METHOD AND APPARATUS FOR TESTING INTERCONNECT NETWORKS

Abstract: Method and apparatus for testing interconnect networks, by generating at least two plasma jets, each in the vicinity of a distinct point (12, 13) of the circuit to be tested, applying a voltage difference whereby to cause an electric current to flow between the points through the circuit, to each point through the corresponding plasma jet and maintaining the current flow to carry out the testing process. Each plasma jet is generated by producing a discharge between two electrodes within a cavity, said plasma being ejected out through an orifice. The apparatus comprises at least two plasma injector-electrodes (14, 16) to drive an electric current between two points of the circuit to be tested, each so positioned or positionable as to direct the plasma generated thereby to one of the two points, a first electric circuit (21, 27) to supply voltage to each plasma injector-electrode, means (18, 20) for supplying gas to each plasma injector-electrode, and a second electric circuit (21, 27) similar or different from the first electric circuit, to sustain electric discharge.

Description: -1- WO 99/56137 PCT/IL99/00091 METHOD AND APPARATUS FOR TESTING INTERCONNECT NETWORKS Field of the Invention The present invention relates to the electrical testing of interconnect networks. More particularly, the invention relates to the use of plasma for testing interconnect networks. Background of the Invention It is known in the art to test interconnect networks, particularly printed circuit boards, by applying a voltage difference between two pads of the circuit, generating an electric current between them α.xi determining the electrical resistance of the net that connects the two La s. It is desirable to do this without physically contacting the pads with electrodes connected to the voltage sources. Lasers have been used in the art in order to generate conductive paths for this purpose. Thus, the generation of plasma, which impinges on a surface to create a conductive path therewith, is exploited, e.g., in US Patent No. 5,587,664, of the same assignee hereof, for the non- contact inspection of electric parts. The prior art has concentrated mainly on generating a conducting pathway by ablating a metallic plasma from the target conductor, which ablated metal generates, under the conditions employed in the art, a metallic plasma which is of conductive nature. This approach, while useful in some cases, suffers from some drawbacks: The metallic plasiii is very short living and difficult to control. It also requires a relatively high amount of laser energy that is sufficient to produce substantial amount of metallic plasma, and therefore requires relatively powerful and expensive lasers. Co-pending application No. 122654 of the same applicant discloses and claims a method for generating and guiding an electric pathway from one electrode to another, if desired in order to test electrical circuits, and an apparatus for carrying it out, which method comprises applying a laser pulse along at least a section of the path where it is desired to create the electric pathway, the energy of said laser pulse being sufficient to generate a plasma within said medium along said pathway and continuing to apply a voltage or current, after the end of the laser pulse, of a magnitude sufficient to sustain an electric discharge in said pathway. While said method and apparatus constitute a valuable improvement on the art, they require the generation and control of laser beams. They also require transmission of the high power laser beam to the contacting heads and re-imaging it into a small spot on the tested board. All these requirements impact the apparatus cost and complexity. Methods and apparatuses involving the generation £ a confined plasma cloud were disclosed in the art for several other application, under various names such as: plasma jet, plasma (or arc) torch, plasma transfer, etc. This method has been effected, for instance, in U.S. Patent 3,553,422, in which a method and apparatus for plasma arc welding was disclosed. This method has also been effected, for instance, in U.S. Patent 3,619,549, in which a method and apparatus for arc torch cutting was disclosed. In both these applications, very high power arcs are used as the plasma source. The very hot gaseous plasma is injected upon the workpiece through a nozzle. In these patents and many following a'xnts, methods and apparatuses for directing and constricting the plasma cloud were disclosed, aimed to improve the welding or cutting quality. This includes various combinations of types of plasma gas, shielding gas that is injected in a concentric geometry around the plasma jet, various shapes of injection nozzle, and also water vortex swirling around the jet in order to constrict it even more. Plasma injection was also used for applications of surface treatment and coating, where the coating material in form of a powder is mixed into the injected plasma. Plasma jet was also used in marking and printing applications. The same approach was used for imaging lithographic plates, for instance, in U.S. Patent 4,911,075. A plasma