KR20230021667A - high resolution soldering - Google Patents

Abstract

A circuit manufacturing method includes defining solder bumps comprising a specified solder material and having a specified bump volume to be formed at target locations on an acceptor substrate. A transparent donor substrate having a donor film containing the designated solder material is positioned, the donor film positioned proximate to a target location on the acceptor substrate. a sequence of pulses of laser radiation directed to pass through a first surface of the donor substrate and impinge on the donor film to induce ejection of a plurality of molten droplets of solder material from the donor film onto a target location on the acceptor substrate; This causes the droplets deposited at the target location to cumulatively reach the designated bump volume. The target location is heated so that the deposited droplets melt and reflow to form solder bumps.

Description

high resolution soldering

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

In laser direct-write (LDW) technology, a laser beam is used to create a patterned surface with a spatially resolved three-dimensional structure by controlled material ablation or deposition. Laser-Induced Forward Transfer (LIFT) is a LDW technique that can be applied to deposit micropatterns on surfaces.

In LIFT, laser photons provide the driving force to catapult a small volume of material from a donor film towards an acceptor substrate. Typically the laser beam interacts with the inner side of the donor film that is coated on the non-absorbable 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. Above a certain energy threshold, material is ejected from the donor film towards the surface of the acceptor substrate. Given an appropriate choice of donor film and laser beam pulse parameters, the laser pulse causes molten droplets of the donor material to eject from the film and then land on the acceptor substrate and cure.

LIFT systems are particularly (but not exclusively) useful for printing conductive metal droplets and traces for the purpose of electronic circuit fabrication. 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 describes a printing device comprising a donor supply assembly having opposing first and second surfaces, the second surface for positioning a donor film proximate to a target area on an acceptor substrate. It is configured to provide a transparent donor substrate having a donor film formed thereon. An optical assembly to simultaneously direct multiple output beams of laser radiation into a predefined spatial pattern to impinge on the donor film and pass through the first surface of the donor substrate to direct ejection of material from the donor film onto the acceptor substrate. is configured, thereby recording a predefined pattern on the target area of the acceptor substrate.

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

Therefore, according to an embodiment of the present invention, there is provided a circuit manufacturing method, which defines a solder bump having a specified bump volume and containing a specified solder material to be formed at a target location on an acceptor substrate. include that A transparent donor substrate having a donor film containing the designated solder material is positioned, the donor film positioned proximate to a target location on the acceptor substrate. a sequence of pulses of laser radiation directed to pass through a first surface of the donor substrate and impinge on the donor film to induce ejection of a plurality of molten droplets of solder material from the donor film onto a target location on the acceptor substrate; This causes droplets deposited at the target location to cumulatively reach the specified bump volume. The target location is heated so that the deposited droplets melt and reflow to form solder bumps.

Typically, droplets have their own droplet volume that depends on the intensity of the pulses of laser radiation, and directing the pulse sequence involves setting the number of pulses in the sequence and the intensity of the pulses of laser radiation in response to a specified bump volume. . In the disclosed embodiments, the droplet volume also depends on a set of pulse parameters consisting of the spot size and duration of the pulses of laser radiation, and directing the pulse sequence does not involve adjusting the droplet volume by changing one or more of the pulse parameters. contains more

In some embodiments, defining solder bumps includes 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. comprising directing the pulse sequences to cause droplets to cumulatively reach different first and second bump volumes, respectively, at respective first and second target locations, wherein the different first and second pulse sequences are directed onto the donor substrate; This includes directing it through different points. In one embodiment, defining the first and second solder bumps further comprises specifying different respective first and second compositions of the first and second solder bumps, and positioning the transparent donor substrate comprises first and second solder bumps. and providing one or more donor films comprising a plurality of different solder materials selected to create 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 to create the first and second compositions. and providing one or more donor films comprising a plurality of different solder materials.

