CN115702488A

CN115702488A - High resolution welding

Info

Publication number: CN115702488A
Application number: CN202180039742.3A
Authority: CN
Inventors: Z·科特勒; O·福格
Original assignee: Orbotech Ltd
Current assignee: Orbotech Ltd
Priority date: 2020-06-04
Filing date: 2021-02-24
Publication date: 2023-02-14
Also published as: TW202146255A; JP2023529317A; KR20230021667A; JP7411829B2; WO2021245467A1

Abstract

A method for circuit fabrication includes defining a solder bump to be formed at a target location on an acceptor substrate, the solder bump including a specified solder material and having a specified bump volume. Positioning a transparent donor substrate having a donor film including the designated solder material such that the donor film is close to the target location on the acceptor substrate. A sequence of pulses of laser radiation is directed through the first surface of the donor substrate and impinges on the donor films so as to induce ejection of a number of molten droplets of the solder material from the donor films onto the target locations on the acceptor substrate such that the droplets deposited at the target locations cumulatively reach the specified bump volumes. Heating the target location to melt and reflow the deposited droplets to form the solder bumps.

Description

High resolution welding

Technical Field

The present disclosure relates generally to the manufacture of electronic devices, and in particular, to methods and systems for soldering.

Background

In the Laser Direct Writing (LDW) technique, a laser beam is used to produce a patterned surface with spatially resolved three-dimensional structures by controlled material ablation or deposition. Laser Induced Forward Transfer (LIFT) is a LDW technique that can be used to deposit micropatterns on surfaces.

In LIFT, laser photons provide the driving force to eject small volumes of material from a donor film to an acceptor substrate. Typically, the laser beam interacts with the inner side of the donor film, which is coated onto a non-absorbing carrier substrate. In other words, the incident laser beam propagates through the transparent carrier substrate before the photons are absorbed by the inner surface of the film. Beyond a certain energy threshold, material is ejected from the donor film towards the surface of the acceptor substrate. After appropriate selection of the donor film and laser beam pulse parameters, the laser pulse causes molten droplets of donor material to be ejected from the film and then land and harden on the acceptor substrate.

LIFT systems are particularly, although not exclusively, useful for printing conductive metal drops and traces for electronic circuit fabrication. This LIFT system is described, for example, in U.S. patent 9,925,797, the disclosure of which is incorporated herein by reference. This patent describes a printing apparatus including a donor supply assembly configured to provide a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface to position the donor film near a target area on an acceptor substrate. The optical assembly is configured to simultaneously direct a plurality of output beams of laser radiation in a predetermined spatial pattern through the first surface of the donor substrate and onto the donor film so as to induce ejection of material from the donor film onto the acceptor substrate, thereby writing the predetermined pattern onto a target area of the acceptor substrate.

Disclosure of Invention

Embodiments of the present invention described below provide improved methods and systems for fabricating circuits and devices.

There is thus provided, in accordance with an embodiment of the present invention, a method for circuit fabrication, including: solder bumps are defined to be formed at target locations on a recipient substrate, the solder bumps including a specified solder material and having a specified bump volume. A transparent donor substrate having opposing first and second surfaces and a donor film comprising the specified solder material are positioned on the second surface such that the donor film is proximate to the target location on the acceptor substrate. Directing a sequence of pulses of laser radiation through the first surface of the donor substrate and onto the donor films so as to induce ejection of a plurality of molten droplets of the solder material from the donor films onto the target locations on the acceptor substrate such that the droplets deposited at the target locations cumulatively reach the specified bump volumes. Heating the target location such that the deposited droplets melt and reflow to form the solder bumps.

Typically, the droplets have respective droplet volumes that depend on the intensity of the pulses of the laser radiation, and directing the sequence of the pulses includes setting the intensity of the pulses of laser radiation and the number of the pulses in the sequence in response to the volume of the designated bump. In disclosed embodiments, the drop volume is further dependent on a set of pulse parameters including a spot size and a duration of the pulse of the laser radiation, and wherein directing the sequence of the pulses further includes adjusting the drop volume by changing one or more of the pulse parameters.

In some embodiments, defining the solder bump includes: first and second solder bumps having different, respective first and second bump volumes are defined at different, respective first and second target locations on the same acceptor substrate, and directing the sequence of pulses includes directing the different, first and second sequences of pulses through different points on the donor substrate such that the droplet accumulatively reaches each of the different first and second bump volumes at the respective first and second target locations. In one embodiment, defining the first solder bump and the second solder bump includes designating different, respective first and second compositions of the first solder bump and the second solder bump, and positioning the transparent donor substrate includes providing one or more donor films including a plurality of different solder materials selected so as to produce the first composition and the second composition.

Additionally or alternatively, defining the solder bumps includes defining first and second solder bumps having different, respective first and second compositions, and positioning the transparent donor substrate includes providing one or more donor films including a plurality of different solder materials to produce the first and second compositions.