jet head is placed close to the printing surface of the plate and plasma jet is injected from it. The plasma jet volatilizes a portion of surface metal layer of the plate, or a coating thereof, at the point on which it impinges, thereby changing its affinity for ink and/or water so as to produce an image spot. The plasma jet head comprise means for flowing a gas therethrough, a nozzle, and means for delivering high-voltage pulses of thousands volts to an electrode disposed behind the nozzle to ignite a discharge and an electric current of tens to hundreds amperes to produce the plasma jet. All said methods and apparatuses were not directed to create an electrical pathway by the plasma jet. The plasma jet acts essentially like an intense heat source. It is the thermal and/or chemical reaction between the plasma and the workpiece that is utilized. In US Patent 5,202,623, laser activated plasma was used for non-contact testing of printed circuit boards. The plasma was ge.j xα.od in air in a small chamber, which is subjected to a concentrated laser pulse, the plasma exits the chamber through an orifice, and the electrically conductive plume of plasma is used to probe the circuit. It is an object of the present invention to provide a method for the non- contact inspection of interconnect networks, particularly printed circuit boards, which does not require the generation and application of laser beams. It is another object of the invention to provide such a method that comprises generating and directing a plasma jet to each of the points or pads of the electric circuit to which a voltage is to be applied and applying said voltage through the plasma jet. It is a further object of the invention to provide such a method for precisely and accurately measuring the resistance between pads of the electric circuit, which can be implemented using the said plasma contacts. It is a still further object of the invention to provide an apparatus for the non-contact inspection of electrical circuits, particularly printed circuit boards, which is simpler and less costly than the apparatus of the prior art. It is a still further object of the invention to provide an apparatus wherein electric pathways are created through plasma without the use of laser beams. It is a still further object of the invention to provide an apparatus for generating a most confined and finely controlled pl°snιa jet for high resolution probing of interconnect networks. It is a still further object of the invention to provide an apparatus for generating a delicate plasma jet that will not damage the interconnect networks while probing. Other objects and advantages of the invention will become apparent as the description proceeds. Summary of the Invention The method for testing electrical circuits, according to the invention, comprises the following steps: a) generating at least two plasma jets, each in the vicinity of one point of the circuit to be tested; b) applying a voltage to each of said points through the corresponding plasma jet, said voltages being different whereby to cause an electric current to flow through said circuit between said two points; and c) maintaining the flow of said electric current to carry out the testing process. The voltage and current data are elaborated, to determine the desired characteristics of the tested electric circuit, in a maiiiier which is in principle identical to what was described in the cited USP 5,587,664, the contents of which are introduced herein by reference. One additional step in the data processing, which was not described there, is the contact voltage compensation. There is a voltage drop on the plasma contact, which has to be compensated in order to calculate the resistance between said two points accurately. This voltage drop will be called hereinafter Vc (for Contact Voltage). To enable a precise compensation of Vc, a configuration with a low and stable Vc was established. Vc depends upon the exact jet configuration and operating parameters, the working distance, and the measurement current. We found that, for high resistance measurements, when checking isolation between said two points, the very low measurement range, of less then 5 mA is appropriate and gives low and stable Vc; while for low resistance measurement, when checking continuity between said two points, the current range above 0.1 A (the arc discharge range) is appropriate. In the last case we used a current source as the measurement power source, and measured using constant current. In both cases, for a working distance of 10-1000 μm, a typical value for the contact voltage is up to several tens of volts. The actual contact voltage is calibrated and compensated according to the specific configuration. Each plasma jet is injected, from a corresponding sensing head, through an orifice. The plasma is generated within a cavity behind said orifice. One way to generate said plasma is placing two electrodes in said cavity, between which high voltage pulse is applied, followed by a current pulse to ignite and maintain an electric discharge. Microwave generated plasma can also be used in the same manner. Preferably, the gas in the cavity is a