Test needle for testing printed circuit board, test probe and flying probe tester

Abstract

To manufacture a test needle which can easily, more surely and very accurately measure back-drilled holes on a printed circuit board and measure electrically conductive layers in the holes on the printed circuit board, a corresponding test probe, and flying probe tester.SOLUTION: The invention relates to a test needle (19) for measuring electrically conductive layers in holes on printed circuit boards, and to a test probe (12) having a test needle of this type and a flying probe tester for testing printed circuit boards which has a test needle of this type or a test probe of this type. The test needle has a capacitive measuring element (26) which is connected to a capacitive measuring device by means of a cable (25). The cable is shielded such that only the capacitive measuring element is able to form a capacitive coupling with additional electrically conductive elements. As a result, this capacitive coupling can be detected with high positional resolution.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a test needle for measuring conductive layers in holes in printed circuit boards, a test probe having such a test needle, and a flying probe tester for testing printed circuit boards with such a test needle or test probe. [Background technology]

[0002] When a printed circuit board is manufactured, it is first manufactured with a so-called coupon, which is an area of ​​the printed circuit board with predetermined conductor tracks, on which measurements can be made. After the measurements have been made to check whether the printed circuit board has been manufactured properly, the coupon is separated from the remainder of the printed circuit board, which comprises one or more so-called application areas or panels that are equipped with electrical components and are used in electrical products.

[0003] Different electrical properties can be measured on the coupons. The coupons have special conductor tracks, where it can be easily determined, for example, whether the requirements for producing conductor tracks suitable for high frequencies were met in the manufacturing process. The conductor tracks of the coupons can also be used to perform load tests, which can also be destructive to individual conductor tracks on the coupons. The conductor tracks of the coupons can be specifically optimized for a particular test procedure, since they are not used in the subsequent product.

[0004] However, not all measurements on the coupon are transferable to the conductor tracks in the used area: the conductor tracks in the used area often have different dimensions than the conductor tracks on the coupon, so the measurement results on the coupon are not necessarily transferable to the conductor tracks in the used area.

[0005] For a long time, measuring probes have also existed that apply high-frequency signals to the conductor tracks of the area of ​​use in order to check high-frequency properties. Such high-frequency measurements can only be performed reliably if the contact of the conductor tracks occurs in an electrically compatible manner. If reflections occur, it is difficult to determine the actual location of the defect. Furthermore, high-frequency measurements always require a ground reference point in the vicinity of the contact. The design of a corresponding measuring head is therefore costly.

[0006] For measuring conductive layers in back-drilled holes, a test head with a conductive elastomer is known from US 9459285. Said test head can mechanically sense and electrically measure the conductive layer.

[0007] It is also known from US Patent No. 8,431,834 to provide a printed circuit board with an additional conductor track. An impedance measurement between this additional conductor track and the actual conductor track can be used to verify whether the back-boring has been made deep enough or too deep. Providing additional conductor tracks of this kind is very expensive and significantly increases the cost of the multilayer printed circuit board.

[0008] US 2015 / 047892 discloses a method in which the drill is simultaneously used as a probe so that the electrical contact with the conductor track during back-boring can be measured and the drill controlled accordingly.

[0009] US 9341670, US 9488690 and US 9739825 disclose methods and devices that can be used to measure so-called "stubs" in holes in printed circuit boards. Such stubs are remnants or stumps of a conductive layer in holes in printed circuit boards, which are generally electrically connected to conductor tracks. To remove such stubs, the holes in printed circuit boards are often drilled a second time, called back-drilling. This back-drilling can remove stubs that were unintentionally created during the coating of the individual layers with a conductive layer, generally a copper layer. However, this back-drilling can also interrupt the desired electrical connection of the conductor tracks. The devices comprise a capacitive probe that is inserted into the back-drilled hole in the printed circuit board, the capacitance of which is measured. Any other conductive material present in the back-drilled hole is detected by capacitance. In this way, back-drilling defects can be detected. Summary of the Invention [Problem to be solved by the invention]

[0010] The object of the present invention is to create a test needle, a corresponding test probe and a flying probe tester for measuring conductive layers in holes in printed circuit boards, which can easily, more reliably and very accurately measure back-drilled holes in printed circuit boards according to the method described in U.S. Pat. No. 9,341,670. [Means for solving the problem]

[0011] This object is achieved by the subject matter of the independent claims. Advantageous embodiments of the invention are disclosed in the respective dependent claims.

