GB2582928A

GB2582928A - A laser apparatus for stripping and soldering wires

Info

Publication number: GB2582928A
Application number: GB1904958.4A
Authority: GB
Inventors: Taylor Paul; Proud Harry; Tindle William
Original assignee: Laser Wire Solutions Ltd
Current assignee: Laser Wire Solutions Ltd
Priority date: 2019-04-08
Filing date: 2019-04-08
Publication date: 2020-10-14
Also published as: GB201904958D0

Abstract

A method for stripping/soldering a component (preferably a wire), comprising; a) operating a laser (which may be a continuous wave laser) in a first low power mode to emit a low power beam having a first energy E1; b) directing the low power laser beam at a component; c) using the low power laser beam to visually align the laser with a target location on the component; d) operating the laser in a second high power mode to emit a laser beam (which may be a pulsed laser beam) at the target location having a second laser energy E2 that is greater than first laser energy E1, to strip or solder the component. A second aspect is directed towards an apparatus for laser soldering. A third aspect is directed towards a method of laser soldering comprising, operating an ultraviolet laser in a stripping mode in which the laser is configured to ablate a coating from the surface of the component to expose a surface for soldering, applying solder material and then operating the laser in a soldering mode to control the melt and flow of the solder.

Description

A LASER APPARATUS FOR STRIPPING AND SOLDERING WIRES

The present invention relates to a laser apparatus, and in particular a laser apparatus for the soldering and stripping of insulated wires.

Soldering is a well-established process in the field of electronics for joining two or more electrically conductive elements through the melting of a metallic solder that has a lower melting point than conductive elements being joined. Conventional soldering used a heated soldering iron that transfers heat in contact with the solder through conduction.

The use of a soldering iron places physical restrictions on the process, and the size of the soldering iron itself makes the soldering of miniature, precision electronic devices difficult. At that scale it is not possible to accurately apply the soldering iron the joint and there is a risk contacting and damaging adjacent components.

The creation of small scale solder bridges or joints between adjacent wires is a technically difficult task. The close proximity to other heat sensitive wires adds an additional layer of difficulty and precision required to create the required solder joint and suitably functional parts. For such small scale applications it is known to use laser welding techniques in which a laser is used to impart heat to the solder. A low energy laser is used to ablate the wires to strip away the coating and provide a clean, bare wire for soldering. A quantity of solder material is then deposited at the solder site and a second higher powered laser is used to heat the solder. The higher power laser radiation applied to the solder diffuses into the interior of the material through heat conduction, causing the material to melt and form a molten pool that flows across the joint.

The advantage of using a laser for soldering is that it is that power is closely controllable and the applied area, or 'spot size' is far smaller than is achievable with mechanical means; approximately 0.015 mm diameter. Infrared lasers were initially used for soldering due to their reasonably small spot sizes. However, it proved difficult to control the power of infrared lasers at high speed. Infrared lasers have therefore been replaced by high power laser diodes, which have far stronger absorption and faster power control.

A disadvantage of laser diodes is that there is a size limit imposed by their use, and it is difficult to achieve spot sizes much smaller than 0.1x0.2 mm. It has also proved difficult to accurately align high powered lasers with the solder site within acceptable tolerances at very small scales.

It is therefore desirable to provide an improved laser soldering system, which addresses the above described problems and/or which offers improvements generally.

According to the present invention there is provided a laser apparatus for the stripping and/or soldering of wires. The apparatus comprises a laser configured to emit both continuous wave and pulsed laser beams. Laser optics are arranged to guide and focus the laser to a target location. A controller is provided that is configured and arranged to control operation of the laser. The controller is configured to cause the laser to operate in a low power mode to direct a continuous wave laser beam at the target location having a first laser energy El for enabling the laser to be visually aligned with the target location.

The controller is also configured to operate the laser in a high power mode to direct a pulsed laser beam at the target location having a second laser energy E2 that is greater than laser energy El for soldering and/or stripping the wire. Visual alignment may comprise any means of aligning the laser with the target location by visual reference to the low power laser beam. Visual alignment may include direct viewing of the laser beam by an operator, with the operator manually adjusting the relative positions of the laser beam and the wire. Visual alignment may also include the use of an image capture device such as a digital camera to visually recognise the laser beam. Alignment may be conducted manually by the operator, or automatically based on the image acquired by the image capture device. The wire may for example be a single wire, comprising a conductive core and an insulating coating, or a plurality of conjoined wires, such as a ribbon wire.