gas that requires a relatively low voltage difference to generate plasma, e.g., Helium, Neon, Argon, Xenon, and other inert gases. Additional advantage of these gases is the reduced chemical reaction that may occur with the electrodes and the workpiece. By feeding a gas stream so as to flow through the cavity and out of the orifice, the plasma jet can be farther injected. However, plasma jet can be created by the impulse of the discharge itself without a gas stream. The specific characteristics of the plasma injector closely relate to its geometry, construction and materials; particularly, to the geometry and materials of the electrodes, the spacing between them, the type of gas, and the geometry of the nozzle. The apparatus for testing electrical circuits, according to the invention, comprises at least two plasma injectors, which function also as electrodes to drive an electric current between two points of the circuit to be tested, and which therefore will be called hereinafter "plasma injector-electrodes", or, briefly, "plasma electrodes". Each plasma injector electrode is or can be so positioned as to direct the plasma generated thereby to one of said two points. The apparatus further comprises a first electric circuit to supply voltage to said plasma injector-electrodes, means for supplying a gas to the same, and a second electric circuit, which may be the same as, or different from said first electric circuit, to sustain said plasma discharge. Each of said plasma injector-electrode comprises: 1- an insulating body; 2- two electrodes situated toward a cavity inside said body; 3- a discharge ignition and sustaining circuit connected to said electrodes; 4- said cavity being so structured and oriented as to provide a passage therethrough for a gas stream, and a nozzle orifice for the generated plasma jet, and as to cause said jet to impinge on one of said two points of the tested circuit board; 5- the electrodes of one of said plasma injector-electrodes being at lower voltages than the electrodes of the other plasma injector-electrode by an amount sufficient to generate said electric current. For the sake of brevity, the point of the electric circuit, on which a plasma injector-electrode cause the jet generated by it to impinge, will be called the point "corresponding to" said plasma injector-electrode. Hereinafter, for the sake of clarity, the voltage difference between the two electrodes of a plasma injector-electrode will be called "discharge voltage" and the voltage difference between the two plasma injector-electrodes will be called "measurement voltage". Preferably, the discharge voltages of the two plasma injector-electrodes are the same. In one embodiment of the invention, said plasma injector-electrodes is formed as a multi-layer truncated hollow cone, L ..i insulating body is simply the spacer between two metallic cones which are thread one into the other. The orifice is then the truncation of the cones. Thus, the plasma injector-electrode is formed, in this embodiment, by an inner metal layer which is the first electrode, an insulating layer surrounding it, an outer metal layer which is the second electrode, and an outer insulating layer which covers the second electrode. Another geometry, one that was widely used in other plasma jet applications, is one electrode being a needle, situated inside a cavity in an insulation body, and directed toward a metallic nozzle, which acts also as the other electrode. However, the plasma injector-electrodes might be differently structured, as long as they comprise plasma generation means and they define a passage for the plasma, which terminates in a nozzle. Herein, the term "nozzle" indicates the orifice through which the plasma issues from its electrode. The plasma generating electrodes are connected XΌ [he discharge power sources. The discharge voltage depends on the type of gas used, on the electrodes shape and material, and on the gap between the two electrodes, which determines the length of the electric discharge that is generated through the gas between the two electrodes of the plasma injector-electrode. Said gap will be called "the discharge gap". The plasma injector-electrode is designed so as to minimize the electrical energy that is required in order to ignite and sustain the discharge. Various approaches, that are common in electric discharge technology, are applicable here, such as: working in a discharge gap and gas pressure that corresponds to the minimum of 'Paschen curve', using hollow cathode and/or plasma cathode, etc. Decreasing the discharge energy o . i..usly reduces the load on the discharge circuit, but also improves ilu probing resolution, minimizes the damage to the tested pad, and extends the electrode lifetime. The insulating material of the plasma electrodes should have good dielectric strength to stand the ignition voltage, and a good heat and plasma resistivity. Preferably, it is chosen from among ceramic materials. Long lifetime electrodes were made of a refractory metal, such as tungsten, though other metals performed well. The measurement circuit is connected between one of the plasma generating electrodes of each plasma