Method and device for determining a stub length

Abstract

A first aspect of the invention relates to a method for determining a stub length of a stub in a conductor track. The stub remains in a plated through-hole above an upper conductor layer to a lower conductor layer in the printed circuit board during back-drilling. The method comprises: - providing a specified back-drilling depth, - determining the actual back-drilling depth of a back-drilled borehole using a test finger by mechanically scanning a bottom of a blind hole produced during the back-drilling, and - determining the stub length on the basis of the difference between the actual back-drilling depth and the specified back-drilling depth.

Description

[0001] Method and apparatus for determining a stump length

[0002] The invention relates to a method and a device for determining a stub length.

[0003] A printed circuit board (PCB), also known as a printed circuit board, is a substrate for electronic components. It serves to mechanically mount and electrically connect these components. The PCB's base material is a non-conductive substrate, such as glass-fiber reinforced plastic. Copper traces, which serve as electrical connections between the components, are located on the surface of individual conductor layers. Solder pads or vias made of a conductive material, such as copper, are used at the points where components are to be placed. These provide mechanical mounting and electrical contact for the components. A plastic coating protects the conductors and solder pads from short circuits and contamination.

[0004] Modern printed circuit boards comprise several layers in a vertical direction, with each layer potentially containing conductive layers to electrically connect the components arranged on the circuit boards (FIG. 1 a).

[0005] Conductor layers are layers of the printed circuit board and have conductive traces, for example made of copper, which represent at least partial sections between the components.

[0006] Conductor layers on different vertical planes can be connected to each other using vias. In this process, an upper and a lower conductive layer are connected by a via perpendicular to the plane of the conductive layers. If the printed circuit board (PCB) is flipped over, the upper conductive layer becomes the lower conductive layer, and vice versa. The via, also called a through-hole, is an electrical connection between different conductive layers of a multilayer printed circuit board (PCB). It enables the connection of signal and current-carrying traces between conductor layers on different levels of the PCB. Vias are created by first drilling a hole completely through the PCB at the desired location (FIG. 1b).

[0007] The hole is then lined with copper (FIG. 1c). The walls of the hole are coated with a conductive material, such as copper, using a plating process like electroplating. The metallized wall of the hole forms a plating that allows conductive traces on the various layers of the printed circuit board to be connected.

[0008] The remaining hole can be filled with resin (FIG. 1 d).

[0009] The areas that should not contain copper are drilled out along with the resin. This is also called backdrilling (FIG. 1 e). According to this description, drilling is done from top to bottom due to the vertical orientation. However, drilling from bottom to top is also possible.

[0010] It is important not to drill too deep during this backdrilling process, as otherwise the desired connection between a conductor layer and the via can be interrupted. The copper plating or cladding that protrudes upwards from a conductor layer to be contacted is called a stub (FIG. 1 f). Therefore, a small stub is deliberately left so that the contact between the cladding and the conductor track is not interrupted.

[0011] However, not too little material should be removed during the drilling process, otherwise not enough copper lining of the via will be removed. The length of the stub is usually fixed to a predetermined value.

[0012] A stub, also called a via stub or remnant, refers to the unused portion of a plating in a via on a multilayer printed circuit board (PCB). The stub is the part of the via located above the topmost conductor layer used by the via. The stub has no intended electrical function and does not contribute to signal transmission. Stubs are small, electrically conductive elements connected to this top conductor layer. In many electrical applications, such stubs can be disregarded.

[0013] However, it has been shown that such stubs can be detrimental to high-frequency signals and can interfere with them, especially if the stubs are too large. This is at least partly due to the fact that stubs act as small antennas. Since more and more signals are transmitted in the high-frequency range in current computer development, the systems are more susceptible to interference in these frequency ranges. Long stubs are particularly detrimental here, as the interference occurs even at lower frequencies. The length of the stubs is inversely proportional to the interfering frequencies they generate.

[0014] Small inaccuracies in thickness can occur during the printed circuit board (PCB) manufacturing process. These inaccuracies are typically very small. For PCBs a few millimeters thick, the inaccuracy is a few hundred micrometers. However, these inaccuracies are not taken into account during back-drilling and are the reason why the actual stub length varies.

[0015] Basically, a distinction is made between a top stub, which is located on the top side of the via, and a bottom stub, which is located on the bottom side of the via.

[0016] In addition to these stubs, if the drill bit is used for backdrilling at an angle or off-center, so-called slivers can remain. These are narrow copper strips running vertically in the bore. They can lead to similar problems as the stubs.

[0017] To determine the quality of a printed circuit board, it is therefore important to know the stub length.

[0018] Methods are known for estimating stub length. These methods begin by determining the positions of conductor layers through a test hole drilled at the edge of a printed circuit board (PCB). During this test hole, the drill bit penetrates the PCB. Coupons, which are part of the conductor layers, are positioned at predetermined locations, and a drill bit connected to an electrical detector detects these coupons via electrical measurements. Based on the data acquired, the positions of the conductor layers within the PCB are calculated, along with the required backdrilling depth. The drill bit is then controlled accordingly to achieve a specific stub length.