Control of the laser by the controller may include control of the laser optics and/or control of a laser actuator.

The laser is preferably an ultraviolet laser. One advantage of a UV laser is the small spot size that is achievable. This enables the laser to be used for stripping and soldering wires at a much smaller scale than other lasers. The UV wavelength also creates a visible beam spot on the work surface. The visible light created by the UV laser may be due to reflected light and/or fluorescence at the work surface caused by the UV light. The term 'visible' may mean visible to the human eye. Alternatively or in addition it may mean light that is detectable by an electronic image detection system incorporating a camera.

The laser apparatus may further comprise a laser actuator arranged to guide the laser across a pre-determined area on the wire in the high power mode. This enables the laser to be moved across the surface of the wire for stripping and/or soldering.

The laser actuator is preferably a laser scanner, such as a galvanometer scanner, and is configured and arranged to guide the laser beam in a scanning motion across the pre-determined area. The controller is preferably configured to operate the laser scanner to scan the laser beam across the pre-determined area in the high power mode.

The target location defines a reference point on the surface of the component part to be stripped or soldered. The component part may be a wire, circuit board or microchip for example. The laser scanner is controlled to scan the laser across the pre-determined area with reference to the target location. The target location is any point on the component part with which it is desired for the laser to be aligned.

The laser energy E2 is preferably at least SO times greater than the laser energy El and is selected such that it is suitable to remove the coating from the wire and/or heat solder to a required temperature to cause the solder to melt. Preferably the laser energy E2 is at least 100 times greater than the laser energy El.

The laser is preferably 0-switched in the high power mode to create a series of high powered laser pulses.

The laser energy E2 is preferably approximately 500mW.

The laser energy El preferably has an energy density of 3.14 x 10-13 Jm-2.

Preferably the laser energy El is approximately 1-2mW. This energy is insufficient to strip or otherwise damage a wire or other component part, enabling the laser to be moved across the surface of the component for visual alignment without damaging or otherwise affecting the component part.

The laser energy E2 may have an energy density of 1.57 x 10-10 Jm-2.

The laser apparatus may further comprise an image capture device such as camera configured and arranged to capture an image of the laser in the low power mode. The camera is also arranged to capture an image of the target location, and visual comparison of the laser spot and the target location enables alignment of the two.

The laser apparatus may further comprise a display configured to display the target location and the laser beam to an operator to enable the operator to visually align the laser with the target location by reference to the display.

In another aspect of the invention there is provided a method of stripping and/or soldering a component such as a wire. The method comprises operating a laser in a first low power mode to emit a low power continuous wave laser beam having a first laser energy El; directing the low power continuous wave laser beam at a target location on the component surface; using the low power continuous wave laser beam to visually align the laser with the target location on the component surface; operating the laser in a second high power mode to emit a pulsed laser beam at the target zone having a second laser energy E2 that is greater than the first laser energy El to strip or solder said wire. The lower power beam is suitable for visual alignment of the laser without affecting the wire. Following visual alignment, the laser is switched to high power mode which is suitable for stripping and or soldering the wire. The low power and high power beams are emitted by the same laser and pass through the same optics. Therefore, alignment of the low power beam with the target location ensures highly accurate alignment of the high power beam when the laser is switched.

The method may further comprise guiding the pulsed laser beam in the high power mode across a pre-determined area of the wire to strip or solder said wire. The laser is preferably scanned across the area by an actuator such as a galvo scanner. The laser is preferably an ultraviolet laser.

In another aspect of the invention there is provided a method of laser soldering comprising operating laser in a stripping mode; directing the laser at a target location on the surface of a component while the laser is in the stripping mode to remove a coating material from the component and expose an underlying conductive surface; applying solder to the exposed conductive core of the component; operating the UV laser in a first soldering mode; and directing the laser at the solder while the laser is in the first soldering mode to cause the solder to melt and flow.

Preferably the laser in an ultraviolet laser.

In the stripping mode the laser is preferably operated at a laser energy El and in the first soldering mode the laser is operated at a laser energy E2. The method further comprises operating the UV laser in a second soldering mode in which the laser is operated at a laser energy E3 following the first soldering mode. The laser energy E3 is greater than the laser energy E2. The higher power laser of the second soldering mode ablates the solder rather than simply heating it, causing the solder to spread across the exposed underlying conductive surface. In addition the solder particles to begin impregnating the surface of the wire or wires. In addition the high energy pulsed laser beam acts to clean the surface of the wires and remove any impurities such as residual flux, oxidation that may have formed on the surface following stripping, or remaining insulation from the first heating phase, which prepares the surface to receive the solder, facilitates the flow of solder over the surface and promotes adhesion between the solder and the surface.

