WO2020152352A1

WO2020152352A1 - Laser induced forward transfer with high throughput and recycling of donor material on a transparent drum

Info

Publication number: WO2020152352A1
Application number: PCT/EP2020/051819
Authority: WO
Inventors: Anders EMTHÉN; Fredrik Jonsson
Original assignee: Mycronic AB
Priority date: 2019-01-25
Filing date: 2020-01-24
Publication date: 2020-07-30

Abstract

The technology disclosed relates to high utilization of donor material in a writing process using Laser-Induced Forward Transfer. Specifically, the technology relates to using a transparent cylinder with a light beam projected from inside the cylinder through the transparent wall onto donor material carried on the outer surface of the cylinder.

Description

LASER INDUCED FORWARD TRANSFER WITH HIGH THROUGHPUT AND RECYCLING OF DONOR MATERIAL ON A

TRANSPARENT DRUM

BACKGROUND

[0001] The subject matter discussed in this section should not be assumed to be prior art merely as a result of its mention in this section. Similarly, a problem mentioned in this section or associated with the subject matter provided as background should not be assumed to have been previously recognized in the prior art. The subject matter in this section merely represents different approaches, which in and of themselves may also correspond to implementations of the claimed technology.

[0002] This application relates to material deposition using Laser Induced Forward Transfer, also referred to as LIFT. LIFT is an alternative to screen printing or ejection of solder or other adhesives used to secure semiconductors to substrates such as printed circuit boards.

[0003] LIFT is an industrial process of ejecting a patch of material from a donor drum to an acceptor workpiece. The transferred material can be in the form of a thin layer, a thick layer, a paste, a viscoelastic, or a liquid layer. The transfer does not depend on chemistry so any chemical compound can be transferred. The transferred donor material can be a single fdm, a complex film like an OLED stack, or a functional material like a layer of organic materials, including living cells. The fdm can be transferred as a melted drop, a pellet, a round patch or a complex shape (“decal”). A solid material can liquefy due to heating or due to its rheological properties and return to solid after impact with the workpiece. The impact energy can be high enough to give good contact and sticking to the acceptor surface. One application is to deposit metal for electric conductors, e.g. for repair of circuits. Another application is jetting of “rheological material”, which are not solid and have a complex viscosity, e.g. pastes loaded with ceramic powder, metal particles or nanomaterials. Another application is jetting of biological substances, from reactants in diagnostic screen to living cells for building of two- and three- dimensional tissues and grafts.

[0004] A principal of LIFT is that it is an additive process where material is transferred only where needed and therefore LIFT can save expensive materials in contrast to a blanket coating on a workpiece wherein portions of the blanket coating are etched away or otherwise removed where the coating is not needed. LIFT is also a fast and simple one-step process for creating a coating or a pattern, and is finished immediately after the transfer with no need for pre-coating of the workpiece or resist processing.

[0005] LIFT may have similar applications as inkjet, however due to the flexible nature of LIFT it allows printing of many more materials: viscous, hard, dry, granular, and layered. The possible range of shot sizes is very wide from 1 micron in diameter and a thickness of 0.1 micron to several nanoliters in volume. This is a larger range than can be done by ink jetting. In particular, LIFT can shoot smaller shots than an inkjet system. The term laser jetting is used herein as synonymous to LIFT.

[0006] Despite the benefits of LIFT, it has not reached widespread use in industry. One reason is current LIFT technology does not efficiently use donor plates. Applicant’s prior application PCT/EP2016/052430, incorporated by reference herein, included discussion of how to replenish donor plates with material that the laser induces to forward transfer onto the target substrate. The technology described in that application involved cycling multiple donor plates between production and replenishment. An opportunity arises to introduce improved LIFT deposition technologies by further addressing the donor replenishment problem.

[0007] It is the purpose of this application to devise methods to make the process more industrial by providing architectures with high writing speed and high precision and for easy adaption to different process conditions. Throughput is improved by reduced overhead and by using an uninterrupted feeding of donor to the writing head also for sticky, liquid, or perishable donor material. Methods are disclosed to combine continuous feeding of donor instead of step wise movement and for efficient use of donor material. Printing speed and precision and process flexibility is improved by donor drum design.

SUMMARY

[0008] The technology disclosed addresses problems of low throughput and low utilization of potentially expensive donor material associated with currently available LIFT devices and processes. These problems are addressed by an ongoing process, using a transparent drum that is replenished with donor material in one area while concurrently pulsing a laser beam on another side of the drum to cause laser induced forward transfer of adhesive material, such as solder or glue. [0009] The technology disclosed can provide the effect of efficient and high throughput laser induced forward transfer of donor material, while recoating the donor drum with donor material.

[0010] As a first aspect of the invention, there is provided a device for transfer of a material onto a substrate, comprising:

a pulsed light source for generation of a light beam;

a rotatable hollow cylinder that is transparent to the light beam;

a control unit for the modulation of the light beam;

means for scanning the light beam from the interior of the hollow cylinder along its axis of rotation;

projection optics for the focusing of the light beam to an outer surface of the cylinder;

means for applying the material to be transferred to the outer surface of the cylinder;

wherein said pulsed light source is adapted to generate pulses of electromagnetic energy high enough to initiate a localized spot of lift-off process at an interface between the hollow cylinder and the material to be transferred, as the cylinder continuously rotates, whereby the material to be transferred is ejected from said localized lift-off area.