injector-electrode, preferably the electrode that is closer to the tested circuit. It can also be connected to an independent electrode that is located at the edge of the needle. Brief Description of the Drawings In the drawings; Fig. 1 schematically illustrates the application of an apparatus according to an embodiment of the invention for the testing of a net of a printed circuit board; Figs. 2a, b are schematic axial cross-sections of two embodiments of a plasma injector-electrode according to the invention; Fig. 3 schematically illustrates an electric circuit thpt may be used to carry out the invention; Fig. 4 is a graph of the probability to get a proper electric contact through the plasma jet versus the distance between the plasma injector electrode and the corresponding pad in the circuit under test. Fig. 5 is a graph of the contact voltage (Vc) versus the distance between the plasma injector electrode and the corresponding pad in the circuit under test; Fig. 6 is the Paschen curve for several popular gases. Fig. 7 is the electric discharge I-V curve. Detailed Description of Preferred Embodiments In Fig 1, numeral 10 indicates the printed circuit board, or other interconnects network, to which an embodiment of thy invention is applied. Numeral 11 indicates a circuit from pad 12 to pad 13, which is to be tested by means of the invention. A plasma injector-electrode 14, hereinafter to be described, is placed opposite to pad 12 so as to direct the plasma jet generated by it onto said pad, as shown by arrow 15. Likewise, plasma injector-electrode 16 is placed opposite pad 13, so as to direct the jet generated by it onto said pad, as shown by arrow 17. Evidently, the points can be at any location on any conductor in the circuit, not necessarily pads. They can also be parts of different nets, as is in the case of insulation test. Numeral 18 indicates a source of gas that feeds the two plasma injector- electrodes 14 and 16 through 19 and 20 respectively. An electric circuit 21 is connected to the two electrodes of plasma injector-electrode 14, hereinafter to be described, through two lines 22 and 23. An electric circuit 24 is connected to the two electrodes of plasma injector-electrode 16 through two lines 25 and 26. An electric circuit 27 is connected to one electrode in each plasma injector-electrode. The two plasma electrodes should preferably be identical, but it is not strictly necessary. Fig. 2 shows, in schematic axial cross-section, two possible plasma injector- electrodes according to embodiments of the invention, which could be either plasma electrode 14 or plasma electrode 16 of Fig. 1. 10 is once again the printed circuit board, which rests on a surface 30. The plasma injector- electrode 14 is illustrated as being conical, though it is not necessary. Fig. 2a is an embodiment that comprises an outer layer 31 of insulating material, a metal layer 32, which constitutes the second electrode and is placed immediately inside the insulating layer 31, an intermediate insulating layer 33, placed immediately inside the second electrode 32, and another metal layer 34 which constitutes the first electrode and is placed immediately within the insulating layer 33. Electrode 34 defines a cavity 35, to which a gas is fed through a conduit schematically indicated at 36. The gas stream fed into the plasma injector- electrode flows therefore from top to bottom, as seen in the drawing, and contacts firstly the first electrode 34, and then the second electrode 32, and is transformed into plasma, by the voltage difference that is applied between the two electrodes by the discharge circuit 37. The discharge gap is defined by insulating layer 33, as indicated at 38. The plasma flows out of nozzle 39 and forms a jet 40, which impinges on circuit board 10. Fig. 2b is an embodiment that comprises a needle 41, situated inside the cavity in the insulation body 42. The needle is directed toward a metallic nozzle 43, which acts also as the other electrode. The gap between the needle and the nozzle, 44, is the discharge gap. The gas that flows through this gap transforms into plasma by the discharge 45, and is injected through the nozzle in a jet 46. In the embodiments described so far, few separate plasma injector- electrodes are provided and placed opposite the terminal pads of the circuit to be tested. This requires displacing them when a different point of the circuit is to be contacted. However, if the circuits to be tested, in an interconnect network of any kind, or a plurality cf s ch circuits, are predetermined, the position of the terminal pads of all said circuits are also known, and then a plurality of plasma injector-electrodes can be provided and placed in such positions that the circuit can be tested by selectively activating the injector-electrodes that direct their plasma jets onto its terminal pads. The injector-electrodes then need not be displaced to switch from one point to another, and can be rendered solid or of one piece with one another. In this case, the plurality of injector-electrodes may constitute an injector-electrode system of simplified structure. Such a structure, schematically, comprises four