[0012] A test needle according to the present invention for measuring conductive layers in holes in printed circuit boards comprises a conductor surrounded by a shield, the test needle having a connection end capable of electrically connecting the test needle to a capacitance measurement device and a measurement end capable of being inserted into the hole during measurement.

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

[0014] Because the conductors of the test needle are shielded and only the capacitance measuring body can form capacitive couplings with other conductors, the test needle can very accurately detect the position of the capacitance measuring body based on the capacitance measured relative to the conductive layer.

[0015] The measuring body is preferably surrounded by an insulating layer so that no electrical contact can occur between the measuring body and other conductors.

[0016] The measuring body may be in the form of a hollow cylinder.

[0017] The measurement body may also be one or more turns of the conductor disposed outside the shield.

[0018] The measuring body preferably has a maximum diameter of 1.5 mm, in particular a maximum diameter of 1 mm, preferably a maximum diameter of 0.75 mm.

[0019] The measuring body preferably has a maximum length in the axial direction of the test needle of 0.5 mm, preferably a maximum length of 0.25 mm, in particular a maximum length of 0.15 mm.

[0020] The smaller the measuring body, the higher the resolution of the position sensed by the test needle.

[0021] On the other hand, the larger the area over which the measuring body extends, the greater the capacitive coupling to the conductive layer in the hole and the stronger the measured signal. In order to detect the conductive layer, it is necessary to achieve both position resolution and capacitive coupling.

[0022] The test needle may comprise a conductive tube forming the shield. A cable having the conductor and the insulating layer is guided inside the conductive tube. The cable may protrude from the tube at the connection end to be connected to a capacitance measuring device. The cable may protrude from the measuring end to be arranged around the tube to form the measuring body, which is one or more turns.

[0023] This is a very simple design of the test needle, but nevertheless allows very accurate measurements.The cable is preferably wound with 2, 3, 4 or more turns around the conductive tube to form the measuring body.

[0024] A test probe for measuring a conductive layer in a hole in a printed circuit board may be provided with the above-mentioned test needle, and the test probe includes a touch sensor portion for determining whether the test needle is in contact with another object.

[0025] The touch sensor unit can be used to measure the depth of a blind hole by first contacting the surface of the printed circuit board to be tested with the test probe, then contacting the bottom of the blind hole, and detecting each contact to sense the position of the test probe. The difference between the two positions gives the depth of the blind hole.

[0026] Since the actual capacitance measurement should be performed as non-contact as possible, the touch sensor portion can also be used to determine whether the test needle is touching another object in order to move the test needle so that it does not touch other objects, if necessary.

[0027] The touch sensor portion may include a spring elastic mount that supports the test needle, and a sensor that detects bending of the spring elastic mount.

[0028] The sensor detecting the deflection of the spring-elastic mount can be an optical sensor. The optical sensor can be realized in various ways. For example, a light source can be placed directly on the spring-elastic mount or indirectly by an optical fiber. The light emitted by the light source is detected by an optical sensor. When the spring-elastic mount deflects, a light cone moves away from the optical sensor. This can be detected by a change in brightness. Similarly, a photoelectric beam can be provided.

[0029] The test probe may be provided with a stop against which the test needle and / or the spring-elastic mount rest in the undeflected state. In this way, the position of the test needle is clearly defined relative to the test probe in the undeflected state. It is also possible to deduce the positions of the test needle and the measuring body based on the position of the test probe, if this position is known.

[0030] A flying probe tester for testing printed circuit boards, in particular bare printed circuit boards, preferably comprises test fingers which are freely movable within a predefined test area so as to be able to move into the vicinity of predefined contacts of said printed circuit board, said test fingers being provided with the above-mentioned test needles or with the above-mentioned test probes.

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

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

[0033] The method uses the above-mentioned test needle, the above-mentioned test probe, or the above-mentioned flying probe tester. The test needle with the capacitance measuring body is inserted into the hole. In this case, the position of the capacitance measuring body is detected and at the same time the capacitance of the capacitance measuring body 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 with the capacitance measuring body. The capacitance measuring body can also be inserted into a small hole to more reliably detect the conductive layer on the inner surface of the hole.

[0035] In addition to measuring the actual capacitance, the touch sensor portion described above can also be used to derive the measured blind hole depth, which can be done before, during, or after the capacitance measurement.

