JP2023529317A - high resolution soldering - Google Patents

Abstract

The present invention relates to a circuit manufacturing method that includes defining solder bumps comprising a particular solder material and having a particular bump volume for formation at target locations on an acceptor substrate. A transparent donor substrate with a donor film containing a specified solder material is positioned such that the donor film is proximate a target location on the acceptor substrate. A sequence of pulses of laser radiation is directed through the first surface of the donor substrate to reach the donor film, thereby inducing the ejection of a fixed number of molten droplets of solder material from the donor film to a target location on the acceptor substrate such that the droplets deposited at the target location cumulatively reach a specified bump volume. The target locations are heated so that the deposited droplets melt and remelt to form solder bumps.

Description

The present invention relates generally to electronic device manufacturing, and more particularly to methods and systems for soldering.

In laser direct-write (LDW) technology, a laser beam is used to create patterned surfaces with spatially resolved three-dimensional structures by controlled material ablation or deposition. used for Laser-induced forward transfer (LIFT) is an LDW technique that can be applied in depositing micropatterns on surfaces.

In LIFT, laser photons provide the driving force to catapult small amounts of material from a donor film towards an acceptor substrate. Typically, the laser beam interacts with the inside of a donor film that is coated onto a non-absorbing carrier substrate. In other words, an incident laser beam propagates through the transparent carrier substrate before photons are absorbed by the inner surface of the film. Above a certain energy threshold, material is emitted from the donor film towards the surface of the acceptor substrate. Given proper 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 (but not exclusively) useful for printing conductive metal droplets and traces for electronic circuit manufacturing purposes. A LIFT system of this kind is described, for example, in US Pat. No. 9,925,797, the disclosure of which is incorporated herein by reference. This patent provides a transparent donor substrate having opposed first and second surfaces, and a donor film formed on the second surface for proximity to a target area on an acceptor substrate. A printing apparatus is described that includes a donor supply assembly configured to. The optical assembly simultaneously directs multiple output beams of laser radiation in a predetermined spatial pattern to pass through a first surface of the donor substrate, impinge on the donor film, and pass from the donor film to the acceptor substrate accordingly. It is configured to induce the release of material, thereby writing a predetermined pattern into a target area of the acceptor or substrate.

U.S. Pat. No. 9,925,797

Embodiments of the invention described below provide improved methods and systems for manufacturing electrical circuits and devices.

According to an embodiment of the present invention, a method for circuit fabrication is provided that includes a specific solder material formed at a target location on an acceptor substrate, and a specific bump volume. defining a solder bump having a . A transparent donor substrate having opposing first and second surfaces and a donor film containing a particular solder material on the second surface is positioned such that the donor film is proximate a target location on the acceptor substrate. placed. A series of pulses (a sequence of pulses) of laser radiation (laser beam) passes through the first surface of the donor substrate and strikes the donor film to transfer some of the solder material from the donor film to target locations on the acceptor substrate. Ejection of the molten droplets is induced and directed such that the droplets deposited on the target location cumulatively reach the designated bump volume. The target locations are heated so that the deposited droplets melt and reflow to form solder bumps.

Typically, the droplets have respective droplet volumes that depend on the intensity of the pulse of laser radiation, and directing the sequence of pulses (continued) is the designated bump. Setting the intensity of the pulses of laser radiation and the number of pulses in the sequence in response to the volume. In disclosed embodiments, the droplet volume is further dependent on a set of pulse parameters consisting of the spot size and duration of the pulses of laser radiation, and directing the sequence of pulses further: Including adjusting the drop volume by varying one or more of the pulse parameters.

In some embodiments, defining solder bumps includes 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. Directing the sequence of pulses, including defining two solder bumps, includes directing different things. The first and second sequences of pulses are different on the donor substrate such that the droplet cumulatively reaches each of the different first and second bump volumes at the respective first and second target locations. pass through the points. In one embodiment, defining the first and second solder bumps includes specifying different respective first and second compositions of the first and second solder bumps, and positioning the transparent donor substrate. Doing includes providing one or more donor films comprising a plurality of different solder materials selected to produce the first and second compositions. 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 comprises: , providing one or more donor films comprising a plurality of different solder materials to produce first and second compositions.

Additionally or alternatively, defining the solder bumps includes identifying a composition of the solder bumps comprising different first and second materials, and positioning the transparent donor substrate includes the first and second materials, respectively. Providing first and second donor films comprising a second material. Directing (directing) the sequence of pulses includes directing (directing) the first and second sequences of pulses to impinge the first and second donor films, respectively, resulting in: The droplets deposited at the target location cumulatively reach the specified composition. In one embodiment, identifying the composition includes identifying a gradient of material in the composition of the solder bump, and directing the first and second sequences of pulses comprises identifying the identified gradient depositing droplets of the first and second materials in multiple layers on the target location.