Further additionally or alternatively, defining the solder bump includes designating a composition of the solder bump, the composition including different, first and second materials, and positioning the transparent donor substrate includes providing first and second donor films including the first and second materials, respectively, and directing the pulse sequence includes directing a first and second sequence of the pulses to impinge on the first and second donor films, respectively, such that the droplets deposited at the target location cumulatively reach the designated composition. In one embodiment, specifying the composition includes specifying a gradient of the material in the composition of the solder bumps, and directing the first and second sequences of the pulses includes depositing the droplets of the first and second materials in a plurality of layers on the target location according to the specified gradient.

In some embodiments, directing the sequence of the pulses includes depositing the droplets in multiple layers on the target location so as to reach the specified bump volume.

In a disclosed embodiment, heating the target location includes alternately depositing a layer of droplets a plurality of times and heating the layer to melt the droplets until the specified bump volume is reached.

Additionally or alternatively, defining the solder bumps includes specifying a shape of the solder bumps, and directing the sequence of the pulses includes depositing the molten droplets in a pattern conforming to the specified shape.

In further embodiments, heating the target location includes directing a laser beam to irradiate the target location with sufficient energy to melt and reflow the deposited droplets. Typically, directing the laser beam includes focusing one or more laser pulses onto the target location.

In some embodiments, the method includes printing a conductive pad at the target location on the acceptor substrate using a process of laser-induced forward transfer (LIFT), wherein directing the sequence of the pulses includes depositing the molten droplet of the solder material on the printed conductive pad. In disclosed embodiments, printing the conductive pad includes forming a recessed surface in the conductive pad for depositing the molten droplet therein.

There is also provided, in accordance with an embodiment of the present invention, a system for circuit fabrication, including a controller configured to receive a definition of a solder bump to be formed at a target location on a recipient substrate, the solder bump including a designated solder material and having a designated bump volume. A printing station includes a transparent donor substrate having opposing first and second surfaces and having a donor film including the specified solder material disposed on the second surface, and the transparent donor substrate is positioned such that the donor film is proximate to the target location on the acceptor substrate. A laser is configured to direct a sequence of pulses of laser radiation through the first surface of the donor substrate and impinge on the donor film to induce ejection of molten droplets of the solder material from the donor film onto the target location on the acceptor substrate. The controller is configured to drive the printing station to eject a number of droplets toward the target location such that the droplets deposited at the target location cumulatively reach the specified bump volume. A reflow station is configured to heat the target location such that the deposited droplets melt and reflow to form the solder bumps.

There is additionally provided, in accordance with an embodiment of the present invention, a method for circuit fabrication, including: depositing solder material at one or more target locations on a circuit substrate; and focusing one or more pulses of a laser beam onto each of the target locations with sufficient energy to melt and reflow the deposited droplets in order to form solder bumps.

In disclosed embodiments, depositing the solder material includes ejecting molten droplets of the solder material toward the one or more target locations.

In some embodiments, the pulse has a pulse duration of no more than 1ms, and possibly no less than 100 μ s.

Additionally or alternatively, the pulse has a pulse energy of no greater than 2mJ.

Additionally or alternatively, the pulses have a pulse energy of no more than 3 mJ. In a disclosed embodiment, focusing the one or more pulses includes focusing a single, respective pulse of the laser beam onto each of the target locations.

The invention will be more fully understood from the following detailed description of embodiments of the invention taken in conjunction with the accompanying drawings, in which:

drawings

FIG. 1 is a block diagram schematically illustrating a system for electronic circuit manufacturing, in accordance with an embodiment of the present invention;

FIG. 2A is a schematic front view of a printed circuit substrate on which solder droplets have been deposited in a LIFT process according to an embodiment of the invention;

fig. 2B is a schematic front view of the printed circuit substrate of fig. 2A after solder reflow in accordance with an embodiment of the invention;

FIGS. 3A, 3B, 3C and 3D are schematic cross-sectional views of a circuit substrate showing successive stages in the process of deposition and reflow of solder bumps, according to an embodiment of the invention;

FIG. 4A is a schematic cross-sectional view of a circuit substrate on which droplets of two different solder materials have been deposited in a LIFT process in accordance with an embodiment of the invention;

fig. 4B is a schematic front view of the circuit substrate of fig. 4A after reflow of the solder material in accordance with an embodiment of the invention; and

fig. 5A is a photomicrograph of a contact pad formed by a LIFT process according to an embodiment of the invention; and

fig. 5B is a photomicrograph of a solder bump formed on the contact pad of fig. 5A in accordance with an embodiment of the invention.

Detailed Description

SUMMARY

In electronic circuit fabrication methods known in the art, electrical traces and contact pads are printed on a circuit substrate, and a solder layer is printed onto the contact pads by photolithography. The circuit assembly is then placed on the solder-covered pad and the circuit is heated to melt and reflow the solder, thus creating an electrically conductive bond between the assembly and the pad. In this conventional approach, the pad location and size on each contact pad and the volume of solder material are fixed by a photolithographic mask and solder deposition process.