[0019] This method has two disadvantages. Firstly, the location where the test hole with the coupons is drilled is usually at the edge of a printed circuit board (PCB). A PCB can have a different thickness at the edge than at the location where the vias connecting the conductive layers are located. Therefore, the measurement of the drilling depth at the coupon may not be meaningful for measuring the stub length at another point on the PCB. Secondly, the arrangement of the individual coupons and the non-conductive layers may differ from that of the actual hole, which changes the composition and thus also the compression behavior and thickness. The vertical position of the conductive layer on the PCB can therefore differ between the edge where the coupons are located and where the vias are situated. This can lead to incorrect or inaccurate estimations.

[0020] Furthermore, the data obtained from the test drilling of the coupons must be stored, as the coupons are typically completely removed during the test process. Re-measurement is not possible, and should an error occur or uncertainties arise in the data, these cannot be corrected or traced.

[0021] Such a procedure cannot be carried out on printed circuit boards without coupons.

[0022] Instead of copper, other electrically conductive materials, such as silver, can also be used as a conductive layer and / or lining for a via. In the following, such an electrically conductive layer will be referred to as the "conductor layer" and the electrically conductive material from which the conductor layer and / or lining is formed will be referred to as the "conductor".

[0023] CN 110278660 A describes a method in which a conductive pin, protruding perpendicularly from a metal sheet, is inserted from the back of a printed circuit board into an existing hole, in the opposite direction to the hole being drilled. When the drill bit encounters the pin during drilling, this is electrically detected via the metal sheet and the drill bit, and the drilling process is terminated.

[0024] From DE 102018101031 A1 emerges a test probe with a capacitive measuring element to achieve capacitive coupling with high resolution.

[0025] US patent 2021 / 0153359 A1 discloses the removal of sections of stubs by drilling.

[0026] The object of the present invention is to provide a device and a method for reliably determining the length of conductor remnants in vias in a simple manner.

[0027] The problem is solved by the subject matter of the independent claims. Advantageous further developments and preferred embodiments form the subject matter of the dependent claims.

[0028] A first aspect of the invention relates to a method for determining the stub length of a stub in a conductor track. The stub remains in a via above an upper conductor layer to a lower conductor layer in the printed circuit board during back-drilling. The method comprises:

[0029] Providing a specified back-drilling depth,

[0030] Determining the actual back-drilling depth of a back-drilled hole with a test finger by mechanically probing the bottom of a blind hole created during back-drilling, and determining the stub length based on the difference between the actual back-drilling depth and the target back-drilling depth.

[0031] The circuit boards, conductor layers, vias and stubs are designed as mentioned at the beginning.

[0032] The target back-drilling depth is a predetermined value of the back-drilling depth with which the drilling process with a back-driller can be controlled for drilling the back-drilled hole and / or is returned as the drilled depth after drilling by a back-driller.

[0033] The actual back-drilling depth represents the measured back-drilling depth. It is measured by the test finger through mechanical probing of the bottom of the blind hole created during back-drilling.

[0034] An actual vertical position of the upper conductor layer or the lower conductor layer represents the actual height of the conductor layer relative to a given reference plane, e.g. the surface, within the printed circuit board, measured along the vertical axis of the printed circuit board.

[0035] Determining the actual back-drilling depth, the actual vertical position, or the stub length can involve an exact calculation or the application of mathematical, algorithmic, experience-based, and / or heuristic methods with which the respective quantity is qualifiedly estimated.

[0036] The difference between the target drilling depth V and the actual drilling depth I is denoted by AR in the following: R=VI

[0037] The difference AR between the specified back-drilling depth V and the actual back-drilling depth I is primarily due to an error during the back-drilling process. This can occur, for example, if aluminum dust has become trapped between an applied aluminum layer and the first layer of the printed circuit board during the back-drilling process. This creates an unwanted gap, resulting in the desired drilling depth not being fully achieved.

[0038] There can also be other causes, such as: drill wear, unsuitable drill type, poorly clamped drill, drill breakage, insufficient drill speed and consequently increased friction, excessive axial pressure, incorrectly specified hardness, incorrectly specified toughness, inhomogeneity, fluctuating temperature, external vibration, or similar factors. If the difference AR between the specified back-drilling depth V and the actual back-drilling depth I is known, it can be taken into account to calculate the stub length.

[0039] The difference AR is positive if the actual back-drilling depth is less than the specified back-drilling depth, and the difference is negative if the actual back-drilling depth is greater than the specified back-drilling depth.

[0040] The specified back-drilling depth is usually chosen such that, with a perfect PCB and perfect drilling, a small stub with a predetermined desired length L remains.

[0041] The specified stub length will be denoted by S in the following.

[0042] The difference AR causes a deviation from the desired stub length L:

[0043] S = L + V - I = L + AR.

[0044] The actual back-drilling depth corresponds to an upper starting position of the stub.

[0045] By comparing the actual back-drilling depth I and the target back-drilling depth V, the inaccuracies that arise during back-drilling and have been transferred to the actual length of the stubs are recorded and determined.

