Apparatus and method using photoelectric effect for testing electrical traces

This application is a continuation-in-part of copending application Ser. No. 09/231,410 filed Jan. 14, 1999 titled APPARATUS AND METHOD USING PHOTOELECTRIC EFFECT FOR TESTING ELECTRICAL TRACES.

This invention relates to testing electrical traces, such as on a substrate, such as a circuit board, for characteristics such as opens, shorts, neck-downs, or improper etching; and more specifically relates to a test method using photoelectric effect.

In the manufacture of electronic components, the packaging density has increased considerably, resulting in extremely narrow and thin traces disposed on both sides of the substrates with numerous connections from one side to the other. Fabrication of such fine traces is difficult such that defects are more common. Therefore, testing the quality of fine traces on both sides and connections from one side to the other of a substrate has become increasingly more important.

Most conventional methods of trace testing involve physically contacting the trace with one or two test probes. The physical placement accuracy and the physical size of test probes limit their use in testing in mass. Many current traces are so small or densely packed that they can only be connected with a physical probe individually with a very time consuming and uneconomical process. Even if contact probes are available and can be used in a production mode, the act of contacting, more specifically the force required to make a good electrical contact is high enough to inflict permanent damage to thin traces, rendering them useless.

Therefore there has been a need for a test apparatus and method for opens, shorts, neck-downs, or improper etching in which the trace is not physically contacted and which is not unduly time consuming.

This invention is a tester for electrical traces such as on a circuit board, and the preferred embodiment generally comprises a laser producing an ultraviolet beam, a vacuum chamber, an electrode circuit including electrodes and corresponding electronics including ammeters for measuring photoelectron flow between traces and electrodes, a controller, laser beam optics, an image acquisition system, and a pair of broadband UV lights.

The board containing traces under test is disposed in the vacuum chamber at lowered pressure with grid electrodes lying over the trace area on each side of the board. Electrode electronics selectively maintain a known potential on each electrode. The exact location of traces are determined by an image acquisition system.

The traces are initialized to a known voltage such as by: (1) the induced high voltage method by first applying a relatively high positive voltage to one electrode and a high negative voltage to the other electrode, then reversing the polarity of the voltages; (2) simultaneous photoelectron effect illumination of electrodes by setting both electrodes to a known positive voltage and irradiating electrodes, board and traces by the broadband UV electromagnetic sources; or (3) laser method by setting electrodes to a known voltage and dithering the laser beam so as to strike a portion of electrode and a trace. Shining the laser beam on a location so as to liberate photoelectrons is referred to as “interrogating”.

Continuity between two points on a trace is determined by interrogating the first location until it is charged to a known voltage and then by interrogating the second location.

Shorts between traces can be determined by interrogating a first trace until it is charged to a known voltage and then interrogating the second trace.

Other features and many attendant advantages of the invention will become more apparent upon a reading of the following detailed description together with the drawings wherein like reference numerals refer to like parts throughout.

FIG. 1 is a schematic representation of a preferred embodiment of the trace tester of the invention.

FIG. 2 is an enlarged view of the vacuum chamber and its contents of FIG. 1 .

FIG. 3 is an enlarged representation of the image acquisition elements of FIG. 1 .

FIG. 4 is an enlarged representation of electromagnetic source means and the optical beam path elements of FIG. 1 .

FIG. 5 is an enlarged vertical cross-sectional representation of a representative substrate and nearby elements.

As best seen in FIGS. 1 and 5, a substrate 80 , such as printed circuit board 80 P having an upper side 81 U and a lower side 81 L, contains electrical traces 85 to be tested. A given trace 85 may include upper portions 85 U on board upper side 81 U and lower portions 85 L on board lower side 81 L. Each trace 85 includes pads 87 for attachment of electrical components and, typically, at the end of each trace 85 is an end pad 87 E, such as upper end pad 87 EU and lower end pad 87 EL. Typically, a trace 85 passes from upper side 81 U to lower side 81 L through plated through holes 89 .

Tester 10 , as shown, tests traces 85 on both upper and lower sides 81 U, 81 L of board 80 P. Obvious modifications can be made to tester 10 to test traces 85 on only one side of board 80 P.

Trace tester 10 generally comprises an electromagnetic source means 20 , vacuum chamber means 12 , an electrode circuit 50 including electrodes 51 and corresponding electronics 55 , controller 70 and a plurality of broadband electromagnetic source means 18 , such as pair 18 U, 18 L.

Referring also to FIG. 2, vacuum chamber means 12 may be of conventional, well-known design for creating and controlling the vacuum pressure in a test volume 13 at a specified value. Vacuum chamber means 12 includes: a chamber 12 C enclosing a test volume 13 ; an evacuation means, such as vacuum pump 14 for lowering the pressure in test volume 13 ; doors, not shown, for access; and windows 15 , such as upper window 15 U and lower window 15 L. The use of vacuum chamber 12 aids in the efficiency of tester 10 .

