GB2331038A

GB2331038A - Apparatus for forming holes in sheet material

Info

Publication number: GB2331038A
Application number: GB9723510A
Authority: GB
Inventors: Simon Roy Golightly
Original assignee: Westwind Air Bearings Ltd
Current assignee: Westwind Air Bearings Ltd
Priority date: 1997-11-06
Filing date: 1997-11-06
Publication date: 1999-05-12
Also published as: GB9723510D0

Abstract

A drilling unit for drilling circuit boards comprising a plurality of laser drilling heads arranged in a series, means 1 for production of a common controlled and conditioned beam of laser energy which is directed to each of said plurality of laser heads in the series the conditioned source of laser energy is in the form of laser energy of two different frequencies,each being derived from a fundamental frequency of a laser generator and being independently controllable in output., at each intermediate laser head in the series a beam splitter 18,19 for collection of a required portion of laser energy from the laser beam and for transmission of a remaining portion of energy to a successive laser head in the series, and, at each laser head, control means for controlling the position of the laser energy on a workpiece, shuttering means 20,21 for shuttering off the laser energy when not required and focusing means for focusing the laser energy on the workpiece, and at the last laser head in the series mirror means for collection and for transmission of the remainder of the laser beam energy.

Description

Apparatus for forming holes in sheet material" The present invention relates to apparatus for forming holes in sheet and laminated material, particularly drilling machines for drilling small holes, generally referred to as vias, in printed circuit boards.

Printed circuit boards may consist of a single insulating sheet with an etched copper circuit on a surface and various components may be soldered onto the board and connected by way of the circuit. Multi-layer boards are often used in computer installations and in that case there may be a number of laminated boards with, at each level, a printed circuit, and it will be necessary to provide selective connection to different parts of each circuit board at different levels within the system.

The present invention is specifically concerned with providing a drilling machine which can drill into such boards. In this respect, drilling machines are known which consist of a work table and a plurality of mechanical fine drilling stations which are controlled in spatial relationship with the work table so as to provide the holes in the right places in the circuit board. To this end, each drilling head has to have a control means for locating the drilling head in X and Y co-ordinates as well as control means to control the rate and depth of drilling.

It is known to form holes in circuit boards by use of laser energy, and this is generally referred to as ablation, and in this specification the term will be referred to as laser drilling. However, such arrangements are difficult to incorporate in a production unit where different units have to be separately controlled and located. This is particularly difficult because a laser drilling arrangement requires a laser generator for each such drill together with means for controlling location and power of the laser beam output.

Also, laser drilling is usually only capable of drilling in certain prescribed conditions, i.e. either drilling through the relatively soft insulating material or through the harder conducting material such as copper.

In this respect U.S. Patent 4,839,497 discloses use of two separate lasers at different frequencies whose beams are combined.

Various other prior art arrangements have involved use of UV laser energy particularly quadrupled frequency energy from Nd YAG lasers or doubled frequency energy from copper vapour lasers in addition to that from more traditional excimer laser systems.

The problem with all of these known laser drilling arrangements is that they are not necessarily particularly suited to installation on a multi-head controlled base table operating system. The present invention is concerned with provision of a system which can be so applied.

According to one aspect of the present invention there is provided a multi-head drilling unit for drilling circuit boards comprising a plurality of laser drilling heads, means for production of a common controlled and conditioned beam of unfocussed laser energy which is directed in turn to each of said plurality of laser heads, and at each laser head a beam splitter for collection of a required portion of laser energy from the laser beam, control means for controlling the position of the laser energy on a workpiece, shuttering means for shuttering off the laser energy when not required and beyond the shuttering means in the direction of flow of the laser energy a focusing means for focusing the laser energy on the workpiece.

Preferably the conditioned source of laser energy is in the form of laser energy of two frequencies, a first frequency being a fundamental of the laser generator and being controllable in output and a second frequency being a multiple, preferably a doubling of the fundamental frequency or the sum frequency generation of two fundamental frequencies and being controllable independently of the fundamental frequency laser energy.