Also additionally or alternatively, defining the solder bumps includes specifying a composition of the solder bumps comprising different first and second materials, and positioning the transparent donor substrate comprises first and second materials respectively. comprising providing first and second donor films, wherein directing the pulse sequences comprises directing the first and second pulse sequences to the first and second donor films such that droplets deposited at the target location cumulatively arrive at a specified composition; It includes directing each to impinge on the film. In one embodiment, specifying the composition includes specifying a gradient of the material to the composition of the solder bump, and directing the first and second pulse sequences to a plurality of layers on a target location according to the specified gradient. and depositing droplets of the first and second materials with

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

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

Additionally or alternatively, defining the solder bump includes specifying a shape of the solder bump, such as a non-circular shape, and directing the pulse sequence includes depositing molten droplets in a pattern conforming to the specified shape.

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 the laser beam involves focusing one or more laser pulses on a target location.

In some embodiments, the method includes printing a conductive pad at a target location on an acceptor substrate using a laser induced forward transfer (LIFT) process, directing the pulse sequence to melt the solder material onto the printed conductive pad. This includes depositing the droplets. In the disclosed embodiment, printing the conductive pad includes forming recesses in the conductive pad for deposition of molten droplets therein.

Further, according to an embodiment of the present invention, a circuit manufacturing system is provided, wherein the circuit manufacturing system defines a solder bump having a designated bump volume and containing a designated solder material to be formed at a target location on an acceptor substrate. and a controller configured to receive. The printing station includes a transparent donor substrate, the transparent donor substrate having opposing first and second surfaces and having a donor film comprising a designated solder material disposed on the second surface, the donor film on the acceptor substrate It is positioned close to the target location. The laser directs a sequence of pulses of laser radiation to pass through the first surface of the donor substrate and impinge on the donor film to induce ejection of molten droplets of solder material from the donor film onto target locations on the acceptor substrate. is composed of The controller is configured to drive the printing station to eject a plurality of droplets toward the target location so that the droplets deposited at the target location cumulatively reach the specified bump volume. The reflow station is configured to heat the target location so that the deposited droplets are melted and reflowed to form solder bumps.

Additionally, according to an embodiment of the present invention, a circuit manufacturing method is provided, the circuit manufacturing method comprising depositing a solder material on one or more target locations on a circuit board, melting and reflowing the deposited droplets to form solder bumps. focusing one or more pulses of a laser beam onto each target location with sufficient energy to

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

In some embodiments, the pulse has a pulse duration of 1 ms or less, and possibly 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 the disclosed embodiment, focusing the one or more pulses includes focusing a respective single pulse of the laser beam onto each target location.

The invention will be more fully understood from the following detailed description of embodiments of the invention taken in conjunction with the drawings.

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

survey

In an electronic circuit manufacturing method 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 a photolithography method. A circuit component is then placed on the solder covered pad and the circuit is heated to melt and reflow the solder, thereby creating a conductive bond between the component and the pad. In this conventional approach, the pad location and size and volume of solder material on each contact pad are fixed by a photolithography mask and solder deposition process.

DETAILED DESCRIPTION OF THE INVENTION Embodiments of the present invention provide a LIFT-based method of solder deposition capable of producing solder bumps of substantially any desired size and shape and comprising substantially any suitable solder material or combination of materials as needed. do. This LIFT-based method can create multiple bumps with different volumes, shapes and sizes, and even containing different solder materials and solder material combinations, all 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. Also, in contrast to conventional approaches, the method can print solder bumps on non-uniform substrates, as well as on substrates on which components have already been placed. Thus, the disclosed embodiments provide much greater flexibility and precision in circuit fabrication than techniques known in the art.

In the embodiments described below, solder bumps are defined in terms of a specified 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 comprising a designated solder material on one of its surfaces is positioned proximate to a target location on the acceptor substrate. (For convenience, the surface of the donor substrate proximal to the acceptor substrate is referred to herein as the bottom surface, and the opposite surface of the donor substrate is referred to as the top surface.) To direct the ejection to the target location, the laser directs a sequence of pulses of laser radiation to pass through the top surface of the donor substrate and impinge on the donor film.