Board 80 P is placed in vacuum chamber 12 and supported by any suitable means that does not interfere with the working of tester 10 . Vacuum pump 14 evacuates volume 13 to a rough vacuum.

Image Acquisition. FIG. 3 is an enlarged representation of the image acquisition elements of FIG. 1 by which tester 10 determines the location of test trace 85 T in chamber 12 C relative to the optical path components. Preferably, the initial accuracy of either manual or machine placement of board 80 P within chamber 12 is approximately +/−0.040 inch. The precise location of placement of board 80 P is determined through the use of image acquisition. Controller 70 includes a computer and contains a mapping of traces 85 and other indicia on board 80 P. Controller 70 via path 71 U directs upper servo mechanism 30 U which in turn will rotate upper galvo beam mirror 45 U to a predetermined position that will capture the image of a fiducial, a recognizable feature, on upper side 81 U of board 80 P. This image will be reflected through scan lens 46 U, upper galvo beam mirror 45 U, through upper high reflector 41 U, and through upper telescope vision system 47 U to upper CCD acquisition system 48 U which is connected by line 72 U to controller 70 . The actual position of the fiducial will be compared to the position data supplied by controller 70 . This process will be repeated for a second and perhaps a third known fiducial. The resulting data will provide an X,Y offset and/or scaling factor that will be applied to position data for each of the upper end pads 87 EU of test traces 85 T on board 80 P.

In a like manner, controller 70 via line 71 L will direct lower servo mechanism 30 L, which in turn will rotate lower galvo beam mirror 45 L to a predetermined position that will capture the image of a fiducial on the lower side 81 L of board 80 P. This image will be reflected through scan lens 46 L, lower beam galvo beam mirror 45 L, through first lower high reflector 41 LS, and through the lower telescope vision system 47 L to the lower CCD acquisition system 48 L which is connected by line 72 L to controller 70 . The actual position of the fiducial will be compared to the position data supplied by controller 70 . This process will be repeated for a second and perhaps a third known fiducial. The resulting data will provide an X,Y offset and/or a scaling factor that will be applied to position data for each of the lower end pads 87 EL of test traces 85 T on board 80 P.

Performing this task on both sides of board 80 P not only corrects for inaccuracy of positioning of board 80 P, but, in addition, accounts for any inaccuracy of registration or alignment of the pattern of the traces on the upper side 81 U of board 80 P versus the pattern of the traces on the lower side 81 L of board 80 P, as well as material stretch and shrinkage.

In this manner, controller 70 knows the physical position of traces 85 on board 80 P relative to optical path components.

Optical Beam Path. FIG. 4 is a enlarged representation of the electromagnetic source means 20 and optical beam path elements of FIG. 1 . Electromagnetic source means 20 generally comprises a source of electromagnetic radiation 21 , such as ultraviolet laser 22 , for producing a beam 24 of electromagnetic radiation such as of ultraviolet light, and optical path components 40 , including a beam splitter 42 for splitting beam 24 into upper and lower beams 24 U, 24 L, for directing beams 24 U, 24 L onto chamber 12 C. Although a single source 21 is shown for producing beams 24 U, 24 L, multiple sources could be used. Laser 22 may be continuous wave, pulsed, q-switched or mode-locked. Q-switched is preferred.

Upper beam 24 U passes through upper shutter 43 U and upper beam conditioning optics 44 U, off upper high reflector 41 U, to upper galvo beam mirror 45 U, then through upper scan lens 46 U, window 15 U, and the upper electrode 51 U, to a specific target, such as on test trace 85 T, such as an end pad 87 E.

Scan lenses 46 U and 46 L have been designed for a flat focused field while maintaining both the visual and the UV wave lengths in the same scan area.

Shutters 43 U and 43 L are used to control the time of testing by allowing beams 24 U and 24 L to illuminate the end pads 87 EU and 87 EL of trace 85 T for specific time periods. The shutters 43 U and 43 L may be electromechanical, piezo electrical, acousto-optic or electro-optic.

Lower beam 24 L reflects off first lower high reflector 41 LF, passes through lower shutter 43 L and lower beam conditioning optics 44 L, off second lower high reflector 41 LS, to lower galvo beam mirror 45 L, then through lower scan lens 46 L and window 15 L. Galvo beam mirrors 45 U, 45 L are directed by upper and lower servo mechanisms 30 U, 30 L to deflect beams 24 U, 24 L to the desired target. Servo mechanisms 30 U, 30 L are connected to controller 70 by connectors 71 U, 71 L. Controller 70 has a mapping of all traces 85 on board 80 P and is programmed to sequentially control and move beams 24 U, 24 L at selected times to desired test targets.