In this respect a copper vapour laser is particularly effective because its two fundamental wavelengths are in the region of 500 to 600 nm which is visible light and so its converted frequency is in the region of 250 to 300 nm, that is in the UV region. This then provides a useful arrangement if one of the fundamentals and converted frequency are combined, so that the different components of the beam can ablate different components of the circuit board.

In another development, a Nd YAG laser can be used with its fundamental wavelength operating in the near infra red part of the spectrum and its third or fourth harmonic operating in the UV region. The second harmonic of the Nd YAG laser can be used to generate visible light.

Preferably the two beams are split by a beam splitting device into two beams where they are independently controlled and are then recombined for transmission to the laser head.

Preferably the combined laser energy beam is reflected in its path by a trepanning mirror which is arranged to rotate about a central axis so as to provide a conical motion to the emerging beam and thereby enable holes to be drilled which are larger than the diameter of the laser beam at its focused outlet end.

According to another aspect of the present invention there is provided a drilling unit for drilling multi-layer materials comprising at least one laser drilling head, means for production of a controlled and conditioned beam of unfocussed laser energy which is directed to said laser head, and, at said laser head, a control means for controlling and focussing the laser energy on a workpiece, characterised in that said unit comprises a single laser energy source for provision of laser energy of at least one fundamental frequency, laser energy frequency control means which is arranged to provide from said single laser energy source a composite laser energy beam having two components of different frequencies, which different frequencies being suited to drilling different components of a multi-layer material, means for splitting said composite laser energy beam into a first component in the form of a beam travelling in a first beam path and consisting essentially of laser energy at one of said different frequencies and a second component in the form of a beam travelling in a second beam path spatially separated from said first beam path and consisting essentially of laser energy at the other of said different frequencies, beam control means for controlling said two spatially separated beams independently of one another, and means for recombining said two beams of laser energy for direction to said workpiece.

One of said different frequencies can be the, or one of the fundamental frequencies of the laser energy source and the other frequency can be a multiple of the, or one of the fundamental frequencies or a sum frequency generation of two fundamental frequencies.

In some embodiments neither of said different frequencies is equal to any fundamental frequency of the laser energy source In one development of the invention one of said different frequencies is visible light and the other is UV energy. In another development of the invention one of said different frequencies is infra red light and the other is UV energy.

In some embodiments where one of the different frequencies is visible light, the frequency of the visible light can be a harmonic of the, or one of the fundamental frequencies of the laser energy source.

A separate beam control means can be located in each spatially separated beam.

The beam control means can comprise a mechanical shutter.

The beam control means can comprise an acousto-optic modulator. Where there are two separate beam control means one can be a mechanical shutter and the other can be an acousto-optic modulator.

An embodiment of the invention will now be described by way of example with reference to the accompanying single figure.

A laser unit 1 having a back mirror and polarising unit 2 at its rear end is located to direct laser energy in the visible part of the spectrum through three successive mirrors 3, 4 and 5 to a collimating mirror 6 whereafter the collimated beam is directed on to a frequency conversion unit 7 after passing through a focusing cylindrical lens unit 8 at the inlet of the frequency conversion unit.

In the preferred form of the invention a copper vapour laser is used having two fundamental wavelengths, 510,6 and 578 nm respectively which are within the visible spectrum, but other forms of laser could also be used with similar characteristics.

The frequency conversion unit 7 includes a crystal device into the centre of which the focused beam is directed and which enables the beam to issue in two forms, firstly a beam of visible light at the fundamental frequency of the input beam and secondly an additional beam of double the frequency of the input beam which is therefore in the UV part of the spectrum.

For input beams with more than one frequency in the visible part of the spectrum, the crystal device issues beams at double the frequency of either one of the input beams or a beam at the frequency of the input beams mixed. The frequency converted beam is selected by controlling the vertical rotation angle of the crystal device in a manner known per se. The fundamental frequency beams and the frequency converted beams are then arranged to exit from the crystal device and to be collimated co-linearly by way of a second cylindrical lens unit 9.

The two coincident beams then travel to a system of optical devices which enable the frequency converted beam to be split spatially from the fundamental frequency beams. The fundamental frequency beam is separated from the converted beam in order that control of each beam can be provided independently.