The laser pulse parameters and number of pulses in the sequence are selected so that droplets deposited at the target location cumulatively reach the specified bump volume. Pulse parameters that can be varied to control droplet volume include pulse intensity, i.e. light output per unit area incident on the donor film, as well as spot size and pulse duration. These parameters are set in the donor film to provide a constant droplet volume of the order of 0.1 pl (picoliter), ie, 100 μm ³ , or less, and to ensure ejection of droplets from the donor film at high speed with precise directionality towards the target location. can be adjusted according to the type and thickness of the solder material. Therefore, by proper selection of the pulse parameters and the number of droplets, it is possible to print precisely sized solder bumps having a diameter as small as about 20 μm. Processes according to embodiments of the present invention may be used to print a wide range of solder materials, including low-temperature, medium-temperature and high-temperature solders, onto a variety of substrates and also facilitate fluxless soldering.

After depositing one or more layers of droplets on the acceptor substrate, the target location is heated so that the deposited droplets melt and reflow to form solder bumps. Heating is advantageously performed locally, for example by laser irradiation, to drive fast reflow and minimize damage to the substrate. The laser pulses used in this step can be narrowly focused on the solder bumps, and the duration of the laser pulses is about 1 millisecond or less, in most cases less than 100 microseconds, for example tens of microseconds (or even smaller solder bumps). less cases). Therefore, this laser-driven reflow technique can be performed in air and is suitable for heat-sensitive substrates. It is particularly suitable for use with the LIFT-based solder printing techniques described above, but may alternatively be applied to reflow solder materials deposited by other methods such as inkjet type solder printing and photolithographic techniques. Alternatively, however, the reflow step can be performed by heating the entire acceptor substrate, for example in a high temperature oven. After the solder bumps are formed, circuit components may be placed on the bumps and soldered in place by conventional methods.

system description

1 is a schematic diagram of a system 20 for manufacturing electronic circuits in accordance with an embodiment of the present invention. The system 20 includes a printing station 22, wherein the printing station 22 includes a solder bump 60 containing a specified solder material and having a specified bump volume, to be formed at a target location on an acceptor substrate 34. receive the definition of The printing station deposits multiple droplets 32 of the desired solder material at each target location, such that the droplets cumulatively reach the specified bump volume. The reflow station 24 heats the target location so that the deposited droplets 32 are melted and reflowed to form solder bumps 60 . This heating process may be locally concentrated at the bump locations as shown in FIG. 1, or may be performed entirely over the entire substrate 34, depending on the characteristics of the solder material and substrate and other application requirements.

Typically, after formation of the solder bumps 60, the placement station 26 is placed on the solder bumps using, for example, a pick-and-place machine 62 as is known in the art. Place the component 64 on. A heat source 66 at the final reflow station 28 then heats the solder bumps to form a permanent bond 68 between the component and the substrate 34 . Heat source 66 may apply local heating using, for example, a laser, or heat source 66 may include a reflow oven or any other suitable type of heater known in the art. Bonds 68 typically form both electrical and mechanical connections between component 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 create a mechanical seal around the edges of component 64 . Seals of this kind can be used, for example, for hermetic packaging of sensitive devices such as Micro-Electromechanical Systems (MEMS) devices.

Referring again to the printing station 22, the optical assembly 30 at the printing station includes a laser 38, which under the control of the controller 51 is directed towards the donor foil 44 at a rate of about 1 ns. Directs short pulses of optical radiation with a pulse duration of (As used herein and in the claims, the term "optical radiation" refers to electromagnetic radiation in any range of the visible, ultraviolet, and infrared ranges, and "laser radiation" refers to optical radiation emitted by a laser.) Controller 51 typically includes a general purpose computer or special purpose microcontroller, which has an appropriate interface to other elements of system 20 and is software driven to perform the functions described herein. The donor foil 44 comprises a thin, flexible sheet of a donor substrate 46, typically transparent, coated on the side proximate the acceptor substrate 34 with a donor film 48 comprising a designated solder material or materials. Alternatively, the donor substrate may comprise a rigid or semi-rigid material. Acceptor substrate 34 may include any suitable material, such as glass, ceramic, or polymer, as well as other dielectric, semiconductor, or even conductive materials.

