICT tester 115 with probing access to the PCA 105 and allowing the tester to perform traditional in-circuit tests on the PCA under test 105. Traditional in-circuit tests may include, for example, analog tests that measure characteristics (e.g., resistance, capacitance, current, etc.) of analog components to verify that the component characteristics are within desired tolerance ranges. In-circuit tests may also include functional tests to determine whether components on the PCA operate according to the design specification for those components or the PCA.
The test fixture 103 includes a fixture top 42 and a fixture bottom 44. The fixture bottom 44 includes a plurality of double-ended spring probes 48 that are inserted through precisely aligned holes in the fixture bottom 44. For convenience of illustration and clarity of the invention, only five such double-ended spring probes 48 are shown; however, it will be appreciated by those skilled in the art that a conventional in-circuit tester will typically have thousands of such probes.
The fixture top 42 is configured with a number of digital cameras 20 a, 20 b, one each corresponding to a component under test 6 a, 6 b on the top side 4 of the PCA 105 under test. The components 6 a, 6 b might be capacitors such as the capacitor 207 shown in FIGS. 2 and 3, for example. Each of the cameras 20 a, 20 b is mounted to the fixture top 42 such that it precisely aligns over its corresponding component under test 6 a, 6 b within non-contacting but predetermined distance from the expected location of the top surface of the component under test 6 a, 6 b (if present) when the PCA 105 is properly mounted in the test fixture 103.
In the illustrative embodiment, the PCA 105 includes components under test 6 a, 6 b, 6 c mounted on both sides of the board. Accordingly, accommodation for digital cameras 20 must be made on both sides of the board 105. In this regard, the fixture bottom 44 may also be configured with a number of digital cameras 20 c, one each corresponding to each component under test 6 c on the bottom side 5 of the PCA 105 under test. The cameras 20 c are mounted to the fixture bottom 44 such that the each camera 20 c precisely aligns beneath its corresponding component under test 6 c within non-contacting but predetermined distance from the surface 8 c of the component under test 6 c (if present) when the PCA 105 is properly mounted in the test fixture 103.
The digital cameras 20 can be the same as the cameras 107, 109 described with respect to FIGS. 1 and 5.
In one preferred embodiment, the test fixture 103 includes one digital camera 20 for each capacitor, resistor, or other component or feature of interest on the PCA 105. Accordingly, a large number of digital cameras 20 may be required. For this reason, it may be desirable to multiplex the control signals 38 from the controller/tester hardware of ICT tester 115 going to each digital camera 20 to reduce the number of control lines between the ICT tester 115 and test fixture 103. In the illustrative embodiment, a single 8- bit multiplexer card 46 a , 46 b may be used to address up to 256 different digital cameras 20.
Of course, it will be appreciated that the digital cameras 20 may alternatively be wired in a one-to-one correspondence with the ICT tester 115 or external PC without the use of multiplexers 46 a, 46 b, 46 c, or other control line reduction schemes. In yet another alternative embodiment, shown at 52, the input 21 and output 23 ports of the digital cameras may be connected to nodes on the fixture, which may be probed by tester interface pins 31. In this alternative configuration 52, the digital cameras may be driven by the tester resources 35 through the tester interface pins 31.
The light source s 113, 119 of FIG. 1 can be included in both the fixture top 42 and fixture bottom 44. Multiple light sources can also be used in both the fixture top 42 and fixture bottom 44 to provide more even illumination. The light sources can also be controlled by the multiplexed control signals 38. The light sources may also be probed by tester interface pins 31 and driven by the tester resources 35 through the tester interface pins 31.
It should be understood that there can be various numbers and arrangements of cameras 20 on either the top side 4 or the bottom side 5 of the PCA 105 under test.
In some embodiments, one or more of the cameras 20 can acquire images of multiple components of interest on the PCA 105. A single camera can also acquire an image of the entire top side 4 or bottom side 5 of the PCA 105. In these cases the acquired images can be compared to stored templates which include the multiple components of interest.
FIG. 8 shows an example of one such embodiment wherein a single digital camera, such as one of the cameras 20, comprising a single light focusing means 119, which can be a single lens or a single pinhole, and a single imaging array 107′ is used to image both of the features of interest 121 and 123. Images 210 a, 210 c of the features of interest 121, 123 are focused by the light focusing means 119 onto the image sensor array 107′. The image sensor array 107′ generates an analog electrical signal 537 which is fed to the one or more programmable amplifiers 515 which are coupled to the image sensor array 107′. The amplified signals pass through the sampling stage 560 to the A/D converter 520 which converts the analog signals 537 from the image array 107′ into digital signals 525. The digital signals 525 then pass to the ICT tester 115 or to an external PC.
In other embodiments a digital camera 20 can be placed on a moveable arm so that it can be automatically moved by the tester software to a position over a component of interest. The camera 20 can also be moved across the board in a raster pattern to provide coverage of multiple areas of interest.
The imaging of the present invention can be performed before, during or after the other electrical tests. For example, the acquisition and analysis of the images can be performed at the same time the ICT electrical measurements are performed in order to save time.
The digital cameras 20 of the present invention can be placed in many different kinds of test fixtures, for example in-circuit test fixtures or functional test fixtures.
The digital cameras 20 can work in the visible light spectrum or can work in other spectra such as IR, UV or even x-ray. Combinations of multiple digital cameras having different spectra can also be used. The light sources 113, 119 can also produce light in spectra other than the visible light region. When using high energy radiation, such as x-rays, the one or more cameras 20 and one or more light sources 113, 119 can be on opposite sides of the PCA 105.