Preferably, beams 24 U, 24 L entering windows 15 U, 15 L are of small cross-section, such as focused to approximately 0.003 inch or less in diameter over the entire area of board 80 P, and are capable of producing the photoelectric effect on targets, thereby liberating electrons, sometimes referred to as photoelectrons, therefrom.

Electrode circuit 50 includes electrodes 51 , such as upper electrode 51 U and lower electrode 51 L, and associated electronics 55 U, 55 L. In the preferred embodiment, electrodes 51 U, 51 L are grids 52 U, 52 L of 0.001 inch or smaller wire 53 interlaced on a 0.015 inch or tighter pitch. Grids 52 U, 52 L are situated above and below board 80 P at distance of approximately 5 mm and lie over and under traces 85 . Testing can be accomplished with both larger and shorter separation distances.

Alternate types of electrodes 51 include etched plates or film sheets. A plate electrode comprises a transparent plate, such as of glass, having a conductive grid pattern, such as of chromium, deposited and etched on the side facing board 80 P. A film sheet electrode comprises a thin, transparent conductive film sheet, having the surface that is facing board 80 P coated (e.g. sputtered) with a controlled amount of conductive metal in the form of a thin conductive, such as metallic, coating that is sufficiently transparent to the beam.

Electrode electronics 55 U, 55 L provide voltages to electrodes 51 U, 51 L on lines 56 U, 56 L respectively as well as analyze currents through electrodes 51 , such as with meters 59 , such as ammeters 59 U, 59 L. Results amassed from the grid electronics are forwarded to the controller 70 on lines 57 U, 57 L respectively for display and/or recording.

In some applications one electrode 51 , such as upper electrode 51 U acts as a collector, i.e. a collector of electrons, if the near trace 85 is of lower potential, and one electrode 51 , such as lower electrode 51 L acts as an emitter, i.e. an emitter of electrons, if the near trace 85 is of higher potential.

Initialization of circuit board prior to test. Typically, board 80 P and traces 85 are initially at an unknown voltage. The initial board and trace voltages may be due to static electricity build-up during handling, movement, or vacuum pump-down. For test purposes, it is desirable to provide board 80 P and traces 85 with a known voltage initial condition so that reliable, predictable and repeatable tests can be performed. It has been found that well-known means in the art do not work reliably. For example, the common practice of passing the board 80 P through grounding rollers to give board 80 P and traces 85 a zero or neutral voltage does not work reliably because subsequent operations, performed on board 80 P before test, such as movement into chamber 12 C and vacuum pump-down, may induce static electricity to be formed on board 80 P and/or traces 85 . Therefore the following methods are used to place all traces 85 at a known initial voltage.

One method of voltage initialization of board 80 P and traces 85 is by induced high voltage. In this method, all traces 85 are given a known voltage by applying a relatively high positive voltage in the range of 500 to 2,000 volts to one electrode 51 making it a collector 51 C while at the same time applying high negative voltage in the range 500 to 2,000 volts to the other electrode 51 making it an emitter 51 E such that the resulting high potential field in the presence of natural background electrons initiate an electron flow to be emitted from the emitter electrode 51 E and travel towards the more positive collector electrode 51 C. Since board 80 P is interposed in the space between electrodes 51 , many of the electrons strike board 80 P, and many electrons are then emitted by board 80 P to travel towards collector 51 C. If the voltages on collector 51 C and emitter 51 E are switched off rapidly, some electrons will remain on board 80 P and will collect on traces 85 , thereby charging traces 85 to a known voltage, which is proportional to the applied voltages. Reversing the polarity of the voltages applied to electrodes 51 ensures that all traces 85 on both sides of board 80 P will be charged to the same voltage. Circuitry to accomplish such switching is straightforward and well known in the art. The voltage at which photoelectron currents, as measured by the electrode electronics 55 , drops to zero is the residual voltage remaining on traces 85 .

Another method of voltage initialization of board 80 P and traces 85 is by simultaneous photoelectron effect illumination of electrodes 51 and traces 85 . Electrodes 51 are set to a known positive voltage in the range 5 to 300 volts. Upper broadband UV electromagnetic source 18 U is positioned to irradiate board upper side 81 U and above. Lower broadband UV electromagnetic source 18 L is positioned to irradiate board lower side 81 L and below. The broadband UV electromagnetic sources 18 U, 18 L are flashed so that the photoelectric effect causes photoelectrons to be liberated from all traces 85 , electrodes 51 and all other metal surfaces in the vicinity, such that electrons will flow in various directions as determined by the known voltages on electrodes 51 and the unknown voltage on traces 85 until traces 85 are charged to the same voltage as electrodes. Satisfactory broadband UV electromagnetic sources 18 are UV lamps emitting light with wavelengths of 300 nanometers or less.