Optical assembly 30 includes beam deflector 40 and focusing optics 42, which direct one or more output beams of radiation from laser 38 to a donor substrate according to a spatial pattern determined by controller 51. It passes through the top surface of (46) and is therefore directed to impinge on the donor film (48) on the bottom surface. In an exemplary embodiment, beam deflector 40 includes an acousto-optic modulator, as shown in FIG. 2A or FIG. 2B of the aforementioned U.S. Patent No. 9,925,797 and described in columns 7-8 of this patent. The laser is typically a series of pulses of suitable wavelength, duration and energy to direct ejection of molten droplets 50 of solder material from the donor film 48 onto designated target locations on the acceptor substrate 34. It is controlled to output. Since the droplets 50 are ejected from the donor film 48 at high speed in a direction perpendicular to the donor substrate 46, the donor foil 44 is not in contact with the acceptor substrate but a small distance from the acceptor substrate 34. , for example, with a gap of up to about 0.5 mm between the donor film 48 and the acceptor substrate 34 . Due to the high velocity ejection of the droplet 50 (typically greater than 10 m/s), the droplet's flight time is less than the time it takes for the droplet to solidify, and the print station 22 can operate in ambient atmospheric conditions rather than under vacuum. there is.

Donor film 48 may include virtually any suitable type of solder material or combination of solder materials, including low-temperature, moderate-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 to fabricate high-power electronic devices contain alloys of silver (typically 45-90%) with other metals such as copper, zinc, tin and cadmium, typically in the range of 700-950 °C. melts at the temperature The thickness and composition of the film 48, as well as the pulse parameters of the optical assembly 30, are the parameters of the solder material to provide stable jetting of the molten droplet 50 of the solder material towards a target location on the acceptor substrate 34. Adjusted according to your choice.

In some embodiments, multi-layered and structured donor films 48 may be used to deposit the mixed composition droplets 32 . For example, donor foil 44 may include multiple layered donor films comprising different respective solder compositions to produce molten droplets 50 containing bulk mixtures of different materials. A multi-composition 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 foil 44 may include donor films 48 comprising different materials at different locations on the donor foil. The optical assembly 30 directs a sequence of laser pulses to impinge on different donor locations, respectively, so that droplets 32 of different materials deposited at a given target location cumulatively reach a specified volume and composition. Mixed composition schemes of this kind are further described below with reference to FIGS. 4A and 4B.

The controller 51 controls the pulse parameters of the laser 38 and the optical assembly to deposit an appropriate number of droplets 32 in a desired volume on each target location where a solder bump is to be formed on the acceptor substrate 34. Adjust the scanning and focusing parameters of (30). As described above, 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. By adjusting the laser pulse parameters the droplet volume can be varied so that a given bump volume can be created by depositing a smaller number of larger volume droplets or a larger number of smaller volume droplets. The thickness of the donor film 38 also contributes to droplet size. However, given the inherent tolerances in actual droplet volume, it can be advantageous to rely on logarithmic statistics, especially when dealing with very small bumps, to obtain a large number of smaller droplets rather than a small number of larger droplets. It may be advantageous to use

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

Reflow station 24 includes optical assembly 52 , which directs a beam of radiation to locally melt droplet 32 , thus causing the droplet to coalesce into solder bump 60 . Localized heating of this kind is advantageous in avoiding damage to the particularly sensitive acceptor substrate 34 . The optical assembly 52 in the illustrated example includes a laser 54 along with a beam deflector 56 and focusing optics 58, which melt and reflow the deposited droplet to the solder bump 60. Direct the laser radiation to illuminate the target location with sufficient energy to Reflow station 24 also includes a positioning assembly that can be based on the same stage 36 as in print station 22, or a different stage or other motion device.