[0011] As a configuration of the first aspect, there is provided a device for transfer of solder paste from a deposited film/reservoir onto a substrate, comprising:

a pulsed light source for generation of a light beam;

a rotating hollow cylinder that is transparent to the light beam;

a control unit for the modulation of the light beam;

means for applying solder paste or other adhesive material to the outer surface of the cylinder; a substrate holder moving with the same pace as the outer surface of the cylinder and along an axis orthogonal to the axis of rotation;

wherein said pulsed light source comprises pulse of electromagnetic energy high enough to initiate a localized spot of lift-off process at an interface between the glass cylinder and the solder paste or other adhesive material, as the cylinder continuously rotates, whereby the solder paste ejected from said localized lift-off area. [0012] As a second aspect of the invention, there is provided a method of depositing material in a pattern on a workpiece by transfer of donor material by laser induced forward transfer, including:

providing a donor drum including a laser transparent supports and an initial coating including donor material;

at an exposure station, pulsing a laser beam through a first area of the donor drum causing portions of donor material to be transferred from the first area of the donor drum to form a portion of the pattern on the workpiece;

concurrently with pulsing the laser beam through the first area of the donor drum, at a recoating station, recoating with donor material a second area of the donor drum, wherein, prior to recoating, the second area of the donor drum includes portions from which donor material was previously transferred; and

at the exposure station, pulsing the laser beam through the second area of the donor drum causing portions of donor material to be transferred from the second area of the donor drum to form a portion of the pattern on the workpiece.

[0013] In embodiments of the invention, the material that is deposited or transferred to the substrate is a solid material, such as a powder, or a liquid material. As an example, the material may be selected from solder paste and conductive glue.

[0014] In embodiments of the invention, the material that is deposited or transferred is s in the form of a thin layer, a thick layer, a paste, a viscoelastic, or a liquid layer. The transferred donor material can be a single film, a complex film like an OLED stack, or a functional material like a layer of organic materials, including living cells. The film can be transferred as a melted drop, a pellet, a round patch or a complex shape (“decal”). A solid material can liquefy due to heating or due to its rheological properties and return to solid after impact with the workpiece. The material may be a“rheological material”, which are not solid and have a complex viscosity, e.g. pastes loaded with ceramic powder, metal particles or nanomaterials.

[0015] In embodiments, the device comprises a substrate holder arranged to move with the same pace as the outer surface of the cylinder and along an axis orthogonal to the axis of rotation.

[0016] The device may comprise a rotation member, such as an electrical motor, for rotating the rotatable cylinder around its axis of rotation. The axis of rotation may extend along the height of the cylinder, i.e. the along perpendicular distance between its bases. The rotational axis may thus extend through the center of the cylinder. The cylinder is an open cylinder, and may be a right, circular cylinder.

[0017] The means for applying the material to be transferred to the outer surface of the cylinder may be in the form of a squeegee, blade or roller.

[0018] The means for scanning the pulsed laser beam may be positioned within the rotatable cylinder or outside of the rotatable cylinder. The scanning means may be configured to scan the interior surface of the hollow cylinder with the modulated laser beam, such as along a direction that is parallel to the rotational axis of the rotatable cylinder.

[0019] The device of the present invention may also comprise a transporting member arranged for moving the rotatable cylinder across a surface, such as the surface onto which the material to be transferred is deposited. As an alternative, the substrate on to which the material is transferred may be moved in relation to the rotatable cylinder.

[0020] The device of the present invention may comprise an exposure station, arranged at the interior surface of the rotatable cylinder and configured to receive the pulsed laser beam, thereby causing portions of the donor material to be transferred from the outer cylinder to the substrate or workpiece onto which the donor material is transferred. The device may further comprise a recoating station, arranged at the outer surface of the rotatable cylinder and further arranged for recoating the outer surface of the cylinder with material that is to be transferred. Rotation of the rotatable cylinder may thus position a replenishment area of the outer face of the cylinder facing upwards relative to gravity at the recoating station and positions an exposure area of the outer face facing downwards relative to gravity at the exposure station.

[0021] The proposed method and device may thus involve a transparent, rotating cylinder that is covered by a thin layer of e.g. electronic material (solder paste, conductive adhesive etc) that is the donor material for a laser-induced forward transfer process. The rotating cylinder is covered with the layer of electronic material through e.g. an external squeegee process that continuously refreshes the material layer as the LIFT process is underway.

[0022] As an example, the device and method of the present invention may be used for constructing or repairing circuits on a printed circuit board (PCB).

[0023] Other aspects and advantages of the technology disclosed can be seen on review of the drawings, the detailed description and the claims, which follow. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In the drawings, like reference characters generally refer to like parts throughout the different views. Also, the drawings are not necessarily to scale, with an emphasis instead generally being placed upon illustrating the principles of the technology disclosed. In the following description, various implementations of the technology disclosed are described with reference to the following drawings, in which:

[0025] Fig. 1 illustrates a laser pulse delivery system that uses a rotating polygon mirror, such as a hexagonal or octagonal mirror.

[0026] Fig. 2 illustrates a laser pulse delivery system that uses an angled mirror.

[0027] Fig. 3 illustrates a laser pulse delivery system that uses a more shallowly angled diffraction grating.

[0028] Figs. 4A and 4B depict a laser pulse focused through the transparent drum.

[0029] Figs. 5A, 5B and 5C depict a gas bubble caused by absorption of laser energy between the transparent carrier and donor material.

[0030] Figs. 6A and 6B show methods of forming preformed dots on a transparent carrier.

