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

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 thePC board105 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. 7ais a representation of a stored template.
• FIG. 7bis 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. 10aand10b, illustrate a test access point structure.
DETAILED DESCRIPTION
• FIG. 1 shows one embodiment of the in-circuit test fixture with an integralvision inspection system101 of the present invention. Atest fixture103 of an in-circuit tester115 is shown positioned around a printed circuit assembly (“PCA”)105. Only a small portion of the in-circuit tester115 is shown inFIG. 1.
• Mounted on thetest fixture103 are one or moredigital cameras107,109. Each of the digital cameras has one or more light focusing means117 which can be a lens or, in the case of pinhole cameras, can be a pinhole. Thelight focusing means117 is used to image a feature of interest from thePCA105. In this example, the features of interest are through-holes121,123. 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 thedigital cameras107,109 can include a separateimage sensor array107′, which can be a semiconductor image sensor that records light electronically.
• Looking at thecamera109, thelight focusing means117 is mounted at a position between theimage sensor array107′ and the feature ofinterest121 so that a focused image of the feature ofinterest121 is imaged onto theimage sensor array107′. The distances between the feature ofinterest121,light focusing means117, andimage sensor array107′ can be adjustable to aid in focusing the image onto theimage sensor array107′. This feature allows thecamera109 to be focused on features above, below or co-planar with the surface of thePCA105. In the case of a pinhole camera, focusing can mean assuring that the size of the image projected onto theimage sensor array107′ corresponds to a desired region and area size of theimage sensor array107′. 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 theimage sensor array107′ is used to acquire an image of a single feature of interest.
• Theimage sensor array107′ and light focusing means117 can be enclosed in a common digital camera housing to be part of thedigital camera109. Alternatively, they can be separate. Thelight focusing means117 andimage sensor array107′ 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 thetest fixture103 can be alight source113 for providing general illumination to thePCA105 to allow for the acquisition of brighter and clearer images by the image sensor array. Thelight source113 can be a lamp, for example. In addition or alternatively to using thelight source113, the invention can make use ofring lights119 to illuminate features of interest. A ring light provides diffused illumination over a small area. With the light focusing means117 axis through the center opening of the ring light assembly, the ring light illuminates the area directly in front of the camera. Extending from thering light119 is a communications line which can connect tocommunications paths125 connecting the lights and cameras to a vision engine andcontroller111, thereby providing control signals and power.
• Rather than mounting theimage sensor array107′ ordigital cameras107,109 on thetest fixture103, they can be mounted directly on the in-circuit tester115 or at another location so long has they can acquire an image of a feature of interest on thePCA105. For example, theimage sensor array107′ can be located quite some distance apart from thetest fixture103 and apart from the in-circuit tester115 if fiber optics or another light-transmission method is used to bring the image from thePCA105 to theimage sensor array107′.
• 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 thecameras107,109 andlight sources113,119 can be mounted in the conveying system rather than on thetest fixture103.
• Theimage sensor array107′ 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 andcontroller111 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 thecommunications paths125. The vision engine andcontroller111 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. Thelight sources113,119 can also be controlled by the vision engine andcontroller111 or an external computer. The vision engine and controller can be part of or separate from the in-circuit tester115.
• 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 ofFIG. 5, a more detailed description is provided of alight image sensor500 for the acquisition of the digital image signals, or image information, using theimage sensor array107′. The elements in theimage sensor500 may be implemented as discrete components or may be components integrated in a semiconductor package. Theimage sensor500 includes theimage sensor array107′ for capturing light signals. A representative CMOS image array is integrated into the ADCS-2021 manufactured by AGILENT TECHNOLOGIES. Theimage sensor array107′ can include a plurality ofconventional sensing elements510. Thesensing elements510 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 theimage sensor array107′. The size of theimage sensor array107′ can be 640×480 to permit VGA resolution. In response to a detectedimage signal535 from one of the features of interest, for example the through-holes121,123 on the PCA105 (seeFIG. 1), theimage sensor array107′ will generate an analogelectrical signal537 that has a coded value indicating the color and intensity of the detectedlight signal535.
• One or moreprogrammable amplifiers515 are coupled to theimage array107′. Theamplifiers515 provide a suitable gain to improve signal to noise ratio of theelectrical signals537 generated by theimage array107′.
• An analog-to-digital converter (AID converter)520 converts the analog signals537 from theimage array107′ intodigital signals525, 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.
• Thesensor500 further includes atiming controller530 for providing proper synchronization between the analogelectrical signal537 output of theimage sensor array107′ and thedigital signals525 output from theAID converter520. Typically, theAID converter520 has a10-bit parallel output. The timing controller function is integrated into the AGILENT ADCS-2021 CMOS image sensor device, for example.