The controller 51 scans the pulse parameters of the laser 54 and the optical assembly 52 to apply sufficient energy to melt and reflow each solder bump 60 while avoiding damage to the substrate 34. and adjusting focusing parameters. The pulse duration and energy are chosen such that the solder material under each bump is completely melted without vaporizing the solder material over the bump. The actual power and pulse duration required depends on the melting temperature and thermal conductivity of the solder material. Short laser pulses are generally preferred for this purpose as they minimize the time the solder material melts, thus minimizing oxidation and avoiding damage to the substrate 34. Thus, the reflow station 24 can operate in ambient atmospheric conditions. Short, high-power laser pulses are particularly advantageous in enabling flux-free 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 usually sufficient to achieve complete reflow of small solder bumps, but multiple pulses may alternatively be used, especially for larger solder bumps. do. The resulting rapid local reflow process is also beneficial in reducing the formation of intermetallics between the solder bumps and contact pads, thus creating stronger solder bonds compared to thermal reflow methods known in the art. advantageous to

For a small solder bump made of a stack of droplets 32 of tin-based solder with a thickness of, for example, 20-30 μm, a laser pulse with an optical power of approximately 10 W and a duration of 50-100 μs is typically applied to the substrate. This is sufficient to achieve full reflow while avoiding substantial heat diffusion into the The optimum laser wavelength, pulse power, duration and focus size in each case can be selected to match the absorption spectrum, volumetric and thermal characteristics of the solder material in each case. For small to medium sized solder bumps, the pulse energy should be no greater than about 2 mJ. 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.

Laser 54 in reflow station 24 in the following example may be a high power CW Nd:YAG laser operating at 1064 nm. Alternatively, laser 54 may be a diode pumped fiber laser, for example a Ytterbium Continuous Wave Fiber Laser in the range of 976 nm to 1075 nm and in the tens of Watts of power (eg available from IPG). can Alternatively, laser 54 may be a high power diode laser module, for example a diode laser module manufactured by BTW. Other types of lasers will become apparent to those skilled in the art after reading this specification.

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

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

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

In one embodiment, print station 22 and reflow station 24 are combined into a single operational unit with optical assemblies providing laser radiation for both LIFT and reflow processes. As long as the laser source can provide the different ranges of pulse energy and duration required for LIFT and reflow, the same laser source can be used for both applications. Alternatively, a combined station may include two or more different laser sources, with a shared positioning assembly and possibly shared optics.

Formation of solder bumps

Reference is now made to FIGS. 2A and 2B , which schematically illustrate a process of forming solder bumps on a printed circuit board 70 in accordance with an embodiment of the present invention. 2A is a front view of a substrate 70 onto which drops of solder 32 have been deposited, for example by a LIFT process at a printing station 22 (FIG. 1), and FIG. 2B is a front view of the substrate 70 after reflow of solder. is a schematic front view of This embodiment is useful herein in defining and creating solder bumps having different respective bump volumes, shapes, and/or compositions of solder materials at different target locations on the same acceptor substrate (i.e., substrate 70 in this example). Illustrates the use of the techniques described.

As shown in FIG. 2A, before 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 onto substrate 70 using a photolithography process as known in the art, or alternatively written directly onto substrate 70 using, for example, a LIFT process. there is. LIFT printing of the contact pads may be beneficial to improve adhesion of the solder material to the contact pads as further described below with reference to FIGS. 5A and 5B . Controller 51 is programmed to specify different solder bump volumes to be created on different solder pads. The controller drives the optical assembly 30 to direct different sequences of laser pulses through different points on the donor substrate so that the deposited droplets 32 cumulatively reach designated bump volumes on the various pads. Thus, for example, only a single droplet 32 or a small number of droplets are deposited on each pad 72, while larger droplet collections 74 are deposited on pad 75. When a very fine contact is required, as in the case of pad 72, it is also possible to directly solder the component to the trace by depositing a droplet directly at a target location on the trace 73 without a dedicated contact pad.

