Method for measuring the conductive layer within holes in a printed circuit board

Abstract

The present invention relates to an inspection needle for measuring the conductive layer within a hole in a printed circuit board, an inspection probe equipped with such an inspection needle, and a flying probe tester for inspecting a printed circuit board, equipped with such an inspection needle or inspection probe. [Solution] The inspection needle (19) is equipped with a capacitance measuring body (26) connected to a capacitance measuring device via a cable (25). The cable is shielded so that only the capacitance measuring body forms a capacitance coupling with other conductors. This makes it possible to detect the capacitance coupling with high position resolution.

Description

Technical Field

[0001] The present invention relates to an inspection needle for measuring a conductive layer in a hole of a printed circuit board, an inspection probe including such an inspection needle, and a flying probe tester for inspecting a printed circuit board by such an inspection needle or inspection probe.

Background Art

[0002] When a printed circuit board is manufactured, the printed circuit board is first manufactured with so-called coupons. A coupon is a region of a printed circuit board having predetermined conductor tracks, and measurements can be performed in that region. After measurements are made to inspect whether the printed circuit board is properly manufactured, the coupon is separated from the remaining part of the printed circuit board. The remaining part of the printed circuit board has one or more so-called use areas or panels with electrical components and is used in electrical products.

[0003] In coupons, different electrical characteristics can be measured. Coupons have special conductor tracks. In special conductor tracks, for example, it can be easily determined whether requirements for manufacturing conductor tracks suitable for high frequencies in the manufacturing process are met. The conductor tracks of the coupon can also be used for load testing. Load testing can also break individual conductor tracks on the coupon. Since the conductor tracks of the coupon are not used in subsequent products, they can be particularly optimized for specific inspection procedures.

[0004] However, not all measurements in the coupon can be transferred to the conductor tracks in the use area. Conductor tracks in the use area often have different dimensions from the conductor tracks of the coupon. Therefore, the measurement results in the coupon are not necessarily transferable to the conductor tracks in the use area.

[0005] For a long time, measuring probes that apply high-frequency signals to the conductor tracks in the area of ​​use have also existed for inspecting high-frequency characteristics. Such high-frequency measurements can only be reliably performed when contact between the conductor tracks occurs in an electrically compatible manner. If reflections occur, it becomes difficult to pinpoint the actual location of defects. Furthermore, high-frequency measurements always require a grounding reference point near the contact. Therefore, the design of the corresponding measuring head is costly.

[0006] A test head having a conductive elastomer is known from U.S. Patent No. 9,459,285 for measuring the conductive layer in a back-drilled hole. The test head can mechanically sense and electrically measure the conductive layer.

[0007] Furthermore, U.S. Patent No. 8431834 describes the provision of additional conductor tracks on printed circuit boards. Impedance measurements between these additional conductor tracks and the actual conductor tracks can be used to verify whether back-boring is sufficiently deep or too deep. Providing this type of additional conductor track is very expensive and significantly increases the cost of multilayer printed circuit boards.

[0008] U.S. Patent Application Publication No. 2015 / 047892 discloses a method in which a drill is simultaneously used as a probe so that electrical contact with a conductor track can be measured during back-boring and the drill can be controlled accordingly.

[0009] U.S. Patents 9,341,670, 9,488,690, and 9,739,825 disclose methods and apparatus that may be used to measure so-called "stubs" in holes of printed circuit boards. Such stubs are remnants or stumps of the conductive layer in the holes of the printed circuit board, and are generally electrically connected to the conductor tracks. To remove such stubs, the holes of the printed circuit board are often drilled a second time, called backdrilling. This backdrilling can remove stubs that were unintentionally created while the individual layers were coated with the conductive layer, which is generally a layer of copper. However, this backdrilling may also disconnect the desired electrical connection of the conductor tracks. The apparatus comprises a capacitive probe inserted into the backdrilled hole of the printed circuit board, and the capacitance of the capacitive probe is measured. Other conductive materials present in the backdrilled hole are detected by capacitance. In this way, defects in backdrilling can be detected. [Overview of the project] [Problems that the invention aims to solve]

[0010] The object of the present invention is to manufacture a test needle, a corresponding test probe, and a flying probe tester for measuring the conductive layer within a back-drilled hole in a printed circuit board, which can be easily, more reliably, and very accurately measured according to the method described in U.S. Patent No. 9,341,670. [Means for solving the problem]

[0011] The aforementioned objective is achieved by the subject matter of the independent claim. Advantageous embodiments of the present invention are disclosed in each dependent claim.

[0012] The inspection needle according to the present invention for measuring the conductive layer within a hole in a printed circuit board comprises a conductor surrounded by a shield. The inspection needle has a connection end that can be electrically connected to a capacitance measuring device and a measuring end that can be inserted into the hole during measurement.