The method may further comprise operating the UV laser in a third soldering mode in which the laser is operated at a laser energy E4 following the second soldering mode, wherein the laser energy E4 is less than the laser energy E3. This third solder mode applies energy to the solder at a lower level than the more violet ablation of the second soldering mode, and causes the solder to coalesce and form an evenly distributed, unified solder joint.

The method may further comprise repeatedly scanning the laser across a predetermined soldering area in the first soldering mode for a number of passes N1 to impart heat energy to the solder to cause the solder to melt and coalesce. A UV laser is not conventionally used for laser soldering because pulsed lasers are more difficult to control. While a pulsed UV laser is suitable for the ablation of wire coatings, the use of a pulsed laser is problematic when seeking to apply energy to heat solder, and therefore continuous wave lasers, such as diode lasers, are conventionally used for this task.

Although UV lasers can be operated as a continuous wave, the laser energy is far too low to be used for soldering. However, the Applicant has identified the benefits of using the same laser for stripping and soldering the wire. In order to sufficiently heat the solder using a pulsed UV laser, without the high pulse energies causing localised ablation, the laser is scanned over the soldering area multiple times in multiple passes. The duration of each pass, and the number of passes determines the energy imparted to the solder.

The method may further comprise repeatedly scanning the laser across the predetermined soldering area in the second soldering mode for a number of passes N2 to ablate and distribute the solder over the soldering area. As the laser energy in the second soldering mode is higher than the in the first soldering mode, fewer passes are required. Therefore, the number of passes N2 is fewer than the number of passes Ni.

The laser energy E3 is preferably selected such that in the second soldering mode the laser cleans the surface of the wire in the soldering area to facilitate bonding between the solder and the wire.

The method preferably further comprises repeatedly scanning the laser across a predetermined soldering area in the third soldering mode for a number of passes N3 to heat the solder and cause the solder to flows across the surface of the wire and form a solder joint. N3 is preferably greater than N2. It has been found that the time that the laser is applied to the solder in the third soldering mode is as important as the amount of energy imparted to the solder, so N3 is selected to ensure the laser passes over the solder for a predetermined period of time.

The laser is preferably scanned across the soldering area in a predetermined pattern, which may for example be a hatched pattern.

The present invention will now be described by way of example only with reference to the following illustrative figures in which: Figure 1 shows a schematic illustration of a laser apparatus according to an embodiment of the invention; Figure 2 shows a ribbon wire with a stripped ablation window; and Figure 3 shows the arrangement of Figure 2 with a solder bridge spanning two of the exposed wires.

Referring to Figure 1, a combined laser wire stripping and soldering system 1 comprises a UV laser 2. The UV laser 2 is preferably a diode-pumped solid-state (DPSS) laser. The UV laser 2 comprises a laser diode source. The output of the laser source is focused into a gain medium, which in the case of a DPSS laser is typically a vanadate crystal. The gain medium is located between two mirrors to form a light amplifier or resonator, which causes the laser light to be generated as the light bounces back and forth between the two mirrors of the resonator extracting energy from the gain medium on each pass.

Energy is supplied or 'pumped' into the gain medium by the laser diode, which is preferably an 808nm laser diode. The 808nm light from the laser diode is absorbed by the gain medium and converted to light of approximately 1064nm in wavelength. The generated wavelength is defined by an atomic energy transition of the Neodymium ions present in the host Vanadate medium. The Vanadate gain medium provides adequate amplification to overcome the losses in the laser resonator (from the mirrors, gain medium and other optical elements) and deliver useful power.

The 808nm laser diode emits light in a continuous wave (CW) mode. That is, the light is emitted continuously as long as the electrical power to the diode is maintained. The Vanadate and laser resonator convert the continuous wave output of the diode into a laser output that is more useful output for the end user. The generated 1064nm light from the resonator is continuous and therefore low in peak power. In order to generate a more useful output with higher peak powers, an acousto-optical 0-switch is placed in the resonator. The '0' or 'quality' of the resonator determines the ability of the cavity to generate light, which sets the threshold level at which the laser will operate for a given gain in the gain medium. The Q-switch operates as an electronically controlled light valve within the resonator that prevents operation of the resonator by briefly disturbing its quality.