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

In disclosed embodiments, heating the target location includes depositing alternating layers of droplets and heating the layers to melt the droplets multiple times until a specified bump volume is reached. including.

Additionally or alternatively, defining the solder bump includes specifying a shape of the solder bump, such as a non-circular shape, and directing the sequence of pulses conforms to the specified shape. depositing molten droplets in a pattern that
In a further embodiment, 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 (directing) a laser beam involves focusing one or more laser pulses at a target location.

In some embodiments, the method includes printing conductive pads at target locations on an acceptor substrate using a process of laser-induced forward transfer (LIFT), directing a sequence of pulses ( The commanding step includes depositing molten droplets of solder material on the printed conductive pads. In disclosed embodiments, printing the conductive pads includes forming recesses in the conductive pads for depositing molten droplets therein.

According to an embodiment of the present invention, a controller configured to receive a definition of a solder bump comprising a particular solder material and having a particular bump volume to be formed at a target location on an acceptor substrate; A system for circuit manufacturing is also provided. The print station includes a transparent donor substrate having opposed first and second surfaces and having a donor film containing a particular solder material disposed on the second surface. and the donor film is positioned adjacent to the target location on the acceptor substrate. The laser directs a series of pulses (a sequence of pulses) of laser radiation through the first surface of the donor substrate and impinges on the donor film, forming molten droplets of solder material. It is configured to release from the donor film onto target locations on the acceptor substrate. The controller is configured to drive the print 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. be. The reflow station is configured to heat the target locations so that the deposited droplets melt and reflow to form solder bumps.

Additionally, according to some embodiments of the present invention, the steps of depositing a solder material onto one or more target locations on a circuit board, and melting and reflowing the deposited droplets to form solder bumps. and focusing one or more pulses of a laser beam onto each of the target locations with sufficient energy for circuit fabrication.

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

In some embodiments, the pulse has a pulse duration of 1 ms or less, and in some cases less than 100 μs.

Additionally or alternatively, the pulse has a pulse energy of 2 mJ or less.
Additionally or alternatively, the pulse has a pulse energy of 3 mJ or less. In disclosed embodiments, 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 its embodiments in conjunction with the drawings:

FIG. 1 is a block diagram that schematically illustrates a system for electronic circuit manufacturing, according to one embodiment of the present invention; FIG. 2A is a schematic front view of a printed circuit board with droplets of solder deposited in a LIFT process, according to one embodiment of the present invention; 2B is a schematic front view of the printed circuit board of FIG. 2A after solder reflow, according to one embodiment of the present invention; FIG. 3A is a schematic cross-sectional view of a circuit board showing successive stages in the solder bump deposition and reflow process, according to one embodiment of the present invention; FIG. 3B is a schematic cross-sectional view of a circuit board showing successive stages in the solder bump deposition and reflow process, according to one embodiment of the present invention; FIG. 3C is a schematic cross-sectional view of a circuit board showing successive stages in the solder bump deposition and reflow process, according to one embodiment of the present invention; FIG. 3D is a schematic cross-sectional view of a circuit board showing successive stages in the solder bump deposition and reflow process, according to one embodiment of the present invention; FIG. 4A is a schematic cross-sectional view of a circuit board on which droplets of two different solder materials have been deposited with a LIFT process, according to one embodiment of the present invention; 4B is a schematic front view of the circuit board of FIG. 4A after reflow of the solder material, according to one embodiment of the present invention; FIG. 5A is a photomicrograph of a contact pad formed by a LIFT process, according to one embodiment of the invention; and FIG. 5B is a photomicrograph of solder bumps formed on the contact pads of FIG. 5A, according to one embodiment of the present invention.

<Overview>
In a method of electronic circuit fabrication known in the art, electrical traces and contact pads are printed on a circuit board, and a solder layer is printed on the contact pads by photolithographic methods. A circuit component is then placed on the solder-covered pad and the circuit is heated to melt and reflow the solder, thus creating a conductive bond between the component and the conductive pad. In this conventional approach, the pad position and size and the volume of solder material on each contact pad are determined by a photolithographic mask and solder deposition process.