[0036] To measure the capacitance, an electrical signal having a frequency of at least 1 kHz is applied. Higher frequencies, for example at least 2 kHz or at least 4 kHz, can also be used. A measurement signal can be applied to the electrical conductor of the test needle and to the capacitance measuring body. In a conductor track connected to a part of the electrical conductor to be measured, the measurement signal induced therein is sensed. However, it is preferred that the measurement signal is applied to the conductor track connected to the electrical conductor and sensed by the capacitance measuring body. The measurement signal of the electrical conductor is induced in the capacitance measuring body and can be detected and evaluated accordingly by a measuring device. This is particularly advantageous when the electrical conductor is connected to a number of conductive parts arranged in different holes. The test finger is then necessarily placed on the electrical conductor and the test probe can sense the electrical conductor in several holes one after the other.

[0037] When determining whether the conductor is in the vicinity of the measurement object, a comparison can be made with the capacitance profile of the appropriate hole. A "suitable hole" is a hole whose inner surface is provided with the conductive layer connected to the conductor to which the measurement signal is applied in a desired area, while other areas of the inner surface are not provided with the conductive layer. Based on the comparison, the position of the conductive layer on the inner surface of the hole can thus be determined. When the measured capacitance profile matches the predetermined capacitance profile, the hole is suitable. That is, the conductive layer is provided only in the desired places. However, when there is a deviation, it means that the conductive layer is missing in a certain place or is provided in a place where it should not be.

[0038] In this method, the holes are formed essentially perpendicular to the surface of the printed circuit board, so that the test needles are always kept as perpendicular as possible to the surface of the printed circuit board being tested, in this way the test needles are axially aligned with each hole and the risk of the test needles colliding with the inner surface of the hole is small. [Brief description of the drawings]

[0039] The invention is explained in more detail below on the basis of an exemplary embodiment shown in the drawing.

[0040] [Figure 1] FIG. 1 is a perspective view of a flying probe tester. [Figure 2a] FIG. 2a is a perspective view of a test probe having a test needle according to the present invention. [Figure 2b] FIG. 2b is a perspective view of a test probe having a test needle according to the present invention. [Figure 2c] FIG. 2a is a side view of a test probe having a test needle according to the present invention. [Diagram 3] FIG. 3 is a detailed cross-sectional view of a test needle with a test tip. [Figure 4] FIG. 4 is a schematic diagram of a measurement setup for measuring conductive layers in plated-through holes of a printed circuit board, including a printed circuit board shown in cross-section and a ground conductor track connected to electrical ground. [Diagram 5] FIG. 5 is a schematic diagram of a measurement setup for measuring conductive layers in plated-through holes of a printed circuit board, including a printed circuit board shown in cross-section and a ground conductor track that is not connected to electrical ground. [Figure 6] FIG. 6 is an equivalent circuit diagram of the measurement device of FIGS. 4 and 5 for non-contact measurement of the conductive layer in the plated through-hole. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] The invention is realized by a flying probe tester 1 (FIG. 1), which comprises a number of test fingers 2. Each test finger 2 is provided with a test probe 3 capable of making electrical contact with a printed circuit board 4 to be tested in order to apply or sense measurement signals on conductor tracks.

[0042] A test probe 3 of this kind is described, for example, in WO 03 / 048787, which document is hereby referred to in this respect.

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

[0044] Each guide rail 6 is provided with at least one slider 9 arranged in sliding manner thereon. Each slider 9 supports one inspection finger 2. In the present exemplary embodiment, two sliders 9 are provided on each guide rail 6. However, in principle, more than two sliders 9 can also be provided on one guide rail 6.

[0045] The test finger 2 is pivotally fixed at one end to a slider 9. Such a design of a flying probe tester can be inferred, for example, from WO 2014 / 140029, to which reference is made in this respect in its entirety.

[0046] The test probes 3 are arranged at the ends remote from the slider 9, i.e. at the free ends of the test fingers 2. Each test probe 3 comprises a test needle 10 with a contact tip 11 oriented towards the printed circuit board 4 to be contacted. The test fingers 2 together with the test probes 3 are movable towards and away from the printed circuit board 4 in order to place 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 invention, at least one or several test fingers 2 each include a test probe 12 (FIGS. 2a to 2c) for contactlessly sensing capacitance in holes (particularly blind holes) in a printed circuit board 4. Such a test probe 12 includes a base plate 13 which is a printed circuit board.