superimposed layers, two of them insulating and two of them conducting, placed, from bottom to top, in the succession insulating-conducting-insulating-conducting. The conducting layers constitute the two electrodes, and the required voltages are applied by circuit means as described in connection with the piev iously described embodiments. Registered openings are provided through the said layers, to serve as plasma nozzles, opposite each terminal pad, and conduits are provided to feed gas through said openings towards said terminal pad. The gas, flowing through an opening, contacts firstly the first electrode (viz. the electrode more distant from the circuit to be tested), and then the second electrode, and is transformed into plasma, which flows out of the nozzle and forms a jet which impinges on the terminal pad opposite to it. Such a structure may be considered, for example, as a plurality of injector- electrodes similar to that of Fig. 2a, flattened out and rendered solid with PCT/IL99/00091 one another. Another approach can be also used, where all said openings are placed on a dense matrix, dense enough so that the spacing between two adjacent openings will be the same as the closest pads to be tested. An electric addressing mechanism is used to activate two injector-electrodes at the time to perform a required test between the two pads above which they are located. Fig. 3 is an example of an electric circuit that may be used to carry out the invention. The segments that are denoted by numeral 50 and 51 are the discharge ignition and sustaining circuits, each of them is connected to the two electrodes in one plasma injector-electrode. Preferably, they are identical. The discharge circuits 50 and 51 comprise a high voltage pulse source, VI and V3 respectively, for the ignition, and a current source, 12 and 14 respectively, for sustaining the discharge. The two circuits are connected in parallel through the switches Sl-4. Typical values that are required are 100-1000 volts for the ignition. Immediately after the ignition phase, the discharge voltage falls to several tens volts, and a current level of 0.1-10 Amperes is required to in order to sustain it. The segment that is denoted by numeral 52 is the measurement circuit. It is connected between one electrode of each of the plasma injector-electrodes, but can be connected through an independent electrode. The circuit is closed through the plasma contacts and the net being tested. This circuit comprises a source V5, and voltage and current measuring means. In the high resistance measurement mode (insulation test), the source acts as a voltage source, and the low current level (several microamperes) is measured accordingly. In the low resistance measurement mode (continuity test), the source acts as a current source. In both cases, the net resistance Rx is calculated by dividing the net voltage (after compensating for the contact voltage) by the current through it. In the same manner, types of impedance other than pure resistance can be measured. Fig. 4 shows the probability in percent of failing to obtain a proper electric contact with the pad through the plasma jet versus the distance between the plasma injector-electrode and the pad. By 'proper' is meant a contact that enables the measurement of the net resistance. This obviously depends on the specific injector geometry and operating parameters; nevertheless, in the specific case very good results were obtained up to a distance of 0.5 mm. Fig. 5 shows the contact voltage versus the distance between the plasma injector-electrode and the pad. Obviously, the voltage increases with the distance, because the plasma has some resistance. Yet. this dependence is not too strong. This implies that there is some υ-. tuut initial value of contact voltage. This effect agrees well with the cathode and anode falls that are described in the literature. The Paschen curve that is presented in Fig. 6 gives the voltage that is needed in order to ignite a discharge in various gases, versus the pxd value, which is the discharge gap times the gas pressure. The most important characteristic of this curve is its minimum, implying that the 'breakdown voltage' is reduced when the electrodes are brought closer and the discharge gap decreases, but below certain value this voltage increases again. In argon in an atmospheric pressure, the minimum of Paschen is at a discharge gap of about 50 microns. Preferably, the discharge gap in our plasma injector electrode is designed to operate at the nri iinum. of Paschen curve. Fig. 7 presents the I-V curve (load curve) for electric discharge. For our application, a region with low voltage fall is required. For the high resistance measurements we are using the leftmost region, while for the low resistance measurements we are using the rightmost region (the arc region) which is also the region in which our plasma source works. Although embodiments of the invention have been described by way of illustration, it will be understood that the invention may be carried out with many variations, modifications and adaptations, without departing from its spirit or exceeding the scope of the claims.