Embodiments of the present invention can produce solder bumps of virtually any desired size and shape on demand, including virtually any suitable solder material or material combination, LIFT ( A laser-induced forward transfer) based solder deposition method is provided. This LIFT-based method can fabricate multiple bumps with different volumes, shapes, sizes, and even different solder materials and combinations of solder materials, all on the same substrate with the same process steps. Solder bump volume and composition (including non-uniform composition) can be precisely controlled by setting the LIFT parameters, the donor film material, and the number of droplets deposited at each target location. Moreover, in contrast to conventional approaches, the method is capable of printing solder bumps on non-uniform substrates as well as on substrates with components already placed thereon. Thus, the disclosed embodiments provide much greater circuit fabrication flexibility and precision than techniques known in the art.

In the embodiments described below, solder bumps are defined with respect to a particular solder material and bump volume, and target locations where the bumps are to be formed on the acceptor substrate. A transparent donor substrate having a donor film containing a specific solder material on one of its surfaces is placed with the donor film proximate a target location on the acceptor substrate. (For convenience, the surface of the donor substrate proximate to the acceptor substrate is referred to herein as the bottom surface, and the surface opposite the donor substrate as the top surface.) A laser emits a series of pulses of laser radiation (a laser beam) to the donor substrate. A few molten droplets of solder material are directed (directed) to pass over the top surface of the substrate and impinge on the donor film to induce the ejection of several molten droplets of solder material from the donor film to target locations on the acceptor substrate.

The laser pulse parameters and the number of pulses in the sequence are selected such that the droplet deposited on the target location cumulatively reaches the specified bump volume. Pulse parameters that can be varied to control droplet volume include pulse intensity, ie, optical power per unit area incident on the donor film, as well as spot size and pulse duration. These parameters are adjusted according to the type and thickness of ^the solder material in the donor film to yield consistent droplet volumes on the order of 0.1 pl (picoliters), i. Accurately directional ejection of droplets from the donor film at high speed can be ensured. In this way, by appropriately selecting the pulse parameters and the number of droplets, it is possible to print solder bumps of an accurate size with a small diameter of about 20 μm. Processes according to embodiments of the present invention can be used to print a wide variety of solder materials, including low, medium, and high temperature solders, on a wide variety of substrates, and facilitate fluxless soldering.

After depositing one or more layers of droplets on the acceptor substrate, the target location is heated such that the deposited droplets melt and reflow to form solder bumps. Heating is advantageously done locally, for example by laser irradiation, to facilitate fast reflow and minimize damage to the substrate. The laser pulses used at this stage can be narrowly focused onto the solder bumps, with laser pulse durations not exceeding about 1 millisecond and in most cases less than 100 microseconds, e.g. Must be microseconds (or even shorter for small solder bumps). Therefore, this laser-driven reflow technique can be performed in ambient air (air atmosphere) and is suitable for thermally sensitive substrates. It is particularly well suited for use in conjunction with the LIFT-based solder printing technique described above; may apply. Alternatively, however, the reflow step may be performed by heating the entire acceptor substrate in, for example, a high temperature oven. After the solder bump shave is formed, circuit components can be placed on the bumps and soldered in place by conventional methods.

<Description of the system>
FIG. 1 is a schematic diagram of a system 20 for electronic circuit manufacturing, according to one embodiment of the present invention. The system 20 includes a printing station 22 that receives a definition of solder bumps 60 containing a particular solder material and having a particular bump volume to be formed at target locations on the acceptor substrate 34 . The print station deposits several droplets 32 of the desired solder material onto each target location such that the droplets cumulatively reach the specified bump volume. A reflow station 24 heats the target location so that the deposited droplets 32 melt and reflow to form solder bumps 60 . This heating process can be localized at the bump locations, as shown in FIG. 1, or can be global across the substrate 34, depending on the properties of the solder material and substrate and other application requirements.

Typically, after forming the solder bumps 60, a placement station 26 places components 64 on the solder bumps using, for example, a pick and place machine 62, as is known in the art. do. A heat source 66 in final reflow station 28 then heats the solder bumps to form permanent bonds 68 between the component and substrate 34 . Heat source 66 may apply localized heating using, for example, a laser, or may comprise a reflow oven or any other suitable type of heater known in the art. Bonds 68 typically form both electrical and mechanical connections between components 64 and conductive traces on substrate 34 . Alternatively or additionally, solder bumps 60 may be arranged to form a frame having a rectangular, circular or other shape to form a mechanical seal around the edges of component 64. . This type of encapsulation can be used, for example, for hermetic packaging of sensitive devices such as micro-electro-mechanical systems (MEMS) devices.