[0013] The inspection needle is characterized in that the capacitance measuring body connected to the conductor is positioned outside the shield at the measuring end, and can form a capacitance coupling with the conductive layer within the hole.

[0014] Since the conductive material of the inspection needle is shielded and only the capacitance measuring element can form a capacitance coupling with other conductors, the inspection needle can detect the position of the capacitance measuring element with very high accuracy based on the capacitance measured in relation to the conductive layer.

[0015] Preferably, the measuring body is surrounded by an insulating layer so that no electrical contact occurs between the measuring body and other conductors.

[0016] The measuring body may be a hollow cylindrical shape.

[0017] Furthermore, the measuring body may be one or more windings of the conductive material disposed on the outside of the shield.

[0018] The measuring body preferably has a maximum diameter of 1.5 mm, particularly a maximum diameter of 1 mm, and more preferably a maximum diameter of 0.75 mm.

[0019] The measuring body preferably has a maximum length of 0.5 mm, more preferably 0.25 mm, and particularly 0.15 mm, in the axial direction of the inspection needle.

[0020] The smaller the object being measured, the higher the resolution of the position detected by the inspection needle.

[0021] On the other hand, the larger the area over which the measurement object extends, the greater the capacitive coupling with the conductive layer within the hole, resulting in a stronger measured signal. In order to detect the conductive layer, it is necessary to achieve both positional resolution and capacitive coupling.

[0022] The inspection needle may include a conductive tube that forms the shield. A cable having the conductor and the insulating layer is routed inside the conductive tube. The cable may protrude briefly from the tube at the connection end for connection to a capacitance measuring device. The cable may protrude briefly at the measuring end for arrangement around the tube to form the measuring body, which is one or more windings.

[0023] This is a very simple design for the inspection needle, yet it nevertheless enables very accurate measurements. Preferably, the cable is wound around the conductive tube with two, three, or four or more windings to form the measuring body.

[0024] An inspection probe for measuring the conductive layer within a hole in a printed circuit board may be provided with the inspection needle described above. The inspection probe also includes a touch sensor unit for determining whether the inspection needle is in contact with another object.

[0025] The touch sensor unit can be used to measure the depth of a blind hole. In this measurement, first, the inspection probe contacts the surface of the printed circuit board being inspected. Next, the inspection probe contacts the bottom of the blind hole. These contacts are detected, and the position of the inspection probe is detected. The depth of the blind hole is obtained from the difference between the two positions.

[0026] Actual capacitance measurements should be performed non-contact whenever possible. Therefore, the touch sensor unit may also be used, if necessary, to determine whether the inspection needle is in contact with other objects in order to move the inspection needle so that it does not come into contact with other objects.

[0027] The touch sensor unit can include a spring elastic mount that supports the inspection needle and a sensor that detects the deflection of the spring elastic mount.

[0028] The sensor that detects the deflection of the spring elastic mount can be an optical sensor. The optical sensor can be realized in various ways. For example, the light source can be arranged directly on the spring elastic mount or indirectly by an optical fiber. The light output by the light source is detected by the optical sensor. When the spring elastic mount deflects, the light cone moves away from the optical sensor. This can be detected by a change in brightness. Similarly, it is also possible to provide a photoelectric beam.

[0029] The inspection probe can be provided with a stop portion that abuts in a state where the inspection needle and / or the spring elastic mount is not deflected. In this way, the position of the inspection needle is clearly determined with respect to the inspection probe in a non-deflected state. Also, if the position of the inspection probe is known, it is possible to estimate the positions of the inspection needle and the measurement object based on this position.

[0030] A flying probe tester for inspecting a printed circuit board, particularly a bare printed circuit board, preferably includes an inspection finger that can move freely within a predetermined inspection area so as to be able to move near predetermined contacts of the printed circuit board. The inspection finger can be provided with the above-described inspection needle or the above-described inspection probe.

[0031] The flying probe tester can include a plurality of the inspection fingers. The plurality of the inspection fingers include one or more additional inspection fingers having the inspection needles for electrically contacting specific contacts of the printed circuit board.

[0032] In another aspect, the present invention relates to a method for measuring the conductive layer within a hole in a printed circuit board.

[0033] This method uses the inspection needle, the inspection probe, or the flying probe tester described above. The inspection needle, equipped with a capacitance measuring element, is inserted into the hole. In this case, the position of the capacitance measuring element is detected, and at the same time, the capacitance of the capacitance measuring element with respect to its surroundings is measured. Based on the measured capacitance, it is determined whether a conductor is present near the detected position.

[0034] The position of the conductor can be determined by simultaneously detecting the position of the capacitance measuring body and the capacitance measured together with the capacitance measuring body. Furthermore, the capacitance measuring body can be inserted into small holes to more reliably detect the conductive layer on the inner surface of the hole.