[0046] This allows the reliable determination of the length of copper stubs in vias on printed circuit boards. This is a very simple method that does not require capacitive measurement of the upper or outer end position of the stub. Furthermore, the target back-drilling depth is known in many printed circuit board production facilities, as it is used as a control parameter for the drilling machine. If this target back-drilling depth is individually adjusted for each drilling operation, this individual adjustment is also taken into account when calculating the stub length.

[0047] The procedure described above applies to both types of stubs, except that for the bottom stub, the terms top and bottom are reversed.

[0048] Stubs can alter the signal, particularly by causing reflections in the signal, which can lead to signal distortion, timing problems, and other signal integrity issues.

[0049] Stubs can also act as antennas and cause electromagnetic interference. Stubs can compromise the mechanical stability of the printed circuit board.

[0050] A test finger, also known as a probe, is an electrical component used to test printed circuit boards (PCBs). A test finger can be moved in a plane across any point within a test area where a PCB under test is located, scanning a contact point. A test finger can incorporate a variety of sensors to inspect different aspects of a PCB.

[0051] At least one of the test fingers has a touch sensor for mechanically probing the bottom of the backhole. This touch sensor may include a light barrier, which is triggered as soon as the tip of the test finger comes into contact with a surface, for example, the bottom of the borehole.

[0052] When determining the stub length, the difference between the target vertical position of the upper conductor layer and the specified back-drilling depth can be taken into account.

[0053] The difference AF between the target vertical position O of the upper conductor layer and the specified back-drilling depth V is usually equal to the length L of the ideal stub length.

[0054] The specific stub length S is then determined by:

[0055] S = L+Vl = L +AR = (OV)+AR = (OV)+Vl = AF + AR.

[0056] Here it can be seen that the specified back-drilling depth V is subtracted:

[0057] S = 0-1.

[0058] The determined stub length S is therefore the difference between the target vertical position O and the actual back-drilling depth I.

[0059] The actual thickness of the printed circuit board can be determined, in particular, by probing with test fingers from both sides of the circuit board.

[0060] The actual thickness may differ from the specifications provided by the PCB manufacturer. This difference affects the stub length remaining after back-drilling, as too much or too little material may have been removed.

[0061] Once the actual thickness has been determined, the difference to a corresponding specified thickness can be communicated to the person performing the back-drilling, in order to adjust the relevant parameters.

[0062] A target back-drilling depth, which can be a specification of the depth to be drilled in the back-drilled hole on a drill bit, can be determined based on the determined actual thickness.

[0063] The target back-drilling depth thus takes into account the actual thickness of the printed circuit board at the location of the back-drilling hole, while the hole itself is not considered. Depending on the thickness of the printed circuit board, the vertical position of the upper and lower conductor layers relative to each other and / or to the beginning of the back-drilling hole changes.

[0064] The target drilling depth can serve as a guideline for future drilling. If the target drilling depth is maintained, then the stub length is equal to the specified length L.

[0065] If the circuit board is exactly as thick as specified by the manufacturer, then the target back-drilling depth is equal to the specified back-drilling depth.

[0066] If the back-drilling is perfect, then the target back-drilling depth will be equal to the actual back-drilling depth.

[0067] If the circuit board is exactly as thick as specified by the manufacturer and the back drilling is perfect, then the target back drilling depth will also be the same as the actual back drilling depth.

[0068] The target drilling depth is referred to as B in the following.

[0069] When determining the stub length, the difference between the target back-drilling depth and the specified back-drilling depth can be taken into account.

[0070] By determining the target back-drilling depth, the inaccuracies that arise during the manufacturing process of the printed circuit boards and that have been transferred to the actual length of the stubs are detected and determined.

[0071] Since the actual thickness is also taken into account in the target drilling depth, this can increase the accuracy of the stub length.

[0072] The difference between the target drilling depth B and the specified drilling depth V is AN.

[0073] The difference AN between the target back-drilling depth B and the specified back-drilling depth V is mainly due to different positions of the conductor layers in the printed circuit board, which can vary, for example, due to the different compression of the individual layers of the printed circuit board during the manufacturing of the printed circuit board.

[0074] The definite stub length S is determined by:

[0075] S = L+AR + (BV)= L+ AR +AN.

[0076] The difference AF between the target vertical position O and the specified back-drilling depth V can also be taken into account here:

[0077] S = AR + AF + (BV) = AR + AF + AN. From the actual thickness, the actual vertical position of the upper conductor layer and / or the lower conductor layer can be estimated.

[0078] Typically, the vertical position of a conductor layer is specified by the printed circuit board (PCB) manufacturer. However, due to various inaccuracies, the thickness of individual layers and the overall thickness of the PCB can vary, meaning the actual vertical position may differ from the manufacturer's specified vertical position.

[0079] When determining the stub length, the difference between the actual vertical position and the target vertical position of the upper conductor layer can be taken into account.

[0080] The qualified estimation of the actual vertical position also allows for the detection and determination of inaccuracies arising during the manufacturing process of the printed circuit boards, which have been transferred to the actual length of the stubs.