Referring back to the print station 22, an optical assembly 30 within the print station, under the control of a controller 51, emits short pulses of optical radiation having a pulse duration of approximately 1 ns (nanoseconds) onto the donor foil. 44 is provided with a laser 38. (As used in the context of this specification and claims, the term "optical radiation" refers to electromagnetic radiation in any of the visible, ultraviolet and infrared ranges; "laser radiation" , refers to the optical radiation emitted by a laser.) Controller 51 typically comprises a general purpose computer or dedicated microcontroller, which is a suitable interface to other elements of system 20 and is software driven. , perform the functions described herein. Donor film 44 typically comprises a thin flexible sheet of transparent donor substrate 46 which is coated adjacent to acceptor substrate 34 with donor film 48 containing a particular solder material. Alternatively, the donor substrate may comprise rigid or semi-rigid materials. Acceptor substrate 34 may comprise any suitable material such as glass, ceramic, or polymer, as well as other dielectric, semiconductor, or even conductive materials.

Optical assembly 30 directs beam deflector 40 and one or more output beams of radiation from laser 38 according to a spatial pattern determined by controller 51 over the top surface of donor substrate 46 and thus the donor substrate on the bottom surface. and focusing optics 42 directed to impinge on film 48 . In an exemplary embodiment, beam deflector 40 is shown in FIG. 2A or 2B of US Pat. Including vessel. The laser typically emits a train of pulses of appropriate wavelength, duration and energy to induce the ejection of molten droplets 50 of solder material from the donor film 48 to designated target locations on the acceptor substrate 34. controlled to output. Droplets 50 are ejected from donor film 48 at high velocity in a direction perpendicular to donor substrate 46 such that donor film 44 is a small distance, e.g. and acceptor substrate 34 with a spacing of up to about 0.5 mm. Due to the high speed ejection of the droplets 50 (typically 10 m/sec or greater), the flight time of the droplets is less than the time it takes for the droplets to solidify, and the print station 22 operates in ambient air rather than under vacuum. Able to work under conditions.

Donor film 48 may comprise virtually any suitable type of solder material or combination of solder materials, including low temperature solders, medium temperature solders, 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 contain alloys of silver (typically 45-90%) with other metals such as copper, zinc, tin and cadmium and typically melts at temperatures in the range of 700-950°C. The thickness and composition of film 48 and the pulse parameters of optical assembly 30 are adjusted according to the choice of solder material to stabilize molten droplet 50 of solder material to a target location on acceptor substrate 34 . resulting in a smooth jet.

In some embodiments, a multi-layered donor film 48 can be used to deposit droplets 32 of mixed composition. For example, donor film 44 may include multiple layered donor films containing different respective solder compositions to produce melt droplets 50 containing bulk mixtures of different materials. A multi-component LIFT scheme of this kind is described, for example, in US Pat. No. 10,629,442, the disclosure of which is incorporated herein by reference.

Alternatively or additionally, donor film 44 may include donor film 48 comprising different materials at different locations on the donor foil. The optical assembly 30 directs a sequence of laser pulses to each impinge on a different donor location such that droplets 32 of different materials deposited at a given target location cumulatively reach a specified volume and composition. to (direct). This type of mixed composition scheme is further described below with reference to Figures 4A/B.

The controller 51 controls the pulsing parameters of the laser 38 and the scanning and scanning of the optical assembly 30 to deposit the appropriate number of droplets 32 of the desired volume onto each target location where solder bumps are formed on the acceptor substrate 34 . Adjust the focusing parameters. As previously explained, the controller 51 sets the laser pulse parameters and the number of melted droplets of solder material so that the droplets deposited at each target location cumulatively reach the specified bump volume at that location. to reach Drop volume can be varied by adjusting the laser pulse parameters, so that a given bump volume produces a smaller number of larger volume drops, or a larger number of smaller volume drops. It can be made by depositing. The thickness of the donor film 44 also depends on the droplet size. The actual drop volume has inherent tolerances. It may be advantageous to rely on large-numbers statistics. Especially when dealing with small bumps, it may be preferable to have a large number of small droplets rather than a small number of large droplets.

The print station 22 also includes a positioning assembly that may include, for example, an XY stage 36 on which an acceptor substrate 34 is mounted. Stage 36 shifts acceptor substrate 34 relative to optical assembly 30 and donor film 44 at print station 22 to deposit droplets 32 at different target locations across the surface of the acceptor substrate. Additionally or alternatively, the positioning assembly can include motion components (not shown) to shift the optical assembly 30 as well as the donor film 44 over the surface of the acceptor substrate, where appropriate.

Reflow station 24 includes an optical assembly 52 that directs a beam of radiation to locally melt droplets 32 , thereby coalescing the droplets into solder bumps 60 . This type of localized heating is particularly advantageous in avoiding damage to the sensitive acceptor substrate 34 . The optical assembly 52 in the depicted example comprises a laser 54, a beam deflector 56 and focusing optics 58, which provide sufficient energy to melt and reflow the deposited droplets onto the solder bumps 60. Direct the laser radiation to illuminate the target location at . Reflow station 24 also includes a positioning assembly, which may be based on the same stage 36 as printing station 22, or a different stage or other motion (moving, driving) device.

