WO2022084642A1

WO2022084642A1 - A method for forming at least one through-hole in a metal workpiece

Info

Publication number: WO2022084642A1
Application number: PCT/GB2021/000125
Authority: WO
Inventors: Kenneth Matthew DZURKO; Yidong Zhou
Original assignee: Trumpf Laser Uk Limited
Priority date: 2020-10-23
Filing date: 2021-10-21
Publication date: 2022-04-28
Also published as: GB202016923D0

Abstract

A method for forming at least one through-hole (8) in a metal workpiece (5), which through-hole (8) has a first diameter (11) on a first surface (6) of the metal workpiece (5), a second diameter (12) on a second surface (7) of the metal workpiece (5), and a surface finish (15) on at least the first surface (6), and which method comprises: providing the metal workpiece (5) and a pressure retaining member (17); providing a laser (1) for emitting a laser beam (4) comprising at least one laser pulse (21) defined by a peak power (22); focussing the laser beam (4) from the laser (1) onto the first surface (6) to form a spot (31) having a spot diameter (34) and a pulse fluence (36); selecting the peak power (22) and the spot diameter (34) to cause a plume (9) comprising material (16) from the metal workpiece (5) to be ejected from the first surface (6) while the through-hole (8) is being formed, with the ejection being caused by vapour pressure in the through-hole (8) being formed; selecting the peak power (22), the spot diameter (34), and the pulse fluence (36) to form the through-hole (8); and causing the metal workpiece (5) to be such that it has had the pressure retaining member (17) applied to the second surface (7) prior to the plume (9) being ejected from the metal workpiece (5); and wherein after the laser beam (4) pierces the second surface (7), at least a portion of the vapour pressure within the through-hole (8) being formed is retained by the pressure retaining member (17) in order to maintain the plume (9) until the through-hole (8) is formed.

Description

A method for forming at least one through-hole in a metal workpiece

Field of Invention

This invention relates to a method for forming at least one through-hole in a metal workpiece. The invention has particular application for rapidly forming through-holes in articles where the holes are untapered, tightly spaced together, and where the quality of the surface finish is important.

Background to the Invention

It is very desirable and commercially very important in consumer goods to be able to use lasers to form through-holes in metal workpieces in reliable and repeatable processes that are amenable to high-throughput high-yield production. Arrays of through-holes find application in speakers, for example for earphones and mobile telephones, and in filters and screens used in medical and automotive applications. Such arrays need to comprise small diameter holes, for example 50μm to 200μm, with minimal tapering, such that the holes can be packed closely together.

Prior art methods of forming high-quality through-holes in metal workpieces include using ultra-short pulse lasers such as femtosecond and picosecond lasers. Each hole is micromachined with tens of thousands of individual laser pulses, and the laser beam is scanned around each hole. The results are extremely high-quality holes, which can be tightly packed to form the desired arrays. However, ultra-short pulse lasers are expensive, and more importantly, the time taken to form the arrays of through-holes is too long, making the finished article expensive.

Prior art methods of forming high-quality through-holes in metal workpieces also include the use of continuous wave, pulsed, or quasi continuous wave lasers. Such lasers can form through-holes far more quickly. However, the holes tend to be tapered, it is difficult to space the holes closely together, and it is difficult to avoid thermal damage. Commercial solutions to control or eliminate the taper include complex and precision beam manipulation and trepanning techniques. These are expensive and not ideally suited to very-high throughput manufacture.

There is a need for a method for forming at least one through-hole in a metal workpiece that reduces or avoids the aforementioned problems.

The Invention

Accordingly, in one non-limiting embodiment of the present invention there is provided a method for forming at least one through-hole in a metal workpiece, which through-hole has a first diameter on a first surface of the metal workpiece, a second diameter on a second surface of the metal workpiece, and a surface finish, and which method comprises:

• providing the metal workpiece and a pressure retaining member;

• providing a laser for emitting a laser beam comprising at least one laser pulse defined by a peak power;

• focussing the laser beam from the laser onto the first surface to form a spot having a spot diameter and a pulse fluence;

• selecting the peak power and the spot diameter to cause a plume comprising material from the metal workpiece to be ejected from the first surface while the through-hole is being formed, with the ejection being caused by vapour pressure in the through-hole being formed;

• selecting the peak power, the spot diameter, and the pulse fluence to form the through-hole;

• causing the metal workpiece to be such that it has had the pressure retaining member applied to the second surface, prior to the plume being ejected from the metal workpiece; and wherein: • after the laser beam pierces the second surface, at least a portion of the vapour pressure within the through-hole being formed is retained by the pressure retaining member in order to maintain the plume until the through-hole is formed.

The method of the present invention is particularly attractive because it is able to form high-quality through-holes in metal workpieces faster and more economically than has hitherto been possible. The invention departs from the prior art where though-holes are laser drilled because in the prior art, a pressure retaining member is not provided to block the exit hole on the second surface. The invention enables the rapid manufacture of arrays of tightly-spaced holes, with little or no tapering, that can be produced in high quality materials such as stainless steel with excellent surface quality and very little thermal damage. Such arrays find application in consumer electronic products such as speakers, mobile phones, smart phones, watches and other wearables, tablet, laptop and desktop computers, and filters and screens used in medical and automotive applications. The advantages of having no tapering are that arrays of small holes can be formed that allows for sound or light to propagate through the holes with minimal attenuation, and also that it maximises the hole to material ratio for a given strength specification. Small holes prevent water ingress into electronic devices. The pattern of the holes is the same on both sides of the material. This can improve the performance of filters, speakers and other devices.