[0035] In addition to measuring the actual capacitance, the depth of the blind hole to be measured can also be derived using the touch sensor unit described above. This measurement of the blind hole depth can be performed before, during, or after the capacitance measurement.

[0036] To measure capacitance, an electrical signal having a frequency of at least 1 kHz is applied. For example, higher frequencies of at least 2 kHz or at least 4 kHz may also be used. The measurement signal can be applied to the conductor and the capacitance measuring body of the test needle. The measurement signal induced therein is sensed on a conductor track connected to a portion of the conductor being measured. However, it is preferable that the measurement signal is applied to the conductor track connected to the conductor and sensed by the capacitance measuring body. The measurement signal from the conductor is induced on the capacitance measuring body and can be detected and evaluated accordingly by the measuring device. This is particularly advantageous when the conductor is connected to a number of conductors located in different holes. In that case, the test finger is always positioned on the conductor, and the test probe can sense the conductors in the multiple holes one after the other.

[0037] When determining whether the conductor is present near the measurement object, a comparison may be made with the capacitance profile of a suitable hole. A "suitable hole" is one in which the conductive layer connected to the conductor to which the measurement signal is applied is provided in a desired area of ​​its inner surface, while the conductive layer is not provided in other areas of its inner surface. Therefore, the position of the conductive layer on the inner surface of the hole can be determined based on the comparison. When the measured capacitance profile matches a predetermined capacitance profile, the hole is suitable. That is, the conductive layer is provided only in the desired locations. However, if a discrepancy exists, it means that the conductive layer is missing in a particular location, or that the conductive layer is provided in a location where it should not be present.

[0038] In this method, since the holes are formed perpendicular to the surface of the printed circuit board in principle, the inspection needle is always kept as perpendicular as possible to the surface of the printed circuit board being inspected. In this way, the inspection needle is aligned axially with each hole, and the risk of the inspection needle colliding with the inner surface of the hole is minimal. [Brief explanation of the drawing]

[0039] The present invention will be described in detail below based on exemplary embodiments shown in the drawings.

[0040] [Figure 1] Figure 1 is a perspective view of a flying probe tester. [Figure 2a] Figure 2a is a perspective view of an inspection probe having an inspection needle according to the present invention. [Figure 2b] Figure 2b is a perspective view of an inspection probe having an inspection needle according to the present invention. [Figure 2c] Figure 2a is a side view of an inspection probe having an inspection needle according to the present invention. [Figure 3] Figure 3 is a detailed cross-sectional view of the test needle with a test tip. [Figure 4] Figure 4 is a schematic diagram showing a measuring device for measuring the conductive layer within plated through-holes of a printed circuit board, and includes a printed circuit board depicted in cross-section and a grounding conductor track connected to electrical ground. [Figure 5] Figure 5 is a schematic diagram showing a measuring device for measuring the conductive layer within plated through-holes of a printed circuit board, and includes a printed circuit board depicted in cross-section and a ground conductor track that is not connected to electrical ground. [Figure 6] Figure 6 is an equivalent circuit diagram of the measuring apparatus shown in Figures 4 and 5 for non-contact measurement of the conductive layer within a plated through-hole. [Modes for carrying out the invention]

[0041] The present invention is realized by a flying probe tester 1 (Figure 1). The flying probe tester 1 comprises a plurality of test fingers 2. Each test finger 2 is provided with a test probe 3 that can electrically contact the printed circuit board 4 to be tested in order to apply or sense a measurement signal on the conductor track.

[0042] This type of test probe 3 is described, for example, in International Publication No. 03 / 048787, and this document is referenced in this regard.

[0043] In this exemplary embodiment, the flying probe tester comprises two cross units 5, each having a plurality of guide rails 6. The cross units 5 extend across a test area capable of accommodating a printed circuit board 4 to be tested. Each cross unit 5 forms a frame with a rectangular through-opening. The two cross units 5 stand vertically on a support member 7 such that their through-openings are at the same height as each other. The test area for accommodating the printed circuit board 4 extends through the through-openings of the cross units 5. In this exemplary embodiment, each cross unit has four guide rails 6, two on each side of the cross unit 5. On each side of the cross unit, one set of guide rails 6 is positioned above the through-opening 8, and the other set of guide rails is positioned below the through-opening 8.

[0044] Each guide rail 6 is provided with at least one slider 9 arranged to slide on it. Each slider 9 supports one inspection finger 2. In this exemplary embodiment, two sliders 9 are provided on each guide rail 6. However, basically, two or more sliders 9 can also be provided on a single guide rail 6.

[0045] The inspection finger 2 is pivotally fixed to the slider 9 at one end. Such a design for a flying probe tester can be inferred, for example, from International Publication No. 2014 / 140029. In this regard, refer to the entire document.

