WO1997009630A1

WO1997009630A1 - A method for determining the relative positions of a plurality of layers of a multilayer circuit board, a device suitable for carrying out such a method and also a measuring pin and a circuit board suitable for being used with such a method

Info

Publication number: WO1997009630A1
Application number: PCT/NL1996/000339
Authority: WO
Inventors: Henricus Dethmer Ubbo Ubbens
Original assignee: Henricus Dethmer Ubbo Ubbens
Priority date: 1995-09-01
Filing date: 1996-08-27
Publication date: 1997-03-13
Also published as: NL1001113C2; AU6757696A

Abstract

A method for determining the relative positions of a plurality of layers of a multilayer circuit board, whereby each layer is provided with a number of marks. Reference holes are drilled through superimposed marks of several layers and the position of the mark of each layer relative to the reference hole is measured at each reference hole by means of a measuring pin on the basis of material transitions in said mark, after which the relative positions of the layers with respect to each other are calculated. The position of a mark is measured by means of a measuring pin provided with a sensor, whereby the sensor is rotated and translated in the reference hole by means of the measuring pin.

Description

A method for determining the relative positions of a plurality of layers of a multilayer circuit board, a device suitable for carrying out such a method and also a measuring pin and a circuit board suitable for being used with such a method.

The invention relates to a method for determining the relative positions of a plurality of layers of a multilayer circuit board, whereby each layer is provided with a number of marks, according to which method reference holes are drilled through superimposed marks of several layers and the position of the mark of each layer relative to the reference hole is measured at each reference hole by means of a measuring pin on the basis of material transitions in said mark, after which the relative positions of the layers with respect to each other are calculated, whereby the positions of the marks are measured by means of a measuring pin provided with a sensor, whereby the sensor is rotated and translated in the reference hole by means of the measuring pin.

The invention also relates to a device suitable for carrying out such a method.

The invention furthermore relates to a measuring pin and a circuit board suitable for being used with the method and device according to the invention.

A method, device and measuring pin of this type are known from the ^"multilayer Meβ- und Bohrmaschine MLV-92" manufactured by the firm of essel GmbH of Detmold, and also from European Patent Application EP-A2-0 506 217.

Reference holes are formed in a circuit board by means of the known device, after which the positions of the marks with respect to the reference holes are determined by means of a kind of periscope.

A multilayer circuit board, also called multilayer, comprises a plurality of stacked-together and attached-together layers, which are provided with printed electric circuits. Such a circuit board is provided with a plurality of drilled holes, whereby said drilled holes are coated with an electrically conducting material. In this manner the electric circuits of the various layers are interconnected. Wiring errors occur because the layers of the circuit board have shifted relative to each other due to manufacturing tolerances, and because the bores are not positioned optimally in each layer. This problem is further aggravated by an increasing number of layers and decreasing dimensions of the copper surfaces in which the holes are to be drilled. Knowing the relative positions of the layers before the drilled holes are provided makes it possible to adapt the drilling pattern to the circuit board. To this end each layer of the circuit board is provided with a number of marks, whereby the marks of the various layers must be vertically aligned as well as possible during production. Due to tolerances there will always be deviations, however, and the layers will never be aligned 100% with respect to each other. A mark for example comprises a recognizable copper pattern, which is provided simultaneously with the electric circuit. The mark proposed by the firm of Wessel GmbH and known from the aforesaid European Patent Application comprises a plurality of copper conductors extending parallel and transversely to each other.

A reference hole is drilled through the superimposed marks, after which a measuring pin is slid into the reference hole that has been drilled. The measuring pin used comprises a mirror positioned at an angle and a camera, with which a kind of periscope has been formed, as it were, by means of which the wall of the reference hole is studied. The measuring pin is thereby moved through the reference hole at four different angular positions. Material transitions and changes in the wall of the reference hole as a consequence of the presence or absence of the copper conductors of the mark. A deviation of the position of the mark of a layer with respect to the reference hole is determined on the basis of the expected positions of the material transitions at a reference hole located in the centre of the mark and the actually measured positions of the material transitions. The displacement, rotation and stretch or shrink of a layer with respect to an expected position of a layer are calculated on the basis of the positions of a number of marks with respect to a number of reference holes and the positions of the reference holes with respect to each other. Then the relative positions of the layers are calculated on the basis of the positions of all layers with respect to the reference holes, and the place where the drilled holes must be provided is determined in such a manner that the desired interconnections between the layers can be provided. A drawback of the known device is that the measuring pin is relatively costly, because a highly accurately positioned mirror and a separate camera are required. The mirror must be capable of precise rotating and translating movement with respect to the camera in order to be able to observe the complete marks of each layer. If the camera and the mirror are not properly aligned with respect to each other incorrect measurements will be the result. Another drawback is the fact that the position of a single mark must be filtered from information on four separate measurements at different angular positions, with the attendant risk of measurements being coupled in an incorrect manner.