Controller 51 adjusts the pulsing parameters of laser 54 and the scanning and focusing parameters of optical assembly 52 to apply sufficient energy to melt and reflow each solder bump 60 while avoiding damage to substrate 34 . do. The pulse duration and energy are selected to completely melt the solder material on the bottom of each bump without vaporizing the solder material on the top of the bump. The actual power and pulse duration required will depend on the melting temperature and thermal conductivity of the solder material. A short laser pulse is generally advantageous for this purpose as it minimizes the time the solder material is melted, thus minimizing oxidation and avoiding damage to the substrate 34 . Therefore, reflow station 24 can operate in ambient atmospheric conditions. Short, high power laser pulses are particularly advantageous in enabling fluxless reflow and supporting the use of high temperature solder materials. A single laser pulse of this kind focused on the location of each solder bump is typically sufficient to achieve complete reflow of small solder bumps, but for larger solder bumps in particular, multiple laser pulses are sufficient. pulses may be used instead. The resulting high speed localized reflow process also reduces the formation of intermetallic compounds between the solder bumps and contact pads, thus reducing thermal reflow methods known in the art. to produce a stronger solder bond.

For example, for a small solder bump made from a pile (one set) of tin-based solder droplets 32 with a thickness of 20-30 μm, a laser pulse with an optical power of about 10 W and a duration of 50-100 μs Typically, it is sufficient to achieve complete reflow while avoiding substantial heat diffusion into the substrate. Optimal laser wavelength, pulse power, duration, and focal spot size can be chosen to match the absorption spectrum, volume, and thermal properties of the solder material in each case. For small to medium size solder bumps, the pulse energy should be about 2 mJ or less. Optimal laser parameters can be determined empirically and/or based on thermal and fluid dynamics simulations, for example using finite element analysis tools known in the art.

In the example below, laser 54 in reflow station 24 may be a high power CW Nd:YAG laser operating at 1064 nm. Alternatively, laser 54 may be a diode-pumped fiber laser, such as an Ytterbium continuous wave fiber laser in the range of 976 nm to 1075 nm, and a few watts of power (eg, available from IPG). Alternatively, 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 description.

In an exemplary implementation, print station 22 prints the bumps using tin-based solder. Laser 54 has a pulse energy of about 0.45-1.6 mJ and a duration of 50-150 μs to reflow a bump having a volume of about 40 pL (picoliters, corresponding to a bump diameter of about 50 μm). It is set to output a pulse. Optical assembly 52 focuses the beam to a spot size of approximately 35-50 μm on the solder bumps. On the other hand, for smaller bumps, eg, having a volume of about 15 pi (corresponding to a diameter of about 25 μm), the laser 54 in the reflow station 24 has a pulse energy of about 0.2-0.45 mJ and a duration of 10-30 μs. It is set to output a timed pulse, focused to a spot size of about 15-25 μm on the solder bump.

In another example, to reflow a bump having a volume of about 85 pL (corresponding to a bump diameter of about 100 μm), laser 54 emits pulses with pulse energies of about 1-3 mJ and durations of 50-150 μs. set to output. Optical assembly 52 focuses the beam to a spot size of approximately 50-100 μm on the solder bumps.

Alternative selections of laser-driven reflow parameters will be apparent to those skilled in the art after reading this description.

In one embodiment, print station 22 and reflow station 24 are combined into a single operating unit along with an optical assembly that provides laser radiation for both the LIFT and reflow processes. The same laser source can be used for both purposes as long as the laser source can provide different ranges of pulse energies and durations required for LIFT and reflow. Alternatively, the docking 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 Figures 2A/B. Figures 2A/B schematically illustrate a process of forming solder bumps on a printed circuit board 70, according to an embodiment of the present invention. FIG. 2A is a front view of a substrate 70 on which droplets of solder 32 have been deposited by a LIFT process, for example at the print station 22 (FIG. 1), and FIG. 2B is a schematic front view of the substrate 70 after solder reflow. be. This embodiment defines and fabricates 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). It shows the use of the techniques described herein in practice.