Embodiments of the present invention provide a LIFT-based solder deposition method that is capable of producing solder bumps of substantially any desired size and shape, and comprising substantially any suitable solder material or combination of materials, as desired. This LIFT-based method is capable of producing multiple bumps of different volumes, shapes and sizes, and even including different solder materials and combinations of solder materials, on the same substrate in the same process step. The volume and composition of the solder bumps, even including non-uniform compositions, can be precisely controlled by setting the LIFT parameters, the donor film material, and the number of droplets deposited at each target location. Furthermore, in contrast to conventional methods, the present method enables the printing of solder bumps on non-uniform substrates as well as on substrates on which components have been placed. Accordingly, the disclosed embodiments provide greater flexibility and accuracy in circuit fabrication than techniques known in the art.

In the embodiments described below, the solder bumps are defined in view of a specified solder material and bump volume and a target location where the bumps are to be formed on the acceptor substrate. A transparent donor substrate is positioned, the donor substrate having on one of its surfaces a donor film comprising a specified solder material, wherein the donor film is located in proximity to a target location on an acceptor substrate. (for convenience, the surface of the donor substrate adjacent to the acceptor substrate is referred to herein as the lower surface, and the opposite surface of the donor substrate is referred to as the upper surface.) a laser directs a series of pulses of laser radiation through the upper surface of the donor substrate and onto the donor film so as to induce ejection of a number of molten droplets of solder material from the donor film onto a target location on the acceptor substrate.

The laser pulse parameters and the number of pulses in the sequence are selected such that droplets deposited at the target location cumulatively reach a specified bump volume. The pulse parameters that can be varied to control the drop volume include the pulse intensity, i.e., the optical power per unit area incident on the donor film, as well as the spot size and pulse duration. These parameters can be adjusted to give orders of magnitude of 0.1pl (picoliters), i.e. 100 μm, depending on the type and thickness of the solder material in the donor film ³ Or smaller, and ensures that the ejection of the droplets is accurately positioned from the donor film at high speed towards the target location. Thus, by appropriate selection of pulse parameters and droplet numbers, precisely sized solder bumps down to about 20 μm in diameter can be printed. Processes according to embodiments of the invention can be used to print a variety of solder materials, including low, medium, and high temperature solders, onto a variety of substrates, and also facilitate fluxless soldering.

After depositing one or more layers of droplets on the receiver substrate, the target location is heated such that the deposited droplets melt and reflow to form solder bumps. Heating is advantageously performed locally, e.g., by laser irradiation, to drive rapid reflow and minimize damage to the substrate. The laser pulses used in this stage may be narrowly focused on the solder bumps, and the duration of the laser pulses need not exceed about one millisecond and in most cases need less than 100 microseconds, such as tens of microseconds (for small solder bumps or even less). Thus, this laser-driven reflow technique can be performed in ambient air and is applicable to heat-sensitive substrates. It is particularly suitable for use in conjunction with the LIFT-based solder printing techniques described above; it can also be applied to reflowed solder materials that have been deposited by other methods, such as inkjet solder printing and photolithographic techniques. Alternatively, however, the reflow stage may be performed by heating the entire receptor substrate, for example, in a high temperature furnace. After the solder bumps are formed, the circuit components can be placed on the bumps and soldered in place by conventional methods.

Description of the System

Fig. 1 is a schematic illustration of a system 20 for electronic circuit fabrication according to an embodiment of the present invention. The system 20 includes a printing station 22, the printing station 22 receiving a definition of a solder bump 60 to be formed at a target location on the acceptor substrate 34, the solder bump 60 comprising a specified solder material and having a specified bump volume. The printing station deposits several droplets 32 of the desired solder material at each target location so that the droplets cumulatively reach the specified bump volume. Reflow station 24 heats the target location such that deposited droplets 32 melt and reflow to form solder bumps 60. This heating process may be locally focused on the bump locations, as shown in fig. 1, or may be performed globally across the entire substrate 34 depending on the properties of the solder material and substrate and other application requirements.

Typically, after the solder bumps 60 are formed, the placement station 26 places the components 64 on the solder bumps, for example, using a pick and place machine 62, as is known in the art. The heat source 66 in the final reflow station 28 then heats the solder bumps to form a permanent bond 68 between the component and the substrate 34. The heat source 66 may apply localized heating using, for example, a laser, or it may comprise a reflow oven or any other suitable type of heater known in the art. The bond 68 typically forms both an electrical and mechanical connection between the component 64 and conductive traces on the substrate 34. Alternatively or additionally, the solder bumps 60 may be arranged to form a frame having a rectangular, circular, or other shape in order to create a mechanical seal around the edges of the component 64. Such seals may be used, for example, for hermetic packaging of sensitive devices, such as microelectromechanical system (MEMS) devices.

Referring back to the printing station 22, the optical assembly 30 in the printing station comprises a laser 38 which, under the control of a controller 51, directs short optical radiation pulses towards the donor foil 44 with a pulse duration on the order of 1 ns. (As used in the context of this specification and the claims, the term "optical radiation" refers to electromagnetic radiation in any of the visible, ultraviolet, and infrared ranges; while "laser radiation" refers to optical radiation emitted by a laser.) the controller 51 generally comprises a general purpose computer or special purpose microcontroller having suitable interfaces to the other elements of the system 20 and driven in software to perform the functions described herein. The donor foil 44 typically comprises a thin, flexible sheet of transparent donor substrate 46 coated on the side near the acceptor substrate 34 with a donor film 48 comprising one or several specified solder materials. Alternatively, the donor substrate may comprise a rigid or semi-rigid material. The receptor substrate 34 may comprise any suitable material, such as glass, ceramic, or polymer, as well as other dielectric, semiconductor, or even conductive materials.