Another method to give known voltage to traces 85 is to use the laser method where the ultraviolet laser 22 and optical path components 40 are used to simultaneously illuminate a spot on electrode 51 and an individual trace 85 T with beam 24 U or 24 L, after setting electrode 51 to a known voltage in the range of 10 to 100 volts, such that photoelectrons are liberated from electrode 51 and trace 85 T and flow until trace 85 T is at the same potential as electrode 51 . In practice, when electrodes 51 are wire grids 52 or equivalent, beam 24 L, 24 U may be dithered using controller 70 and galvo beam mirror 45 U or 45 L to insure that beam 24 U, 24 L strikes a portion of electrode 51 while the center of motion is aimed at target test pad 87 EU or 87 EL. This method charges only one trace 85 at a time, so the process described must be repeated for each trace 85 . Additionally, each end 87 E of each trace 85 must be illuminated by beam 24 U or 24 L to ensure that a uniform voltage is applied to traces 85 which may have an open circuit.

Test for Continuity. In a preferred method of testing trace 85 for continuity between two points, electrode electronics 55 U maintain both electrodes 51 as collectors 51 C at a given potential for collecting electrons liberated by the photoelectric effect of beams 24 U, 24 L on trace 85 . A first location on trace 85 , such as on upper end pad 87 EU, is interrogated by beam 24 U. This charges the first location to a voltage level equal to that of the upper collector 51 C. When this voltage level has been achieved as best evidenced by no further current flow between trace 85 and collector 51 C, beam 24 U or 24 L is then directed to another location, such as opposite end pad 87 EL of trace 85 . If the second end point is charged, to the same voltage level such that no current is detected, then it may be assumed that continuity exists between the first and second locations. Conversely, if a current is detected then the second location does not exhibit a charge level equal to that of collector 51 C and it may be assumed that an open circuit has been detected and the system will display and/or record this as a defect. Each trace 85 is tested for continuity in this manner.

Test for shorts. The test for shorts is performed in a manner similar to that described above of determining continuity or opens. Immediately after determining the validity of a trace for continuity, i.e. no opens, locations, such as end pads 87 E of adjacent traces may be interrogated to determine their voltage levels. If the adjacent trace does not exhibit a voltage level equal to that of the collector then it may be assumed that the adjacent trace is isolated from the initial trace. If an adjacent trace exhibits a voltage level equal to that of the initial trace, such as by meter 59 showing no current flow, then it may be assumed that a short exists between the two traces allowing the second trace to charge to collector voltage level simultaneously with the initial trace. Tester 10 will display and/or record this as a defect.

All adjacent traces must be tested in a like manner to assure that the initial trace tested is not shorted to any other trace on Board 80 P.

A further advantage of the invention is the ability to quickly retest individual traces 85 T which may have failed initial testing. In this case, the method is as described earlier except that initial charging to a known voltage is performed with the laser method using the beam 24 U or 24 L since not all traces must be recharged, only those that failed initial testing.

Measurements. During measurements of opens or shorts, when collecting electrons emitted from a trace 85 on the side of board 80 P facing electrodes 51 , the voltage on electrode 51 must be set to a positive value higher than the residual voltage left on traces 85 during initial charging. It has been found that during the test of a trace 85 when beam 24 U or 24 L is directed to top pad 87 EU or to bottom pad 87 EL, photoelectrons will be liberated faster and travel to a collector electrode 51 C faster if the voltage on the opposite electrode 51 is switched to a relatively high negative value, such as −100 to −300 volts, creating a large electric field. The increase in collection speed means more tests per unit of time can be performed.

When using either the UV lamps 18 U and 18 L or UV laser 22 for charging of all traces 85 to a known initial voltage, voltages on collectors 51 C may be in the range 5 to 300 volts during measurements of opens and shorts. When beam 24 U is directed to top end pad 87 EU preferably upper electrode 51 U is set to positive 100 volts while opposite electrode 51 L, is set to negative 150 volts. When beam 24 L is directed to bottom end pad 87 EL, preferably lower electrode 51 U is set to positive 100 volts while upper electrode 51 U is set to negative 150 volts.

Although a particular embodiment of the invention has been illustrated and described, various changes may be made in the form, composition, construction, and arrangement of the parts herein without sacrificing any of its advantages. Therefore, it is to be understood that all matter herein is to be interpreted as illustrative and not in any limiting sense, and it is intended to cover in the appended claims such modifications as come within the true spirit and scope of the invention.