As shown in FIG. 2A, prior to depositing solder bumps on 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 onto the substrate 70 using a photolithographic process, as is known in the art, or alternatively printed onto the substrate 70 using, for example, a LIFT process. May be written directly. LIFT printing of the contact pads can be advantageous to enhance adhesion of the solder material to the contact pads, as further described below with reference to FIGS. 5A/B. Controller 51 is programmed to specify different solder bump volumes to be produced on different solder pads. The controller drives the optical assembly 30 to pulse the laser through different points on the donor substrate such that the deposited droplets 32 cumulatively reach the specified bump volumes on the various pads. Orient a different sequence of . Thus, for example, only a single droplet 32 or a few droplets are deposited on each of the pads 72 and a larger cluster 74 of droplets is deposited on the pad 75 . If very fine contact is required, as is the case with pads 72, it is also possible to deposit droplets directly onto target locations on traces 73 without dedicated contact pads, thus leaving components on the traces. Solder directly. As previously mentioned, by proper selection and configuration of donor film 48, printing station 22 can be controlled to print different respective compositions of solder material onto different contact pads. For example, the print station 22 may apply a suitable low temperature solder to the delicate contacts on the pads 75 while printing higher temperature solder on the pads 75 designed to carry higher operating currents in the operation of the circuit on the substrate 70. 72 can be printed. Optical assembly 30 directs laser pulses through appropriate points on donor substrate 46 to deposit solder material of the appropriate composition on each of the contact pads or locations.

Controller 51 can also be programmed to specify different shapes of solder bumps, including non-circular shapes such as elongated shapes defined by 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. make it An elongated mass 76 of droplets is thus deposited on the contact pad 77 . Solder contacts can be printed in this manner in virtually any desired shape, including, for example, annular and angled shapes.

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. The tendency of the droplets at this stage is to coalesce into a spherical shape that minimizes surface energy. To minimize this tendency, particularly when making non-circular shaped solder bumps, the reflow station 24 applies short, intense laser pulses to locally melt the solder bumps, as described above. can do. Laser pulse parameters and irradiation patterns at reflow station 24 can be adjusted to achieve desired shape characteristics.

3A, 3B, 3C, and 3D are schematic cross-sectional views of a circuit board showing successive stages in the process of depositing and reflowing solder bumps 94 on substrate 70, according to another embodiment of the present invention. This embodiment addresses reflow problems that can occur, especially with large solder bumps. If 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 damage to the substrate surrounding the solder bumps. On the other hand, if the laser pulse energy is not sufficient, the droplet at the bottom of the bump may not melt completely, resulting in poor contact integrity and increased electrical resistance.

To address this issue, the droplets 32 are deposited in multiple layers on the target location to reach the specified bump volume. The substrate 70 is shuttled between the print station 22 and the reflow station 24 multiple times to deposit alternating layers of droplets and then layers to melt the droplets until the specified bump volume is reached. heat up. Alternatively, LIFT printing and fusing of droplets may be performed in a single station, where the optical assembly has the capabilities required for both LIFT printing and reflow. In either case, the energy that must be applied to melt each successive layer of droplets is relatively small, thus reducing the risk of damage.

Thus, in the illustrated 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). This layer is heated and thus melted to form reflow layer 92, as shown in FIG. 3B. A further layer of droplets 32 is deposited over the reflowed layer 92, as shown in FIG. 3C, and then heated to reflow again, as shown in FIG. 3D. This process is repeated for the number of cycles required to produce solder bumps 94 .

Referring now to Figures 4A/B, Figures 4A/B schematically illustrate a process for making mixed composition solder bumps 100 on a substrate 70, according to an embodiment of the present invention. FIG. 4A is a cross-sectional view showing droplets 96 and 98 of two different respective solder materials deposited by the LIFT process at print station 22 . 4B is a front view of solder bump 100 after reflow of the solder material at reflow station 24. FIG.

Controller 51 receives specifications for solder bumps 100 that indicate that the solder bumps contain two (or more) different materials in fixed proportions. For example, the solder bumps may contain particles of copper mixed with tin solder or palladium mixed with SAC solder to enhance mechanical strength and/or conductivity. In some cases, it may also be advantageous to unevenly distribute different materials within the solder bumps with a particular gradient of materials. For example, one of the materials, such as the material in droplet 96, may have a higher concentration at the bottom of the solder bump and a concentration at the top of the solder bump compared to the material in droplet 98. decrease towards This type of gradient composition of palladium and copper with higher palladium concentration at the bottom of the bump is believed to improve the strength of solder joints, as described, for example, in US Pat. No. 9,607,936.