A camera 20 can be an IR camera for measuring the IR radiation emanating from the PCA 105 when stimulated by application of a power source and possibly some combination of signal and/or control inputs. This IR radiation is known as a thermal signature and this signature can be compared with that of a defect-free PCA to provide a measure of the proper functioning of the PCA.
By using a camera operating in the IR spectrum, it is also easy to detect defects in hidden solder junctions of area array devices. Another major PCA defect is the on-board power regulation areas of a device under test such as a power FET. For this type of defect, the PCA might power up properly and pass in-circuit testing, but the defective FET might run hot. The extra heating can be detected by the IR camera the defect reported to an operator by the in-circuit tester.
In general the in-circuit test fixture with an integral vision inspection system 101 of the present invention can include sensors covering any region of the electromagnetic spectrum serving as the cameras.
As described above, generally in in-circuit testing, a PCA under test is mounted in a fixture such that the fixture probes electrically contact various nodes of interest on the PCA under test. In the example of FIG. 4, the test fixture 103, via the fixture probes 48 or the combination of fixture probes 48 and test adapter 50, provides electrical continuity between tester interface pins 31 of the ICT tester 115 and conductive pads of interest 3 a, 3 b, 3 c, 3 d, 3 e of the PCA under test 105, thereby providing the ICT tester 115 with probing access to the PCA 105 and allowing the tester to perform traditional in-circuit tests on the PCA under test 105. The conductive pads of interest correspond to nodes of interest. Of course the conductive pads of interest corresponding to nodes of interest can be on either side 4, 5 of the PCA under test 105.
A problem can arise when the conductive pads 3 a, 3 b, 3 c, 3 d, 3 e are not properly formed on the surface of the PCA under test 105. For example, one of the conductive pads might be missing or might only be partially formed. This can lead to a defective electrical contact between the fixture probes and the conductive pads. For example, there might be no electrical contact, partial electrical contact, or intermittent electrical contact. Any of these defects can result in erroneous in-circuit test measurements.
The present invention can prevent such erroneous measurements caused by conductive pad problems. In an embodiment of the present invention, one or more of the digital cameras, for example the digital camera 107 of FIG. 1 or any of the digital cameras digital cameras 20 of FIG. 4 is used to capture an image of one or more of the conductive pads. The general method is the same as that described with respect to FIG. 9. In this embodiment, the feature of interest of the PCA 105 is a particular conductive pad of interest 3 and a distinguishing feature might be the presence, absence, or other characteristics a mark indicating whether or not the conductive pad is present. The distinguishing feature might also be an indication of whether or not portions of the substrate are visible through missing portions of the conductive pad.
US Patent Application Publication US 2005/0061540A1 to Parker et al., owned by the assignee of the present application, and published on Mar. 24, 2005, describes test access point structures used for better contact between the probe and the nodes of interest on a trace of a PCA while taking up less surface area and providing less interference with electrical matching along the circuit traces. Parker et al.'s test access point structures are typically formed from a solder bead on the trace with a length larger than the width of the trace.
FIGS. 10 a and 10 b, illustrate an exemplary embodiment of Parker et al's test access point structures 1008. A printed circuit board 1001 includes a substrate 1005, a ground plane 1004, and at least one dielectric layer 1003 with a trace 1002 printed, deposited, or otherwise attached thereon. A solder mask 1006 with a hole 1007 formed over the trace 1002 at a location where a test access point structure 1008 is positioned is layered over the exposed surfaces of the dielectric layer 1003 and trace layer 1002. A test access point structure 1008 is conductively attached to the trace 1002 within the solder mask hole 1007 at the test access point. The test access point structure 1008 projects above the exposed surrounding surfaces of the solder mask 1006 to form an exposed localized high point on the trace 1002 that may be used as a test target by a fixture probe, such as the probes 3 in FIG. 4, during testing of the printed circuit board 1001. In the preferred embodiment, the test access point structure 8 is a solder bead with a length (in the y-dimension) larger than the width (in the x-dimension) of the trace to provide maximum probe access success.
One problem is that during the manufacturing process one or more of these structures 1008 can be missing, or might be deformed from the ideal shape, therefore preventing good electrical contact with the probe (for example one of the probes 3 of FIG. 4).
The in-circuit test fixture with an integral vision inspection system 101 can detect missing or deformed structures 1008. In an embodiment of the present invention, one or more of the digital cameras, for example the digital camera 107 of FIG. 1 or any of the digital cameras digital cameras 20 of FIG. 4 is used to capture an image of one or more of the structures 1008. The general method is the same as that described with respect to FIG. 9. In this embodiment, the feature of interest of the PCA 105 is the structure 1008 and the distinguishing features of the feature of interest might be a mark indicating whether or not the structure 1008 is present. Because the structure 1008 is a three-dimensional structure rising some distance above the substrate, it is particularly to be able to use the “focusing” feature of the system 101 to focus on the top of the structure 1008.
The image capture and analysis of the structures 1008 can be performed before the ICT electrical tests so that the wasted time and unknown cause of a missing or faulty probe-node electrical connection can be prevented.
In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.