The step of applying the pressure retaining member prior to forming the through-hole enables the vapour pressure that ejects material from the first surface as the through-hole is being formed to be maintained after the laser pierces the second surface. This enables the laser beam to continue forming the through-hole, enabling holes having a desired taper, or no taper at all, to be formed. This enables arrays of tightly-packed small-diameter holes to be produced more efficiently and with less heat input into the workpiece than has hitherto been possible in the prior art. The reduced heat input is advantageous because it leads to less thermal damage to the workpiece, which is important in the manufacture of high-quality consumer electronics products. Without the pressure retaining member, the vapour pressure is reduced when the laser beam pierces the second surface, resulting in a different and a less efficient laser processing regime thereafter. The vapour pressure is sometimes called recoil pressure. Without the pressure retaining member, further processing of the hole with the laser beam may cause the diameter of the hole on the first surface to expand more rapidly than the diameter of the hole on the second surface, which can increase rather than decrease the taper ratio. This makes the production of arrays of tightly-packed small-diameter holes difficult if not impossible to manufacture. These disadvantages can be avoided by using ultra-short pulse lasers, but at the expense of slower processing speeds, or using complex and precision beam manipulation techniques. These disadvantages of the prior art methods are able to be overcome in the present invention due to the use of the pressure retaining member.

The metal workpiece can be provided with the pressure-retaining member attached ready for laser processing. Alternatively, the pressure-retaining member may be attached to the metal workpiece, for example by clamping them together, after the provision of the metal workpiece, for example as a separate step in the method of the invention. The metal workpiece is preferably removed from the pressure retaining member after the holes have been formed.

The laser beam may be focussed with a lens. Other focussing means such as mirrors and diffraction optics can also be used.

The spot diameter may be selected such that the second diameter is at least one point two times larger than the spot diameter. The second diameter may be at least two times larger than the spot diameter. The pressure retaining member enables the vapour pressure to be maintained when the laser beam first pierces the second surface, enabling through-holes to be drilled that either have no tapers or have very small tapers. If the pressure retaining member is not present, the vapour pressure is released when the laser beam first pierces the second surface, resulting in a second diameter that is often equal to, or less than, the spot diameter. The resulting through-hole may then be tapered and/or be surrounded by a heat-affected zone. A heat affected zone (thermal damage) is associated with discolouration and/or a change in a surface texture of a workpiece, and is undesirable in consumer electronic devices.

The through-hole may have a pre-determined taper ratio, and the laser may be turned off once the pre-determined taper ratio has been achieved. The pre-determined taper ratio can be determined from the design requirements of the through-hole. For example, arrays of through-holes may be required for speakers, and it may be preferred that in a workpiece having a thickness, there is a required difference between the first diameter and the second diameter. The pre-determined taper ratio would then be equal to the required difference between the first diameter and the second diameter, divided by the thickness of the metal workpiece. The taper ratio is positive if the first diameter is larger than the second diameter. The pre-determined taper ratio may be zero.

The first diameter may be within twenty micrometres of the second diameter. The first diameter may be within ten micrometres of the second diameter. The first diameter may be within five micrometres of the second diameter. The first diameter may be equal to the second diameter.

The first surface and the second surface may be separated by between fifty micrometres and five hundred micrometres. The first surface and the second surface may be separated by between fifty micrometres and two hundred and fifty micrometres. Thicker materials can also be processed.

The first diameter may be between fifty micrometres and two hundred micrometres. Larger diameter holes can also be produced.

The method may include the step of moving the laser beam on the first surface and turning the laser beam on and off to form an array of the through-holes. Adjacent ones of the through-holes may be separated by less than half the first diameter. Non-adjacent holes may be formed in sequence.

The laser may be a continuous wave laser. The laser may be a quasi continuous wave laser or a pulsed laser. Laser pulses can be obtained from a continuous wave laser using an external optical modulator, or by turning pumps that pump the continuous wave laser on and off. The pulsed laser may be a master oscillator power amplifier, a Q-switched laser, or a mode-locked laser.

The laser beam may comprise a fundamental mode. The laser beam may have a beam quality M² factor less than 1.3.

The laser beam may comprise a higher-order mode of the laser.

The laser beam may have a top hat or annular beam profile.

The laser beam preferably has a cross-section with circular symmetry. Circular symmetry occurs when the intensity of a laser beam is independent of its azimuth. Guided optical modes of optical fibres and optical fibre lasers that have circular symmetry include the fundamental mode which has a single circular spot, and higher-order modes that have a single circular spot surrounded by one or more concentric rings. A laser beam with circular symmetry enables uniform vapour pressure to be created at each depth in the through-hole while it is being created resulting in a symmetric plasma-melt reaction and thereby a circular through-hole. Other higher-order modes can be used, including modes having a plurality of lobes in a concentric ring around the centre line of the laser beam.

The pressure retaining member may have a contact with the second surface, which contact is sufficient to retain all of the vapour pressure within the through-hole being formed. The laser processing is thus not interrupted by a sudden release of vapour pressure through the second surface, and the plume can continue to be ejected from the first surface. The pressure retaining member may be clamped to the metal workpiece, adhered to the metal workpiece using an adhesive, or coated onto the metal workpiece in order to form the contact. Other contact configurations may be employed.

The first surface may comprise at least one of copper, aluminium, gold, silver, platinum, palladium, nickel, titanium, tin, iron, chromium, and stainless steel.