[0031] Figs. 7A and 7B show an embodiment of a donor drum including a grid pattern of preformed dots wherein the preformed dots are smaller than the focus area of the laser beam used to eject the preformed dots.

[0032] Figs. 8 A, 8B & 8C, Fig, 9, Figs. 10A & 10B, Figs. 11 A, & 1 IB. Figs. And 12A, & 12B depict methods using continuous motion of the donor drum relative to the workpiece to achieve precise deposition of donor material and high utilization of donor material.

[0033] Fig. 8A depicts a method of selecting a pre-assigned dot location to be ejected onto a target spot on a workpiece

[0034] Fig. 8B shows an on-demand laser that may be fired when the selected pre-assigned dot is in alignment with the target spot in the direction of relative movements

[0035] Fig 9 shows the first pre-assigned dot location within the error threshold selected for ejection onto the target spot where first pre-assigned dot location has already been ejected .

[0036] Fig. 10A depicts a grid of target spots to be patterned on a PCB workpiece.

[0037] Fig. 10B shows the positions of deposited material from dot locations.

[0038] Figs.11 A, 1 IB, 12A and 12B illustrate orienting the geometry of the PCB askew to the direction of relative motion of the donor drum and workpiece. [0039] Fig.13 A shows a circuit board is aligned with the Manhattan geometry features parallel to the direction of movement.

[0040] Fig. 13B shows a histogram of the distribution of required donor material to be deposited.

[0041] Figs. 14A and 14B shows oblique relative scanning of the donor drum and workpiece. Fig. 14A shows the donor drum oriented at an oblique angle relative to the workpiece. Fig. 14B shows the donor drum 1702 is oriented parallel to the workpiece.

[0042] Figs. 15A and 15B shows a laser pulse delivery system including an SLM or DMD.

OF ATT /FT) DESCRIPTION

[0043] The following detailed description is made with reference to the figures. Sample implementations are described to illustrate the technology disclosed, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows.

[0044] LIFT is a promising technology because adhesive spots can be rapidly deposited, at a rate limited primarily by laser scanning. A weak point of the technology is the donor substrate. A substantial part of the donor material substrate will only be used if the substrate can be applied to production of multiple boards. Even then, use of donor material is not uniformly distributed, because successive boards require the same pattern donor material.

[0045] Donor replenishment, as described in applicant's prior work, involves filling expended donor spots instead of supplying fresh donor plates. The previously disclosed technology, with cycling donor plates between production and replenishment, required an elaborate mechanism.

[0046] The newly disclosed technology uses a transparent drum as a donor substrate, instead of a flat plate. The target substrate or board rests on a stage that moves below the drum.

Alternatively, the drum could move on a gantry above the stage, but the workpiece has less mass and is less sensitive to acceleration. Forward transfer is accomplished by projecting laser pulses from inside the drum to the bottom or near the bottom of the drum, as the drum rotates to bring fresh donor areas into position. This technology replenishes donor material on the top half of the drop drum. Gravity assists both the forward transfer and the replenishment.

[0047] A variety of laser pulse delivery systems are candidates. They include a rotating polygon mirror shown in Fig. 1, an angled mirror shown in Fig. 2, and more shallowly angled diffraction grating shown in Fig. 3. Each of these mechanisms scans laser pulses along the length of the drum.

[0048] The laser pulse delivery system can vary the size, geometry and pulse length of the forward transfer laser pulses. The size of donor spot transferred depends on the size of the projected laser spot. Geometry of the donor spot transferred can, potentially, be determined by using lenses to shape the laser spot or an SLM or DMD to select a shape to project. The laser pulse length is selected based on the type of donor material, power of the laser source, and area of the spot to be transferred.

[0049] Fig. 1 illustrates a laser pulse delivery system that uses a rotating polygon mirror, such as a hexagonal or octagonal mirror. Rotation of the polygon naturally scans projected laser pulses along the length of the drum. Timing of pulses and scanning a light beam along the length of the drum, coordinated with movement of the target relative to the drum, determines positioning of transferred adhesive spots. In some designs, an anamorphic lens is positioned between the rotating mirror and the drum, to focus the laser spot and refract the projection angle towards perpendicular to the drum’s axis of rotation or outer surface. A perpendicular incidence makes the relationship between laser pulse and forward transfer of the donor material more nearly uniform across scanned legs.

[0050] An alternative, adapted from laser printing, is to combine a telecentric lens assembly outside the drum with a reflective mirror or grating inside the drum. The assembly outside the drum scans laser pulses along the mirror or grating, which reflects pulses approximately perpendicularly onto the drum in the transfer region, as shown in Figs. 2-3. A reflective mirror can be flat or arched. An arched mirror or grating can help compensate for scanning from a fixed point.

[0051] The drum is a transparent donor source. Laser light from inside the drum pulses against donor material on the outer surface of the drum, super heats the donor material, and causes the donor material to be ejected, to be forwarded transferred from the drum to the target.

[0052] A seamless glass cylinder works as a transparent drum material. Plastic can be used if it is sufficiently transparent and heat resistant. Alternatively, a fused crystal structure could be grown. Transparency to laser pulses reduces heat buildup and potential deformation of the cylinder. The drum material should absorb no more than 10% of laser pulse. Alternatively, it should absorb no more than 5% of the laser pulse. [0053] The diameter of the drum is large enough to hold at least one optical component of the laser pulse delivery system. It is large enough to facilitate replenishment of donor materials. The smaller it can be made, the more rigid it will be for particular thickness of transparent material.