[0046] The inspection probe 3 is positioned at the end away from the slider 9, i.e., at the free end of the inspection finger 2. Each inspection probe 3, along with a contact tip 11, includes an inspection needle 10 oriented toward the printed circuit board 4 to be contacted. The inspection finger 2, together with the inspection probe 3, is movable toward and away from the printed circuit board 4 in order to position the contact tip 11 on the surface of the printed circuit board or to insert the contact tip 11 into a hole in the printed circuit board 4.

[0047] According to the present invention, at least one or more inspection fingers 2 are each equipped with an inspection probe 12 (Figures 2a to 2c) that non-contactively senses capacitance in holes (particularly blind holes) of a printed circuit board 4. Such an inspection probe 12 is equipped with a base plate 13 which is a printed circuit board.

[0048] A detent body 14 is provided on the base plate 13. The detent body 14 has two detent arms 15. The two detent arms 15 allow the inspection probe 12 to be attached to one of the inspection fingers 2 in the form of a detent.

[0049] Two spring arms 16 and 17 are fixed to the claw body 14. One of the two spring arms rests directly on the base plate 13 and is sandwiched between the base plate 13 and the claw body 14. The other spring arm 17 is fastened by two screws to the surface of the claw body 14, which faces away from the base plate 13.

[0050] The spring arms 16 and 17 are oriented parallel to each other. Viewed from above, the base plate 13 and the spring arms 16 and 17 are tapered, starting from the claw body 14, with the two spring arms 16 and 17 each tapering toward a short, protruding free end 18. At these ends of the spring arms 16 and 17, each inspection needle 19 is fixed such that the elastic spring arms 16 and 17, the claw body 14, and a portion of the inspection needle 19 connecting the two spring arms 16 and 17 form a parallelogram. The free ends of the two spring arms 16 and 17 are pivotable away from the base plate 13. Therefore, the inspection needles are elastically supported by the inspection probe 12. Hereinafter, the spring arm 16 directly mounted on the base plate will be referred to as the base spring arm 16, and the spring arm 17 that is separated from the base plate 13 will be referred to as the free spring arm 17. In this manner, the base plate 13 constitutes a stopping point for the base spring arm 16 and a stopping point for the movement of the inspection needle 19 relative to the support of the inspection probe 12 by its body.

[0051] Viewed from above, the two spring arms 16 and 17 form a roughly triangular frame. A measuring tag 20, directed toward the base plate 13, is positioned on the free spring arm 17.

[0052] A photoelectric beam device 21 is provided on the base plate 13. The photoelectric beam device 21 includes a light source and a light sensor, and generates a photoelectric beam that is blocked by the measurement tag 20 when the spring arms 16 and 17 are not bent. When the spring arms are bent, the measurement tag 20 moves away from the photoelectric beam. As a result, the light sensor detects an increase in brightness. In this way, the photoelectric beam device 21, together with the measurement tag 20, constitutes a touch sensor unit, and the inspection needle 19 detects that the spring arms 16 and 17 are bent when they come into contact with another object.

[0053] The inspection needle has a connecting end 22 and a measuring end 23. In the region of the connecting end 22, the free spring arm 17 is connected to the inspection needle 19. The inspection needle 19 is connected to the base spring arm 16 such that when the spring arms 16 and 17 are not bent, i.e., when the base spring arm 16 is leaning against the base plate 13, the inspection needle 19 is positioned perpendicular to the plane of the base plate 13.

[0054] The inspection probe 12 and the corresponding inspection finger 2 are positioned perpendicular to the inspection area and the printed circuit board 4 located within the inspection area, with the spring arms 16 and 17 not bent when the inspection needle is attached to the inspection finger 2.

[0055] The inspection needle 19 on the inspection probe 12 extends a short distance beyond the base plate 13. The inspection needle 19 has a metal tube. The metal tube is conductive and preferably has an outer diameter of up to 0.2 mm, and particularly preferably up to 0.18 mm. The cable 25 has a conductor surrounded by an insulating layer and is led into the metal tube 24. In this exemplary embodiment, the cable 25 is connected to a conductor track on the base plate 13 and electrically connected to a capacitance measuring device (not shown). The conductor of the cable 25 is connected to a capacitance measuring body 26 at the measuring end 23. The capacitance measuring body is a metal annular body disposed around the tube and electrically insulated from the tube. The capacitance measuring body 26 may be a metal ring with an insulating layer and is electrically connected to the conductor of the cable 25. However, the capacitance measuring body 26 may also consist of one or more windings of the cable 25.

[0056] The metal tube 24 helps to shield the cable 25. As a result, only the capacitance measuring element 26 can form a capacitance coupling with the conductors placed in its vicinity, and only the capacitance coupling of the capacitance measuring element 26 with other conductors is sensed by the cable 25.