• Thetiming controller530 can be part of the vision engine andcontroller111 ofFIG. 1.
• A conventional clock source (not shown inFIG. 5) provides clock signals536 to thetiming controller530.
• The AGILENT ADCS-2021 CMOS image sensor provides an 12Cserial bus interface540 to facilitate external read and write of the ADCS-2021 internal registers. Thebus interface540 is a summation of an output bus (DRDY, nFRAME_nSYNC, nROW, nIRQ_nCC).
• Theconventional ICT unit115 receives thedigital signals525 for analysis. Onesuitable ICT unit115 is the3070 ICT from AGILENT TECHNOLOGIES.
• In another embodiment thedigital signals525 are received by an external personal computer for processing and in yet another embodiment the vision engine andcontroller111 is also implemented using the personal computer.
• In one embodiment, thesensor500 includes asampling stage560 that reads theanalog output537 of theimage array107′. Thesampling stage560 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 theimage sensor array107′ in thesensor500 records thelight image535, each pixel is translated into an electronic signal that can be ported via the analog-to-digital converter (ADC)520 to theICT unit115 or external PC. This electronic signal (converted byADC520 to a digital signal525) is analyzed by the In-Circuit Tester unit115 or processed by the PC which communicates with thesensor500. Conventional software tools are typically used by theICT unit115 or personal computer to analyze thesignal525 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 thesignal525 representation of features of interest.
• Thesampling stage560 advantageously permits programmability for window size, panning, and gain. Thus, to select the window size, or to change panning and/or gain, thesampling stage560 will sample particular subsets of thesensing elements510 in theimage array107′.
• It is further noted that thesampling stage560 can be implemented in or integrated in theimage array107′.
• FIGS. 2 and 3 show sample images taken of anelectrolytic capacitor203 which might, for example serve as a bypass capacitor, using thepresent invention101 ofFIG. 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 thedigital camera107 is mounted over thecapacitor203. Thecamera107 is focused on a top surface of thecapacitor203 so that distinguishing features within rectangular target areas orimaging windows205,207 and209 are in focus. In this case one distinguishing feature is apolarity marker210 along the outer edge of the top surface of thecapacitor203. Thepolarity marker210 is within thetarget area209 inFIG. 2 and within thetarget area205 inFIG. 3. Also, inFIGS. 1 and 2 within thetarget area207 aregrooves211 passing through the center of the top surface of thecapacitor203. Image processing and analysis is performed on the markings within these target areas. The image inFIG. 2 shows thecapacitor203 loaded onto thePC board105 with the correct orientation whileFIG. 3 shows thecapacitor203 loaded onto thePC board105 with a reversed orientation.
• Thevision inspection system101 analyzes the images within theimaging windows205,207 and209 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 thecapacitor203 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. 7ais a representation of a storedtemplate701. The stored template is actually a table of digital values stored in the memory of, for example, the vision engine andcontroller111. IfFIG. 7brepresents an acquiredimage703, then a processor of the vision engine andcontroller111 can compare the digital values of the acquiredimage703 with those of the storedtemplate701 and in this particular case would recognize that they are substantially different. The processor searches for distinguishingfeatures703a,703b,703cwithin the image and compares these distinguishing features to distinguishingfeatures701a,701b,701cwithin the storedtemplate701. 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 thesensor array107′ is sensitive to and used to detect infrared radiation.
• In the image ofFIG. 2, the images within theimaging windows205,207 and209 all match the stored template within a threshold value. However, in the image ofFIG. 3, thecapacitor203 is loaded onto thePCA105 with an incorrect reversed orientation and so the images within theimaging windows205,207 and209 do not match the stored template. Therefore, in the case ofFIG. 3, thevision inspection system101 will indicate the orientation of thecapacitor203 is incorrect. This information is transferred to theICT unit115, or external PC, and the machine operator so that appropriate action can be taken.
• Thevisual inspection system101 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.
• Thevisual inspection system101 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 thevisual inspection system101 is described by the operational flowchart ofFIG. 9. The steps include:
• STEP901: Load thePCA105 into thetest fixture103 of the in-circuit tester115.
• STEP903: Focus thecamera107 on distinguishing features of a feature of interest of thePCA105, such as the top surface of thecapacitor203, so that distinguishing features are in focus within target areas.
• STEP905: Capture animage701 of the feature of interest having distinguishing features with thecamera107.
• STEP907: Analyze the images by comparing the captured image to a storedtemplate703.
• STEP909: Determine the similarity of the distinguishing features in the capturedimage701 and storedtemplate703.
• STEP911: Output results of the visual inspection of whether or not the feature of interest of thePCA105 has a defect.
• It will be appreciated thatSteps907 and909 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.
• InFIG. 6, the following signals embody a hardware interface between the sensing elements510 (seeFIG. 5) and theICT tester115 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,pin17, is an I2C, 100 khz, SCLK that acts as a transfer sequencer of the data, SDATA_TxD which ispin18 inFIG. 6.