The optical assembly 30 includes a beam deflector 40 and focusing optics 42 that direct one or more output beams of radiation from the laser 38 through the upper surface of the donor substrate 46 and thus impinge on the donor film 48 on the lower surface after a spatial pattern determined by a controller 51. In an example embodiment, the beam deflector 40 comprises an acousto-optic modulator, as shown in fig. 2A or 2B of U.S. patent 9,925,797 referenced above and described in columns 7-8 of this patent. The laser is typically controlled to output a pulse train of appropriate wavelength, duration and energy so as to induce ejection of molten droplets 50 of solder material from the donor film 48 onto designated target locations on the acceptor substrate 34. Because the droplets 50 are ejected from the donor film 48 in a direction perpendicular to the donor substrate 46 and at a high velocity, the donor foil 44 can be positioned at a small distance from the acceptor substrate 34, e.g., up to about 0.5mm separation between the donor film 48 and the acceptor substrate 34, rather than in contact with the acceptor substrate. Due to the high velocity ejection of droplets 50 (typically 10m/sec or greater), the flight time of the droplets is less than the time it takes for the droplets to solidify, and the printing station 22 can operate at ambient atmospheric conditions rather than under vacuum.

The donor film 48 can include substantially any suitable type of solder or combination of solder materials, including low temperature, medium temperature, and high temperature solders. Low and medium temperature solders include, for example, tin-lead and tin-silver-copper (SAC) alloys. The high temperature solders most commonly used in the manufacture of high power electronic devices comprise alloys of silver (typically 45% to 90%) with other metals such as copper, zinc, tin and cadmium, and typically melt at temperatures in the range of 700 ℃ to 950 ℃. The thickness and composition of the film 48 and the pulse parameters of the optical assembly 30 are adjusted depending on the choice of solder material in order to provide a stable ejection of the molten droplets 50 of solder material toward the target location on the receptor substrate 34.

In some embodiments, multiple layers and structured donor films 48 may be used in order to deposit droplets 32 of the mixed composition. For example, donor foil 44 may comprise a multi-layer donor film comprising different, respective solder compositions so as to produce molten droplets 50 containing a bulk mixture of different materials. Such a multi-combination LIFT scheme is described, for example, in U.S. patent 10,629,442, which is incorporated herein by reference.

Alternatively or additionally, the donor foil 44 may comprise a donor film 48, said donor film 48 comprising different materials at different locations on the donor foil. Optical assembly 30 directs laser pulse sequences to impinge on different donor locations, respectively, such that droplets 32 of different materials deposited on a given target location cumulatively reach a specified volume and composition. Such a hybrid compositional scheme is further described below with reference to fig. 4A/B.

Controller 51 adjusts the pulse parameters of laser 38 and the scanning and focusing parameters of optical assembly 30 so that the appropriate number of droplets 32 of the desired volume are deposited at each target location where a solder bump is to be formed on acceptor substrate 34. As explained earlier, the controller 51 sets the laser pulse parameters and the number of molten droplets of solder material so that the droplets deposited at each target location cumulatively reach the specified bump volume at that location. Since the drop volume can be varied by adjusting the laser pulse parameters, a given bump volume can be created by depositing a smaller number of larger volume drops or a larger number of smaller volume drops. The thickness 38 of the donor film also contributes to the droplet size. However, in view of the inherent tolerance of actual drop volumes, relying on a large number of statistics, and using a larger number of smaller drops instead of a small number of larger drops may be advantageous, particularly when dealing with very small bumps.

The printing station 22 also includes a positioning assembly, which may include, for example, an X-Y table 36 on which the receptor substrate 34 is mounted. Stage 36 displaces acceptor substrate 34 relative to optical assembly 30 and donor foil 44 in printing station 22 so as to deposit droplets 32 at different target locations across the surface of the acceptor substrate. Additionally or alternatively, the positioning assembly may include a motion component (not shown) that displaces the optical assembly 30 and, if appropriate, the donor film 44 over the surface of the acceptor substrate.

Reflow station 24 includes an optics assembly 52 that directs a beam of radiation to locally melt droplets 32, thus causing the droplets to coalesce into solder bumps 60. Such localized heating is particularly advantageous to avoid damage to the sensitive receptor substrate 34. The optical assembly 52 in the depicted example includes a laser 54 along with a beam deflector 56 and focusing optics 58 that direct laser radiation to irradiate a target location with sufficient energy to melt and reflow deposited droplets into solder bumps 60. The reflow station 24 also includes a positioning assembly, which may be based on the same step 36 as in the print station 22, or a different step or other movement device.