In the example shown in Figure 4A, donor film 44 includes two donor films 48, which include two different donor materials, such as the different types of materials described 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 at each of the donor films is selected such that droplets 96 and 98 are deposited in the proper proportions to cumulatively reach the specified composition and total solder bump volume. To generate a gradient composition, the ratio of pulses directed at the two donor films, and thus the ratio of droplets 96 to droplets 98, is adjusted layer by layer from the bottom to the top of the droplets, as shown in FIG. 4A. Change. Rapid heating of the collection of droplets 96 and 98 in the reflow station 24 causes the droplets to solder bumps 100 with minimal mixing such that the specified gradient is maintained, as shown schematically in FIG. 4B. will coalesce into

This multi-material deposition of solder bumps may also be useful in other applications. For example, the bottom layer of the solder bumps can be made by printing droplets of a material that improves solder wetting or, alternatively, limits solder wetting. As another example, the bottom layer may be selected to improve the thermal expansion coefficient match between the substrate and the solder material. This property can be fine-tuned by mixing two materials with different coefficients of thermal expansion to match the properties of the substrate.

FIG. 5A is a photomicrograph of contact pads 110 formed on substrate 112 by a LIFT process, according to one embodiment of the present invention. In other words, the contact pads are written directly onto the substrate 112 by LIFT using, for example, a suitable copper donor film 48, rather than creating contact pads by conventional photolithographic printing. LIFT printing of contact pads 110 is useful for controlling the shape and texture of the contact pads to improve adhesion of solder bumps to the pads. Thus, as shown, the contact pad 110 has a roughened surface with a recess 114 in the center of the pad.

FIG. 5B is a micrograph showing solder bumps 116 formed on contact pads 110, according to one embodiment of the present invention. Solder bumps 116 are formed by print station 22 as described above by deposition of droplets of solder material in recesses 114 followed by melting of the droplets in reflow station 24 . The contact pad roughness provides an increased surface area for adhesion of the solder material to the pad and, together with the recesses, helps ensure good electrical and mechanical contact.

It will be appreciated that the above-described embodiments are given by way of example, and the invention is not limited to what has been specifically shown and described above. Rather, the scope of the invention encompasses both combinations and subcombinations of the various features described above, as well as variations and modifications thereof not disclosed in the prior art that may occur to those skilled in the art upon reading the foregoing description. include.

Claims (30)