[0057] The measuring end 23 of the inspection needle 19 is provided with a measuring tip 27. This measuring tip is used only when mechanically contacting another object. The measuring tip may be made of a non-conductive material or may have an insulating layer. In this exemplary embodiment, the measuring tip 27 is not used to electrically contact contacts on a printed circuit board.

[0058] However, in the present invention, the measuring tip can also be realized as a conductor so that it can be used to create an electrical connection with a contact on a printed circuit board. When such a conductive contact tip is electrically connected to the capacitance measuring body 26, the contact tip can also be considered in the capacitive coupling that the capacitance measuring body 26 forms with the conductive object being measured.

[0059] An exemplary embodiment of the measuring tip 27 is comprised of a cannula tube 28. One end of the cannula tube 28 is mechanically connected to the tube or shield 24 in a conductive manner by a solder joint 29. In the region of the solder joint 29, the cannula tube has a diameter of, for example, 0.3 mm. The axis of the cannula tube is oriented toward the measuring tip 27, and in the region of the measuring tip 27, it has a diameter of, for example, 0.2 mm or less. In the region of the measuring tip, the cannula tube 28 is cut in an inclined shape to obtain an outlet opening 30 that extends in an inclined shape.

[0060] The cable 25 passes through the tube 24 and the cannula tube 28, and exits the cannula tube 28 through the outlet opening 30. In the region of the measuring tip 27, the cable is wound three times around the cannula tube 28, forming the capacitance measuring body 26.

[0061] The measuring tip 27 itself is conductive and is connected to the tube 24 in a conductive manner.

[0062] Such a conductive measuring tip has the advantage that, for example, the inspection needle 19 can come into contact with a calibration surface having a conductive surface and a non-conductive surface, making it possible to calibrate the position of the inspection needle 19 and the position of the inspection finger 2 to which the inspection needle 19 is fixed.

[0063] The inspection needle 19 shown in Figure 3 is easy to manufacture and has a very small measuring body 26, thus providing good positional resolution.

[0064] A typical measurement procedure for measuring the conductive coating inside a blind hole in a printed circuit board using a flying probe tester 1 is as follows:

[0065] The inspection finger 2 of the inspection probe 12 according to the present invention mechanically contacts the surface of the printed circuit board 4 to be inspected in order to determine its height.

[0066] An inspection probe equipped with an inspection needle is inserted into the blind hole of the printed circuit board 4 to be measured until its measuring tip 27 contacts the bottom of the blind hole in the printed circuit board 4. The touch sensor detects the position of the inspection probe 12 at the moment it contacts the bottom of the blind hole, and the depth of the blind hole is derived based on the height difference between this position and the position where the inspection probe (more precisely, its measuring tip 27) contacts the surface of the printed circuit board.

[0067] A capacitance measuring element 26 placed in a blind hole measures the capacitance between the capacitance measuring element 26 and a conductor placed in the blind hole. In this measurement, a predetermined measurement signal is applied to the conductor by another inspection finger 2 that is in contact with a contact of a printed circuit board connected to the conductor. The measurement signal preferably has a frequency of at least 1 kHz, particularly at least 4 kHz, or at least 10 kHz.

[0068] The signal induced in the capacitance measuring body 26 is sensed by the capacitance measuring body 26 via the cable 25 and relayed to the measuring device. The amplitude of the signal measured in this way is used to derive the capacitance between the capacitance measuring body 26 and the adjacent conductor. At the same time, the position of the capacitance measuring body 26 is detected. The position of the capacitance measuring body 26 is predetermined by the position of the test probe 12. The position of the test probe 12 is determined by the operation of the corresponding test finger 2 and is known in the flying probe tester 1. By simultaneously detecting the position and capacitance while the capacitance measuring body 26 is moving through the blind hole, the capacitance that the capacitance measuring body 26 has with respect to its surroundings can be derived in a position-dependent manner. This creates a capacitance profile. Based on this, it is possible to estimate whether the inner surface of the blind hole is covered with a conductor.

[0069] The flying probe tester 1 preferably comprises a plurality of inspection fingers 2, each having an inspection probe 12 according to the present invention. This allows for simultaneous measurement of multiple blind holes or through holes. During simultaneous measurement, it is preferable to use measurement signals with different frequencies. This, along with a corresponding bandpass filter, prevents crosstalk from one measurement to another.

[0070] It is preferable that each guide rail 6 be provided with at least one inspection finger 2 equipped with an inspection probe 3 having a conventional inspection needle 10 for electrically contacting the contacts of a printed circuit board, and another inspection finger 2 equipped with the inspection probe 12 according to the present invention. It may also be advantageous to provide the guide rail 6 with two inspection fingers 2 equipped with conventional inspection probes 3 and another inspection finger 2 equipped with the inspection probe 12 according to the present invention.