[0054] The alternative laser pulse delivery systems have different physical requirements for the drum diameter. A rotating polygon mirror with an anamorphic lens requires a relatively large drum. A mirror at a 45° angle requires a drum that has a larger diameter than its scan length, to accommodate the mirror geometry. A grating can have a smaller reflection angle, allowing a smaller diameter of drum to accommodate a grating than needed for a mirror.

[0055] The smaller the diameter of the drum, the more rigid it is for a given thickness of transparent glass. Replenishment of donor material requires some force to be applied, if only the weight of depositing material combined with pressure applied by a squeegee, blade or roller used to spread the material. The mechanism that spreads donor replenishment material will affect the chosen diameter of the drum.

[0056] Preliminary design indicates a drum diameter in a range of 2.5 cm to 10 cm. Larger drums may be necessary when a scan length longer than 2.5 cm is used.

[0057] The technology disclosed is particularly well adapted to small target boards. With batteries, antenna and other components, the circuit boards in watches, cell phones and similarly sized devices are often narrow, such as 2 cm wide. The drum has a length that accommodates the selected scan length of donor material. A scan length of 1-2 cms is anticipated for some implementations. A range of lengths from 1 to 5 cm also will practice the technology disclosed. The scan length of the cylinder need not match the width of the target. Multiple passes can be used for transfer of the required number of thoughts.

[0058] A shorter scan length simplifies optics. Using a rotating mirror in a particular diameter of cylinder, a shorter scan length involves a smaller range reflecting angles and, consequently, a smaller and less complex anamorphic lens. For either a mirror or grating combined with telecentric optics, a shorter scan length requires less accommodation in the optical path, due to a reduced difference in distance between paths over near and far ends of the reflecting or diffracting surface onto the target workpiece.

[0059] Two figures of merit that can be applied when selecting the cylinder size are delivery rate of ejected spots and number of scans required to cover the width of the target substrate or board. Present ejectors can deposit up to about 300 adhesive spots per second. The LIFT system disclosed is expected to deposit between 300 and 1,000 adhesive spots per second. The number of adhesive spots per minute is likely to dominate the time required for reversing movement of the target on the stage and for executing a second scan of the target, and so it is likely to be the more important figure of merit.

[0060] Fig. 1 depicts an embodiment of a donor drum. In the embodiment shown, the donor drum includes a transparent drum wall 100, also referred to as a donor substrate and a layer of donor material 102. The transparent drum 100 provides support for the donor material 102 and is configured to be transparent to the laser beam to be absorbed by the donor material which causes LIFT to occur. In embodiments, the transparent carrier comprises clear plastic, polyimide, PC, PET, PEN, thin glass, or a glass sheet; or a combination thereof. The thickness of the donor drum may be between 20 microns and 500 microns for plastic film and up to several millimeters for hard plastics and glass plates. The thickness of the transparent carrier is determined based on the support and flexibility requirements of the donor material and specific LIFT application.

[0061] More detail of the example shown in Fig. 1 appears in Figs. 4A-B and 5A-C. In the additional figures, the layer of donor material 102 is a single layer film. However, in some embodiments, the film may include multiple layers of donor material. For example, the donor material may comprise multiple layers, each of different composition, for example an OLED stack. Further, in some embodiments the donor layer may include a functional film including organic materials or living cells.

[0062] As shown in Figs. 4A-4B, in a single layer donor material embodiment, a laser pulse 108 is focused through the transparent drum 100 and a portion 110 of the donor material at the interface of the transparent carrier absorbs the laser energy and is evaporated. The vapor pressure of the evaporated donor material ejects a section of the donor material across a gap 106 to the acceptor surface 112 of a workpiece 104.

[0063] Absorption of laser energy causes a gas bubble 206 between the transparent carrier 100 and donor material 120 which ejects out a patch 208 of the donor material, as shown in Figs. 5A- 5C. The ejected patch flies across the gap to the acceptor surface 104 of the workpiece and sticks there as a deposit of donor material 210.

[0064] In some embodiments, the gap may be dependent on the thickness of the donor material on the donor drum and the number of overlapping layers of donor material to be deposited in a pattern on the workpiece. For a particular process, the gap between the donor and the acceptor needs to be held constant within a reasonable error, e.g. within +/- 10%. The nominal gap depends on the process. The gap should be small for micron-sized shots, e.g. around 50-100 microns, while for large shots with moderate precision requirements the gap may be around 1 -2 mm. Many applications will fall somewhere in the middle in the span 100 - 1000 microns.

[0065] In Fig. 6A, the donor substrate 902 includes pits 904 in a surface of the substrate. The pitted donor substrate can be produced by injection molding, heat embossing or nano-imprinting. In some embodiments, the pits in the surface form a grid, for example in a Cartesian or hexagonal pattern. The donor material 906 is easily added to the pitted substrate as is shown in Fig. 6B and form isolated preformed dots 908 similar to as shown in Fig. 8 A. For example, a paste or soft donor material may be deposited on the surface of the donor. The excess material may be scraped off by a sharp blade 910, e.g. a“doctor blade”, or it may be scraped off by a soft edge, e.g. a“squeegee”, depending on the mechanical properties of the donor.