The controller 51 adjusts the pulse parameters of the laser 54 and the scanning and focusing parameters of the optical assembly 52 in order to apply sufficient energy to melt and reflow each solder bump 60 while avoiding damage to the substrate 34. The pulse duration and energy are selected so that the solder material at the bottom of each bump is completely melted without evaporating the solder material at the top of the bump. The actual power and pulse duration required depends on the melting temperature and thermal conductivity of the solder material. For this reason, short laser pulses are generally preferred because they minimize the time for the solder material to melt, and thus minimize oxidation and avoid damage to the substrate 34. Thus, the reflow station 24 is capable of operating at ambient atmospheric conditions. Short, high power laser pulses are particularly advantageous in achieving unassisted reflow and in supporting the use of high temperature solder materials. Such a single laser pulse focused on the location of each solder bump is typically sufficient to achieve complete reflow of the small solder bumps, but multiple pulses may alternatively be used, particularly for larger solder bumps. The resulting fast local reflow process is also beneficial in reducing the formation of intermetallics between the solder bumps and the contact pads, and thus results in stronger solder bonds, as compared to thermal reflow methods known in the art.

For small solder bumps made of mound droplets 32 of tin-based solder having a thickness of 20 to 30 μm, for example, a laser pulse having an optical power of approximately 10W and a duration of 50 to 100 μ s is generally sufficient to achieve complete reflow while avoiding significant heat diffusion to the substrate. The optimal laser wavelength, pulse power, duration, and focal length can be selected in each case to match the absorption spectrum, volume, and thermal properties of the solder material. For small to medium sized solder bumps, the pulse energy needs to be no greater than about 2mJ. The optimal laser parameters may be determined empirically and/or based on thermal and hydrodynamic simulations, such as using finite element analysis tools known in the art.

In the following example, the laser 54 in the reflow station 24 may be a high power CW Nd: YAG laser operating at 1064 nm. Alternatively, the laser 54 may be a diode-pumped fiber laser, such as a continuous wave fiber laser in the range of 976nm to 1075nm and tens of watts of power (such as may be obtained from an IPG). Alternatively, the laser 54 may be a high power diode laser module, such as a diode laser module manufactured by BTW. Other types of lasers will be apparent to those skilled in the art upon reading this specification.

In an example embodiment, the printing station 22 prints the bumps using tin-based solder. To reflow a bump having a volume of about 40pl (corresponding to a bump diameter of about 50 μm), the laser 54 is set to output a pulse having a pulse energy of about 0.45mJ to 1.6mJ and a duration of 50 μ s to 150 μ s. The optical assembly 52 focuses the beam to a spot size of about 35 to 50 μm on the solder bumps. On the other hand, for smaller bumps, for example, having a volume of about 15pl (corresponding to a diameter of about 25 μm), the laser 54 in the reflow station 24 is set to output a pulse having a pulse energy of about 0.2mJ to 0.45mJ and a duration of 10 μ s to 30 μ s focused at a spot size of about 15 μm to 25 μm on the solder bump.

In another example, a reflow bump having a volume of about 85pl (corresponding to a bump diameter of about 100 μm), the laser 54 is set to output a pulse having a pulse energy of about 1mJ to 3mJ and a duration of 50 μ s to 150 μ s. The optical assembly 52 focuses the beam to a spot size of about 50 μm to 100 μm on the solder bumps.

Alternative choices for laser driven reflow parameters will be apparent to those skilled in the art after reading this specification.

In one embodiment, the print station 22 and reflow station 24 are combined into a single operating unit, with an optical assembly that provides laser radiation for both LIFT and reflow processes. The same laser source can be used for both purposes as long as it is capable of providing different ranges of pulse energies and durations needed for LIFT and reflow. Alternatively, the combining station may include two or more different laser sources, with shared positioning assemblies and possibly shared optics.

Formation of solder bumps

Reference is now made to fig. 2A/B, which schematically illustrate a process of forming solder bumps on a printed circuit substrate 70, in accordance with an embodiment of the present invention. Fig. 2A is a front view of a substrate 70, such as in the print station 22 (fig. 1), on which droplets 32 of solder have been deposited by the LIFT process, while fig. 2B is a schematic front view of the substrate 70 after reflow of the solder. This embodiment illustrates the use of the techniques described herein in defining and producing solder bumps having different, respective bump volumes, shapes, and/or compositions of solder material at different target locations on the same acceptor substrate (i.e., substrate 70 in this example).

As shown in fig. 2A, prior to depositing solder bumps on the substrate 70, electronic traces 73 and

various contact pads

72, 75, 77 of different sizes and shapes are formed on the substrate. These pads and traces may be printed on the substrate 70 using a photolithographic process, as is known in the art, or they may alternatively be written directly onto the substrate 70, for example using a LIFT process. LIFT printing of contact pads may be advantageous to enhance adhesion of solder material to the contact pads, as explained further below with reference to fig. 5A/B. The controller 51 is programmed to specify different solder bump volumes to be produced on different solder pads. The controller drives optical assembly 30 to direct different sequences of laser pulses through different points on the donor substrate so that deposited droplets 32 cumulatively reach the specified bump volumes on the various pads. Thus, for example, only a single drop 32 or a small number of drops are deposited on each of pads 72, while a larger collection 74 of drops is deposited on pad 75. When very fine contact is required, as is the case with pad 72, the droplets can also be deposited directly onto the target location on trace 73 without the need for a dedicated contact pad, and thus the component is soldered directly to the trace.