[0048] The base plate 13 is provided with a detent body 14. The detent body 14 has two detent arms 15 by means of which the test probe 12 can be attached to one of the test fingers 2 in the manner of a detent.

[0049] Two spring arms 16, 17 are fixed to the claw body 14. One of the two spring arms is placed 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 to the surface of the claw body 14 facing away from the base plate 13 by two screws.

[0050] The spring arms 16, 17 are oriented parallel to each other. Viewed from above, the base plate 13 and the spring arms 16, 17 are tapered starting from the claw 14 towards the free ends 18 from which the two spring arms 16, 17 each protrude for a short distance. At these ends of the spring arms 16, 17, each test needle 19 is fixed in such a way that the elastic spring arms 16, 17, the claw 14 and the part of the test needle 19 connecting the two spring arms 16, 17 form a parallelogram. The free ends of the two spring arms 16, 17 can be pivoted away from the base plate 13. The test needles are thus supported in an elastic manner on the test probe 12. In the following, the spring arms 16 resting directly on the base plate are called base spring arms 16 and the spring arms 17 that are away from the base plate 13 are called free spring arms 17. The base plate 13 thus provides a stop for the base spring arm 16 and therefore for the movement of the test needle 19 relative to its support by the body of the test probe 12 .

[0051] When viewed from above, the two spring arms 16 and 17 form a substantially triangular frame. A measurement tag 20 facing the base plate 13 is disposed 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, 17 are not deflected. When the spring arms are deflected, the measurement tag 20 moves so as to be out of the photoelectric beam. As a result, the light sensor detects an increase in brightness. In this way, the photoelectric beam device 21 and the measurement tag 20 form a touch sensor unit, and detect that the spring arms 16, 17 are deflected by the contact of the test needle 19 with another object.

[0053] The test needle has a connection end 22 and a measuring end 23. In the region of the connection end 22, the free spring arm 17 is connected to the test needle 19. The test needle 19 is connected to the base spring arm 16 in such a way that when the spring arms 16, 17 are in the unflexed state, i.e. when the base spring arm 16 rests against the base plate 13, the test needle 19 is arranged perpendicular to the plane of the base plate 13.

[0054] The test probe 12 and corresponding test finger 2, when the test needle is attached to the test finger 2, are positioned perpendicular to the test area and the printed circuit board 4 placed in the test area with the spring arms 16, 17 undeflected.

[0055] The test needle 19 of the test probe 12 protrudes to a short extent relative to the base plate 13. The test needle 19 comprises a metal tube, which is electrically conductive and has an outer diameter of preferably at most 0.2 mm, particularly preferably at most 0.18 mm. A cable 25, which comprises an electrical conductor surrounded by an insulating layer, is led into the metal tube 24. In the present exemplary embodiment, the cable 25 is connected to a conductor track on the base plate 13 and is electrically connected to a capacitance measuring device (not shown). The conductor of the cable 25 is connected at the measuring end 23 to a capacitance measuring body 26. The capacitance measuring body is a metal ring arranged around the tube and is electrically insulated from it. The capacitance measuring body 26 can be a metal ring provided with an insulating layer and is electrically connected to the conductor of the cable 25. However, the capacitance measuring body 26 can also consist of one or more turns of the cable 25.

[0056] The metal tube 24 serves to shield the cable 25. This allows only the capacitance measuring body 26 to form a capacitive coupling with conductors placed in its vicinity, and only the capacitive coupling of the capacitance measuring body 26 with other conductors is sensed by the cable 25.

[0057] The measuring end 23 of the test needle 19 is provided with a measuring tip 27. This measuring tip is used only for mechanical contact with other objects. The measuring tip may be made of a non-conductive material or may have an insulating layer. In the present exemplary embodiment, the measuring tip 27 is not used for electrical contact with contacts on a printed circuit board.

[0058] However, in the present invention, the measuring tip can also be realized as an electrical conductor so that it can be used to create an electrical connection with a contact of 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 taken into account 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 consists of a cannula tube 28. One end of the cannula tube 28 is mechanically connected in a conductive manner to the tube or shield 24 by means of a solder connection 29. In the region of the solder connection 29, the cannula tube has a diameter of, for example, 0.3 mm. The cannula tube points with its axis towards the measuring tip 27 and in the region of the measuring tip 27 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 oblique shape, resulting in an outlet opening 30 extending in an oblique shape.