[0066] The shape of the preformed dots on the donor drum defines the shape of the transferred donor features and therefore the optics requirements are relaxed compared to embodiments in which the shape of the deposited material is primarily dependent on the properties, e.g. shape, power and duration, of the laser beam pulsed through the donor drum. Figs. 7A-7B show an embodiment of a donor drum including a grid pattern of preformed dots 430 wherein the preformed dots are smaller than the focus area 432 of the laser beam used to eject the preformed dots. This allows both the shape and location of the laser beam to be less precise while still resulting in a well-aimed ejection of donor material. As shown in Fig. 7A, the size of the focus area 432 of the laser beam is small enough and the spacing of the preformed dots is large enough so that ejecting a first preformed dot 434 does not result in ejection of the preformed dots 436 surrounding the first preformed dot 434.

[0067] Fig. 7B illustrates a donor drum including preformed features 438 having different shapes, sizes and orientations, for example first preformed feature 440 has a“T” shape and second preformed feature 442 has a square shape. To use a donor drum as shown in Fig. 7C the system controller assigns the donor features to pattern elements on the workpiece. Further, similar to discussed above relating to Fig. 7A, to eject a preformed feature 438 using LIFT, the focus area 432 of the laser beam used to eject the preformed feature is larger than the preformed feature. [0068] While the examples shown in Fig. 7 A and 7B include X-Y Cartesian grids, in some embodiments the donor drum may be coated with donor material in any number of discrete dot configurations. For example, a hexagonal pattern. Further, as noted, the discrete dots may be of any shape, for example circular, oval, square, triangular, or a shape corresponding to a structure of an electronic component to be formed on the workpiece, for example a transistor or a diode. The shape of the discrete dot may include a 2-dimensional outline with uniform thickness throughout the dot. In some embodiments, the shape may be 3 -dimensional with portions of a dot having different thickness.

[0069] In embodiments wherein the laser beam focus area on the preformed dots is larger than the outline of the preformed dots, such as shown in Figs. 7A and 7B, the donor drum is configured so that the required energy of the laser to eject a discrete dot does not damage the workpiece when the portions of the laser beam that do not hit the preformed dot hit the workpiece. Further, in some embodiments, the donor drum may include a light absorbing layer around the preformed dots which absorbs the laser beam around the preformed dots.

[0070] In some embodiments, the pre-assigned dot locations form a tight pattern, with very little or no donor material between pre-assigned dots. A tight pattern achieves high utilization of the donor material. In some embodiments, the pattern of pre-assigned dots may be divided into sub-patterns, wherein all the dots in a sub-pattem are ejected prior to ejecting dots in other sub patterns. For example, dots in a first sub-pattern are each ejected in a first pass prior to ejecting the dots in a second sub-pattern in a second pass. The sub-patterns each contain a plurality of pre-assigned dots which will have symmetrical attachment to surrounding donor material after the donor material in the pre-assigned dot locations of the previous passes are ejected.

Therefore, the use of patterns, with sub-patterns, of pre-assigned dots is both efficient and leads to high quality ejections due to avoidance of the ejection problems caused by ejection of dots with non-uniform/symmetrical attachment to surrounding donor material.

[0071] Figs. 8A-12B depict methods using continuous motion of the donor drum relative to the workpiece to achieve precise deposition of donor material and high utilization of donor material. In some embodiments, the donor drum is caused to move relative to the workpiece and with relative motion a laser causes ejection of donor material onto the workpiece.

[0072] Aspects of the technology discussed above may be used with the continuous motion technology disclosed herein. Fig. 8 A depicts a method of selecting a pre-assigned dot location to be ejected onto a target spot 1002 on a workpiece. As discussed above, pre-assignment allows for greater utilization of donor material as opposed to ejection at random locations. As shown, the donor drum including a grid pattern of pre-assigned dot locations 1004 are aligned at an oblique angle relative to the relative motion 1006 of the donor drum and workpiece. In this example, the donor drum includes a fdm with pre-assigned dots. Similar embodiments may include a donor drum with a grid pattern of discrete dots which may be aligned at an oblique angle relative to the relative motion of the donor drum and workpiece. Due to the oblique angle alignment, a plurality of pre-assigned dot locations, including locations 1008, 1010, 1012, 1014, 1016 and 1018, will overlap the central portion of the target spot 1002, as the donor drum moves along the path of relative motion 1006 over the workpiece. As illustrated, some of the plurality of pre-assigned dot locations 1008, 1010, 1012, 1014, 1016 and 1018 will overlap the target spot 1002 more than others. Due to the preassigned nature of the dots, there may not be a dot that will exactly overlap with the target spot. The control system of the writer may include a predefined error threshold relating to the allowable error between a target spot on the workpiece and the location of deposited material from a pre-assigned dot location. If ejection of a pre-assigned dot will overlap the target spot within the error threshold then the pre-assigned dot may be selected to be ejected. Fig. 10B shows the positions of deposited material from dot locations 1008, 1010, 1012, 1014, 1016 and 1018 relative to the target spot 1002. As shown, dot location 1018 results in the smallest error and if within the preselected error threshold may be selected. However, if another dot, for example dot location 1008, overlaps target spot 1002 prior to dot location 1018 and is within the error threshold, then dot location 1008 may be selected because it overlaps first in time even though it does not have the smallest error. In embodiments, both order of dots and error deviation from the target spot may be used to determine the selection of pre-assigned dot to be ejected.