As mentioned earlier, by appropriate selection and configuration of the donor film 48, the printing station 22 can be controlled to print different, respective compositions of solder material onto different contact pads. For example, the printing station 22 may print low temperature solder suitable for fine contact onto the pad 72 and high temperature solder onto the pad 75, the pad 75 being designed to carry higher operating currents on the substrate 70 in operation of the circuit. The optical assembly 30 directs laser pulses through appropriate points on the donor substrate 46 in order to deposit a solder material of appropriate composition onto each of the contact pads or locations.

The controller 51 may additionally be programmed to designate different shapes of solder bumps, including non-circular shapes, such as an elongated shape defined by the contact pads 77. Controller 51 then drives optical assembly 30 to direct a sequence of laser pulses through the donor substrate such that droplets 32 are deposited on each contact pad in a pattern conforming to the specified shape. Thus, an elongated collection 76 of droplets is deposited on the contact pads 77. The solder contacts can be printed in this manner in substantially any desired shape, including annular and angled shapes, for example.

After deposition of the droplets 32, the substrate 70 is heated, causing the droplets to melt and reflow, thus coalescing into solder bumps 82, 84, 86, as shown in fig. 2B. At this stage, the droplets tend to coalesce into a spherical shape, which minimizes the surface energy. To minimize this tendency, particularly when producing non-circular solder bumps, the reflow station 24 may employ short, intense laser pulses to locally melt the solder bumps. As explained above. The laser pulse parameters and shot pattern in the reflow station 24 may be adjusted in order to achieve the desired shape characteristics.

Fig. 3A, 3B, 3C and 3D are schematic cross-sectional views of a circuit substrate according to another embodiment of the invention, showing successive stages in the process of deposition and reflow of solder bumps 94 on substrate 70. This embodiment solves the reflow problem that may occur especially in large solder bumps: when the deposition process is performed in a single stage, a high energy laser pulse may be required to melt the droplet 32 at the bottom of the solder bump. High pulse energies increase the risk of damaging the substrate around the solder bumps. On the other hand, if the laser pulse energy is insufficient, the droplet at the bottom of the bump may not completely melt, resulting in poor contact integrity and increased electrical resistance.

To address this problem, droplets 32 are deposited in multiple layers on the target location to achieve a specified bump volume. The substrate 70 is shuttled between the print station 22 and the reflow station 24 multiple times in order to alternately deposit layers of droplets and then heat the layers in order to melt the droplets until a specified bump volume is reached. Alternatively, LIFT printing and melting of droplets may be performed within a single station, with the optics assembly having the capabilities required for LIFT printing and reflow. In either case, the energy that must be applied to melt each successive layer of droplets is relatively small and thus reduces the risk of damage.

Thus, in the depicted example, an initial layer of droplets 32 is deposited on substrate 70, as shown in fig. 3A (or more precisely, on contact pads on the substrate). As shown in fig. 3B, this layer is heated and thus melted to form the reflow layer 92. Another layer of droplets 32 is deposited over the reflow layer 92 as shown in fig. 3C, and then heated to reflow again as shown in fig. 3D. This process is repeated for as many cycles as are required to create the solder bumps 94.

Reference is now made to fig. 4A/B, which schematically illustrate a process for producing a hybrid composition solder bump 100 on a substrate 70, in accordance with an embodiment of the present invention. Fig. 4A is a cross-sectional view showing two different,

respective droplets

96 and 98 of solder material deposited by the LIFT process in the print station 22. Fig. 4B is a front view of the solder bump 100 after reflow of the solder material in the reflow station 24.

The controller 51 receives specifications of the solder bump 100 indicating that the solder bump will include two (or more) different materials in a certain ratio. For example, to improve mechanical strength and/or conductivity, the solder bumps may include copper particles mixed with tin solder, or palladium particles mixed with SAC solder. In some cases, it may also be advantageous to distribute the different materials unevenly within the solder bumps at a specified material gradient. For example, one of the materials (e.g., the material in the droplet 96) may have a higher concentration at the bottom of the solder bump, with a decreasing concentration toward the top of the solder bump relative to the material in the droplet 98. Such a gradient composition of palladium and copper (with a higher palladium concentration at the bottom of the bump) is seen as improving the strength of the solder joint, as disclosed, for example, in us patent 9,607,936.