The frequency converted beam and the fundamental frequency beam are separated by a beam splitter 10 which has a specially selected optical coating which allows the fundamental frequency beam to be deflected downwards while the converted beam path carries straight on through the mirror. The frequency converted beam path then proceeds through a high speed mechanical shutter 11 and then is reflected by a mirror 12 to a beam combining optic 13 which again has a selected form of coating which allows the converted beam to pass straight through while the fundamental frequency beam is reflected into the same path.

In the meantime the fundamental frequency beam reflected from the dividing beam splitter 10 passes a further mirror 14 and then passes through an acousto-optic modulator 15 which acts as a high speed shutter and beam attenuator to control the output power of this beam.

These two separate shutter devices enable independent control of the two laser beams (fundamental frequency and converted frequency) to be achieved. They are then combined in the combining optic 13 and thereafter all optics have multi-layer optical coatings which are efficient at both wavelengths.

In the next phase the two beams are passed through a motorised beam expander 16 to reach a rotating tilted mirror trepanning head 17. The trepanning head 17 rotates at a small angle to a principal axis, the amount of angle being controllable and proportional to the tilt of the trepanning head mirror, thereby causing the locus of the beam to become very slightly conical so as to adjust the final required hole size at the ultimate output end of the beam path.

After emerging from the trepanning head mirror, the combined beam (fundamental and converted) passes to a series of galvanometer drilling heads.

These heads are known as galvanometer drilling heads because they are essentially controlled in the manner of a galvanometer by use of a mirror in each case which is adjustable by very fine angles to control the co-ordinate locations (X and Y) of the beam on a workpiece.

In practice the conditioned laser beam exits the laser beam conditioning unit 10, 11, 12, 13, 14, 15, 16 and 17, and travels parallel to a cross gantry of the machine to a series of such galvanometer scanning heads with each head positioned above each drilling station.

The galvanometer head in each case comprises a partial beam splitter 18, 19, which splits off the correct percentage of the dual frequency beam, a mechanical shutter 20 and 21 respectively to shut off the beam from the galvanometer head, twin high speed swinging mirrors (22, 23) each mounted on the galvanometer for fine angular adjustment to control X and Y traversing of the laser beam at the work piece and an f-theta telecentric scanning lens assembly.

In the examples shown two galvanometer drilling heads are shown and in this case the partial beam splitter of the first galvanometer head will take half of the energy while the remainder of the beam energy carries on to the second drilling head. In the case where there are three drilling heads, the first would take a third of the energy, the second would take half of the remaining energy and the last one would take the last remaining energy, whereas with four scanning heads the first scanning head would take a quarter of the energy, the second scanning head would take a third of the remaining energy, the third head would take a half of the then remaining energy and the fourth head would take the rest of the energy.

In regard to the f-theta telecentric scanning lens assembly 24, 25, each contains an additional crystal material which enables the focal lengths of the two frequencies of the beams to be matched, causing the focus spot of each beam to be coincident at the workpiece. The rotating axis of the laser beam created by the trepanning head tilted mirror 17 causes the focus spot to describe a circle on the workpiece, the diameter of which dictates the diameter of the required ablated via. It should also be noted that it is only at the final stage that the beam is correctly focused to provide its action in ablating into the printed circuit board whereas at earlier stages in its path it is still out of focus and therefore parts of the beam can be shut off or deflected without risk of damage to the components used at that stage. Finally it is explained that the system is controlled by a P.C. based control system which optimises the frequency selection that is to say the ratio between fundamental and the converted frequency (which will usually be in the UV region) so that the laser beam is fully controlled.

Thus there is control of each frequency, the rotational frequency of the trepanning mirror, the trepanning mirror tilt angle, the pulse repetition rate of the laser, the ablation time at each frequency, and the respective focal lengths. Thus, the focal position of the laser beam spot relative to the workpiece surface is in each case controlled to achieve the highest quality via possible in the shortest time. Moreover, control as between the fundamental and the converted frequency beams is selected to ensure that the required drilling through one or more layers of a multi-layer board, copper and dielectric material, can be effected as required.