[0073] The embodiment shown in Fig.8B utilized an on-demand laser that may be fired when the selected pre-assigned dot is in alignment with the target spot in the direction of relative movements. This is evidenced by the dots in Fig. 8B only having an error deviation from the target spot in one direction. In some embodiments, the laser may be a pulsed laser that may not be fired on demand. In these embodiments, the selected dot may have an error in both the direction of relative motion and the direction perpendicular to relative motion as is shown in Fig. 8C. As discussed above relating to Fig. 8B, the pre-assigned dot that is within the error threshold in both directions is selected to be ejected.

[0074] During the exposure process, the first pre-assigned dot location on the donor drum within the error threshold for a target spot may already have been ejected and therefore cannot be used again. Therefore, in embodiments, a pre-assigned dot location that subsequently overlaps the target spot may be selected and ejected. In embodiments, a larger error threshold allows for greater redundancy and further allows for optimization for efficient use of the pre-assigned spots. For example, as shown in Fig. 9, the first pre-assigned dot location 1102 is within the error threshold to be selected for ejection onto the target spot 1104, however first pre-assigned dot location 1102 has already been ejected. Therefore, another pre-assigned dot within the error threshold must be selected. As shown, a plurality of pre-assigned dot locations 1106, 1108,

1110, 1112, 1114, 1116, and 1118 are within the error threshold and may be selected to be ejected onto the target spot. The redundancy makes it possible to select different dots and optimize other parameters, e.g. time or utilization of donor area. In some embodiments, the finer the grid of preassigned dots, the smaller the error threshold that may be selected and still allow for redundancy, as well as better utilization of donor material since many dots will be within an error threshold to be ejected. In some embodiments, prior to selection it may be determined that the next dot to overlap the target spot within the error threshold would be better for a different target spot because it will result in overall better utilization of the donor material, and another dot within the error threshold may be selected.

[0075] In embodiments, the laser cannot issue two pulses in immediate adjacency but must have time to build up the pulse energy. Therefore, in embodiments sequential pre-assigned dots for ejection are selected taking into account the pulse timing parameters of the laser.

[0076] The donor material has a limited number of possible positions and they must be used efficiently. If the donor material has a pre-defined grid of dots, either physical patches of donor material or assigned spots on the surface, each dot can be pre-assigned to a position on the workpiece and a job plan calculated which satisfies the different restrictions and gives efficient use of time and donor material. In embodiments, the target locations matched with the pre assigned dots are selected prior to any ejection in order to maximize the utilization of the donor drum. However, in some embodiments, the assignment may be done on the fly and the next dot within the next predetermine number of dots that has the smallest error which is lower than the error threshold is selected to be ejected onto the target location.

[0077] In some embodiments, with certain geometries of patterns on a workpiece it is beneficial to rotate the axes of the pattern relative to the direction of relative movement of the donor drum and the workpiece. Fig. 10A depicts a grid of target spots 1202 to be patterned on a PCB workpiece 1204. The grid of target spots 1204 has an X and Y axes, which may be referred to as Manhattan geometry of the PCB. PCBs frequently include Manhattan Geometry wherein lines and pads are configured in a grid-like pattern with 90-degree angles.

[0078] Figures 11 A, 1 IB, 12A and 12B further illustrate the concept of orienting the geometry of the PCB 1302 askew to the direction of relative motion 1304 of the donor drum and workpiece in order to more evenly distribute the usage of donor material across the donor drum in a direction perpendicular to the relative direction of motion. As shown in Fig.13 A a circuit board is aligned with the Manhattan geometry features parallel to the direction of movement 1304. Fig. 13B shows a histogram of the distribution of required donor material to be deposited, where the Y-axis shows required donor material and the X-axis corresponding to paths parallel paths in the direction of motion across the donor drum in a direction particular to the direction of motion. As shown, this parallel alignment causes paths that include high usage of donor material where stripes of contact lines are formed and further causes areas where very little donor material is required to be deposited. This creates very poor utilization of donor material because many paths along the direction of relative motion will hardly be used at all, while a very large area of donor material along a path on a donor drum will be needed in order to deposit the material need in the spikes.

[0079] Therefore, as shown in Figs. 12A and 12B, by aligning the Manhattan geometry 1302 askew to the direction of relative movement 1304, the features that previously required high amounts of donor material along a single path are now spread over several paths and therefore the distribution of required donor material is evened out.

[0080] In some embodiments, donor drums may have a short shelf live due to properties of the donor material. For example, if the donor material is solid and dry or if it is perishable, it may be beneficial to coat the transparent carrier only a short time before it is used. Example materials include perishable materials which dry or harden, such as solder paste, nanopaste, and conductive adhesive, and various glues and paints. The solder paste used in surface mounting needs to be tacky since components are pressed into the paste and sticks by the tackiness. The paste also has a limited useable time when exposed to air. Other perishable materials are foods and biological substances or structures. Also, some chemical compounds have a limited life in air, e.g. organic electronic materials.

[0081] As noted above, Figs. 6A and 6B show methods of forming preformed dots on a transparent carrier, which may be performed at the regeneration stations disclosed herein. In some embodiments, at the regeneration station the donor drum may be coated with a fdm of donor material and in some embodiments the donor substrate may include features which aid in the recoating process.

[0082] In some embodiments, the pattern to be deposited on the workpiece may have a low density, and a repeating sweeping writing pattern as shown in figure 13A may be an inefficient use of time. Therefore, in some embodiments, the relative movement of the donor drum 1602 and workpiece 1604 may be irregular, for example as shown in figure 13B, in order to pattern the workpiece in an optimal time.