In the example shown in fig. 4A, the donor foil 44 comprises two donor films 48 comprising two different donor materials, such as the different kinds of materials mentioned above. Optical assembly 30 directs laser pulses toward one of the donor films to deposit droplets 96 on substrate 70 and toward the other donor film to deposit droplets 98. The number of pulses directed toward each of the donor films is selected so that

droplets

96 and 98 are deposited and cumulatively reach the specified composition and total solder bump volume in the appropriate ratio. To generate a gradient composition, the pulse ratio directed towards the two donor films, and thus the ratio of droplets 96 to 98, varies layer by layer from the bottom to the top of the droplets, as shown in fig. 4A. The rapid heating of the collection of

droplets

96 and 98 in the reflow station 24 will cause the droplets to coalesce into solder bumps 100 with minimal mixing so that a specified gradient is maintained, as schematically illustrated in fig. 4B.

Such solder bump deposition of multiple materials may also be used in other applications. For example, the bottom layer of the solder bump may be fabricated by printing droplets of a material that improves or alternatively limits solder wetting. As another example, the underlayer may be selected to improve the matching of the coefficients of thermal expansion between the substrate and the solder material. This property can be tuned by mixing two materials with different coefficients of thermal expansion in order to match the coefficient of thermal expansion of the substrate.

Fig. 5A is a photomicrograph of contact pads 110 formed on a substrate 112 by a LIFT process in accordance with an embodiment of the invention. In other words, the contact pads are written directly onto the substrate 112 by LIFT using a suitable copper donor film 48, for example, rather than being produced by conventional photolithographic printing. LIFT printing of contact pads 110 may be used to control the shape and texture of the contact pads in order to improve solder bump-to-pad adhesion. Thus, as shown in the figure, the contact pad 110 has a rough surface with a concave surface 114 in the center of the pad.

Fig. 5B is a photomicrograph showing solder bumps 116 formed on contact pads 110, according to an embodiment of the invention. Solder bumps 116 are formed by the print station 22 in the manner described above by depositing droplets of solder material in the recessed surfaces 114, followed by melting the droplets in the reflow station 24. The roughness of the contact pad increases the surface area for adhesion of the solder material to the pad and, together with the concavity, helps ensure good electrical and mechanical contact.

It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Claims

1. A method for circuit fabrication, comprising:

defining a solder bump to be formed at a target location on an acceptor substrate, the solder bump comprising a specified solder material and having a specified bump volume;

positioning a transparent donor substrate having opposing first and second surfaces and a donor film comprising the specified solder material on the second surface such that the donor film is proximate to the target location on the acceptor substrate;

directing a pulse sequence of laser radiation through the first surface of the donor substrate and onto the donor film so as to induce ejection of a number of molten droplets of the solder material from the donor film onto the target location on the acceptor substrate such that the droplets deposited at the target location cumulatively reach the specified bump volume; and

heating the target location such that the deposited droplets melt and reflow to form the solder bumps.

2. The method of claim 1, wherein the droplets have respective droplet volumes that depend on an intensity of the pulses of the laser radiation, and wherein directing the sequence of the pulses comprises setting the intensity of the pulses of laser radiation and a number of the pulses in the sequence in response to a volume of the specified bump.

3. The method of claim 2, wherein the drop volume is further dependent on a set of pulse parameters including a spot size and a duration of the pulses of the laser radiation, and wherein directing the sequence of the pulses further comprises adjusting the drop volume by changing one or more of the pulse parameters.

4. The method of claim 1, wherein defining the solder bump comprises: defining first and second solder bumps having different, respective first and second bump volumes at different, respective first and second target locations on the same acceptor substrate, and

wherein directing the pulse sequence comprises directing different, first and second pulse sequences of the pulses through different points on the donor substrate such that the droplets cumulatively reach each of the different first and second bump volumes at the respective first and second target locations.

5. The method of claim 4, wherein defining the first and second solder bumps comprises: different, respective first and second compositions of the first and second solder bumps are specified, and wherein positioning the transparent donor substrate comprises providing one or more donor films comprising a plurality of different solder materials selected so as to produce the first and second compositions.

6. The method of claim 1, wherein defining the solder bump comprises: first and second solder bumps having different, respective first and second compositions are defined, and wherein positioning the transparent donor substrate comprises providing one or more donor films comprising a plurality of different solder materials to produce the first and second compositions.

7. The method of claim 1, wherein defining the solder bump comprises: specifying a composition of the solder bump, the composition comprising different, first and second materials, and

wherein positioning the transparent donor substrate comprises providing a first donor film and a second donor film comprising the first material and the second material, respectively, and

wherein directing the pulse sequence comprises directing a first sequence and a second sequence of the pulses to impinge on the first donor film and the second donor film, respectively, such that the droplets deposited at the target location cumulatively reach the specified composition.

8. The method of claim 7, wherein specifying the composition comprises: specifying a gradient of the material in the composition of the solder bump, and wherein directing the first and second sequences of the pulses comprises depositing the droplets of the first and second materials in a plurality of layers on the target location according to the specified gradient.

9. The method of claim 1, wherein directing the sequence of the pulses comprises: depositing the droplets in a plurality of layers on the target location so as to reach the specified bump volume.

10. The method of claim 9, wherein heating the target location comprises: alternately depositing a layer of droplets a plurality of times and heating the layer to melt the droplets until the specified bump volume is reached.

11. The method of claim 1, wherein defining the solder bump comprises: specifying a shape of the solder bump, and wherein directing the sequence of the pulses comprises depositing the molten droplets in a pattern that conforms to the specified shape.