[0081] Since the actual thickness of the printed circuit board (PCB) also influences the target back-drilling depth, careful consideration must be given to where this thickness is factored into the calculation. It can still be advantageous to consider both differences, as other factors can be taken into account when determining the actual vertical position, such as the PCB composition in the area of ​​the via. For example, if this area contains a large number of layers of conductive material, such as copper, the actual vertical position may differ from the target vertical position.

[0082] The difference between the actual vertical position P and the target vertical position O is AZ.

[0083] The specific stub length S is then determined by:

[0084] S = L+AR + (PO) = L+AR + AZ.

[0085] The difference AF between target vertical position O and target back-drilling depth V and / or the difference AN between target back-drilling depth B and target back-drilling depth V can also be taken into account.

[0086] A surface position of a surface of the printed circuit board can be determined and the actual back-drilling depth can be determined relative to the surface position, with the surface position being determined particularly in the vicinity of the back-drilling hole.

[0087] This establishes a fixed reference point for the actual backdrilling depth, enabling precise determination of the actual backdrilling depth. The area surrounding a backdrill hole refers to a zone around the hole. The diameter of this area is larger than the diameter of the backdrill hole itself. The shape of the area can be arbitrary, for example, circular, square, or rectangular. The radius is preferably no greater than 1 cm, preferably no greater than 5 mm, and particularly no greater than 1 mm.

[0088] The surface position can be determined by mechanical and / or optical scanning of the test finger. Mechanical scanning is performed using the same procedure as the mechanical scanning of the bore and blind hole described above.

[0089] Optical scanning of a test finger is a non-contact method for determining the surface position. The test probe can, for example, be equipped with a laser sensor that projects a spot of light onto the surface of the circuit board. A light sensor detects the position of the reflected light, and based on this position and the known properties of the test finger, the relative position of the test finger to the surface is calculated.

[0090] The surface position can form a level for the entire printed circuit board.

[0091] This level can be used as a reference level for the entire circuit board to determine the vertical positions and hole depths of other holes relative to this reference. This improves the accuracy of height measurements across the entire circuit board.

[0092] The surface position can be scanned at multiple locations on the printed circuit board surface. The surface position can represent an average of these measurements and / or a surface model can be determined based on the measurement.

[0093] Measuring multiple points can increase accuracy, as inaccuracies in a single measurement can be compensated for. Misalignments and / or warping of the circuit board can be corrected in this way.

[0094] An average of this measurement can represent an improvement over a single measurement, since the single measurement may be an outlier that compromises the entire stub length determination.

[0095] A surface model, created using multiple surface positions and refined and interpolated, for example, by a spline function, can also represent deformations of the printed circuit board, such as curvature, which can then be taken into account when determining the stub length. The test finger can include a capacitance sensor for measuring capacitance, which detects nearby metallic structures, thereby determining the initial position of a sliver, with the endpoint being the target vertical position of the upper conductor layer.

[0096] A sliver is a thin piece of material left over from back-drilling a printed circuit board. It forms at the edge of the hole when a drill bit drills at an angle and / or offset from the center of the hole. It is essentially a narrow extension of a stub.

[0097] A capacitance sensor can determine the vertical position of such a sliver, which then allows the total length of the sliver and / or stub to be determined.

[0098] Another aspect of the invention relates to a method for determining the length of a stub and / or a sliver that remains as a protrusion above the upper conductor layer in a via from an upper conductor layer to a lower conductor layer in a printed circuit board when backdrilling is performed at an angle and / or offset. The method comprises providing a predefined backdrilling depth and / or a target backdrilling depth, determining an initial position of a stub and / or sliver using a capacitance sensor of a test finger, and determining the stub or sliver length based on the difference between the initial position and at least one of the provided predefined backdrilling depths and / or target backdrilling depths.

[0099] This method corresponds to the first aspect described above, whereby the initial position of the stub or sliver is measured by the capacitance sensor and not by mechanically probing the bottom of a blind hole.

[0100] By determining the initial position of the stub or sliver, the inaccuracies that arise during the manufacturing process of the printed circuit boards and that have been transferred to the actual length of the stubs are detected and determined.

[0101] This allows the length of copper remnants in the form of stubs or silver strands in vias of printed circuit boards to be reliably determined.

[0102] The procedure can execute one of the procedures described above.

[0103] Another aspect of the invention relates to a system for determining a stub and / or a sliver in a printed circuit board, which is designed to carry out a previously described method.

[0104] The invention is explained in more detail below by way of example, using the examples shown in the drawings. The drawings schematically show:

[0105] FIG. 1 a)-f) a method for creating a through-hole via with subsequent back-drilling,

[0106] FIG. 2 a finger tester in perspective view,

[0107] FIG. 3a)-c) a test probe with a test needle according to the invention in perspective views and in a side view,

[0108] FIG. 4 shows a section of a test needle with a test tip in cross-section,

[0109] FIG. 5 a block diagram for a first embodiment for measuring the length of a stub,

[0110] FIG. 6 shows a block diagram for a second embodiment for measuring the length of a stub, and

[0111] FIG. 7 shows a block diagram for a third embodiment for measuring the length of a stub.