The pressure retaining member may transmit more than 50% of the peak power of the laser beam. The pressure retaining member may comprise a glass. The metal workpiece may be clamped onto the glass. Glass may be a preferred pressure retaining member because it can be both transparent, and thermally non-conducting, and can survive modest drilling by the laser beam. Also, the glass will not weld to the workpiece, and a glass can be selected that does not absorb a substantial proportion of the laser energy, thus avoiding the glass cracking or being damaged by the laser beam.

The pressure retaining member may comprise a ceramic. The metal workpiece may be clamped onto the ceramic. The ceramic may comprise sapphire.

The pressure retaining member may comprise a metal having a higher melting point than a melting point of the workpiece.

The pressure retaining member may comprise a metal having a higher reflectivity than a reflectivity of the workpiece.

The pressure retaining member may be of any configuration, for example a layer, a piece of tooling, a plate, a slide, or a slip such as a microscope cover slip. A layer of a glass, ceramic, or a metal that preferably will not weld to the metal workpiece can be affixed to the workpiece before the laser processing commences by a weld, adhesive, wax or with clamps. The workpiece can be attached to tooling, such as a base plate, surface, or a component of a clamp or mechanical table. Preferably the base plate, surface, or component comprises glass. The workpiece can be attached to a slide such as a microscope slide, a microscope cover slip, a shim, or other item that can be used as the pressure retaining member. Preferably the pressure retaining member is such that it can be used for more than one workpiece before being disposed of or recycled.

The method may include a step of removing the pressure retaining member.

The method of the present invention may include the step of providing an apparatus for providing the pressure retaining member.

The invention also provides an article comprising at least one through-hole and which hole is formed according to the method of the invention. The article may comprise the pressure retaining member. Alternatively, the pressure retaining member may have been removed. Examples of articles are mobile phones, smart phones, tablet computers, lap top computers, desk top computers, speakers, watches, televisions, machinery, jewellery, and filters and screens used in medical and automotive applications.

The invention also provides an apparatus for forming at least one through-hole in a workpiece according to the method claimed in any one of the preceding claims.

Brief Description of the Drawings

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:

Figure 1 shows apparatus for use in the method according to the present invention;

Figure 2 shows a pulsed laser waveform;

Figure 3 shows a laser beam that has been focussed onto a surface;

Figure 4 shows a plume being ejected from the first surface; and

Figure 5 shows an array of through-holes made according to a method of the present invention.

Detailed Description of Preferred Embodiments of the Invention

Figure 1 shows an apparatus 10 for forming through-holes 8 in a metal workpiece 5, which apparatus comprises a laser 1 , a scanner 2, and a lens 3. The through-holes 8 have a first diameter 11 on a first surface 6 of the metal workpiece 5, a second diameter 12 on the second surface 7 of the metal workpiece 5, and a surface finish 15 on at least the first surface 6. The laser 1 emits a laser beam 4 in the form of at least one laser pulse 21 defined by a peak power 22. The laser beam 4 from the laser 1 is focussed onto or near the first surface 6 to form a spot 31 , shown with reference to Figure 3, having a spot diameter 34 and a pulse fluence 36. The peak power 22 and the spot diameter 34 are selected to cause a plume 9 comprising material 16 from the metal workpiece 5 to be ejected from the first surface 6 while the through-hole 8 is being formed, with the ejection being caused by vapour pressure within the through-hole 8 being formed. The peak power 22, the spot diameter 34, and the pulse fluence 36 are selected to form the through-hole 8. The workpiece 5 has a pressure retaining member 17 applied to the second surface 7 prior to the plume 9 being ejected from the metal workpiece 5, The pressure-retaining member 17 retains at least a portion of the vapour pressure in the through-hole 8 being formed after the laser beam 4 pierces the second surface 7 in order to maintain the plume 9 until the through-hole 8 is formed.

The laser beam 4 is shown being focussed by a lens 3. Other focussing means such as mirrors and diffraction optics can also be used to focus the laser beam 4.

The metal workpiece 5 can be provided with the pressure-retaining member 17 attached ready for laser processing. Alternatively, the pressure-retaining member 17 may be attached to the metal workpiece 5, for example by clamping them together, after the provision of the metal workpiece 5.

The scanner 2 would typically include mirrors attached to galvanometers (not shown), and be controlled by a controller 19 which may also control the laser 1. The scanner 2 is for moving the workpiece 5 with respect to the laser beam 4 in order to form a plurality of the holes 8 which may be in the form of an array. Alternatively or additionally, the workpiece 5 may be moved with respect to the laser beam 4 using a translation stage.

The laser 1 can be a fibre laser, a solid state rod laser, a solid state disk laser, or a gas laser such as a carbon dioxide laser. The laser 1 can be a continuous wave laser, a pulsed laser, or a quasi continuous mode QCW laser. Laser pulses can be obtained from a continuous wave laser using an external optical modulator, or by turning pumps that pump the continuous wave laser on and off. For the rapid formation of the holes 8, the laser 1 is preferably a continuous mode laser or a quasi continuous mode laser that emits the laser beam 4 as a fundamental mode.