As mentioned above, by proper selection and configuration of donor film 48, print station 22 can be controlled to print different respective compositions of solder material on different contact pads. For example, print station 22 prints suitable low-temperature solder for delicate contacts onto pad 72, while pad 75 is designed to carry a higher operating current during operation of circuitry on board 70. You can print high temperature solder on it. The optical assembly 30 directs the laser pulses through the appropriate points on the donor substrate 46 to deposit the appropriate composition of solder material on each of the contact pads or locations.

Controller 51 may also be programmed to specify solder bumps of different shapes, 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. Thus, an elongated droplet assembly 76 is deposited on the contact pad 77 . Solder contacts can be printed in this manner in virtually any desired shape, including, for example, annular and angular shapes.

After deposition of droplet 32, substrate 70 is heated to melt and reflow the droplet, 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 spherical shapes that minimize surface energy. To minimize this tendency, especially when creating non-circularly shaped solder bumps, the reflow station 24 may apply short, intense laser pulses to locally melt the solder bumps as described above. Laser pulse parameters and irradiation patterns at the reflow station 24 may be adjusted to achieve desired shape characteristics.

3A, 3B, 3C and 3D are schematic cross-sectional views of a circuit board showing successive steps in a reflow process and deposition of solder bumps 94 on a substrate 70 according to another embodiment of the present invention. This embodiment addresses the problem of reflow, which can occur especially with large solder bumps. When the deposition process is performed in a single step, a high energy laser pulse may be required to melt the droplet 32 at the bottom of the solder bump. Higher pulse energies increase the risk of damage to the board 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 melt completely, resulting in poor contact integrity and increased electrical resistance.

To solve this problem, droplet 32 is deposited in multiple layers on a target location to reach a designated bump volume. Substrate 70 is shuttled back and forth between print station 22 and reflow station 24 multiple times to alternately deposit layers of droplets until a specified bump volume is reached and then heat the layers to melt the droplets. Alternatively, LIFT printing and melting of droplets can be performed within a single station where the optical assembly is capable of both LIFT printing and reflow. In either case, the risk of damage is reduced because the energy that must be applied to melt each successive layer of droplets is relatively small.

Thus, in the illustrated example, as shown in FIG. 3A , an initial layer of droplets 32 is deposited on the substrate 70 (or more precisely, on contact pads on the substrate). As shown in FIG. 3B, this layer is heated and thus melted to form a reflowed layer 92. An additional layer 32 of droplets 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 as many cycles as necessary to create solder bumps 94.

Reference is now made to FIGS. 4A and 4B , which schematically illustrate a process for creating a mixed composition solder bump 100 on a substrate 70 in accordance with an embodiment of the present invention. 4A is a cross-sectional view showing two different respective droplets 96 and 98 of solder material being deposited by the LIFT process at print station 22 . 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 contain two (or more) different materials in a specified percentage. For example, for improved mechanical strength and/or conductivity, the solder bumps may include intergranular copper mixed with tin solder or palladium mixed with SAC solder. In some cases, it may be advantageous to have different materials distributed non-uniformly in a defined gradient of material within the solder bumps. 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 with decreasing concentration relative to the material in droplet 98 towards the top of the solder bump. As described for example in US Pat. No. 9,607,936, this type of gradient composition of palladium and copper with a higher concentration of palladium at the bottom of the bump is believed to improve the strength of the solder joint.

In the example shown in FIG. 4A , donor foil 44 includes two donor films 48 , which include two different donor materials, such as the different types of materials mentioned above. Optical assembly 30 directs laser pulses towards one of the donor films to deposit droplet 96 on substrate 70 and towards the other donor film to deposit droplet 98 . The number of pulses directed towards each of the donor films is selected such that droplets 96 and 98 are deposited at the proper rate and cumulatively reach the specified composition and total solder bump volume. To create a gradient composition, the ratio of pulses directed towards the two donor films, and thus the ratio of droplets 94 to droplets 98, varies layer by layer from the bottom to the top of the droplet, as shown in FIG. 4A. do. Rapid heating of the collection of droplets 96 and 98 at reflow station 24 will cause the droplets to coalesce into solder bump 100 with minimal mixing, thus maintaining the specified gradient as schematically illustrated in FIG. 4B. will make it happen

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

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

5B is a photomicrograph showing solder bumps 116 formed on contact pads 110 according to an embodiment of the present invention. The solder bumps 116 are formed by the printing station 22 in the manner described above by depositing droplets of solder material in the recesses 114 and then melting the droplets in the reflow station 24 . The roughness of the contact pad provides increased surface area for adhesion of the solder material to the pad and helps ensure good electrical and mechanical contact with the indentation.