[0083] Figs. 14A-14B show examples of how oblique relative scanning of the donor drum and workpiece can be accomplished. In Fig. 14A, the donor drum 1702 is oriented at an oblique angle relative to the workpiece 1704. In Fig. 14B, the donor drum 1702 is oriented parallel to the workpiece 1704, the donor drum 1702 is oriented at an angle relative to the workpiece 1704 and the relative motion 1706 is oblique to the alignment of the donor drum and workpiece.

[0084] In Fig. 15B an SLM 2406 is included. The SLM can be a coherent MEMS SLM, or an LCD SLM as is manufactured by HoloOr (Berlin, Germany) and other companies. In some embodiments, the SLM may also be a non-coherent micromirror device as the DMD chip from Texas Instruments. LIFT with structured light from a DMD mirror has recently been described by Raymond Auyeung et al. in Optics Express, Vol. 23, Issue 1, pp. 422-430 (2015). They describe using the de-magnified image from a DMD to transfer shapes. Comparing possible SLMs, the DMD has a useful update rate of about 30 kHz, higher than both the coherent MEMS SLM developed by the assignee and the LCD SLMs that are commercially available. In the example embodiment in Fig. 15B the light is stricter near the laser source 2408 and the shape that is impressed on the light beam follows it through the scanning optics. Therefore, the SLM can make a shape and change it 30 000 times per second and the scanning system can make 200 000 prints of the shape on the workpiece 2410. Fig. 15A shows in conceptual form a writer with shaped light and fast scanner for LIFT. The writer includes a laser 2412, a beam shaper 2414 (e.g. SLM), a second beam shaper 2416, a donor drum movement stage 2418, and a workpiece movement stage 2420, all attaches to a digital controller 2422. The Digital controller accepts a pattern to be produced and controls the stage with the acceptor and the stage with the donor, the optical scanner, the beam shaper, and the emission of laser pulses. Note that the addition of the SLM makes the system much faster since complicated patterns can be built from shapes, not dots.

[0085] The systems in Figs. 15A and 15B can be combined with several other aspect of the technology disclosed herein. The DMD does not withstand high energy pulses, but the pulse energy can be handled across a large area of the DMD.

STING OF EMBODIMENTS

1. A device for transfer of solder paste from a deposited film/reservoir onto a substrate, comprising:

a pulsed light source for generation of a light beam;

a rotating hollow cylinder that is transparent to the light beam;

a control unit for the modulation of the light beam;

wherein said pulsed light source comprises pulse of electromagnetic energy high enough to initiate a localized spot of lift-off process at an interface between the glass cylinder and the solder paste or other adhesive material, as the cylinder continuously rotates, whereby the solder paste ejected from said localized lift-off area.

2. The device of item 1, wherein said pulsed light source is modulated by the control unit to produce a pattern of solder paste at the substrate;

3. The device of any of items 1 -2, wherein the means for scanning the light beam comprises:

a polygon mirror of an of 3-, 4-, 5-, 6-, or 8-fold symmetry; and

an anamorphic lens positioned in an optical path between the polygon mirror and the outer surface of the cylinder and refracting the light beam down to the outer surface of the cylinder, wherein the refracted beam is essentially orthogonal to the axis of rotation of the cylinder.

4. The device of any of items 1 -3, where the means for scanning is essentially located inside the transparent cylinder.

5. The device of any of Items 1 -2, wherein the means for scanning the light beam comprises:

a stationary mirror inside the cylinder positioned to reflect the light beam through the cylinder to the outer surface; and

telecentric optics that sweep the light beam along the stationary mirror. 6 The device of item 5, wherein the stationary mirror is substantially flat.

7. The device of item 5, wherein the stationary mirror is arched.

8 The device of any of items 1 -2, wherein the means for scanning the light beam comprises:

a stationary diffraction grating inside the cylinder positioned to diffract the light beam through the cylinder to the outer surface; and

telecentric optics that sweep the light beam along the stationary diffraction grating.

9. The device of any of items 5-8, wherein the refracted or diffracted light beam is essentially orthogonal to the axis of rotation of the cylinder.

10. The device of any of items 1 -9, where the projection optics comprises actuation of lenses in order to allow for a dynamic adjustment and fine-tuning of focus and the spot position along the axis of scanning.

11. The device of any of items 1 -10, where the projection optics comprises actuation of lenses in order to allow for a dynamic adjustment and fine-tuning of focus and the spot position along the axis of scanning.

12. A method of depositing material in a pattern on a workpiece by transfer of donor material by laser induced forward transfer, including:

at an exposure station, pulsing a laser beam through a first area of the donor drum causing portions of donor material to be transferred from the first area the donor drum to form a portion of the pattern on the workpiece;

13. The method of item 12, wherein the donor drum includes an outer face including the donor material; wherein at the recoating station, the outer face is oriented upwards relative to gravity and is recoated with donor material; and

wherein at the exposure station, the outer face is oriented downwards relative to gravity.

14. The method of any of items 12-13, wherein recoating includes:

depositing additional donor material on the outer face of the donor drum; and

recoating a layer of donor material on the outer face of the donor drum plate using a mixture of the deposited additional donor material and donor material remaining on the second donor drum plate after the previous exposure from which donor material was transferred from the donor drum.

15. The method of any of items 12-14, wherein recoating further includes:

scraping, using at least one blade, scraper or squeegee, to create an even layer of donor material on the outer face of the donor drum.

16. The method of any of items 12-15, wherein the second donor drum plate includes trenches filled with the donor material; and

wherein recoating includes filling in trenches from which donor material was previously transferred.