Claims

1. An in-circuit test fixture for performing both electrical tests on a Printed Circuit Assembly (“PCA”) and for imaging distinguishing features of the PCA comprising:

an image sensor array supported by the in-circuit test fixture;

a light focusing means having a position relative to the distinguishing features and the image sensor such that a focused real image of the distinguishing features is imaged onto the image sensor array, the image sensor array outputting image information of the distinguishing features; and

a processor for performing image analysis based on the image information of the distinguishing features to determine if defects exist.

2. The in-circuit test fixture of claim 1, wherein the light focusing means is a lens.

3. The in-circuit test fixture of claim 1, wherein the light focusing means is a pinhole.

4. The in-circuit test fixture of claim 1, wherein the feature is selected from the set consisting of: an electrical component, a trace, a through-hole connector, a PC board, a PC board assembly and a solder joint.

5. The in-circuit test fixture of claim 1, wherein the distinguishing features are selected from the set consisting of: a crack, a barcode, a design, alpha numeric characters, a color and a hot-spot.

6. The in-circuit test fixture of claim 1, wherein the defects are selected from the set consisting of: missing component, incorrect type of component, improperly oriented component, solder open, and solder short.

7. The in-circuit test fixture of claim 1, wherein the image sensor array and light focusing means are part of a digital camera enclosed in a common digital camera housing.

8. The in-circuit test fixture of claim 1, further comprising a light source supported by the in-circuit test fixture for shining light on the distinguishing features to allow acquisition of a brighter image by the image sensor array.

9. The in-circuit test fixture of claim 1, further comprising an a vision engine and controller which sends signals to adjust the focusing of the distinguishing features on the image sensor array.

10. The in-circuit test fixture of claim 1, wherein the processor for performing image analysis performs pattern recognition to identify the distinguishing features within target areas and compares the distinguishing features to stored templates.

11. The in-circuit test fixture of claim 1, wherein the image sensor array acquires images using the IR spectrum of light.

12. The in-circuit test fixture of claim 1, wherein one of the distinguishing features is a solder bead serving as a test access point structure.

13. The in-circuit test fixture of claim 1, wherein one of the distinguishing features is a conductive pad corresponding to a node of interest.

14. The in-circuit test fixture of claim 1, further comprising additional image sensor arrays and wherein each image sensor array acquires an image of a different distinguishing feature.

15. The in-circuit test fixture of claim 1, wherein the image sensor array acquires an image of multiple distinguishing features.

16. The in-circuit test fixture of claim 1, wherein the image sensor array is supported by a top section of the in-circuit test fixture for imaging distinguishing features of a feature on a top face of the PCA and further comprising a second image sensor array supported by a bottom section of the in-circuit test fixture for imaging distinguishing features of a feature on a bottom face of the PCA.

17. The in-circuit test fixture of claim 1, wherein the processor performs image analysis based on the image information of the distinguishing features to determine if defects exist at the same time the ICT electrical tests are performed.

18. The in-circuit test fixture of claim 1, wherein the in-circuit test fixture also performs functional tests on the PCA.

19. A method for performing both in-circuit electrical tests on a Printed Circuit Assembly (“PCA”) and for imaging distinguishing features of a feature of interest of the PCA using a single test fixture of an in-circuit tester comprising the steps of:

loading the PCA into the test fixture;

focusing a digital camera on the distinguishing features of the feature of interest of the PCA;

capturing an image of the distinguishing features of the feature of interest with the camera;

analyzing the image; and

outputting results of whether or not the feature of interest of the PCA shows a defect.

20. The method of claim 19, wherein the step of analyzing the image further comprises determining the similarity of the distinguishing features in the captured image and a stored template.

21. The method of claim 19, wherein the step of analyzing the image further comprises performing pattern recognition on the captured image to recognize the distinguishing features.

22. The method of claim 19, wherein the step of focusing further comprises adjusting the focus to focus on a feature of interest above or below the surface of the PCA.

23. The method of claim 19, wherein the step of focusing further comprises the step of focusing on a top surface of a solder bead serving as a test access point structure and wherein the analyzing the image is performed prior to performing ICT electrical measurements on the PCA.

24. The method of claim 19, further comprising the step of performing ICT electrical measurements on the PCA at the same time the step of analyzing the image is performed.

25. The method of claim 19, further comprising the step of performing functional tests on the PCA at the same time the step of analyzing the image is performed.