Claims: CLAIMS 1. Method for testing interconnect networks, which comprises the steps of: a) generating at least two plasma jets, each in the vicinity of a distinct point of the circuit to be tested; b) applying a voltage to each of said points through the corresponding plasma jet, said voltages being different whereby to cause an electric current to flow through said circuit between said points; and c) maintaining the flow of said electric current to carry out the testing process. 2. Method according to claim 1, comprising generating each plasma jet by producing a discharge between two electrodes within a cavity, said plasma being ejected out through an orifice. 3. Method according to claim 1, comprising generating each plasma jet by using microwave-generated plasma, said plasma being ejected through an orifice. 4. Method according to claim 1, 2, and 3 comprising feeding a gas stream so as to flow past said cavity, transformed into plasma state and ejected upon a point of the circuit to be tested. 5. Method according to claims 1,2,3, and 4, wherein a low breakdown voltage gas is employed. 6. Method according to claims 1,2,3, and 4, wherein an inert gas, such as Helium, Neon, Argon, and Xenon is employed. 7. Method according to claim 2, wherein a high discharge voltage is employed for the ignition and is thereafter reduced. 8. Method according to claim 7, the ignition discharge voltage is from 100 to 1000 volts and falls after ignition to several tens of volts. 9. Method according to claim 1, further comprising compensating the voltage drop on the plasma contact (contact voltage). 10. Method according to claim 9, wherein the contact voltage is up to several tens of volts. 11. Method according to claim 2, wherein the same discharge voltage is applied to generate each plasma jet. 12. Apparatus for testing electrical circuits, which comprises at least two plasma injector-electrodes to drive an electric current between two points of the circuit to be tested, each so positioned or positionable as to direct the plasma generated thereby to one of said two points, a first electric circuit to supply voltage to each of said plasma injector-electrodes, means for supplying a gas to each of said plasma injector-electrodes, and a second electric circuit, which may be the same as, or different from, said first electric circuit, to sustain said electric discharge. 13. Apparatus according to claim 12, further compri. lug sources of power connected to the electrodes to maintain the flow of the electric current through the electric circuit as long as this is being tested. 14. Apparatus according to claim 12, wherein each plasma injector-electrode comprises: a- an insulating body; b- two electrodes situated toward a cavity inside said body; c- a discharge ignition and sustaining circuit connected to said electrodes; d- each of said two plasma injector-electrodes being so structured and oriented as to provide a passage therethrough for a gas stream, and a nozzle orifice for the generated plasma jet, and as to cause said jet to impinge on one of said two points of the tested circuit board; e- the electrodes of one of said plasma injector-electrodes being at lower voltages than the electrodes of the other plasma injector-electrode by an amount sufficient to generate the electric current. 15. Apparatus according to claim 14, wherein the insulating body of each plasma injector-electrode is hollow, the first electrode thereof is formed by a metal layer or coating covering at least part of the inner surface of said insulating body and the second electrode thereof is formed by a metal layer or coating of similar shape as the first electrode, but positioned outwardly of it and separated from it by insulating material. 16. Apparatus according to claim 14, comprising voltaic sources for initially applying to the first electrode a breakdown voltage and later a lower voltage to maintain the formation of plasma. 17. Apparatus according to claim 14, wherein the insulating material of the plasma injector-electrodes is a ceramic material. 18. Apparatus according to claim 14, wherein the electrodes are made of tungsten. 19. Apparatus according to claim 12, which comprises a plurality of plasma injector-electrodes, each so positioned as to direct the plasma generated thereby to a terminal pad of a circuit to be tested. 20. Apparatus according to claim 19, wherein a fixed array of plasma injector-electrodes is employed, and each plasma injector-electrode is selectively activated to contact its corresponding terminal pads. 21. Method for testing electrical circuits, substantially as described and illustrated. 22. Apparatus for testing electrical circuits, substantially as described and illustrated.

Inventors: BANITT DAVID, BEN SHLOMO MOSHE, BLAU PINHAS, LEVY YAAKOV, NEGRI SHABTAI, BANITT, DAVID, BEN SHLOMO, MOSHE, BLAU, PINHAS, LEVY, YAAKOV, NEGRI, SHABTAI
Applicants: BANITT DAVID, BEN SHLOMO MOSHE, BLAU PINHAS, EXSIGHT ELECTRO OPTICAL SYSTEM, LEVY YAAKOV, NEGRI SHABTAI, BANITT, DAVID, BEN SHLOMO, MOSHE, BLAU, PINHAS, EXSIGHT ELECTRO-OPTICAL SYSTEMS LTD., LEVY, YAAKOV, NEGRI, SHABTAI
Application year: 1999
Main classification: G01R1
IPC codes: G01R1, G01R31