[0060] The cable 25 passes through the tube 24 and the cannula tube 28 and leaves the cannula tube 28 at an exit opening 30. In the region of the measuring tip 27, the cable is wound three times around the cannula tube 28 to form the capacitance measuring body 26.

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

[0062] Such a conductive type measuring tip has the advantage that it is possible to calibrate the position of the test needle 19 and the position of the test finger 2 to which the test needle 19 is fixed, for example, since the test needle 19 can be contacted with a calibration surface having a conductive surface portion and a non-conductive surface portion.

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

[0064] A typical measurement procedure for measuring the conductive coating inside a blind hole in a printed circuit board by means of a flying probe tester 1 is carried out as follows.

[0065] A test probe 12 according to the invention is provided with a test finger 2 which mechanically contacts the surface of the printed circuit board 4 to be tested in order to derive its height.

[0066] The test probe with the test needle is inserted into the blind hole of the printed circuit board 4 to be measured until the measuring tip 27 touches the bottom of the blind hole in the printed circuit board 4. The touch sensor detects the position of the test probe 12 at the time of contacting 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 test probe (more precisely, its measuring tip 27) touches the surface of the printed circuit board.

[0067] By means of the capacitance measuring body 26 arranged in the blind hole, the capacitance between the capacitance measuring body 26 and a conductor arranged in the blind hole is measured. In the measurement, a predetermined measurement signal is applied to this conductor by means of the other test finger 2, which is in contact with a contact of the printed circuit board connected to this conductor. The measurement signal preferably has a frequency of at least 1 kHz, in particular 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 generated between the capacitance measuring body 26 and the adjacent conductor. At the same time, the position of the capacitance measuring body 26 is sensed. 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 defined by the movement of the corresponding test finger 2 and is known in the flying probe tester 1. By simultaneously sensing the position and the capacitance while the capacitance measuring body 26 moves in 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. A capacitance profile is thereby created. On the basis of this, it is possible to estimate whether the inner surface of the blind hole is covered by a conductor or not.

[0069] The flying probe tester 1 preferably comprises several test fingers 2, each with a test probe 12 according to the invention. This allows several blind or through holes to be measured simultaneously. During the simultaneous measurements, preferably different measurement signals are used in frequency. This, together with corresponding band-pass filters, makes it possible to prevent crosstalk from one measurement to another.

[0070] Each guide rail 6 is preferably provided with at least one test finger 2 equipped with a test probe 3 having a conventional test needle 10 for electrical contact with a contact of the printed circuit board and another test finger 2 equipped with a test probe 12 according to the invention. It may also be advantageous to provide a guide rail 6 with two test fingers 2 equipped with conventional test probes 3 and another test finger 2 equipped with a test probe 12 according to the invention.

[0071] However, the invention can also be used in conventional flying probe testers having several separate cross beams which are not arranged on a combined traversing unit 5. In a flying probe tester of this kind, it is likewise advantageous to provide each cross beam with at least one test finger 2 equipped with a test probe 12 according to the invention and one or more test fingers 2 equipped with a conventional test probe 3.

[0072] In this manner, the inspection probe of the present invention makes it very easy to accurately determine the dimensions of a blind or through hole, and at the same time, to determine whether the inner surface of the blind or through hole is coated with a conductive material.

[0073] Furthermore, the test probe 12 according to the invention can be used in conventional flying probe testers. No other mechanical modifications are required. The flying probe tester only needs to be equipped with a suitable control program capable of applying suitable measurement signals, processing the measurement signals generated by the test probe 12 according to the invention, and controlling the operation of the test finger 2 equipped with the test probe 12 according to the invention.

[0074] In the following, a method for measuring the plated through hole 31 in the hole 37 of the printed circuit board 38 is described. The plated through hole 31 is a hole with a conductive coating applied to the inner surface of the hole. When the printed circuit board 38 is manufactured, the hole is first coated over the entire hole. The hole is then back-drilled a second time to a depth such that the coating of the hole 37 is again removed in a predetermined area. In this connection, defects can occur when the back-drilling is performed in such a way that it is not deep enough or when it is performed in such a way that the edge of the coating in the hole is slightly offset from the desired position because the second hole is slightly offset from the first hole. Also, the two holes can no longer be concentric because the second hole is slightly offset from the first hole. As a result, an unwanted streak of thin coating can remain in the area of ​​the hole. The streak extends in a direction approximately parallel to the central axis of the hole. Such a streak can also occur when the second hole is slightly inclined with respect to the first hole.