[0112] The invention can be implemented using a finger tester 1, which has several test fingers 2, each equipped with a test probe 3, with which the positions of points on or in a printed circuit board 4 to be tested can be determined. According to this embodiment, the test probes 3 can also be electrically contacted with the printed circuit board 4 in order to inject or tap off a corresponding measurement signal in the conductor tracks (Figure 2).

[0113] Such test probes 3 are described, for example, in WO 03 / 048787 A1, to which reference is made in this respect.

[0114] In the present embodiment, the finger tester 1 has two crossbeam units 5, on which several guide rails 6 are arranged. These guide rails extend over a test area into which the circuit board 4 to be tested can be received. Each crossbeam unit 5 forms a frame with an elongated through-opening 8. The two crossbeam units 5 are mounted perpendicularly on a support 7, so that their through-openings 8 are aligned. The test area for receiving the circuit board 4 extends through the through-openings 8 of the crossbeam units 5. In the present embodiment, each crossbeam unit 5 has four guide rails 6, with two arranged on each side of the crossbeam unit 5. On each side of the crossbeam unit 5, one of the guide rails 6 is located above and the other below the through-opening 8.At least one slide 9 is slidably arranged on each guide rail 6, each slide carrying one of the test fingers 2. In the present embodiment, two slides 9 are provided on each guide rail 6. However, in principle, more than two slides 9 can also be arranged on a guide rail 6.

[0115] The test fingers 2 are pivotally attached to the slide 9 at one end. Such a configuration of a finger tester 1 is described, for example, in WO 2014 / 140029 A1. This document is hereby incorporated by reference in its entirety.

[0116] The test probes 3 are arranged at the ends 18 furthest from the carriages 9, the free ends 18 of the test fingers 2, and each has a test needle 10 which is directed with a contact tip 11 towards the circuit board 4 to be contacted. The test fingers 2 can be moved with the test probes 3 towards and away from the circuit board 4 in order to position the contact tip 11 either on the surface of the circuit board 4 or to insert it into a hole in the circuit board 4.

[0117] At least one or more of the test fingers 2 each have a test probe 12 (Figure 3a-c), which is designed for both contact scanning and non-contact, capacitive scanning of bores, and in particular blind holes, of the printed circuit board 4. Such a test probe 12 has a base plate 13, which is configured as a printed circuit board 4. A locking element 14 is arranged on the base plate 13, which has two locking arms 15 with which the test probe 12 can be locked onto one of the test fingers 2. Two spring arms 16, 17 are attached to the locking element 14. One of the two spring arms 16 rests directly on the base plate.

[0118] The first spring arm 13 is clamped between the base plate 13 and the locking element 14. The second spring arm 17 is attached to the surface of the locking element 14 furthest from the base plate 13 by means of two screws. The spring arms 16 and 17 are arranged parallel to each other. From the locking element

[0119] Starting at point 14, the base plate 13 and the spring arms 16, 17 taper in plan view to a free end 18, from which the two spring arms 16, 17 each project a short distance. A test needle 19 is attached to each of these ends 18 of the spring arms 16, 17, so that the elastic spring arms 16, 17, the detent body 14, and the section of the test needle 19 connecting the two spring arms 16, 17 form a parallelogram. The two spring arms 16, 17 can be pivoted away from the base plate 13 with their free ends 18, so that the test needle 19 is elastically mounted on the test probe 12. The spring arm 16 directly adjacent to the base plate 13 is referred to below as the base spring arm 16, and the spring arm 17 furthest from the base plate 13 is referred to as the free spring arm 17. The base plate 13 thus forms a stop for the base spring arm 16 and therefore for the movement of the test needle 19 relative to the rest of the body of the test needle 12.In plan view, both spring arms 16, 17 form an approximately triangular frame. A measuring flag 20, pointing towards the base plate 13, is arranged on the free spring arm 17.

[0120] On the base plate 13 is a light barrier assembly 21, which, together with a light source and a light sensor, forms a light barrier into which the measuring flag 20 engages when the spring arms 16, 17 are not deflected. When the spring arms 16, 17 are deflected, the measuring flag 20 is moved out of the light barrier, so that the light sensor detects a greater brightness. This light barrier assembly 21, together with the measuring flag 20, thus forms a touch sensor, which detects when the spring arms 16, 17 are deflected by contact of the test needle 19 with another body.

[0121] The test 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 test needle 19. The test needle 19 is connected to the base spring arm 16 such that, in the non-displaced state of the spring arms 16 and 17, i.e., when the base spring arm 16 rests on the base plate 13, the test needle 19 is arranged perpendicular to the plane of the base plate 13.

[0122] This test probe 12 and the corresponding test finger 2 are designed such that, in the state mounted on the test finger 2, the test needle is arranged perpendicular to the test area or to a circuit board 4 located in the test area in the non-deflected state.