• Turning now toFIG. 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 integralvision inspection system101 is shown within a portion of anICT tester115 employing severaldigital cameras20a,20b,20cimplemented in accordance with the invention. In addition to theICT tester115, also illustrated is thetest fixture103, and the PCA undertest105. 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.
• TheICT tester115 includes a plurality of tester interface pins31 arranged in an array (or “bed-of-nails”) along the top side of thetester115. TheICT tester115 includestester hardware35 which operates under the control of acontroller36. Thecontroller36 may be controlled bytester software37, which may execute within theICT tester115 itself, or remotely via a standard communication interface. The controller can be the vision engine andcontroller111 ofFIG. 1 which can be within thetester115 or within an external PC. One function of thecontroller36 is to configure thehardware35 to make or not make electrical connections between measurement circuits within the tester and each of the test interface pins31. To this end, eachtest interface pin31 is connectable to or isolated from the tester hardware by arelay34. Electrical contact between the test resources and a respectivetest interface pin31 may be made by closing itscorresponding relay34; conversely, thepin31 may be isolated from the test hardware by opening itscorresponding relay34.
• Mounted on top of theICT tester115 and over the bed-of-nails test interface pins31 is thetest fixture103. Thetest fixture103 may directly interface the test interface pins31 to fixture probes48, or as shown, may indirectly interface the test interface pins31 to fixture probes48 through atest adapter50. Thetest fixture103 is mounted over the tester interface pins31 of theICT tester115 such that the bottom tips of its double-ended spring probes48 make electrical contact with the top tips of corresponding test interface pins31 of theICT tester115, either directly, or through atest adapter50 as shown. The top tips of the double-ended spring probes48 align with and make electrical contact with conductive pads ofinterest3a,3b,3c,3d,3eon the bottom side of a PCA undertest105. Thetest fixture103, via the fixture probes48 or the combination of fixture probes48 andtest adapter50, provides electrical continuity between tester interface pins31 of theICT tester115 and conductive pads ofinterest3a,3b,3c,3d,3eof the PCA undertest105, thereby providing theICT tester115 with probing access to thePCA105 and allowing the tester to perform traditional in-circuit tests on the PCA undertest105. 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.
• Thetest fixture103 includes afixture top42 and afixture bottom44. Thefixture bottom44 includes a plurality of double-ended spring probes48 that are inserted through precisely aligned holes in thefixture bottom44. For convenience of illustration and clarity of the invention, only five such double-ended spring probes48 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.
• Thefixture top42 is configured with a number ofdigital cameras20a,20b, one each corresponding to a component undertest6a,6bon thetop side4 of thePCA105 under test. Thecomponents6a,6bmight be capacitors such as thecapacitor207 shown inFIGS. 2 and 3, for example. Each of thecameras20a,20bis mounted to thefixture top42 such that it precisely aligns over its corresponding component undertest6a,6bwithin non-contacting but predetermined distance from the expected location of the top surface of the component undertest6a,6b(if present) when thePCA105 is properly mounted in thetest fixture103.
• In the illustrative embodiment, thePCA105 includes components undertest6a,6b,6cmounted on both sides of the board. Accordingly, accommodation for digital cameras20 must be made on both sides of theboard105. In this regard, the fixture bottom44 may also be configured with a number ofdigital cameras20c, one each corresponding to each component undertest6con thebottom side5 of thePCA105 under test. Thecameras20care mounted to the fixture bottom44 such that the eachcamera20cprecisely aligns beneath its corresponding component undertest6cwithin non-contacting but predetermined distance from the surface8cof the component undertest6c(if present) when thePCA105 is properly mounted in thetest fixture103.
• The digital cameras20 can be the same as thecameras107,109 described with respect toFIGS. 1 and 5.
• In one preferred embodiment, thetest fixture103 includes one digital camera20 for each capacitor, resistor, or other component or feature of interest on thePCA105. Accordingly, a large number of digital cameras20 may be required. For this reason, it may be desirable to multiplex the control signals38 from the controller/tester hardware ofICT tester115 going to each digital camera20 to reduce the number of control lines between theICT tester115 andtest fixture103. In the illustrative embodiment, a single 8-bit multiplexer card46a,46bmay be used to address up to 256 different digital cameras20.
• Of course, it will be appreciated that the digital cameras20 may alternatively be wired in a one-to-one correspondence with theICT tester115 or external PC without the use ofmultiplexers46a,46b,46c, or other control line reduction schemes. In yet another alternative embodiment, shown at52, theinput21 andoutput23 ports of the digital cameras may be connected to nodes on the fixture, which may be probed by tester interface pins31. In thisalternative configuration52, the digital cameras may be driven by thetester resources35 through the tester interface pins31.
• The light source s113,119 ofFIG. 1 can be included in both thefixture top42 andfixture bottom44. Multiple light sources can also be used in both thefixture top42 and fixture bottom44 to provide more even illumination. The light sources can also be controlled by the multiplexed control signals38. The light sources may also be probed by tester interface pins31 and driven by thetester resources35 through the tester interface pins31.