When the resonator is not lasing, the Vanadate gain medium continues to receive energy from the continuous wave laser diode and a build-up of energy in the Vanadate occurs in the form of an electron state population inversion. Once the gain medium is saturated with the 808nm energy, the 0-switch is electronically opened and lasing is allowed in the resonator. As a result of the high amount of stored energy in the gain medium, laser light is generated very rapidly by the resonator and at very high peak powers. The resulting output is a near infrared 1064nm pulse of light approximately 40 nanoseconds long. Following the pulse the gain medium is depleted of energy and the 0-switch is closed to allow the gain to build up again in preparation for the next pulse. The rate at which the switch opens and closes determines the repetition rate of the laser. For the DPSS laser, this is typically 30 kilo-Hertz (30,000 pulses/second).

The infrared 1064nm laser light is converted to a shorter wavelength laser light in the UV spectrum for laser wire stripping. This is achieved using harmonic conversion. Crystals are placed within the resonator that combine photons of light to produce different wavelengths. In a first stage some of the 1064nm infrared light is converted into 532nm green light using a crystal commonly known as a 'doubler', which combines two photons of 1064nm to form one photon of 532nm light. Two superimposed beams exiting the doubler crystal. One of the beams has a wavelength of 1064nm and other is at 532nm. These superimposed beams then enter a second 'tripler' crystal in the cavity and in a similar process to the doubler crystal, a photon of residual 1064nm and a photon of 532nm light are combined to form a new photon of 355nm light. The three beams, -residual 1064nm, residual 532nm and useful 355nm -exit the tripler crystal superimposed. A prism is then used to physically separate the beams and the 355nm UV laser beam is directed to the output port of the laser by a pair of mirrors and defines the laser output 4. The 532nm beam is dumped and the 1064nm beam stays within the resonator to form the laser cavity.

A series of laser optics, including lenses and prisms 6 and a focussing telescope 8, are used to form the high power 355nm UV laser beam pulse into the correct beam shape and size for the laser stripping. During an 'off state, being the period when the Q-switch is not in operation, the continuous wave 1064nm laser beam is converted to a continuous wave UV beam using the same process, the continuous wave UV beam defining the laser output 4 in the 'off state.

Laser wire stripping relies on a strong interaction between the laser and the layer to be removed, while at the same time the laser allows the underlying layer to remain unaffected. Different materials interact with laser at different wavelengths and frequencies. The laser wavelength and the duration of the laser pulses can be varied to suit different materials and processes. UV laser light is particularly applicable to the stripping of enamels such as polyimide from small gage wires down to 50 AWG and even smaller, which are used in applications such as medical device manufacture.

A ribbon wire 10 is positioned beneath the focussed laser beam 12 on a support platform 14. A mechanical spool feed 16 comprising a first feed spool 18 and a second collection spool 20 is used to move the ribbon wire in a first longitudinal direction x, parallel to the platform surface 14, and to longitudinally position the ribbon wire 10 on the platform 14.

The wire 10 is fed to the collection spool 16 from the feed spool 14 and the feed spool 14 and collection spool 16 are spaced such that a length of wire 10 extends between the two spools. The spools 14,16 are arranged such that the exposed section of the wire lOis fed through a stripping zone 18 on the platform. The laser 12 is focussed at the stripping zone 18. The wire 10 is controlled such that the section of wire to be stripped is positioned at the stripping zone 18. A further actuator is provided that engages the wire 10 and moves in a second axis y, transverse to the longitudinal axis x of the wire and parallel to the platform 14.

The ribbon wire 10 comprises a series of parallel wires covered and separated by an insulating material. In order to electrically connect two adjacent wires with a solder bridge a window must be formed in the insulation that exposes the two wires to be bridged at a common location along their lengths, while leaving the remaining wires covered by insulation to ensure that the solder only makes an electrical connection with the designated wires.

In order to form an ablation window the laser must be aligned with a designated target location along the length and width of the ribbon wire coinciding with the two wires to be connected. Alignment of wires with a stripping laser has previously been achieved using techniques such as 'shadowing' in which a photosensor is located on a stage or 'fixture plate' beneath the wire and aligned with the laser. As the wire is brought into alignment with the laser on the stage it occludes the photosensor and the light energy received by the photosensor is reduced. This reduction in light energy is used as a measure of the alignment of the wire with the laser. While this technique is acceptable for larger wires, the accuracy of the technique is not suitable for very small scale wires and is particularly unsuitable for ribbon wires where the laser must be aligned with specific wires across a ribbon of multiple wires. While it is possible to attempt to determine the position of individual wires in a ribbon based on the know distance of the wire from the edge of the ribbon, with the laser being moved inwardly from the edge position determined by the shadowing technique, any inaccuracy in the initial alignment is then compounded as the laser tracks across the ribbon.