In one example an optical platform can be provided which is matched to the base of an existing mechanical drilling machine so forming a multihead PCB laser drilling machine.

A laser which emits light in the visible part of the spectrum with very short pulse length (in the region of 5-100 nano seconds and typically 20 nano seconds) and high repetition rate (in the order of kilohertz and typically 6 - 10 kilohertz) is used. This light is ideal for ablating small holes in metals. The light is sent through the UV conversion optics where high power, deep UV light is generated. The fundamental, visible light and the UV light travel collinearly across the machine into the beam conditioning optics which incorporate an optical system to enable both light beams to be separately switched and attenuated. The two controlled beams reflect off the trepanning spindle, which sets the required hole size, across the machine into the drilling heads. The heads take a share of the available light power and direct this to the workpiece through the focusing scan lens being accurately positioned within a square tile of predetermined size (typically 25mm x 25mm) by the X - Y galvanometer mirrors.

The system controller determines the selection of the correct wavelength light dependent on the material being ablated; the visible will ablate the top copper layer, followed by the UV light to ablate the substrate and clean up the next layer of copper. This is carried out at high speed for all the holes in the square tile, and then the standard X - Y table moves the workpiece to the next tile.

The laser, the UV conversion kit, the beam conditioning optics, the X Y scanning heads, system controller (suited for interface with an existing control system), all necessary cables etc and detail design drawings for the necessary machine framework can be provided to allow the system to be used with an existing drilling machine.

Claims

CLAIMS 1. A drilling unit for drilling circuit boards comprising a plurality of laser drilling heads arranged in a series, means for production of a common controlled and conditioned beam of laser energy which is directed to each of said plurality of laser heads in the series, at each intermediate laser head in the series a beam splitter for collection of a required portion of laser energy from the laser beam and for transmission of a remaining portion of energy to a successive laser head in the series, and, at each laser head, control means for controlling the position of the laser energy on a workpiece, shuttering means for shuttering off the laser energy when not required and focusing means for focusing the laser energy on the workpiece, and at the last laser head in the series mirror means for collection and for transmission of the remainder of the laser beam energy.
2. A drilling unit according to claim 1 in which the conditioned source of laser energy is in the form of laser energy of two different frequencies, each being derived from a fundamental frequency of a laser generator and being independently controllable in output.
3. A drilling unit according to claim 2 in which the laser generator is a copper vapour laser utilising a first wavelength selected from two fundamental wavelengths in the region of 500 to 600 nm which is visible light and a second wavelength which is a summation frequency of the two fundamental wavelengths, or a frequency doubling of either of the two fundamental wavelengths providing a wavelength in the region of 250 to 300 nm, that is in the W region whereby a visible light component and a W component of the beam can ablate different components of the circuit board.
4. A drilling unit according to claim 2 in which said laser generator is a Nd YAG laser having a fundamental wavelength operating in the near infra red part of the spectrum and a third or fourth harmonic operating in the W region.
5. A drilling unit according to claim 4 in which a second harmonic of the Nd YAG laser is used to generate visible light.

6. A drilling unit according to any one of claims 2 to 5 in which the conditioned beam is split by a beam splitting device into two beams of different frequency which are independently controlled and are then recombined for transmission to the laser head.
6. A drilling unit according to any one of claims 2 to 5 in which the combined laser energy beam is reflected in its path by a trepanning mirror which is arranged to oscillate about a central axis so as to provide a conical motion to the emerging beam and thereby enable holes to be drilled which are larger than the diameter of the laser beam at its focused outlet end.
7. A drilling unit for drilling multi-layer materials comprising at least one laser drilling head, means for production of a controlled and conditioned beam of laser energy which is directed to said laser head, and, at said laser head, a control means for controlling and focusing the laser energy on a workpiece, characterised in that said unit comprises a single laser energy source for provision of laser energy of at least one fundamental frequency, laser energy frequency control means which is arranged to provide from said single laser energy source a composite laser energy beam having two components at different frequencies, which different frequencies being suited to drilling different components of a multi-layer material, means for splitting said composite laser energy beam into a first component in the form of a beam travelling in a first beam path and consisting essentially of laser energy at one of said different frequencies and a second component in the form of a beam travelling in a second beam path spatially separated from said first beam path and consisting essentially of laser energy at the other of said different frequencies, beam control means for controlling said two spatially separated beams independently of one another, and means for recombining said two beams of laser energy for direction to said workpiece.
8. A drilling unit for drilling multi-layer materials in which one of said different frequencies is fundamental frequency of the laser energy source and the other frequency is a summation, for example a multiple, of fundamental frequencies of the laser energy source.
9. A drilling unit for drilling multi-layer materials according to claim 7 in which one of said different frequencies is visible light and the other is W energy.
10. A drilling unit for drilling multi-layer materials according to claim 7 in which there are two separate beam control means one being a mechanical shutter and the other being an acousto-optic modulator.
11. A drilling unit substantially as herein described with reference to the accompanying drawings.