• It should be understood that there can be various numbers and arrangements of cameras20 on either thetop side4 or thebottom side5 of thePCA105 under test.
• In some embodiments, one or more of the cameras20 can acquire images of multiple components of interest on thePCA105. A single camera can also acquire an image of the entiretop side4 orbottom side5 of thePCA105. 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 cameras20, comprising a singlelight focusing means119, which can be a single lens or a single pinhole, and asingle imaging array107′ is used to image both of the features ofinterest121 and123.Images210a,210cof the features ofinterest121,123 are focused by the light focusing means119 onto theimage sensor array107′. Theimage sensor array107′ generates an analogelectrical signal537 which is fed to the one or moreprogrammable amplifiers515 which are coupled to theimage sensor array107′. The amplified signals pass through thesampling stage560 to the A/D converter520 which converts the analog signals537 from theimage array107′ intodigital signals525. Thedigital signals525 then pass to theICT tester115 or to an external PC.
• In other embodiments a digital camera20 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 camera20 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 cameras20 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 cameras20 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. Thelight sources113,119 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 cameras20 and one or morelight sources113,119 can be on opposite sides of thePCA105.
• A camera20 can be an IR camera for measuring the IR radiation emanating from thePCA105 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 integralvision inspection system101 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 ofFIG. 4, thetest fixture103, via the fixture probes48 or the combination of fixture probes48 andtest adapter50, provides electrical continuity between tester interface pins31 of theICT tester115 and conductive pads ofinterest3a,3b,3c,3d,3eof the PCA undertest105, thereby providing theICT tester115 with probing access to thePCA105 and allowing the tester to perform traditional in-circuit tests on the PCA undertest105. 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 eitherside4,5 of the PCA undertest105.
• A problem can arise when theconductive pads3a,3b,3c,3d,3eare not properly formed on the surface of the PCA undertest105. 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 thedigital camera107 ofFIG. 1 or any of the digital cameras digital cameras20 ofFIG. 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 toFIG. 9. In this embodiment, the feature of interest of thePCA105 is a particular conductive pad of interest3 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. 10aand10b, illustrate an exemplary embodiment of Parker et al's testaccess point structures1008. A printedcircuit board1001 includes asubstrate1005, aground plane1004, and at least onedielectric layer1003 with atrace1002 printed, deposited, or otherwise attached thereon. Asolder mask1006 with ahole1007 formed over thetrace1002 at a location where a testaccess point structure1008 is positioned is layered over the exposed surfaces of thedielectric layer1003 andtrace layer1002. A testaccess point structure1008 is conductively attached to thetrace1002 within thesolder mask hole1007 at the test access point. The testaccess point structure1008 projects above the exposed surrounding surfaces of thesolder mask1006 to form an exposed localized high point on thetrace1002 that may be used as a test target by a fixture probe, such as the probes3 inFIG. 4, during testing of the printedcircuit board1001. In the preferred embodiment, the test access point structure8 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 thesestructures1008 can be missing, or might be deformed from the ideal shape, therefore preventing good electrical contact with the probe (for example one of the probes3 ofFIG. 4).
• The in-circuit test fixture with an integralvision inspection system101 can detect missing ordeformed structures1008. In an embodiment of the present invention, one or more of the digital cameras, for example thedigital camera107 ofFIG. 1 or any of the digital cameras digital cameras20 ofFIG. 4 is used to capture an image of one or more of thestructures1008. The general method is the same as that described with respect toFIG. 9. In this embodiment, the feature of interest of thePCA105 is thestructure1008 and the distinguishing features of the feature of interest might be a mark indicating whether or not thestructure1008 is present. Because thestructure1008 is a three-dimensional structure rising some distance above the substrate, it is particularly to be able to use the “focusing” feature of thesystem101 to focus on the top of thestructure1008.
• The image capture and analysis of thestructures1008 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 (25)

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 ofclaim 1, wherein the light focusing means is a lens.

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

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

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

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

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

16. The in-circuit test fixture ofclaim 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 ofclaim 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 ofclaim 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 ofclaim 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 ofclaim 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 ofclaim 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 ofclaim 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 ofclaim 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 ofclaim 19, further comprising the step of performing functional tests on the PCA at the same time the step of analyzing the image is performed.

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