12. The method of claim 1, wherein heating the target location comprises: directing a laser beam to irradiate the target location with sufficient energy to melt and reflow the deposited droplets.

13. The method of claim 1, comprising printing conductive pads at the target locations on the acceptor substrate using a process of laser-induced forward transfer (LIFT), wherein directing the sequence of the pulses comprises depositing the molten droplets of the solder material on the printed conductive pads.

14. The method of claim 13, wherein printing the conductive pad comprises: forming a recessed surface in the conductive pad for depositing the molten droplet therein.

15. A system for circuit fabrication, comprising:

a controller configured to receive a definition of a solder bump to be formed at a target location on a recipient substrate, the solder bump comprising a specified solder material and having a specified bump volume;

a printing station, comprising:

a transparent donor substrate having opposing first and second surfaces and having a donor film comprising the specified solder material disposed on the second surface, and positioned such that the donor film is proximate to the target location on the acceptor substrate; and

a laser configured to direct a sequence of pulses of laser radiation through the first surface of the donor substrate and impinge on the donor film to induce ejection of molten droplets of the solder material from the donor film onto the target location on the acceptor substrate,

wherein the controller is configured to drive the printing station to eject a number of droplets toward the target location such that the droplets deposited at the target location cumulatively reach the specified bump volume; and

a reflow station configured to heat the target location such that the deposited droplets melt and reflow to form the solder bumps.

16. The system of claim 15, wherein the droplets have respective droplet volumes that depend on an intensity of the pulses of the laser radiation and a set of pulse parameters consisting of a spot size and duration of the pulses of the laser radiation, wherein the controller is configured to set the intensity of the pulses of laser radiation and a number of the pulses in the sequence in response to a volume of the specified bump, and wherein the controller is configured to adjust the droplet volumes by changing one or more of the pulse parameters.

17. The system of claim 15, wherein the controller is configured to receive, at different, respective first and second target locations on the same acceptor substrate, definitions of first and second solder bumps having different, respective first and second bump volumes, and wherein the controller is configured to drive the laser to direct different, first and second sequences of the pulses through different points on the donor substrate such that the droplets cumulatively reach each of the different first and second bump volumes at the respective first and second target locations, and

wherein the definition of the first and second solder bumps specifies different, respective first and second compositions of the first and second solder bumps, and wherein one or more donor films comprising a plurality of different solder materials are disposed on the second surface of the donor substrate, wherein the solder materials are selected to produce the first and second compositions.

18. The system of claim 15, wherein the controller is configured to receive definitions of first and second solder bumps having different, respective first and second compositions, and wherein one or more donor films comprising a plurality of different solder materials are disposed on the second surface of the donor substrate, wherein the solder materials are selected to produce the first and second compositions.

19. The system of claim 15, wherein the definition specifies a composition of the solder bumps comprising different, first, and second materials, and

wherein the transparent donor substrate includes a first donor film and a second donor film respectively including the first material and the second material,

wherein the controller is configured to drive the printing station to direct the first and second sequences of pulses to impinge on the first and second donor films, respectively, such that the droplets deposited at the target location cumulatively reach the specified composition, and

wherein the defining specifies a gradient of the material in the composition of the solder bumps, and wherein the controller is configured to drive the printing station to deposit the droplets of the first material and the second material in multiple layers on the target locations according to the specified gradient.

20. The system of claim 15, wherein the controller is configured to drive the printing station to deposit the droplets in multiple layers on the target location so as to reach the specified bump volume.

21. The system of claim 20, wherein the controller is configured to drive the printing station and the reflow station to alternately deposit a layer of droplets a plurality of times and heat the layer to melt the droplets until the specified bump volume is reached.

22. The system of claim 15, wherein the definition specifies a shape of the solder bump, and wherein the controller is configured to drive the printing station to direct the sequence of the pulses so as to deposit the molten droplets in a pattern conforming to the specified shape.

23. The system of claim 15, wherein the printing station is configured to print a conductive pad at the target location on the acceptor substrate using a process of laser-induced forward transfer (LIFT), and deposit the molten droplet of the solder material on the printed conductive pad.

24. The system of claim 23, wherein the printing station is configured to print the conductive pad having a concave surface for depositing the molten droplet therein.

25. A method for circuit fabrication, comprising:

depositing solder material at one or more target locations on a circuit substrate; and

one or more pulses of a laser beam are focused onto each of the target locations with sufficient energy to melt and reflow the deposited droplets in order to form solder bumps.

26. The method of claim 25, wherein depositing the solder material comprises: ejecting the molten droplets of solder material toward the one or more target locations.

27. The method of claim 25, wherein the pulses have a pulse duration of no greater than 1 ms.

28. The method of claim 27, wherein the pulse has a pulse duration of not less than 100 μ β.

29. The method of claim 25, wherein the pulse has a pulse energy of no greater than 3 mJ.

30. The method of claim 25, wherein focusing the one or more pulses comprises: a single, respective pulse of the laser beam is focused onto each of the target locations.