[0075] The printed circuit board 38 has several layers 32 between which conductor tracks 33. There are large and small conductor tracks. As a rule, the so-called ground conductor tracks are the largest conductor tracks of the printed circuit board and can often extend over several layers 32. In the measurement method described below, the ground conductor tracks are connected to an electrical ground 34 (FIG. 4). As an alternative to each ground conductor track, individual conductor tracks that are smaller and connected simultaneously to the electrical ground 34 can also be used. In the following description, the term "ground conductor track" is used to describe one or more conductor tracks that can be connected simultaneously to an electrical ground and that can branch out over a large area of ​​the printed circuit board 38 (in particular over several layers). This connection to the electrical ground 34 is preferably generated by placing a contact finger of a flying probe tester against the conductor track. This contact finger is equipped with a conductive contact tip that is connected to the electrical ground.

[0076] When the test probe 3 is inserted into the hole, the capacitance measuring body 26 forms a capacitance with the surrounding conductors. The purpose of the present 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 oscillating signal relative to electrical ground to the plated through-hole 31. This measurement signal flows along the plated through-hole 31, via 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 via the cable 25 to the current measuring device 36. In the current measuring device 26, the voltage and the capacitance C1, or more precisely the change in the capacitance, are measured when the capacitance measuring body 26 is moved in the vicinity of the plated through-hole 31.

[0078] A parasitic capacitance C2 is generated between the plated through hole 31 and the adjacent conductor track. The 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 induces a measurement signal in the other conductor track. The measurement signal is transferred to the ground conductor track via a capacitive coupling (capacitance C3). As a result, the capacitance measuring body observes a measurement signal via a capacitive coupling (capacitance C4) between the capacitance measuring body 26 and the conductor track located outside the plated through hole (FIG. 5). As a result, it is not possible to determine exactly 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] In the following, the function of the measuring device of Figures 4 and 5 is explained on the basis of an equivalent circuit diagram (Figure 6), which has the following elements: measuring body 26, plated through hole 31, electrical ground 34 (switchable ground finger), signal generator 35, current measuring device 36.

[0080] Capacitor C1 is the junction between plated through hole 31 and measurement body 26.

[0081] The capacitor C2 is located between the plated through hole 31 and the ground conductor track, as defined in the above description. The capacitor C3 is the coupling between the ground conductor track and the other conductor track. The capacitor C4 is the coupling between the conductor track and the measuring body 26. The coupling between the conductor track and the measuring body 26 can be made directly 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 made between the ground conductor track and the measuring body 26, the capacitor C3 is eliminated. This is why the capacitor C3 is short-circuited in the equivalent circuit diagram as shown by a dashed line and why it does not need to be taken into account in the following discussion.

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

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

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

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

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

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

[0088] In this way, no current flows through capacitors C3 and C4 because they are located between two points at electrical ground potential, and therefore capacitors C3 and C4 do not show up in the measurement.

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

[0090] As a result, by connecting the ground 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 is derived, making it possible to gather information about the geometrical size of defects in the plated through-hole.

[0091] This method is so accurate that it can be modified to apply the signal of the signal generator to a single conductor track that runs into the through-hole, so that on the basis of this signal it is possible to detect the capacitive coupling of the measurement body to said conductor track, and in this way determine the position of the measurement body within the through-hole and determine that said conductor track is correctly conducting the measurement signal to the region of the plated through-hole.

[0092] Typically, the position of the test probe relative to the position of the printed circuit board to be tested is calibrated by touching the contact tip of the test probe 12 to the surface of the printed circuit board. This mechanical calibration can be replaced by an electrical calibration. When an electrical calibration is performed, an oscillating signal is applied to the conductor track adjacent to the through-hole and the signal is detected by the test probe (more precisely, by the measuring body of the test probe).