The laser 1 is shown as being connected to the scanner 2 via an optical fibre cable 13 and collimation optics 14. The laser 1 is shown as emitting the laser beam 4 at a wavelength 20. The laser 1 is preferably a single-mode, ytterbium-doped fibre laser, and the wavelength 20 is preferably in the range 1000nm to 1100nm. The laser beam 4 may have a beam quality M² factor less than 1.3. The fundamental mode may be gaussian. The laser beam 4 may have a beam profile that has a top hat or annular shape. Top hat beam profiles can be formed by exciting many transverse optical modes of the laser 1 or the optical fibre cable 13 such that the beam quality M² factor is greater than 6 or preferably greater than 15. The laser beam 4 may comprise a higher order mode of the laser 1 or the optical fibre cable 13. Preferably the laser beam 4 has a cross section with circular symmetry in order to enable the production of circular through-holes. Circular symmetry occurs when the intensity of a laser beam is independent of its azimuth. Guided optical modes of optical fibres and optical fibre lasers that have circular symmetry include the fundamental mode which has a single circular spot, and higher-order modes that have a single circular spot surrounded by one or more concentric rings. A laser beam with circular symmetry enables uniform vapour pressure to be created at each depth in the through-hole while it is being created resulting in a symmetric plasma-melt reaction and thereby a circular through-hole. Other higher-order modes can be used, including modes having a plurality of lobes in a concentric ring around the centre line of the laser beam 4.

Referring now to Figure 2, there is shown a series of pulses 21. The series of pulses 21 may be obtained from the laser 1 wherein the laser 1 is a pulsed laser, or a continuous mode or quasi continuous mode laser where the laser beam 4 is turned on and off either using an external modulator, or by turning pumps (such as laser diodes) that pump the laser on and off. The series of pulses 21 is characterized by a peak power 22, an average power 23, a pulse shape 24, a pulse energy 25, a pulse width 26, and a pulse repetition frequency F_R 27.

Figure 3 shows a spot 31 formed by focussing the laser beam 4 onto the metal workpiece 5. The optical intensity 32 is the power per unit area of the laser beam 4. The optical intensity 32 varies across the diameter of the spot 31 from a peak intensity 39 at its centre 37, to a 1/e² intensity 33 and to zero. The diameter 34 of the spot 31 is typically taken as the 1/e² diameter, which is the diameter at which the optical intensity 32 falls to the 1/e² intensity 33 on either side of the peak intensity 39. The area 35 of the spot 31 is typically taken as the cross-sectional area of the spot 31 within the 1/e² diameter 34. Figure 3 shows the optical intensity 32 varying with a Gaussian or bell-shaped profile. The optical intensity 32 may have other profiles, including a top hat profile that is substantially uniform within the diameter 34. Pulse fluence 36 is defined as the energy per unit area of the pulse 21 . Pulse fluence is typically measured in J/cm², and is an important parameter in laser processing because certain processes, such as drilling a particular hole with specific dimensions, requires a minimum amount of energy to be provided. Thus if the peak power 22 is reduced, then the time required to drill the hole would be expected to increase. If a plurality of pulses 21 are required to form the hole 8, then the pulse fluence 36 refers to the total fluence of the plurality of pulses that are required to form the hole 8.

A method according to the invention for forming at least one through-holes 8 in a metal workpiece 5 will now be described solely by way of example and with reference to Figure 1 . The through-hole 8 has the first diameter 11 on the first surface 6 of the metal workpiece 5, the second diameter 12 on the second surface 7 of the metal workpiece 5, and the surface finish 15 on at least the first surface 6. The method comprises:

• providing the metal workpiece 5 and a pressure retaining member 17;

• providing the laser 1 for emitting the laser beam 4 comprising at least one laser pulse 21 defined by the peak power 22;

• focussing the laser beam 4 from the laser 1 onto the first surface 6 to form the spot 31 having the spot diameter 34 and the pulse fluence 36;

• selecting the peak power 22 and the spot diameter 34 to cause the plume 9 comprising the material 16 from the metal workpiece 5 to be ejected from the first surface 6 while the through-hole 8 is being formed, with the ejection being caused by vapour pressure in the through-hole 8 being formed;

• selecting the peak power 22, the spot diameter 34, and the pulse fluence 36 to form the through-hole 8; and • causing the metal workpiece 5 to be such that it has had the pressure retaining member 17 applied to the second surface 7 prior to the plume 9 being ejected from the metal workpiece 5; and wherein:

• after the laser beam 4 pierces the second surface 7, at least a portion of the vapour pressure within the through-hole 8 being formed is retained by the pressure retaining member 17 in order to maintain the plume 9 until the through- hole 8 is formed.

Figure 4 shows the process part-way through forming the hole 8. In order to initiate the process, the laser beam 4 can have an intensity on the first surface 6 higher than an ablation threshold of the metal workpiece 5. Once the process is initiated, the laser beam 4 can couple into the metal workpiece 5. An overshoot in the output power of a continuous wave laser, for example an overshoot caused by relaxation oscillations, can be sufficient to provide such process initialisation. Once the process has commenced, the plume 9 comprising the material 16 from the metal workpiece 5 is emitted from the first surface 6, and a vapour capillary 41 , or key hole, is formed, and the laser beam 4 drills down to the second surface 7. The plume 9 comprises a combination of plasma, vapour, molten metal, and oxides of the material 16. The process relies upon vapour pressure within a vapour capillary 41. When the laser beam 4 pierces through the second surface 7, the presence of the pressure retaining member 17 enables at least a proportion of the vapour pressure to be retained, thus enabling additional of the material 16 from the metal workpiece 5 to be emitted from the first surface 6 until the laser beam 4 from the laser 1 is turned off. By controlling the pulse fluence 36, it is possible to control the hole 8 such that it is parallel sided (zero taper) or has a predetermined taper. It is possible to control the hole 8 such that the first diameter 11 is substantially the same as the second diameter 12. When the hole 8 has a taper, then the hole 8 may be positive taper (entry hole 101 larger than exit hole 102) or a negative taper (exit hole 102 larger than entry hole 101). The method of the present invention is particularly attractive because it is able to form high-quality through-holes 8 through metal workpieces faster, and therefore more economically, than has hitherto been possible. The invention departs from the prior art where though-holes are laser drilled without the exit hole in the second surface 7 being blocked by the pressure retaining member 17. The invention enables the rapid manufacture of arrays of tightly-spaced holes 8, with little or no tapering, that can be produced in high quality materials such as stainless steel with excellent surface quality and very little thermal damage on both the first surface 6 and the second surface 7, Such arrays find application in consumer electronic products such as speakers, mobile phones, smart phones, watches and other wearables, tablet, laptop and desktop computers, and filters and screens used in medical and automotive applications.