[0071] However, the present invention can also be used in conventional flying probe testers having a plurality of separate crossbeams that are not arranged on a combined transverse unit 5. In this type of flying probe tester, it is equally advantageous to provide each crossbeam with at least one inspection finger 2 equipped with an inspection probe 12 according to the present invention, and one or more inspection fingers 2 equipped with a conventional inspection probe 3.

[0072] Thus, with the inspection probe according to the present invention, it is very easy to accurately determine the dimensions of a blind hole or through hole, and at the same time, it is very easy to determine whether the inner surface of the blind hole or through hole is coated with a conductor.

[0073] Furthermore, the inspection probe 12 according to the present invention can be used in conventional flying probe testers. No other mechanical modifications are required. The flying probe tester only needs to have a suitable control program that can apply an appropriate measurement signal, process the measurement signal generated by the inspection probe 12 according to the present invention, and control the operation of the inspection finger 2 equipped with the inspection probe 12 according to the present invention.

[0074] The following describes a method for measuring plated through-holes 31 within holes 37 of a printed circuit board 38. A plated through-hole 31 has a conductive coating applied to its inner surface. When manufacturing the printed circuit board 38, the coating is first applied to the entire hole. Next, the hole is back-drilled a second time to a depth in a predetermined area where the coating on the hole 37 is removed again. In this regard, defects may occur if the back-drilling is not performed in a sufficiently deep manner, or if the back-drilling is performed in a manner that causes the edge of the coating inside the hole to be slightly offset from the desired position. Also, if the second hole is slightly offset from the first hole, the two holes may no longer be concentric. As a result, thin, unwanted streaks of coating may remain in the area of ​​the hole. These streaks extend in a direction approximately parallel to the central axis of the hole. Such streaks may also occur when the second hole is slightly inclined relative to the first hole.

[0075] The printed circuit board 38 has multiple layers 32 between conductor tracks 33. There are large and small conductor tracks. In principle, the so-called ground conductor track is the largest conductor track on the printed circuit board and may extend across multiple layers 32. In the measurement method described below, the ground conductor track is connected to electrical ground 34 (Figure 4). As an alternative to each ground conductor track, multiple smaller individual conductor tracks that are simultaneously connected to electrical ground 34 may also be used. In the following description, the term “ground conductor track” is used to describe one or more conductor tracks that can be simultaneously connected to electrical ground and can branch over a wide area of ​​the printed circuit board 38 (in particular, across multiple layers). This connection to electrical ground 34 is preferably made by positioning the contact fingers of a flying probe tester relative to the conductor track. These contact fingers have conductive contact tips connected to electrical ground.

[0076] When the test probe 3 is inserted into the hole, the capacitance measuring body 26 forms a capacitance with the surrounding conductor. The purpose of this method is to measure this capacitance, or more precisely, the change in capacitance when the capacitance measuring body is inserted into the hole.

[0077] For this purpose, a signal generator 35 is used to apply an oscillation signal to the plated through-hole 31 relative to the electrical ground. This measurement signal flows along the plated through-hole 31 and through the capacitance C1 between the plated through-hole 31 and the capacitance measuring body 26 to the capacitance measuring body 26. From there, it flows through the cable 25 to the current measuring device 36. The current measuring device 26 measures the voltage and capacitance C1, or more precisely, the change in capacitance, as the capacitance measuring body 26 moves near the plated through-hole 31.

[0078] A parasitic capacitance C2 is generated between the plated through-hole 31 and the adjacent conductor track. This parasitic capacitance C2 is mainly relevant when it is generated between the plated through-hole and the ground conductor track. When the ground conductor track is not grounded, this parasitic capacitance causes a measurement signal in the other conductor track. This measurement signal is transmitted to the ground conductor track via capacitive coupling (capacitance C3). As a result, the capacitance measuring body observes the measurement signal via capacitive coupling (capacitance C4) between the capacitance measuring body 26 and the conductor track located outside the plated through-hole (Figure 5). Consequently, it becomes impossible to accurately determine whether the capacitance C1 between the measuring body 26 and the plated through-hole 31, or the capacitance C4 between the measuring body 26 and one other conductor track, is being measured.

[0079] The functions of the measuring devices shown in Figures 4 and 5 will be explained below based on the equivalent circuit diagram (Figure 6). This equivalent circuit diagram has the following elements: measuring body 26, plated through-hole 31, electrical ground 34 (switchable grounding finger), signal generator 35, and current measuring device 36.

[0080] Capacitor C1 is the connection point between the plated through-hole 31 and the measuring body 26.

[0081] Capacitor C2 is located between the plated through-hole 31 and the ground conductor track, as defined in the above description. Capacitor C3 is the coupling between the ground conductor track and the other conductor tracks. Capacitor C4 is the coupling between the conductor track and the measurement object 26. The coupling between the conductor track and the measurement object 26 may be directly performed by the ground conductor track, or by the capacitance C3 of the other conductor track coupled to the ground conductor track. When a direct coupling is created between the ground conductor track and the measurement object 26, capacitor C3 is excluded. This is why capacitor C3 is short-circuited as shown by the dashed line in the equivalent circuit diagram, and why it does not need to be considered in the following discussion.