The present system provides a significantly more accurate means of aligning the laser with the wire to be stripped and/or soldered. In a first mode the laser 2 is operated in a low power continuous wave mode of operation, with the CZ-switch in the 'off' state, in which the laser is continuously pumped and continuously emits light. As such, while the CZ-switch is the 'off' state, the laser 2 is the continuous 'on' state; i.e. it is not pulsed. This lower energy continuous wave light is not suitable for stripping, and as it is emitted while the 0-switch is in the 'off' state, it is referred to as 'leakage light'. The low power continuous wave laser beam is converted to a UV beam using harmonic conversion. The 355nm wavelength continuous wave 'leakage light' beam illuminates the platform 14 is with a visible light in the form of a blue spot on the platform surface. This marker spot generated by the low power continuous wave beam is a highly accurate indicator of the location of the higher energy CZ-switched beam, as both beams are generated by the same laser 2, are of the same wave length and pass through the same optics 6,8. The marker spot may be captured and magnified by a camera for more accurate visual alignment. The ribbon wire 10 is moved into position beneath the laser 12 and its position is adjusted until the leakage light guide beam is aligned with a specific reference position on the wire, which may for example be a location between two adjacent wires to be soldered, corresponding to the location of the ablation window. The use of a UV laser enables a very small laser spot size to be achieved, which enables highly accurate alignment.

Preferably the reference position corresponds to a corner of the ablation window. The reference position RP in Figure 2 corresponds to the lower left hand corner of the ablation window, having a positional reference xo,yo. The laser is programmed to generate an ablation window relative to the xo,yo reference point. Prior to stripping the wire 10 is moved to a desired position along its length using the spool actuator. The guide light of the laser is activated and the wire 10 is then moved in the y axis using the second actuator to align the laser with a position on the wire 10 corresponding to the desired position of the lower left hand corner of the ablation window; it will of course be appreciated that the reference point may correspond to any suitable coordinate of the ablation window, the desired position of which on the wire can be readily determined.

Once the laser has been aligned with the location on the wire 10 that defines the reference point, the laser may then be switched to the stripping mode, and laser moves across a preprogramed ablation window relative to the reference position RP.

The low energy of the continuous wave UV laser used for the guide beam is insufficient to ablate or otherwise effect the wires, and positioning of the wire may therefore be achieved without damage to the regions of the wire outside the ablation window. In the low power, continuous wave guide mode the laser is operated at an energy El. The energy El of the laser 'leakage light' when operating in the low power guide mode may be approximately 1-2mW, with an energy density of 3.14 x 10-131m-2 and a beam radius of micron. It will however be appreciated that for different applications and/or different materials, the laser energy, laser density and/or beam radius may be varied and the invention is not limited to the above examples, and each of the above examples may be implemented without limitation to its combination with the other values.

Certain lasers, such as diode lasers, by virtue of their mode of operation don't have leakage light. Other lasers generate 'leakage' light, but this is in the non-visible spectrum and the leakage light cannot therefore be used as a guide. It is known to generate an alignment beam using a second laser in the visible spectrum. In such an arrangement the guide laser beam is generated by a separate laser to the primary laser. A second set of optics may be used to guide the secondary guide laser. The use of a second set of optics means that the secondary laser travels a different optical path to the primary laser. While the optics may calibrate to ensure a close approximation between the focal point of the primary and secondary lasers, there will always be error between the two locations.

Alternatively, the secondary alignment laser may be directed through the same optics as the primary laser. Therefore, the secondary and primary lasers are travelling the same optical path. However, the optics will set up to accommodate the wavelength of the primary laser, and as such there will be deviation in the path of the secondary laser beam, which has a different wavelength to the primary laser. In both cases, while the secondary guide laser provides a reasonable approximation of the position of non-visible laser beam, it is not exact. At certain scales, and for certain applications the error may be acceptable.

However, neither method is suitable for applications where precise alignment is required at the very small scale, such as in the soldering of small scale ribbon wire.