Prior art methods of forming through-holes though metal workpieces include using ultra-short pulse lasers such as femtosecond and picosecond lasers, or continuous wave, pulsed, or quasi continuous wave lasers without the use of the pressure retaining member. Ultra-short pulse lasers can produce excellent quality though-holes. However, ultra-short pulse lasers are expensive, and more importantly, the time taken to form an array of through-holes is too long, making the finished article expensive. The time taken to form an array using continuous wave, pulsed, or quasi continuous wave lasers is much shorter. However, the holes tend to be tapered, it is difficult to space the holes closely together, and it is difficult to avoid thermal damage.

The step of applying the pressure retaining member 17 prior to forming the hole 8 enables the vapour pressure that ejects the material 16 from the first surface 6 to be maintained after the laser beam 4 pierces the second surface 7. This enables the laser beam 4 to continue forming the through-hole 8, enabling holes 8 having a desired taper, or no taper at all, to be formed. This enables arrays of tightly-packed small-diameter holes 8 to be produced more efficiently and with less heat input into the workpiece 5 than has hitherto been possible in the prior art. A typical array 51 , in a hexagonal close packed arrangement, is shown in Figure 5. Advantageously, the invention enables rapid manufacture of arrays that have first diameters 11 of approximately 100 μm, with spacings 52 between adjacent holes of less than half the first diameters 11 . Spacings 52 less than 50 μm, preferably less than 25 μm, and more preferably less than 20 μm are achievable. Importantly, given that the method of the invention can produce holes 8 that have negligible tapering such that the first diameter 11 is substantially equal to the second diameter 12, the spacings 52 on the second surface 7 can be the same as the spacings 52 on the first surface 6. Other arrays are also possible, including rectangular close packed arrays, arrays in which the spacings between holes in one dimension are different from the spacings in a different direction, arrays in which at least some of the holes are non-circular, arrays that have holes of different diameters, arrays in the form of logos or art works, and irregular arrays.

The reduced heat input is advantageous because it leads to less thermal damage to the workpiece 5, which is important in the manufacture of high-quality consumer electronics products.

Without the pressure retaining member 17, the vapour pressure is reduced when the laser beam 4 pierces the second surface 7, resulting in a different and a less efficient laser processing regime thereafter. The though-hole 8 is typically tapered when the laser beam 4 first pieces the second surface 7. Further processing of the hole 8 with the laser beam 4 can result in the hole 8 becoming more tapered, with the first diameter 11 of the hole 8 on the first surface 6 expanding at least as rapidly as the second diameter 12 of the hole 8 on the second surface 7, making the production of arrays of tightly-packed small-diameter holes 8 difficult if not impossible to manufacture. These disadvantages can be avoided by using ultra-short pulse lasers, but at the expense of slower processing speeds, or using complex and precision beam manipulation techniques which are expensive. These disadvantages of the prior art methods are able to be overcome in the present invention due to the use of the pressure retaining member 17.

The pressure retaining member 17 enables the vapour pressure within the through- hole 8 to be maintained when the laser beam 4 first pierces the second surface 7, enabling through-holes 8 to be drilled that either have no tapers or have very small tapers. The spot diameter 34 may be selected such that the second diameter 12 is at least one point two times larger than the spot diameter 34. The second diameter 12 may be at least two times larger than the spot diameter 34.

An assist gas can be used to remove the plume 9. Typical assist gases are nitrogen, argon and air.

The through-hole 8 may have a pre-determined taper ratio, and the laser 1 may be turned off once the pre-determined taper ratio has been achieved. The first diameter 11 may be within twenty micrometres of the second diameter 12, that is the magnitude of the difference between the diameters may be less than 20 micrometers. The first diameter 11 may be within ten micrometres of the second diameter 12. The first diameter 11 may be within five micrometres of the second diameter 12.

The first surface 6 and the second surface 7 may be separated by between fifty micrometres and five hundred micrometres. The first surface 6 and the second surface 7 may be separated by between fifty micrometres and two hundred and fifty micrometres. The method of the invention can also be used with thicker materials.

The first diameter 11 may be between fifty micrometres and two hundred micrometres. The method of the invention can also be used to form larger holes.

The method may include the step of moving the laser beam 4 on the first surface 6 and turning the laser beam 4 on and off to form an array of the holes 8. The holes 8 may be formed in sequence. Alternatively, non-adjacent holes 8 may be formed in sequence in order to avoid localised heat input and thus permit faster processing. The non-adjacent holes 8 that are formed in sequence may be separated by at least the thickness 103 of the metal workpiece 5, and more preferably, by at least two times the thickness 103 of the metal workpiece 5. Adjacent ones of the holes 8 may be separated by less than half the first diameter 11 .

The laser 1 may be a continuous wave laser. The laser 1 may be a quasi continuous wave laser or a pulsed laser. The laser beam 4 may comprise a fundamental mode. The laser beam 4 may have a beam quality M² factor less than 1.3.