The object of the invention is to provide a method and a device by means of which it is possible to determine the relative positions of the layers of a multilayer circuit board relatively accurately in a relatively simple manner.

This objective is accomplished with the method according to the invention in that the sensor is successively positioned at each mark by means of the measuring pin, whereby the sensor is rotated in the reference hole at the location of a mark and material transitions of the mark are measured.

Material transitions and changes in the mark are detected immediately by means of the sensor rotating at the location of a mark. All information on a mark is obtained in one uninterrupted measurement, as a result of which the accuracy and reliability of the measurements is relatively high.

One embodiment of the method according to the invention is characterized in that the sensor is an electric contact, whereby an electric signal from the measuring pin is passed to the sensor via the mark, whereby the times and/or duration of interruptions in the electric signal caused by material transitions between parts of the mark which are electrically conducting and parts which are not electrically conducting are measured. During measuring the measuring pin is constantly in contact with electrically conducting parts of the mark. An electric voltage is for example applied to the measuring pin, as a result of which also the copper conductors of the mark are energized. The sensor butts against a local part of the wall of the reference hole and will only pass the voltage present on the mark when the sensor comes into contact with a copper conductor. As soon as the sensor is positioned opposite a part of the layer not provided with a copper conductor by rotation of the measuring pin, the sensor will not pass electric voltage. The position of the mark relative to the reference hole is determined on the basis of the times and/or duration of the interruptions in the electric signal.

The mark on the layers of the circuit board must be of a type which makes it possible to apply electric voltage to all conducting parts of the mark. This is for example possible when the mark comprises a copper ring, whereby the inner side of the ring is provided with notches. The material of the layer present in said notches is not electrically conducting. The material transitions between the notches and the copper of the ring can be measured by means of the sensor.

Another embodiment of a method according to the invention is characterized in that the reference holes are drilled by means of the measuring pin, whereby the material changes in the marks are measured by means of the sensor during drilling. In this manner a reference hole is drilled whilst simultaneously the positions of the marks relative to the reference hole are measured. As a result of this the time required for determining the relative positions of the layers is minimized. The wall of the reference hole is scanned by means of a sensor during the rotation required for drilling, and material transitions are detected. When an electric signal is passed through the measuring pin, said measuring pin is provided with a drill bit, via which the measuring pin comes into electrically conducting contact with a mark.

The invention will be explained in more detail with reference to the drawing, in which:

Figure 1 shows a measuring pin according to the invention;

Figure 2 shows a device provided with the measuring pin shown in Figure 1; Figure 3 is a bottom view of a measuring pin according to the invention;

Figure 4 shows a mark on a layer of a circuit board according to the invention;

Figure 5 shows a measuring pin present in the mark shown in Figure 4;

Figure 6 shows another mark on a layer of a circuit board according to the invention; Figures 7A, B and C show graphs of measuring results; and

Figure 8 is a cross-sectional view of a circuit board with a measuring pin provided therein. Like parts are numbered alike in the Figures.

Figure 1 shows an elongated metal measuring pin 1, which is provided with a drill bit 2 at one end and with a drill coupling 3 at another end. Drill bit 2 comprises two insulated metal wires 4 extending along the helix of drill bit 2, whereby the insulation has been removed from a part 5 of each wire 4 located near the end of the drill bit 2, in such a manner that the metal wire is not in contact with drill bit 2. Said part 5 forms the sensor of measuring head 1. The part 5 of the wire, which has been stripped of its insulation, may extend parallel or transversely to the central axis of measuring pin 1. One end of wire 4 facing away from drill bit 2 is connected to a contact ring 6. Measuring pin 1 is provided with two contact rings 6 for the two wires 4 and one contact ring 6 for drill bit 2.