[0082] The connection point 34 of the grounding conductor track to the electrical ground is depicted as a switch in the equivalent circuit diagram (Figure 6). In the measuring device in Figure 4, the switch is closed, and in the measuring device in Figure 5, the switch is open.

[0083] When the grounding conductor track is not connected to electrical ground (when switch 34 in Figure 6 is open), the following occurs:

[0084] Capacitors C2(,C3) and C4 are connected in parallel with capacitor C1. When the series circuit formed by capacitors C2(,C3) and C4 is larger than that of capacitor C1, the total capacitance between the plated through-hole 31 and the measuring body 26 is mainly determined by the series circuit of capacitors C2(,C3) and C4, and the current measured by the current measuring device 36 is mainly determined by the series circuit formed by capacitors C2(,C3) and C4.

[0085] When the grounding conductor track is connected to electrical ground (when switch 34 in Figure 6 is closed), the connection between capacitor C2 and capacitor C3 is connected to electrical ground.

[0086] The current measuring device 36 is a current-voltage converter and includes an operational amplifier 39 and a measuring resistor 40. The operational amplifier is, for example, an AD549 type. At the output of the current measuring device 36, there is a voltage signal Um that is proportional to the current flowing through the input of the current measuring device.

[0087] One input of the operational amplifier is connected to the measuring body 26, and the other input is connected to electrical ground. Feedback between the input and output sides of the operational amplifier via the measuring resistor 40 maintains the two inputs of the operational amplifier 39 at the potential of electrical ground.

[0088] In this way, since capacitors C3 and C4 are located between two points that are at the potential of electrical ground, no current flows through capacitors C3 and C4. Therefore, capacitors C3 and C4 do not appear in the measurement.

[0089] The voltage signal from signal generator 35 is present on one side of capacitor C2, and the other side of capacitor C2 is connected to electrical ground. Therefore, current flows through this capacitor. This current flows directly to electrical ground and is not measured by the measuring device. This current affects the power output from signal generator 35, but not the voltage output from signal generator 35. The voltage signal U present on capacitor C1 and the current I flowing through the capacitor are known. Therefore, when the grounding conductor track is connected to electrical ground, the voltage signal U and current I can be used to derive the capacitance of capacitor C1 without measurements that are affected by other capacitances C2, C3, and C4.

[0090] As a result, by connecting the grounding conductor track to electrical ground, the capacitive coupling C1 between the measuring body 26 and the plated through-hole 31 can be accurately measured. The capacitance of the capacitive coupling C1 can be derived, making it possible to collect information about the geometric size of the defect in the plated through-hole.

[0091] Because this method is highly accurate, the signal generator's signal can be modified to apply to a single conductor track extending through the through-hole. As a result, the capacitive coupling of the object being measured to this conductor track can be detected based on this signal. This method allows for the determination of the object's position within the through-hole and the confirmation that the conductor track is correctly conducting the measurement signal to the plated through-hole region.

[0092] Typically, the position of the inspection probe relative to the location on the printed circuit board being inspected is calibrated by bringing the contact tip of the inspection probe 12 into contact with the surface of the printed circuit board. This mechanical calibration can be replaced by electrical calibration. When electrical calibration is performed, an oscillation signal is applied to a conductor track adjacent to a through-hole, and this signal is detected by the inspection probe (more precisely, by the measuring element of the inspection probe).

[0093] This method can also be modified to apply multiple measurement signals of different frequencies to different conductor tracks and / or plating through-holes. During measurement, different frequencies are identified. This modification can be made, for example, when a variable bandpass filter is provided before the current measuring device 36 and the variable bandpass filter is connected to each frequency band containing the frequency of one of the measurement signals. This modification allows for the detection of multiple conductor tracks near a through-hole, unless the multiple conductor tracks are shielded by the plating through-hole. [Explanation of Symbols]

[0094] 1. Flying probe tester 2. Inspection Finger 3. Test probe 4 Printed circuit board 5 Cross-sectional unit 6 Guide rails 7 Support material 8 Through-opening 9 Sliders 10 Inspection Needles 11 Contact tip 12 Test probes 13 Base plate 14. Claw body 15 Claw Arm 16. Spring arm (base spring arm) 17. Spring arm (free spring arm) 18 Free end 19. Inspection needle 20 measurement tags 21 Photoelectric beam device 22 Connection end 23 Measuring end 24 tubes 25 Cables 26 Capacitance measuring element 27 Measuring tip 28 Cannula tubes 29 Solder connection 30 outlet opening 31 Plated through-holes 32 layers 33 Conductor Tracks 34 Electrical grounding 35 Signal Generator 36 Current measuring device 37 Hole 38 Printed circuit boards 39 Operational Amplifier 40 Measuring resistance