The laser beam may comprise a higher-order mode. The laser beam 4 may comprise a single higher-order mode, or a plurality of modes. Higher order modes can form a focussed spot 31 that has a larger spot diameter 34, and can be used to form a hole 8 having a first diameter 11 up to 2 mm or larger.

The laser beam 4 may have a top hat or annular beam profile. The use of top hat and annular beam profiles can enable greater accuracy in the hole diameters being formed, particularly when making holes having a first diameter 1 1 greater than 250 μm.

The laser beam 4 preferably has circular cross-sectional symmetry. Circular symmetry occurs when the intensity of a laser beam is independent of its azimuth. Guided transverse optical modes of optical fibres and optical fibre lasers that have circular symmetry include the fundamental mode which has a single circular spot, and higher-order modes that have a single circular spot surrounded by one or more concentric rings. A laser beam having a cross section with circular symmetry enables uniform vapour pressure to be created at each depth in the through-hole 8 while it is being created resulting in a symmetric plasma-melt reaction and thereby a circular through-hole 8. Other higher-order modes can be used, including modes having a plurality of lobes in a concentric ring around the centre line of the laser beam 4.

The pressure retaining member 17 may have a contact with the second surface 7, which contact is sufficient to retain at least a portion of the vapour pressure within the through-hole 8 being formed. Preferably the contact is sufficient to retain the entire vapour pressure while the through-hole 8 is being created such that no vapour pressure is lost through the exit hole 102. The pressure retaining member 17 may be clamped to the metal workpiece 5, welded to the metal workpiece 5, adhered to the metal workpiece 5 using an adhesive or a wax, or coated onto the metal workpiece 5 in order to form the contact. The first surface 6 may comprise at least one of copper, aluminium, gold, silver, platinum, palladium, nickel, titanium, tin, iron, chromium, and stainless steel. The first surface 6 may be coated.

The transmission of the pressure retaining member 17 at the laser wavelength 20 may be greater than 50%. A transparent pressure retaining member 17 is advantageous to avoid absorption of the laser beam 4 by the pressure retaining member 17 and thereby risk damage to the pressure retaining member 17.

The pressure retaining member 17 may comprise a glass which may be clamped to the metal workpiece 5. Glass is a preferred pressure retaining member 17 because it can be both transparent, and in comparison with the metal, thermally non-conducting. Glass can also survive modest drilling by the laser beam 4 without cracking. Also, the glass is a material that will not weld to the workpiece 5, and may be selected such that the glass does not absorb significant amounts of the laser energy and thus avoid the glass cracking or being damaged by the laser beam 4.

The pressure retaining member 17 may comprise a ceramic. The ceramic may comprise sapphire.

The pressure retaining member 17 may comprise a metal having a higher melting point than a melting point of the workpiece 5. The metal may be tungsten.

The pressure retaining member 17 may comprise a metal having a higher reflectivity than a reflectivity of the workpiece 5.

The method may include the step of removing the pressure retaining member 17, for example, by unclamping it from the metal workpiece 5, or by peeling it off the metal workpiece 5.

The method of the invention will now be described with reference to the following Examples.

EXAMPLE 1

The method of the invention was applied to a metal workpiece 5 which was stainless steel having a thickness 103 of 0.12 mm. The laser 1 was a 2 kW singlemode ytterbium- doped fibre laser SP-2000-C-W-025-10-PIQ-014-001-000 manufactured by TRUMPF Laser UK Limited of Southampton, United Kingdom. The laser beam 4 had a beam quality M² factor of 1.1 , and was focussed down to a spot diameter 34 of 42 μm. Each of the holes 8 were formed using output powers of 1 .4 kW in a single laser pulse of 15 ps. The pressure retaining member 17 was glass (a standard microscope slide), which was clamped to the metal workpiece 5. Several hundred holes having a first diameter 11 on the first surface 6 of 100μm, and a second diameter 12 on the second surface 7 of 100μm were formed. The accuracy of the diameters was within +/- 10μm. There was no discernible taper in the through-holes. Referring to Figure 5, adjacent through-holes were separated by a spacing 52 of tens of microns. The process was repeatable.

EXAMPLE 2

The method of Example 1 was repeated with the output power reduced to 1.0 kW. The same number of holes were drilled using single pulses of 18 ps duration. The array of holes could be formed in approximately 20 holes per second by imposing a time delay of 0.05 seconds between the processing of the holes. The delay was designed to provide some relaxation time for the workpiece to cool, and also to allow for the plasma plume to be removed by the exhaust air flow that removes ejected material from the drilling machine and prevents the ejected metal from coating the lens 3.

A further trial reduced the delay time between holes to 0.01 seconds and therefore reduced the drilling time for the array to less than 6 seconds and the resulting holes had similar quality, but with more spatter and discoloration. The increased spatter and discoloration were easily removed using a wire brush agitation.

Further decreases in the overall processing time were achieved by forming non- adjacent holes sequentially. Processing non-adjacent holes permits heat to dissipate before the adjacent hole is processed, and thus reduces the time delay required between laser pulses.

For all aspects of the method of the invention, a desirable feature when forming arrays of closely packed holes is to maintain the structural integrity of the remaining material between holes and thus the durability of the resulting structure. Care has to be taken to avoid a significant heat affected zone which would appear as a melted appearance, discolouration, or change of surface texture of the remaining material between holes. Microscope inspection revealed that the material between the holes in the above experiments clearly showed the original grain structure of the metal workpiece as well as the surface finish 15 of the metal workpiece before drilling. This is a strong indication that the heat affected zone has very small dimensions in this process, and may not be present at all. The heat affected zone may have a width less than 5 μm, and may be less than 1 μm. The absence of a heat affected zone is a major advantage for forming holes in commercial electronic devices.