Figure 2 shows a device 7 which is provided with a drill chuck 8, in which the drill coupling of measuring pin 1 is detachably fitted. Device 7 is furthermore provided with a pressure foot, with respect to which drill chuck 8 is journalled in such manner as to be capable of translating and rotating movement in directions indicated by arrow Pl and arrow P2 respectively. Device 7 is provided with an annular disc 10, which comprises three sliding contacts 11 positioned opposite contact rings 6. Disc 10, together with drill bit 10, is capable of translating movement with respect to pressure foot 9 in the direction indicated by arrow Pl. Sliding contacts 11 butt against contact rings 6 on one side, and are connected, via a slot 13 provided in pressure foot 9, to contact wires 12 leading to a computer (not shown) on another side. Figure 3 shows a bottom side of a measuring pin 1, whereby sensor 5 is embedded in drill bit 2. A reference hole is drilled through superimposed marks by means of drill bit 2, whereby sensors 5 butt against the bottom of the hole to be drilled.

Figure 4 shows a form of a copper mark 14 as provided in the various layers of a multilayer circuit board. Mark 14 is annular and in its inner ring provided with rectangular notches 15 provided in a regular pattern. A reference hole is provided through the circular part 15 of the circuit board surrounded by annular mark 14. If said reference hole has centre M of annular mark 14 as its centre, wall 17 of the reference hole will have a regular pattern of the copper of mark 14 and of the circuit board material, for example plastic material, present in notches 15. If centre M^' of said reference hole deviates from centre M of mark 14, however, wall 17^' will have an irregular pattern of material transitions. Figure 4 shows a shift AX in x-direction relative to the centre M of mark 14.

Figure 5 shows mark 14, in which a reference hole 18 has been drilled by means of drill bit 2. Wall 17 of reference hole 18 is positioned symmetrically with respect to mark 14. Wall 17 of reference hole 18 is scanned by means of sensor 5 during the rotation of drill bit 2 in the direction indicated by arrow P2, and material transitions between the copper of mark 14 and the circuit board material present in notches 15 are detected, as will be explained in more detail hereafter.

Figure 6 shows another mark 19 which may be provided on a circuit board. Annular mark 19 comprises triangular notches 20, which taper from the inner ring in the direction of the outer ring. When a mark 19 of this type is used and reference hole 18 is positioned eccentrically, the change of the pattern of material transitions will be more distinct, because the width W of the notch, among other things, will change more than will be the case with the rectangular notches shown in Figure 4.

Figures 7A-7C show three graphs illustrating measuring results. The time is plotted on the horizontal axis, and the measured electric current through wall 17, 17^' of reference hole 18 is plotted on the vertical axis. Figure 7A shows a graph of a measurement wherein the centre of reference hole 18 coincides with centre M of mark 14, 19. Figures 7B and 7C show graphs of measurements with reference holes which are positioned eccentrically with respect to the mark 14 shown in Figure 4 and the mark 19 shown in Figure 6 respectively.

Figure 8 is a cross-sectional view of a multilayer circuit board 25, showing marks 14 of several layers. Reference hole 18 has already been partially formed in circuit board 25 by means of drill bit 2. The operation of the device will now be explained in more detail . A multilayer circuit board 25 is positioned on an X-Y table (not shown) and moved under device 7 by means of said X-Y table. The locations where the marks of the layers of the multilayer circuit board should be present are stored in a computer (not shown). The pressure foot 9 of device 7 is positioned at the expected location of superimposed marks, and the circuit board is pressed down on the X-Y table by means of pressure foot 9. Then drill chuck 8 is driven and rotated and translated with respect to pressure foot 9. The drill bit 2 coupled to drill chuck 8 is passed through an opening 21 (Figure 2) present in pressure foot 9, and a reference hole is drilled in the circuit board by means of drill bit 2. By means of a computer a voltage difference is applied between drill bit 2 on the one hand and wires 4 on the other hand via wires 12, sliding contacts 11 and the contact rings 6, which rotate with respect to said sliding contacts. Since the reference hole is drilled in the mark by means of drill bit 2, drill bit 2 will at all times butt against mark 14, 19, as a result of which mark 14, 19 will be electrically connected to drill bit 2 at all times. Sensor 5 butts against the wall 18 of the reference hole. Sensor 5 may also butt against the part of the circuit board through which a hole is to be drilled yet. In both cases an electric current will flow from drill bit 2, through mark 14, 19, to sensor 5 (or vice versa) if sensor 5 butts against the copper of mark 14, 19. If sensor 5 butts against the circuit board material present in notches 15, there will be no flow of electric current I. The computer will detect the presence or absence of the electric current I, thus showing material transitions between the copper and the circuit board material. The position of mark 14, 19 with respect to the reference hole is determined on the basis of the position of the drill, the known pattern of current transitions with a reference hole positioned centrically with respect to the mark and the measured pattern of current changes. When the reference hole is drilled deeper in the circuit board, drill bit 2 and sensor 5 are brought into contact with a next layer of the circuit board, and the position of mark 14, 19 of the next layers with respect to the reference hole is determined in the same manner. After the reference hole has been drilled completely, the positions of all marks that have been drilled through with respect to the reference hole are known. In a similar manner all further reference holes 18 are drilled through superimposed marks 14, 19. The relative positions of the layers of the multilayer circuit board can be determined from the positions of the marks with respect to the reference holes and the positions of the reference holes with respect to the X-Y table. This may for example be done in the same manner as with the above-described Wessel GmbH machine. After the relative positions of the layers and the corrections required have been calculated, the measuring pin is replaced by a drill, by means of which a desired pattern of drilled holes is provided in the circuit board.