[0093] The method can also be modified in such a way that multiple measurement signals of different frequencies are applied to different conductor tracks and / or plated through holes. During the measurement, the different frequencies are distinguished. This can be done, for example, when a variable band-pass filter is provided before the current measuring device 36 and is connected to each frequency band that respectively includes the frequency of one of the measurement signals. This modification allows the detection of multiple conductor tracks in the vicinity of a plated through hole, as long as the multiple conductor tracks are not shielded by the plated through hole. [Explanation of symbols]

[0094] 1 Flying Probe Tester 2 Inspection Finger 3 Inspection probe 4 Printed Circuit Board 5 Crossing Unit 6 Guide rail 7 Support material 8 Through opening 9 Slider 10 Test Needle 11 Contact tip 12 Inspection probe 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 Test Needle 20 Measurement Tag 21 Photoelectric beam device 22 Connection end 23 Measuring end 24 tubes 25 Cable 26 Capacitance measuring body 27 Measuring tip 28 Cannula 29 Solder joint 30 outlet opening 31 Plated Through Holes 32 layers 33 Conductor Track 34 Electrical Grounding 35 Signal Generator 36 Current measuring device 37 Hole 38 Printed Circuit Board 39 Operational Amplifier 40 Measuring Resistance

Claims

1. 1. A test needle for measuring a conductive layer within a hole in a printed circuit board, comprising: the test needle comprises a conductor surrounded by a shield (24); the test needle (19) has a connection end (22) for electrically connecting the test needle (19) to a capacitance measuring device and a measurement end (23) for insertion into the bore during measurement; A test needle in which a capacitance measuring body (26) connected to the conductor is positioned outside the shield (24) at the measuring end (23) and forms a capacitive coupling with the conductive layer in the hole.

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

3. 3. The test needle of claim 1 or 2, wherein the measuring body (26) is hollow cylindrical in shape.

4. 3. The test needle of claim 1, wherein the measuring body (26) is one or more turns of the electrical conductor disposed outside the shield (24).

5. The test needle (19) comprises a conductive tube (24) forming the shield; A cable (25) having the conductor and an insulating layer is guided to the inside of the conductive tube (24) and to the measurement end, the cable (25) protrudes short from the tube (24) at the connection end (22) for connection to the capacitance measuring device; the cable (25) protrudes short at the measuring end (23) to form the measuring body (26) which is one or more turns around the tube (24); 5. The test needle of claim 4.

6. the conductive tube (24, 28) is cut in an oblique manner in the region of the measuring tip (27) in an axial direction of the tube (24, 28) such that the outlet opening (30) is formed in an oblique manner, The cable (25) is routed through the outlet opening (30).

6. The test needle of claim 5.

7. 1. A test probe for measuring a conductive layer within a hole in a printed circuit board, comprising: A test needle (12) according to any one of claims 1 to 6, a touch sensor unit (20, 21) for determining whether the test needle (12) is in contact with another object; An inspection probe comprising:

8. 8. The test probe according to claim 7, wherein the touch sensor portion (20, 21) comprises a spring-resilient mount (16, 17) supporting the test needle (12) and a sensor for detecting deflection of the spring-resilient mount (16, 17).

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

10. 10. The test probe according to claim 8 or 9, wherein the test probe (12) is provided with a stop (13) against which the test needle (19) and / or the spring-elastic mount (16, 17) rest in an undeflected state.

11. 1. A flying probe tester for testing printed circuit boards, in particular for testing bare printed circuit boards, comprising: at least one test finger (2) freely movable within a predetermined test area so as to be movable adjacent to a predetermined contact of the printed circuit board (4); A flying probe tester, wherein the test finger comprises a test needle according to any one of claims 1 to 6 or a test probe according to any one of claims 7 to 10.

12. The flying probe tester (1) comprises a plurality of the test fingers (2), one or more of the test fingers have test needles (10) for electrically contacting predetermined contacts of the printed circuit board (4); 12. The flying probe tester of claim 11.

13. 1. A method for measuring a conductive layer within a hole in a printed circuit board, comprising: Using the flying probe tester according to claim 11 or 12, A test needle having a capacitance measuring body (26) is inserted into the hole; The position of the capacitance measuring body (26) is detected and at the same time the capacitance of the capacitance measuring body (26) with respect to its surroundings is measured; A method in which it is determined whether a conductive object is present in the vicinity of a sensed location based on the measured capacitance.

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

15. 15. The method according to claim 13 or 14, wherein the determination of the presence of a conductive object in the vicinity of the measuring body (26) is made by comparison with a capacitance profile of a suitable 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; 17. The method of claim 16, wherein an input of the current measuring device has a potential of electrical ground applied to the current measuring device.

18. A measurement signal is applied to a plurality of said conductive layers in the area of ​​the plated through holes; The signal is detected by the measuring body (26).

18. The method of claim 16 or 17.

19. the plurality of measurement signals are oscillating signals having different frequencies; The different measurement signals are distinguished in the measurement based on the different frequencies.

20. The method of claim 18.

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