[0123] The test needle 19 projects from the test probe 12 a short distance beyond the base plate 13. The test needle 19 has a metal tube 24, which is electrically conductive and has an outer diameter of preferably a maximum of 0.2 mm and particularly a maximum of 0.18 mm. A cable 25, comprising an electrical conductor surrounded by an insulating layer, is guided within the metal tube 24. In the present embodiment, the cable 25 is connected to a conductor formed on the base plate 13, which is electrically connected to a capacitive measuring device (not shown). At the measuring end 23, the conductor of the cable 25 is connected to a capacitive measuring element 26. The capacitive measuring element 26 is a metallic ring body that is arranged around the tube 24 and electrically insulated from it. The capacitive measuring element 26 can be a metal ring provided with an insulating layer, which is electrically connected to the conductor of the cable 25.However, the capacitive measuring element 26 can also be formed from one or more windings of the cable 25.

[0124] The metal tube 24 serves to shield the cable 25, so that only the capacitive measuring element 26 can establish capacitive coupling with a nearby electrical conductor, and only its capacitive coupling to this other electrical conductor can be detected by means of the cable 25. A measuring tip 27 is formed at the measuring end 23 of the test probe 19. This measuring tip 27 serves only for the mechanical contact of another body. The measuring tip 27 can be made of a non-electrically conductive material or be provided with an insulating layer. In the present embodiment, it does not serve for the electrical contact of a contact point of a printed circuit board 4.

[0125] However, within the scope of the invention, the measuring tip 27 can also be electrically conductive, so that it can be used to establish an electrical connection with a contact point of the circuit board 4. If such an electrically conductive contact tip 11 is electrically connected to the capacitive measuring body 26, then the contact tip 11 can be taken into account in a capacitive coupling that the capacitive measuring body 26 forms with an electrically conductive object to be measured.

[0126] One embodiment of the measuring tip 27 is formed from a cannula tube 28, which is electrically and mechanically connected at one end to the tube 24 or the shield 24 by means of a soldered connection 29. The cannula tube 24 has a diameter of, for example, 0.3 mm in the area of ​​the soldered connection 29. The cannula tube 28 is axially oriented towards the measuring tip 27 and has a diameter of 0.2 mm or less in the area of ​​the measuring tip 27. The cannula tube 28 is cut obliquely in the area of ​​the measuring tip 27, resulting in an obliquely oriented outlet opening 30.

[0127] The cable 25 is guided through the tube 24 and the cannula tube 28 and exits the cannula tube 28 at the exit opening 30. In the region of the measuring tip 27, the cable 25 is wound around the cannula tube 28 with, for example, three turns and forms the capacitive measuring element 26.

[0128] The measuring tip 27 itself is electrically conductive and electrically connected to the tube 24.

[0129] Such an electrically conductive measuring tip 27 has the advantage that the test needle 19 can, for example, come into contact with a calibration surface which has electrically conductive and non-electrically conductive surface sections, so that the position of the test needle 19 and thus the position of a test finger 2 in which the test needle 19 is attached can be calibrated.

[0130] The test probe 19 shown in Figure 3 is easy to manufacture and has a very small measuring body 26, so that the spatial resolution is correspondingly precise.

[0131] Preferably, at least one test finger 2 with a test probe 3 having a conventional test needle 10 for electrically contacting a contact point of the printed circuit board 4 and another test finger 2 with a test probe 12 according to the invention are provided on each guide rail 6. It may also be advantageous to provide two test fingers 2 with conventional test probes 3 and one further test finger 2 with a test probe 12 according to the invention on one guide rail 6.

[0132] However, the invention can also be applied to a conventional finger tester 1 that has several separate crossbeams not arranged on a common crossbeam unit 5. In such a finger tester 1, it is again advantageous to provide at least one test finger 2 with a test probe 12 according to the invention and one or more test fingers 2 with conventional test probes 3 per crossbeam.

[0133] The test probe 12 according to the invention thus makes it very easy and precise to determine the geometry of a blind hole or through hole 33 and, on the other hand, to determine the coating with an electrical conductor of the inner surface of the blind hole or through hole 33.

[0134] Furthermore, the test probe 12 can be used in conventional finger testers 1 (English: flying probe tester). No further mechanical modifications are necessary. The finger tester 1 simply needs to be equipped with a suitable control program that can process the measurement signals generated by the test probe 12 according to the invention, apply suitable measurement signals, and control the movement of a test finger 2 with the test probe 12 according to the invention.

[0135] The measuring system also includes an evaluation unit (not shown) that can perform calculations. The evaluation unit can be a software module running on a processing unit. The processing unit can be, for example, a computer. The processing unit can be connected to the finger tester 1 via data links and receive measurement data from it. The processing unit and the evaluation unit are not shown in the figures.

[0136] The following describes a method for determining the stub length of a stub 37 in a printed circuit board 4.

[0137] The process begins with step S1 (Figure 5).

[0138] In the next step (S2), the specified back-drilling depth V and the predetermined desired stub length L are defined / specified.

[0139] The drilling parameters are specified by the person who performed the drilling or by the driller itself. This data can be entered manually or retrieved from saved data. It is also possible for the data to be transmitted from the driller via a network, such as the internet. The data is then sent to an evaluation unit (not shown), which performs the analysis and the procedure described here.

[0140] Next, a surface position is determined (step S3).

[0141] First, a measuring point is selected in the vicinity of the borehole. This measuring point serves as the starting point for further measurements. The selected point is digitally marked, for example by saving its coordinates, before its vertical position is measured.