Claims

1. An inspection needle for measuring the conductive layer inside a hole in a printed circuit board, The inspection needle comprises a conductor surrounded by a shield (24), The inspection needle (19) has a connection end (22) that electrically connects the inspection needle (19) to the capacitance measuring device, and a measuring end (23) that is inserted into the hole during measurement. The capacitance measuring body (26) connected to the conductor is an inspection needle positioned outside the shield (24) at the measuring end (23) and forming a capacitance coupling with the conductive layer in the hole.

2. The inspection needle according to claim 1, wherein the measuring body (26) is surrounded by an insulating layer.

3. The inspection needle according to claim 1 or claim 2, wherein the measuring body (26) is a hollow cylindrical shape.

4. The inspection needle according to claim 1 or claim 2, wherein the measuring body (26) is one or more windings of the conductor disposed outside the shield (24).

5. The inspection needle (19) comprises a conductive tube (24) that forms the shield, A cable (25) having the conductor and an insulating layer is guided inside the conductive tube (24) and to the measuring end. The cable (25) protrudes a short distance from the pipe (24) at the connection end (22) in order to connect to the capacitance measuring device. The cable (25) has a short projection at the measuring end (23) so as to form the measuring body (26), which is one or more windings around the pipe (24). The inspection needle according to claim 4.

6. The conductive tubes (24, 28) are cut in an inclined shape in the region of the measuring tip (27) such that the outlet opening (30) is formed in an inclined shape in the axial direction of the tubes (24, 28). The cable (25) is routed through the outlet opening (30). The inspection needle according to claim 5.

7. An inspection probe for measuring the conductive layer inside a hole in a printed circuit board, An inspection needle (12) according to any one of claims 1 to 6, A touch sensor unit (20, 21) that determines whether the inspection needle (12) is in contact with another object, A test probe equipped with [a specific feature / equipment].

8. The inspection probe according to claim 7, wherein the touch sensor section (20, 21) comprises a spring-elastic mount (16, 17) that supports the inspection needle (12) and a sensor that detects the deflection of the spring-elastic mount (16, 17).

9. The inspection probe according to claim 8, wherein the sensor for detecting the deflection of the spring-elastic mounts (16, 17) is an optical sensor, in particular a photoelectric beam device (21).

10. The inspection probe (12) is provided with a stop portion (13) on which the inspection needle (19) and / or the spring elastic mount (16, 17) rest when they are not bent, according to claim 8 or 9.

11. A flying probe tester for inspecting printed circuit boards, particularly for inspecting bare printed circuit boards, The printed circuit board (4) is equipped with at least one inspection finger (2) that can move freely within a predetermined inspection area so as to be movable near predetermined contacts, A flying probe tester wherein the inspection finger comprises an inspection needle according to any one of claims 1 to 6, or an inspection probe according to any one of claims 7 to 10.

12. The flying probe tester (1) comprises a plurality of the inspection fingers (2), One or more of the inspection fingers have an inspection needle (10) for electrically contacting a predetermined contact on the printed circuit board (4). The flying probe tester according to claim 11.

13. A method for measuring the conductive layer within a hole in a printed circuit board, Using the flying probe tester according to claim 11 or claim 12, An inspection needle equipped with a capacitance measuring element (26) is inserted into the hole. Simultaneously with the detection of the position of the capacitance measuring body (26), the capacitance of the capacitance measuring body (26) with respect to its surroundings is measured. A method for determining whether there is a conductor near the detected location, based on the measured capacitance.

14. The method according to claim 13, wherein, in order to measure the capacitance, an electrical signal having a frequency of at least 1 kHz is applied to a conductive track having a conductive portion near the hole to be measured and / or to which the measuring body (26) is applied.

15. The method according to claim 13 or 14, wherein the determination of whether or not there is a conductor in the vicinity of the measuring body (26) is performed by comparison with the capacitance profile of an appropriate hole.

16. At least one conductor track (33) of the printed circuit board is connected to electrical ground. The conductor track (33) is preferably a conductor track that extends far across the printed circuit board, such as a ground conductor track. The method according to any one of claims 13 to 15.

17. A current measuring device is used to measure the capacitance. The method according to claim 16, wherein the input side of the current measuring device has the potential of the electrical ground applied to the current measuring device.

18. A measurement signal is applied to multiple conductive layers within the region of the plated through-hole. The signal is detected by the measuring body (26). The method according to claim 16 or 17.

19. The multiple measurement signals are oscillation signals having different frequencies. The different measurement signals are identified in the measurement based on the different frequencies. The method according to claim 18.

Images