When laser processing stainless steel, it is common to see a brown discoloration when heat is applied. The discoloration is due to oxides of iron, nickel and chromium redepositing on the metal workpiece, and such discoloration was observed during our trials. However, in other laser related processes, it can be common to see more severe (dark brown or black) discolouration, and this was not observed in Examples 1 and 2.

When the holes are being formed with a laser, liquid and vapor phase metals are ejected from the hole. Some of this ejected metal will cling to the edge of the hole as burrs and others redeposit as splatter or re-attached droplets. Lightly attached burrs and droplets were observed in the above Examples 1 and 2. These were easily removed.

The method of the invention can include selecting the peak power 22, the spot diameter 34, and the pulse fluence 36 to optimise the surface finish 15, and in particular, reduce the heat affected zone, decolourisation, burrs, spatter, and re-attached droplets. The optimisation can be performed by experiment.

The method of the present invention may include the step of providing an apparatus, (not shown) for providing the pressure retaining member 17.

The method of the present invention may be such that a laser beam 4 that is a near- ideal Gaussian beam (M²=1.1) provides a circularly symmetric heat source along the centre line of the drilled hole 8. Moreover, the beam intensity profile (Gaussian) may create a radially symmetric thermal environment for initiating the creation of molten metal and the plasma . The heat induced by the laser beam 4 typically creates both molten melt and metal vapour as the plasma is created, and the creation of vapour and plasma in turn create a shock wave that propels the molten metal and vaporized metal outward from the first metal surface 6. During the first moments of the drilling process, the hole 8 does not penetrate the full thickness of the metal workpiece 5. The ambient air within and near to the hole 8 provides little resistance to the propulsion forces of the shock wave, and therefore the ejected molten metal (the melt) and vapour are typically ejected in a direction normal to the first surface 6 of the metal workpiece 5. The melt formation progresses both in depth and radially from the centre line of the laser beam 4 by heat conduction and continued laser radiation heating.

If allowed to progress through the metal workpiece 5, the creation of an exit hole 102 would immediately change the dynamics of the plasma formation and shock wave generation. This is because there would no longer be an absorbing metal at the centre line of the laser beam 4 to create melt and heat, and also because any expanded gasses would now be able to exit from either the entry hole 101 through which the laser beam 4 enters the hole 8, or the exit hole 102 through which the laser beam 4 now emerges from the hole. The majority of the laser intensity can now pass through the exit hole 102 without being absorbed by the metal workpiece 5, and the process quickly ends, resulting in an exit hole 102 that is smaller than the entry hole 101. The exit hole diameter (the second diameter 12) more closely matches the effective beam diameter of the laser (at an intensity sufficient to melt the target metal), whereas the first diameter 11 of the entry hole 101 is expanded due to heat conduction from the melt and plasma formation.

The method of the present invention blocks the exit hole 102 physically, and such that the vapour pressure is retained, and the plasma and shock wave continue to be directed away from the first surface 6 of the metal workpiece 5. The melt and vaporization can continue to expand radially to create an exit hole 102 of similar diameter to the entry hole

101. As indicated above, it is possible to create either a positive taper (entry hole 101 larger than exit hole 102) or a negative taper (exit hole 102 larger than entry hole 101) by adjusting the energy of the laser pulse and thus the pulse fluence 36. Compared to a zero- taper hole (exit hole 102 the same size as the entry hole 101), reduced laser energy will stop the process before the exit hole 102 is fully formed creating a smaller exit hole 102 and a positive taper angle. Similarly, if the pulse energy is increased compared to a zero-taper hole, the extra energy can continue to expand the exit hole 102 resulting in a negative taper angle.

The pressure retaining member 17 is preferably a material that will not weld to the workpiece 5, nor absorb the laser energy. Glass is a preferred material. The final moments of the metal drilling process can result in partial drilling into the pressure retaining member, however, this partial drilling does not affect the process. If the pressure retaining member 17 were another metal, then the two metals could weld together, making it difficult to recover a usable part. The pressure retaining member 17 may however be a metal that has a higher melting temperature than the metal of the workpiece 5, or one that does not weld to the metal workpiece 5.

The symmetry of the hole 8 may also be dependent on the creation of an environmentally “quiet” zone near the first surface 6 of the workpiece 5, that is, lacking in directional air flow or thermal conduction that would distort the symmetry of the drilling process and thereby the symmetry of the hole 8. When there is a strong airflow direction at the first surface 6, one can see recasted melt attached on the downstream side because the airflow pushes the melt which can re-attach to the colder metal at the edge of the hole 6. Similarly, if the metal workpiece 5 is firmly clamped to a heat conducting body (such as a metal clamp), the first hole diameter 11 will be smaller near to the heat conducting body - indicating that the heat created by the laser beam 4 to generate the hole 8 is being conducted away from the hole 8.

A laser drilling process may be conducted in stainless steel workpieces having a range of thicknesses 103 between 100 μm and 160 μm. First hole diameters 11 in the range of 85 microns to 130 microns may be achieved with virtually zero taper angles. The method of the invention may be used with both thicker and thinner workpieces, though care needs to be taken with thicker workpieces as the melt can damage the scanner optics. An air knife and/or a physical aperture may be incorporated in the apparatus in order to protect the scanner optics from damage when using thicker materials.