[0142] The vertical position for measuring the surface position is determined by the probe 12 of one of the test fingers 2, which is placed on the surface of the circuit board 4 in such a way that it touches it. The surface position can therefore be determined by the touch sensor described above, which is formed by the light barrier arrangement 21 and the measuring flag 20, because at the moment the touch sensor is triggered, the position of the tip of the probe 12 represents the surface. The determined surface position is transmitted to the evaluation unit. The surface position serves as a reference point for all further depth measurements.

[0143] Preferably, the process is repeated, either at the same location or at additional measuring points, to verify the accuracy of the measurement. Consistency of the measurement results can then determine the reliability of the determined surface position.

[0144] Step S4 follows, in which the actual back-drilling depth I is measured.

[0145] For this purpose, the test probe 12 is positioned precisely above the backbore. The test probe 12 is then lowered vertically into the borehole to ensure a precise measurement.

[0146] The lowering speed should be selected so that the contact sensor has time to react to contact with the bottom without causing inaccurate measurements due to inertia. The contact sensor detects the moment of contact with the bottom of the backhole. This contact is indicated by an electrical signal. The depth at which contact occurs is recorded as the actual backhole depth I and transmitted to the evaluation unit.

[0147] In the following step (S5), the stub length S is determined based on the difference AR between the actual back-drilling depth I and the target back-drilling depth V.

[0148] The difference AR between the actual drilling depth I and the target drilling depth V is calculated using the formula: AR = V - l. This calculation reveals how far the actual depth deviates from the planned depth. This difference can then be added to the predetermined length L. It should be noted that the difference AR can also be negative, as explained above.

[0149] The difference, especially if it is similar across multiple back-drilled holes, can indicate a fundamental error or misinterpretation of the data. For example, if a similar difference is found in several back-drilled holes, this could mean that the thickness of circuit board 4 deviates from the specified thickness and / or the drill bit is drilling to a different depth than specified. Appropriate corrective action can then be taken.

[0150] According to a further development of the invention, it is also conceivable that a corresponding warning is issued if the difference AR is greater than a predetermined threshold.

[0151] The procedure ends with step S6.

[0152] The following describes a second embodiment for determining the stub length, where the same elements as in the first embodiment are designated with the same reference numerals. The explanations given above apply to identical elements unless otherwise stated below.

[0153] The procedure begins with step S7.

[0154] The following step S8 corresponds to the step S2 described above, except that here a target vertical position O is additionally defined, which represents a predetermined vertical position of the upper conductor layer 31.

[0155] This target vertical position O can be taken, for example, from the datasheet of the printed circuit board 4 or from the specified thickness of the printed circuit board 4 in conjunction with a relative position of the upper conductor layer 31 .

[0156] Step S9 follows, in which the thickness of circuit board 4 is measured.

[0157] For this purpose, as in step S3 above, a surface position is determined, this being done on both sides of the printed circuit board 4. The vertical distance between these two positions of the two surfaces corresponds to the thickness of the printed circuit board 4. Preferably, the thickness is determined at several locations on the printed circuit board 4 in order to determine an average value or even a surface topography of the printed circuit board 4. Values ​​between the individual measurement points can be determined using a spline method.

[0158] In step S10, a surface position is determined, as described above in step S3. The thickness measurement data from step S9 can also be used for this purpose.

[0159] In the next step S1 1, the actual back-drilling depth I is measured, as was done above in the procedure in step S4.

[0160] Step S12 follows, in which the actual vertical position P is determined. Here, the measured thickness of the circuit board 4 is compared with a predetermined thickness. If these two thicknesses differ, a predetermined target vertical position O must also be proportional to the thickness difference relative to the actual vertical position P. This assumes that the cause of the different thicknesses is unexpected compression behavior during the pressing of the circuit board 4.

[0161] It is assumed here that the position of the upper conductor layer 31 varies proportionally to the thickness of the circuit board 4.

[0162] In step S13, the stub length S is determined based on the difference AR between the actual back-drilling depth I and the target back-drilling depth V, as well as the difference AZ between the actual vertical position P and the target vertical position O. This is initially done as in step S5 above, with the additional addition of the difference AZ. Here too, AZ can take on both positive and negative values, depending on the extent to which the actual vertical position P differs from the target vertical position O.

[0163] The difference AZ thus takes into account a change in the lower end 18 of the stub 37 due to a different thickness than assumed.

[0164] However, measuring the back-drilling depth and the difference AR take into account the upper starting point of the stub 37.

[0165] The procedure ends with step S14.

[0166] A third embodiment is described below, in which identical elements as in the first and second embodiments are designated with the same reference numerals. The explanations given above apply to identical elements unless otherwise stated below. The procedure begins with step S15.

[0167] In the next step S16, a specified back-drilling depth V and a predetermined desired stub length L are provided, as in step S2.

[0168] In step S17, a surface position is determined, as in step S3. The procedure differs particularly from step S18. Here, the initial position of the stub 37 is determined by a capacitance sensor, as described above. Since the stub 37 is electrically conductive, the capacitance sensor of finger test 1 identifies such a stub 37. Slivers that lie parallel to the back-drilled hole can also be detected here.