Accurate alignment of the laser with the reference position RP may be visually verified by the operator using the camera system to magnify the view of the wire 10 and the guide laser spot. Alternatively, the alignment of the laser with the reference position RP may be automated, with the image recognition software controlling the movement of the wire 10 based on physical characteristics of the wire 10 determined by the software. When it is confirmed that the laser beam 12 is accurately aligned with the reference position RP on wire 10, the laser 2 is switched to a second high power mode of operation in which the DPSS laser is Q-switched to create a series of high energy pulses. The power of the high energy laser beam is operated at a second laser energy E2, which may be approximately 500mW, with an energy density of 1.57 x 10-th Jm-2 and a beam radius of 10 micron. The higher energy pulsed laser beam therefore has a power of 100 -250 times the power of the low energy guide beam. It will however be appreciated that for different applications and/or different materials, the laser energy, laser density and/or beam radius may be varied and the invention is not limited to the above examples, and each of the above examples may be implemented without limitation to its combination with the other values. The high energy pulsed laser beam 12 is used to ablate the wire to reveal a stripped area referred to as an ablation window. The high energy pulsed UV laser beam 12 is directed at the region of the coating to be removed. The UV laser beam 12 interacts with and vaporizes the insulation material cleanly and precisely. At the same time the UV laser is reflected off the underlying metal shield or conductor, which remains unaffected.

The pulsed UV laser is scanned across the ablation window using a galvo scanner 21, which receives the focussed laser beam from the focussing telescope 8. The high energy Q-pulsed UV laser beam 12 is scanned across the ablation window in a pre-programmed scanning x,y pattern until the entire ablation window has been ablated to completely remove the insulating material from that area. The strip pattern defined by the scan matrix of the galvo scanner 21, with its dimensions, is executed under program control.

The program, with the encoded strip pattern, is selected via the operator interface.

Different insulation types and thicknesses can be accommodated by adjusting the speed of the laser beam 12 across the ablation window and the selected power. In the case of a ribbon wire, the ablation window will typically reveal two adjacent conducting cores to be connected by a solder bridge. The UV laser stripping process presents clean metal surfaces on both wires in preparation for the application of a solder to electrically bridge the wires.

Figure 2 shows a ribbon wire 22 comprising a plurality of insulated wires 24 arranged in parallel. Each wire 24 comprises an outer layer of insulation 26 and an inner conductive metal core 28. Each wire 24 is connected to and insulated from the adjacent wires 24 by the insulating material 26. Following the UV stripping process an ablation window 30 is formed, in which the insulating material 26 has been removed from at least the upper surface of two adjacent wires 24a and 24b, revealing the inner cores 28a and 28b. Solder is applied to the ablation window following the UV laser stripping process. Soldering is typically undertaken at a second solder stage using a second laser for the solder process.

The reason for this is that a high powered, continuous wave laser, or a high power laser with relatively long pulse widths, is conventionally used for the soldering process. A pulsed UV laser has not been considered suitable for this application as the lower power and short pulse widths of the laser means it is difficult to impart sufficient heat into the solder to cause the required degree of melting. However the Applicant has recognised the advantages in utilising the same laser for both the ablation and soldering process. Firstly, it enables the ablation and soldering stages to be conducted at the same location using the same laser, and hence does not require a second stage and laser set-up for soldering, thereby saving significant cost and space. In addition, the wire is already aligned with the laser from the ablation stage, and it is therefore not necessary to realign the laser with the ablation window, which maintains accuracy (both processes are conducted at exactly the same spot on the wire) and significantly increases processing speed. Furthermore, in the event that it is required to align, or re-align the laser with the ablation window, the above described process of utilising the continuous wave laser as an alignment beam may also be used for alignment of the laser for soldering.

The solder may be applied by any means suitable for achieving the required volume of solder, but is preferably applied by the needle deposition of solder paste. The volume of solder paste is selected to cause the solder to substantially fill the ablation window 30, which is formed to ensure only the wires 24a and 24b to be bridged are exposed. A typical ablation window for a bridge solder on small scale ribbon wire may be 300 x 100 Lim. The solder paste when initially applied does not provide an electrical contact and must be heated to form a conductive solder joint. To achieve this the UV laser is switched to the 0-switched high power pulsed mode. Prior to switching to the high power mode, the leakage light may be utilised to provide visual confirmation that the laser is still aligned with the ablation window, and adjustments may be made is necessary. This re-alignment may be done manually. Alternatively, the camera system may include software configured to detect any misalignment between the guide beam and the ablation window defining the solder zone, and any necessary adjustments to the position of the platform 14 to move the ribbon may be effected automatically.