Instead of detecting an electric current it is also possible to detect capacitive or inductive changes at material transitions, or to detect dark/light transitions at material transitions by means of light conductors.

It is possible to work with a single sensor or with an N-number of sensors. It is preferred for the drill bit to make at least 1/N revolutions in a mark, in order to be able to scan a mark completely.

It is also possible to drill a reference hole first and then examine the wall of the reference hole by means of a separate measuring pin.

It is possible to apply a high-frequency voltage, as a result of which transitions between sensor and mark can still be measured, even in the case of wear. It is possible to use a drill with two, three or more cutting edges. It is also possible to press the sensor against the wall of the reference hole under spring force.

Claims

1. A method for determining the relative positions of a plurality of layers of a multilayer circuit board, whereby each layer is provided with a number of marks, according to which method reference holes are drilled through superimposed marks of several layers and the position of the mark of each layer relative to the reference hole is measured at each reference hole by means of a measuring pin on the basis of material transitions in said mark, after which the relative positions of the layers with respect to each other are calculated, whereby the positions of the marks are measured by means of a measuring pin provided with a sensor, whereby the sensor is rotated and translated in the reference hole by means of the measuring pin, characterized in that the sensor is successively positioned at each mark by means of the measuring pin, whereby the sensor is rotated in the reference hole at the location of a mark and material transitions of the mark are measured.

2. A method according to claim 1, characterized in that the sensor is an electric contact, whereby an electric signal from the measuring pin is passed to the sensor via the mark, whereby the times and/or duration of interruptions in the electric signal caused by material transitions between parts of the mark which are electrically conducting and parts which are not electrically conducting are measured.

3. A method according to claim 1 or 2, characterized in that the reference holes are drilled by means of the measuring pin, whereby the material changes in the marks are measured by means of the sensor during drilling.

4. A method according to any one of the preceding claims, characterized in that capacitive and/or inductive variations at material transitions are measured.

5. A method according to any one of the preceding claims, characterized in that variations in the light intensity at material changes are measured by means of a sensor comprising a light conductor.

6. A device suitable for carrying out the method according to any one of the preceding claims, said device being provided with a measuring pin which is capable of translating and rotating movement, characterized in that said measuring pin is provided with a sensor which is movable in a reference hole, by means of which the times and/or duration of interruptions of material transitions of the mark can be measured during a rotation of the measuring pin at the location of a mark in a layer.

7. A device according to claim 6, characterized in that said measuring pin is provided with a plurality of circumferentially distributed sensors.

8. A device according to claim 6 or 7, characterized in that said measuring pin is provided with a drill bit at a first end comprising said sensor, and with a drill coupling at a second end remote from said first end.

9. A device according to any one of the claims 6, 7 or 8, characterized in that said measuring pin is provided with a contact ring connected to said sensor, which is in sliding contact with a contact surface connected to a computer.

10. A measuring pin suitable for being used with the method and device according to any one of the preceding claims.

11. A circuit board provided with a plurality of layers which each comprise a plurality of marks, characterized in that each mark comprises a copper ring, whereby the inner side of said ring is provided with notches.