The invention also provides an article comprising at least one through-hole 8 and which the hole 8 is formed according to the method of the invention. The article may comprise the pressure retaining member 17. Alternatively, the pressure retaining member 17 may have been removed from the metal workpiece 5. Examples of articles are ear phones, mobile phones, smart phones, tablet computers, lap top computers, desk top computers, speakers, watches, televisions, machinery, filters, screens, and jewellery.

The invention also provides an apparatus for forming at least one through-hole 8 in a workpiece 5 according to the method claimed in any one of the preceding claims.

The method described above may be used to form holes in a wide variety of articles including, for example, ear phones, mobile phones, smart phones, tablet computers, lap top computers, desk top computers, speakers, watches, televisions, machinery, jewellery, and filters and screens used in medical and automotive applications.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications and additional steps and components may be provided to enhance performance. Individual components shown in the drawings are not limited to use in their drawings and may be used in other drawings and in all aspects of the invention. The present invention extends to the above mentioned features taken singly or in any combination.

Claims

Claims A method for forming at least one through-hole (8) in a metal workpiece (5), which through-hole (8) has a first diameter (11) on a first surface (6) of the metal workpiece (5), a second diameter (12) on a second surface (7) of the metal workpiece (5), and a surface finish (15) on at least the first surface (6), and which method comprises:

• providing the metal workpiece (5) and a pressure retaining member (17);

• providing a laser (1) for emitting a laser beam (4) comprising at least one laser pulse (21) defined by a peak power (22);

• focussing the laser beam (4) from the laser (1) onto the first surface (6) to form a spot (31 ) having a spot diameter (34) and a pulse fluence (36);

• selecting the peak power (22) and the spot diameter (34) to cause a plume comprising material from the metal workpiece (5) to be ejected from the first surface (6) while the through-hole (8) is being formed, with the ejection being caused by vapour pressure in the through-hole (8) being formed;

• selecting the peak power (22), the spot diameter (34), and the pulse fluence (36) to form the through-hole (8); and

• causing the metal workpiece (5) to be such that it has had the pressure retaining member (17) applied to the second surface (7) prior to the plume being ejected from the metal workpiece (5); and wherein:

• after the laser beam (4) pierces the second surface (7), at least a portion of the vapour pressure within the through-hole (8) being formed is retained by the pressure retaining member (17) in order to maintain the plume until the through- hole (8) is formed. A method according to claim 1 wherein the spot diameter (34) is selected such that the second diameter (12) is at least one point two times larger than the spot diameter (34). A method according to claim 2 wherein the second diameter (12) is at least two times larger than the spot diameter (34). A method according to any one of the preceding claims wherein the through-hole (8) has a pre-determined taper ratio, and the laser (1) is turned off once the predetermined taper ratio has been achieved. A method according to claim 4 wherein the first diameter (11 ) is within twenty micrometres of the second diameter (12). A method according to any one of the preceding claims wherein the first surface (6) and the second surface (7) are separated by between fifty micrometres and five hundred micrometres. A method according to claim 6 wherein the first surface (6) and the second surface (7) are separated by between fifty micrometres and two hundred and fifty micrometres. A method according to any one of the preceding claims wherein the first diameter (11) is between fifty micrometres and two hundred micrometres. A method according to any one of the preceding claims and including the step of moving the laser beam (4) on the first surface (6) and turning the laser beam (4) on and off to form an array of the through-holes (8). A method according to claim 9 wherein adjacent ones of the through-holes (8) are separated by less than half the first diameter (11). A method according to claim 9 or claim 10 wherein non-adjacent ones of the through- holes (8) are formed in sequence. A method according to any one of the preceding claims wherein the laser (1) is a continuous wave laser. A method according to any one of claims 1 to 11 wherein the laser (1 ) is a quasi continuous wave laser or a pulsed laser. A method according to any one of the preceding claims wherein the laser beam (4) comprises a fundamental mode of the laser (1). A method according to any one of the preceding claims wherein the laser beam (4) has a beam quality M² factor less than 1.3. A method according to any one of claims 1 - 13 wherein the laser beam (4) comprises a higher order mode. A method according to any one of claims 1 - 13 wherein the laser beam (4) has a top hat or annular beam profile. A method according to any one of the preceding claims wherein the laser beam (4) has a cross section with circular symmetry. A method according to any one of the preceding claims wherein the pressure retaining member (17) has a contact with the second surface (7), which contact is sufficient to retain all of the vapour pressure within the through-hole (8) being formed. A method according to any one of the preceding claims wherein the first surface (6) comprises at least one of copper, aluminium, gold, silver, platinum, palladium, nickel, titanium, tin, iron, chromium, and stainless steel. A method according to any one of the preceding claims wherein pressure retaining member (17) transmits more than 50% of the peak power (22) of the laser beam (4). A method according to any one of the preceding claims wherein the pressure retaining member (17) comprises a glass. A method according to any one of claims 1 to 21 wherein the pressure retaining member (17) comprises a ceramic. A method according to claim 23 wherein the ceramic comprises sapphire. A method according to any one of claims 1 to 20 wherein the pressure retaining member (17) comprises a metal having a higher melting point than a melting point of the workpiece (5). A method according to any one of claims 1 to 20 wherein the pressure retaining member (17) comprises a metal having a higher reflectivity than a reflectivity of the workpiece (5). A method according to any one of the preceding claims and including a step of removing the pressure retaining member (17). An article comprising at least one through-hole (8) and which through-hole (8) is formed according to the method claimed in any one of the preceding claims. An apparatus for forming at least one through-hole (8) in a workpiece (5) according to the method claimed in any one of claims 1 to 27.