The pulsed UV laser beam 12 is applied to the solder over a pre-determined area in a first low energy scan stage at a laser energy E3, which is typically half of the stripping ablation energy. The galvo scanner 21 is used to scan the laser beam 12 across the target area. Pulse energy is controlled by varying the repetition rate of the laser, i.e. how many pulses per second are emitted by the 0. switch. Low frequency rep rate generally correlates to higher pulse energy. Using a high speed galvanometer scanner 20 for beam delivery it is possible to make any shaped heating area by rapidly moving the focussed laser beam 12 in a pre-programmed hatch pattern. The laser 12 is scanned over the target area approximately 10 times to impart heat energy to the solder to cause the solder to melt and coalesce into larger agglomerates of solder particles. At the same time, any flux in the solder paste is evaporated.

For certain applications, such as small flex circuit pads having a gold coating that is clean and readily takes solder, this initial low energy phase is sufficient to cause the solder to form an acceptable solder joint. However, for ribbon wires and other applications a sufficient bond between the solder and the core 28 of the wires 24 is not created using the low energy phase alone. For such applications and second high energy phase is utilised.

In order to distribute and bond the solder the laser 2 is operated in a second higher energy phase, in which the laser has an energy E4. The higher energy pulsed laser beam 12 is scanned across the surface of the solder using the galvo scanner 21 in a second preprogramed scanning sequence. The controller uses a lower frequency rep rate for this process which results in a higher energy pulse than the first passes. The higher energy laser pulses begin to ablate the solder rather than simply heating it, causing a violent is reaction that results in the solder being spread across the surface of the stripped wire cores 28. This higher energy ablation action causes the solder particles to begin impregnating the surface of the wires 28. It has also been identified by the Applicant that the high energy pulsed laser beam 12 acts to clean the surface of the wires 28 and remove any residual flux from the first heating phase, which prepares the surface of the wires 28 to receive the solder, facilitates the flow of solder over the surface of the wires 28 and promotes adhesion between the solder and the wires 28. Due to the higher energy of the laser beam 12 in the second high energy phase, a lower number of scan passes are required, and the laser beam 12 may scan the target area only 3-4 times, as compared to around 10 times in the low energy phase. The period of the second higher energy phase must also be limited to minimise solder losses occurring at this higher laser energy.

The laser may then be operated in a third low energy phase, in which the pulsed laser beam 12 is scanned across the target area again at a lower energy than the high energy phase. The lower energy pulses heat the solder causes the solder to begin flow again, and the solder flows across the cleaned surface footprint generated in the second high energy phase. The solder coalesces and forms the solder joint 30 bridging both wires 28a,28b, as shown in Figure 3.

The UV soldering mode of operation may be employed in conjunction with, or as a separate operation to, the preliminary UV wire stripping ablation mode of operation. The soldering mode of operation may for example be used for small scale spot soldering, for example for soldering components on a PCB. In the above described operation for stripping and soldering a ribbon wire the UV laser is used in a first mode of operation to strip the wire, and in a second mode of operation to create a solder joint. Alternatively, the solder may be applied to a component such as a PCB and the UV laser used in the second mode of operation in one of two ways. In a first variant the solder may be applied to the component and the laser then used in the first lower power mode to melt and coalesce the solder. A stripped wire may then be laid onto the solder at that stage. The laser may then be operated in the second high power mode to evaporate the flux and spread the solder, as well as cleaning the surface to which the solder is applied.

Alternatively, the wire may be laid in position initially prior to the solder being applied. Once the solder has been applied the laser may be operated in the first and second modes of operation to melt and spread the solder. The use of the UV laser for soldering provides benefits in its own right, as described above. Further benefits are achieved when the same UV laser is used for conducting a stripping operation and then a soldering operation on the same wire.