[0169] In step S19, the stub or sliver length S is determined based on the difference AR between the initial position and the target back-drilling depth V. The same calculation and procedure are performed as in step S5 above, except that the actual back-drilling depth I is replaced by the initial position.

[0170] The procedure ends with step S 20.

[0171] The modification of the second embodiment can also be applied in the third embodiment, in which a thickness measurement is included.

[0172] Reference sign

[0173] 1 finger tester 20 measuring flag

[0174] 2 Test fingers 21 Light barrier arrangement 3 Test probe 22 Connection end

[0175] 4 Circuit board 25 23 Measuring end

[0176] 5 truss units, 24 tubes

[0177] 6 guide rails, 25 cables

[0178] 7 Holder 26 Capacitive measuring element 8 Through-hole 27 Measuring tip

[0179] 9 sleds 30 28 cannula tubes

[0180] 10 test probe 29 solder joint

[0181] 11 Contact tip 30 Exit opening

[0182] 12 Test probe 31 Upper conductor layer 13 Base plate 32 Lower conductor layer

[0183] 14 Detent body 35 33 Through hole

[0184] 15 Raster arm 34 Copper lining

[0185] 16 Spring arm (base spring arm) 35 Resin plugs

[0186] 17 Spring arm (free spring arm) 36 Backdrilling 18 Free end 37 Stub

[0187] 19 test needles

[0188] V Specifications - Back-drilling depth

[0189] I Actual back-drilling depth

[0190] S Stub length L predetermined desired length

[0191] AR Difference between target drilling depth V and actual drilling depth I

[0192] Target vertical position

[0193] AF Difference between target vertical position O and target back-drilling depth V

[0194] B Target back-drilling depth AN Difference between target back-drilling depth B and specified back-drilling depth V

[0195] P Actual Vertical Position

[0196] AZ Difference between actual vertical position P and target back-drilling depth V

Claims

International patent application atg Luther & Maelzer GmbH P374063WO Claims 1. Method for determining the stub length of a stub (37) in a printed circuit board (4) which remains as a protrusion with respect to the upper conductor layer in a via from an upper conductor layer to a lower conductor layer in the printed circuit board (4) when backdrilling in a borehole, comprising: - Providing the specified back-drilling depth, - Determining the actual back-drilling depth of a back-drilled hole with a test finger (2) by mechanically probing the bottom of a blind hole created during back-drilling, and - Determining the stub length based on the difference between the actual back-drilling depth and the target back-drilling depth.

2. Method according to claim 1, characterized in that the difference between a target vertical position of the upper conductor layer and the specified back-drilling depth is taken into account when determining the stub length.

3. Method according to claim 1 or 2, characterized in that the actual thickness of the printed circuit board (4) is determined, in particular by scanning with test fingers (2) from both sides of the printed circuit board (4).

4. Method according to claim 3, characterized in that a target back-drilling depth, which can be a specification of a depth to be drilled of the back-drilling hole on a drill, is determined on the basis of the determined actual thickness.

5. Method according to claim 4, characterized in that the difference between the target back-drilling depth and the specified back-drilling depth is taken into account when determining the stub length.

6. Method according to one of claims 3 to 5, characterized in that an actual vertical position of the upper conductor layer and / or the lower conductor layer is estimated from the actual thickness.

7. Method according to claim 6, characterized in that the difference between the actual vertical position and a target vertical position of the upper conductor layer is taken into account when determining the stub length.

8. Method according to one of claims 1 to 7, characterized in that a surface position of a surface of the printed circuit board (4) is determined and the actual back-drilling depth is determined relative to the surface position, wherein the surface position is determined in particular in the vicinity of the back-drilling hole.

9. Method according to claim 8, characterized in that the surface position is determined by mechanical and / or optical scanning of the test finger (2).

10. Method according to claim 8 or 9, characterized in that the surface position forms a level for the entire printed circuit board (4).

11. Method according to one of claims 8 to 10, characterized in that the surface position is scanned at several locations on the surface of the printed circuit board (4) and the surface position represents an average of these measurements and / or a surface model is determined based on the measurements.

12. Method according to one of claims 1 to 11, characterized in that the test finger (2) has a capacitance sensor for measuring a capacitance, with which nearby metallic structures are detected, wherein the initial position of a sliver is determined, wherein the endpoint is the target vertical position of the upper conductor layer.

13. Method for determining the length of a stub (37) and / or a sliver which remains as a projection with respect to the upper conductor layer in a via from an upper conductor layer to a lower conductor layer in a printed circuit board (4) when backdrilling at an angle and / or offset in a borehole, comprising: - Providing a predefined back-drilling depth and / or a target back-drilling depth, determining an initial position of a stub (37) and / or sliver using a capacitance sensor of a test finger (2), and - Determining the stub or sliver length based on the difference between the initial position and at least one of the provided target back-drilling depths and / or target back-drilling depths.

14. Method according to claim 13, characterized in that a method according to one of claims 1 to 12 is carried out.

15. System for determining a stub (37) and / or a sliver in a printed circuit board (4), which is designed to perform a method according to any one of claims 1 to 14.

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