17. The method of item 16, wherein the trenches form a grid pattern on the donor drum.

18. The method of any of items 12-13 or 17, wherein recoating includes jet printing of donor material onto the second donor drum plate.

19. A device for depositing material in a pattern on workpieces by transfer of donor material by laser induced forward transfer, including:

donor drum including laser transparent supports and initial coatings including donor material; an exposure station, including a laser, configured to receive the donor drum plates, and pulse a laser beam through each of the donor drum plates to cause portions of the donor material to be transferred from the donor drum plates to the workpieces;

a recoating station configured to receive the donor drum plates and recoat the donor drum plates; and

wherein the donor drum includes an outer face including the donor material, and wherein rotation of the donor drum positions a replenishment area of the outer face facing upwards relative to gravity at the recoating station and positions an exposure are of the outer face facing downwards relative to gravity at the exposure system.

20. A method of efficiently transferring donor material by laser induced forward transfer from a donor drum to the workpiece, including:

providing one surface of a donor drum with a pattern of discrete separated dots of donor material;

orienting the surface with the pattern of discrete separated dots of donor material to face a workpiece; and

pulsing a laser beam through the donor drum, causing the discrete separated dots to be transferred from the donor drum to form a pattern on the workpiece.

21. The method of item 20, wherein the pattern includes a Manhattan geometry, the method further including:

causing the donor drum to move relative to the workpiece in a relative direction of motion; and orienting the pattern on the workpiece askew to the relative direction of motion, wherein the askew orientation angle more evenly distributes donor material requirements, imposed by the Manhattan geometry pattern, across the donor drum in a direction perpendicular to the relative direction of movement.

22. The method of item 21 , further including calculating for the Manhattan geometry an askew orientation angle based at least in part on projection of the donor material requirements for at least a segment of the pattern on the workpiece onto a base line.

23. The method of any of items 20-22, wherein the pattern of discrete separated dots includes dots of different sizes.

24. The method of any of items 20-22, wherein the pattern of discrete separated dots includes dots of different shapes.

25. The method of any of items 20-24, wherein the pattern of discrete separated dots includes a dot including portions of donor material with thicknesses that vary by at least 20 percent.

26. The method of any of items 20-25, wherein pulsing a laser beam through the donor drum includes focusing the laser beam on an area of the donor drum larger than the discrete separated dot to be ejected. 27. The method of any of items 20-26, wherein the pattern of discrete separated dots are formed within recesses in the surface of the donor drum.

28. The method of any of items 20-27, wherein the pattern of discrete separated dots includes a first dot having a first thicknesses and a second dot having a second thickness different than the first thickness.

29. The method of any of items 20-28, wherein the pattern of the discrete separated dots is a hexagonal pattern.

Claims

1. A device for transfer of a material onto a substrate, comprising:

a pulsed light source for generation of a light beam;

a rotatable hollow cylinder that is transparent to the light beam;

a control unit for the modulation of the light beam;

wherein said pulsed light source is adapted to generate pulses of electromagnetic energy high enough to initiate a localized spot of lift-off process at an interface between the hollow cylinder and the material to be transferred, as the cylinder continuously rotates, whereby the material is ejected from said localized lift-off area.

2. The device of claim 1, wherein the control unit is adapted to modulate said pulsed light source to produce a pattern of material to be transferred at the substrate

3. The device of any of claims 1-2, wherein the means for scanning the light beam comprises:

a polygon mirror of an of 3-, 4-, 5-, 6-, or 8-fold symmetry; and

4. The device of any of claims 1-3, where the means for scanning is essentially located inside the rotatable cylinder.

5. The device of any of claims 1 -2, wherein the means for scanning the light beam comprises:

telecentric optics that sweep the light beam along the stationary mirror.

6. The device of claim 5, wherein the stationary mirror is substantially flat.

7. The device of claim 5, wherein the stationary mirror is arched.

8. The device of any of claims 1-2, wherein the means for scanning the light beam comprises:

9. The device of any of claims 5-8, wherein the refracted or diffracted light beam is essentially orthogonal to the axis of rotation of the cylinder.

10. The device of any of claims 1-9, where the projection optics comprises actuation of lenses in order to allow for a dynamic adjustment and fine-tuning of focus and the spot position along the axis of scanning.

11. The device of any of claims 1-10, where the proj ection optics comprises actuation of lenses in order to allow for a dynamic adjustment and fine-tuning of focus and the spot position along the axis of scanning.

12. The device according to any previous claim, wherein the material to be transferred is selected from solder paste and conductive glue.

13. A method of depositing material in a pattern on a workpiece by transfer of donor material by laser induced forward transfer, including:

14. The method of claim 13, wherein the donor drum includes an outer face including the donor material;

wherein at the recoating station, the outer face is oriented upwards relative to gravity and is recoated with donor material; and

15. The method of any of claims 13-14, wherein recoating includes:

depositing additional donor material on the outer face of the donor drum; and

16. The method of any of claims 13-15, wherein recoating further includes:

17. The method of any of claims 13-16, wherein the second donor drum plate includes trenches filled with the donor material; and

18. The method of claim 17, wherein the trenches form a grid pattern on the donor drum.

19. The method of any of claims 13-14 or 18, wherein recoating includes jet printing of donor material onto the second donor drum plate.

20. The method according to any previous claim, wherein the material that is deposited is selected from solder paste and conductive glue.