10. A drilling unit for drilling multi-layer materials according to claim 9 in which the frequency of the visible light is a harmonic of a fundamental frequency of the laser energy source.

11. A drilling unit for drilling multi-layer materials according to claim 8 in which one of said different frequencies is infra red light and the other is W energy.

12. A drilling unit for drilling multi-layer materials according to claim 7 to 11 in which a separate beam control means is located in at least one of the spatially separated beams.

13. A drilling unit for drilling multi-layer materials according to claim 12 in which the beam control means comprises a mechanical shutter.

14. A drilling unit for drilling multi-layer materials according to claim 12 in which the beam control means comprises an acousto-optic modulator.

15. A drilling unit for drilling multi-layer materials according to claim 8 in which there are two separate beam control means one being a mechanical shutter and the other being an acousto-optic modulator.

Amendments to the claims have been filed as follows 1. A drilling unit for drilling circuit boards comprising a plurality of laser drilling heads arranged in a series, means for production of a common controlled and conditioned beam of laser energy which is directed to each of said plurality of laser heads in the series the conditioned source of laser energy being in the form of laser energy of two different frequencies, each being derived from a fundamental frequency of a laser generator and being independently controllable in output, at each intermediate laser head in the series a beam splitter for collection of a required portion of laser energy from the laser beam and for transmission of a remaining portion of energy to a successive laser head in the series, and, at each laser head, control means for controlling the position of the laser energy on a workpiece, shuttering means for shuttering off the laser energy when not required and focusing means for focusing the laser energy on the workpiece, and at the last laser head in the series mirror means for collection and for transmission of the remainder of the laser beam energy.

2. A drilling unit according to claim 1 in which the laser generator is a copper vapour laser utilising a first wavelength selected from two fundamental wavelengths in the region of 500 to 600 run which is visible light and a second wavelength which is a summation frequency of the two fundamental wavelengths, or a frequency doubling of either of the two fundamental wavelengths providing a wavelength in the region of 250 to 300 nm, that is in the W region whereby a visible light component and a UV component of the beam can ablate different components of the circuit board.

3. A drilling unit according to claim 1 in which said laser generator is a Nd YAG laser having a fundamental wavelength operating in the near infra red part of the spectrum and a third or fourth harmonic operating in the UV region.

4. A drilling unit according to claim 3 in which a second harmonic of the Nd YAG laser is used to generate visible light.

5. A drilling unit according to any one of claims 1 to 4 in which the conditioned beam is split by a beam splitting device into two beams of different frequency which are independently controlled and are then recombined for transmission to the laser head.

6. A drilling unit according to any one of claims 1 to 5 in which the combined laser energy beam is reflected in its path by a trepanning mirror which is arranged to oscillate about a central axis so as to provide a conical motion to the emerging beam and thereby enable holes to be drilled which are larger than the diameter of the laser beam at its focused outlet end.

7. A drilling unit for drilling multi-layer materials according to claim 5 in which a separate beam control means is located in at least one of the spatially separated beams.

8. A drilling unit for drilling multi-layer materials according to claim 7 in which the beam control means comprises a mechanical shutter.

9. A drilling unit for drilling multi-layer materials according to claim 8 in which the beam control means comprises an acousto-optic modulator.