A circuit manufacturing method comprising:
defining solder bumps comprising a particular solder material and having a particular bump volume to be formed at target locations on the acceptor substrate;
A transparent donor substrate having opposing first and second surfaces and a donor film comprising a specific solder material on the second surface is placed such that the donor film is proximate a target location on the acceptor substrate. and placing it in
Directing a sequence of pulses of laser radiation through the first surface of the donor substrate and striking the donor film to deposit several molten droplets of solder material from the donor film to target locations on the acceptor substrate. so that droplets deposited at the target location cumulatively reach a specified bump volume;
heating the target location so that the deposited droplets melt and reflow to form solder bumps;
including,
Circuit manufacturing method. The droplets have respective droplet volumes that depend on the intensity of the pulses of laser radiation, and directing the sequence of pulses varies the intensity of the pulses of laser radiation and within the sequence according to the specified bump volume. including setting the number of pulses,
The method of claim 1. The droplet volume is further dependent on a set of pulse parameters consisting of the spot size and duration of the pulses of laser radiation, and directing the sequence of pulses is achieved by varying one or more of the pulse parameters. further comprising adjusting the volume;
3. The method of claim 2. Defining the solder bumps defines 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 directing the sequence of pulses such that the droplets cumulatively reach each of the different first and second bump volumes at the respective first and second target locations. directing the first and second sequences of different pulses through different points on the
The method of claim 1. Defining the first and second solder bumps includes specifying different respective first and second compositions of the first and second solder bumps, and disposing the transparent donor substrate includes: providing a donor film comprising a plurality of different solder materials selected to produce first and second compositions;
5. The method of claim 4. 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 first and second solder bumps. providing one or more donor films comprising a plurality of different solder materials to produce a composition of
The method of claim 1. Defining the solder bumps includes specifying a composition of the solder bumps comprising different first and second materials, and disposing the transparent donor substrate comprises first and second materials comprising the respective first and second materials. directing the sequence of pulses including providing first and second donor films, directing the first and second sequences of pulses to impinge the first and second donor films, respectively; 2. The method of claim 1, comprising cumulatively reaching a particular composition of droplets deposited at the target location. Identifying the composition includes identifying a gradient of the material in the composition of the solder bump, and directing the first and second sequences of pulses is performed on the target location according to the identified gradient. depositing droplets of the first material and the second material on a layer of
8. The method of claim 7. directing the sequence of pulses includes depositing the droplets in multiple layers on the target location to reach a specified bump volume;
The method of claim 1. heating the target location comprises alternating multiple times depositing a layer of droplets and heating the layer to melt the droplets until a specified bump volume is reached;
10. The method of claim 9. defining the solder bump includes specifying a shape of the solder bump, and directing the sequence of pulses includes depositing the molten droplets in a pattern conforming to the particular shape;
The method of claim 1. Heating the target location includes directing the laser to irradiate the target location with sufficient energy to melt and reflow the deposited droplets;
The method of claim 1. A process of laser-induced forward transfer (LIFT) is used to print conductive pads at target locations on the acceptor substrate, and directing a sequence of pulses causes molten droplets of solder material to form on the printed conductive pads. depositing on the pad;
The method of claim 1. printing the conductive pads includes forming recesses for depositing molten droplets in the conductive pads;
14. The method of claim 13. A system for circuit manufacturing, comprising:
a controller configured to receive a definition of a solder bump comprising a specified solder material and having a specified bump volume to be formed at a target location on an acceptor substrate;
a printing station,
A transparent donor substrate having opposed first and second surfaces and a donor film comprising a particular solder material disposed on the second surface, the donor film overlying the acceptor substrate. a transparent donor substrate positioned proximate to the target location;
A sequence of pulses of laser radiation is directed through the first surface of the donor substrate and impacts the donor film to eject molten droplets of solder material from the donor film toward target locations on the acceptor substrate. a laser configured to
a printing station comprising
a reflow station configured to heat a target location such that deposited droplets melt and reflow to form solder bumps;
including
a controller configured to drive the print station to eject droplets toward the target location such that the droplets deposited at the target location cumulatively reach the specified bump volume;
A system for circuit manufacturing. The droplets have respective droplet volumes that depend on the intensity of the pulse of laser radiation and a set of pulse parameters consisting of the spot size and duration of the pulse of laser radiation, and the controller controls the bump volume to the specified bump volume. configured to set the intensity of the pulses of laser radiation and the number of pulses in the sequence accordingly, and the controller configured to adjust the droplet volume by varying one or more of the pulse parameters. to be
16. The system of claim 15. The controller is configured to receive definitions of 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. is,
A controller drives the laser to point in different directions, and the first and second sequences of pulses direct the droplets to different first and second bump volumes at respective first and second target locations. configured to pass through different points on the donor substrate to reach cumulatively;
The definitions of first and second solder bumps specify different respective first and second compositions of the first and second solder bumps, one or more donor films comprising a plurality of different solder materials comprising: disposed on the second surface of the donor substrate, the solder material being selected to produce first and second compositions;
16. The system of claim 15. The controller is configured to receive definitions of first and second solder bumps having different respective first and second compositions, wherein one or more donor films comprising a plurality of different solder materials are applied to the donor substrate. disposed on the second surface, the solder material being selected to produce the first and second compositions;
16. The system of claim 15. the definition specifies a solder bump composition comprising different first and second materials;
a transparent donor substrate comprising first and second donor films comprising first and second materials, respectively;
A controller drives the print station to direct first and second sequences of pulses to impinge the first and second donor films, respectively, so that droplets deposited at target locations accumulate. configured to reach a specifically specified composition,
The definition identifies a material gradient in the composition of the solder bumps, and the controller drives the print station to deposit droplets of the first and second materials in multiple layers on the target location according to the specified gradient. configured to deposit;
16. The system of claim 15. the controller is configured to drive the print station to deposit the droplets in multiple layers on the target location to reach the specified bump volume;
16. The system of claim 15. The controller drives the print station and reflow station to alternately deposit layers of droplets and heat the layers to melt the droplets multiple times until a specified bump volume is reached. configured to do
21. System according to claim 20. the definition defines the shape of the solder bump, and the controller is configured to drive the print station to direct the sequence of pulses to deposit molten droplets in a pattern conforming to the specified shape;
16. The system of claim 15. The printing station prints conductive pads on the target locations on the acceptor substrate using a process of laser-induced forward transfer (LIFT), and melt droplets of the solder material on the printed conductive pads. configured to deposit a
16. The system of claim 15. The printing station is configured to print a conductive pad having recesses for depositing molten droplets;
24. The system of claim 23. A circuit manufacturing method comprising:
depositing a solder material at one or more target locations on a circuit board;
focusing one or more pulses of the laser beam onto each of the target locations with sufficient energy to melt and reflow the deposited droplets to form solder bumps;
A circuit manufacturing method comprising: depositing the solder material includes projecting molten droplets of the solder material toward one or more target locations;
26. The method of claim 25. the pulse has a pulse duration of 1 ms or less;
26. The method of claim 25. the pulse has a pulse duration of less than 100 μs;
28. The method of claim 27. the pulse has a pulse energy of 3 mJ or less;
26. The method of claim 25. focusing the one or more pulses comprises focusing a single respective pulse of the laser beam onto each of the target locations;
26. The method of claim 25.

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