Claims

CLAIMS1. A laser apparatus for the stripping and/or soldering of wires, the apparatus comprising: laser optics arranged to guide and focus the laser to a target location on the surface of a component; a controller configured to control operation of the laser; wherein the controller is configured to operate the laser in a low power mode having a first laser energy El for enabling the laser to be visually aligned io with the target location without causing damage to the component, and to operate the laser in a high power mode to direct a pulsed laser beam at the target location having a second laser energy E2 that is greater than laser energy El for the soldering and/or stripping of the component.
A laser apparatus according to claim 1 wherein the laser is configured to emit both continuous wave and pulsed laser beams, and wherein in the low power mode the controller operates the laser to emit a continuous wave laser beam.
A laser apparatus according to claim 1 or 2 wherein the laser is an ultraviolet laser.
A laser apparatus according to any preceding claim further comprising a laser actuator arranged to guide the laser across a pre-determined area on the wire in the high power mode.
A laser apparatus according to claim 4 wherein the laser actuator comprises a laser scanner configured and arranged to guide the laser beam in a scanning motion across the pre-determined area, and the controller is configured to operate the laser scanner to scan the laser beam across the pre-determined area in the high power mode. is 2. 3. 4. S.
6. A laser apparatus according to claim 5 wherein the target location defines a reference point on the surface of the wire, and the laser scanner is controlled to scan the laser across the pre-determined area with reference to the target location.
7. A laser apparatus according to any preceding claim wherein the laser energy E2 is at least 50 times greater than the laser energy El.
8. A laser apparatus according to claim 7 wherein the laser energy E2 is at least 100 times greater than the laser energy El.
9. A laser apparatus according to any preceding claim wherein the laser is 0-switched in the high power mode.
10. A laser apparatus according to any preceding claim wherein the laser energy E2 is in the range 400-600mW.
11. A laser apparatus according to any preceding claim wherein the laser energy El has an energy density of 3.14 x 10 Jm 2.
12. A laser apparatus according to any preceding claim wherein the laser energy El is 1-2mW.
13. A laser apparatus according to any preceding claim wherein the laser energy E2 has an energy density of 1.57 x 1010 Jm 2.
14. A laser apparatus according to any preceding claim further comprising a camera configured and arranged to capture an image of the laser and the target location in the low power mode, which is used to align the laser with the target location.
15. A laser apparatus according to claim 14 further comprising a display configured to display the target location and the laser beam to an operator to enable the operator to visually align the laser with the target location by reference to the display.
16. A method of stripping or soldering a component, the method comprising: operating a laser in a first low power mode to emit a low power laser beam having a first laser energy El; directing the low power laser beam at a component; using the low power laser beam to visually align the laser with a target location on the component; operating the laser in a second high power mode to emit a laser beam at the target zone having a second laser energy E2 that is greater than the first laser energy El to strip or solder the component. 17. 18. 19. 20. 21.
A method according to claim 16 wherein in the low power mode the laser is operated to emit a continuous wave laser beam and in the high power lode the laser is operated to emit a pulsed laser beam.
A method according to claim 15 or 16 further comprising guiding the pulsed laser beam in the high power mode across a pre-determined area of the wire to strip or solder said wire.
A method according to claims 16 to 18 wherein the laser is an ultraviolet laser.
A method according to any one of claims 16 to 19 wherein the target location defines a reference point for the laser, and the laser is controlled to strip and/or solder a predetermined area relative to the reference point.
A method of laser soldering comprising: operating an ultraviolet laser in a stripping mode in which the laser is configured to ablate a coating from the surface of a component; directing the laser at the surface of the component while the laser is in the stripping mode to remove a coating material from the component and expose an underlying conductive surface; applying solder to the exposed conductive surface; operating the laser in a soldering mode in which the laser is configured to heat and distribute solder; directing the laser at the solder while the laser is in the soldering mode to cause the solder to melt and flow.
22. A method according to claim 20 wherein in the stripping mode the laser is operated at a laser energy El, and in the soldering mode the laser is operated at in a first soldering mode in which the laser has an energy E2, and in a second soldering mode in which the laser is operated at a laser energy E3, wherein the laser energy E3 is greater that the laser energy E2.
23. A method according to claim 21 further comprising operating the laser in a third soldering mode in which the laser is operated at a laser energy E4 following the second soldering mode, wherein the laser energy E4 is less than the laser energy E3.
24. A method according to any one of claims 20 to 23 further comprising repeatedly scanning the laser across a predetermined soldering area in the first soldering mode for a number of passes N1 to impart heat energy to the solder to cause the solder to melt and coalesce.
25. A method according to claim 24 further comprising repeatedly scanning the laser across the predetermined soldering area in the second soldering mode for a number of passes N2 to ablate and distribute the solder over the soldering area.
26. A method according to claim 25 wherein number of passes N2 is fewer than the number of passes Ni.
27. A method according to claim 25 or 26 wherein the laser energy E3 is selected such that in the second soldering mode the laser cleans the surface of the wire in the soldering area to remove impurities and facilitate bonding between the solder and the wire.
28. A method according to any one of claims 22 to 24 further comprising repeatedly scanning the laser across a predetermined soldering area in the third soldering mode for a number of passes N3 to heat the solder and causes the solder to flow across the surface of the wire and form a solder joint, wherein N3 is greater than N2.
29. A method according to any one of claims 20 to 25 wherein the laser is scanned across the soldering area in a predetermined pattern.