It will be appreciated that the embodiments described above are cited by way of example, and that the invention is not limited to that specifically shown and described above. Rather, the scope of the present invention includes all combinations and subcombinations of the various features described above, as well as all variations and modifications that will come to one skilled in the art upon reading the foregoing description and not disclosed in the prior art.

Claims (30)

In the circuit manufacturing method,
defining a solder bump comprising a specified solder material and having a specified bump volume, to be formed at a target location on the acceptor substrate;
Positioning a transparent donor substrate having opposing first and second surfaces and a donor film comprising the designated solder material on the second surface, such that the donor film is proximate to the target location on the acceptor substrate. step;
A pulse sequence of laser radiation is directed to the first portion of the donor substrate to induce ejection of a plurality of molten droplets of the solder material from the donor film onto the target location on the acceptor substrate. directing through the surface and impinging on the donor film, so that droplets deposited at the target location cumulatively reach the designated bump volume; and
heating the target position so that the deposited droplets are melted and reflowed to form solder bumps;
Including, circuit manufacturing method. The method of claim 1,
The droplet has a respective droplet volume that depends on the intensity of the pulse of the laser radiation, and directing the pulse sequence sets the intensity of the pulse of the laser radiation and the number of pulses in the sequence in response to the specified bump volume. A circuit manufacturing method comprising the steps of: The method of claim 2,
The droplet volume also depends on a set of pulse parameters consisting of the spot size and duration of the pulses of the laser radiation, wherein directing the pulse sequence comprises adjusting the droplet volume by changing one or more of the pulse parameters. A method of manufacturing a circuit, further comprising a step. The method of claim 1,
defining the solder bumps includes 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; ,
Directing the pulse sequence comprises directing different first and second pulse sequences such that the droplet cumulatively arrives at each of the different first and second bump volumes at the respective first and second target locations. and directing it through different points on the donor substrate. The method of claim 4,
Defining the first and second solder bumps further includes specifying different respective first and second compositions of the first and second solder bumps, and the step of positioning the transparent donor substrate further comprises specifying different respective first and second compositions of the first and second solder bumps. and providing at least one donor film comprising a plurality of different solder materials selected to create first and second compositions. The method of claim 1,
The step of defining the solder bumps includes defining first and second solder bumps having different respective first and second compositions;
and wherein positioning the transparent donor substrate includes providing one or more donor films comprising a plurality of different solder materials to create the first and second compositions. The method of claim 1,
Defining the solder bump includes specifying a composition of the solder bump including different first and second materials,
positioning the transparent donor substrate includes providing first and second donor films comprising the first and second materials, respectively;
directing the pulse sequences comprises directing first and second pulse sequences to impinge the first and second donor films, respectively, such that droplets deposited at the target location cumulatively reach the specified composition; A circuit manufacturing method comprising a. The method of claim 7,
Specifying the composition includes specifying a gradient of the material in the composition of the solder bump, and directing the first and second pulse sequences onto the target location according to the specified gradient. and depositing droplets of the first and second materials in a plurality of layers. The method of claim 1,
and wherein directing the pulse sequence comprises depositing the droplet in a plurality of layers on the target location to reach the designated bump volume. The method of claim 9,
and heating the target location includes alternately depositing a layer of droplets multiple times until the designated bump volume is reached and heating the layer to melt the droplet. The method of claim 1,
Wherein defining the solder bump includes specifying a shape of the solder bump, and directing the pulse sequence includes depositing the molten droplet in a pattern conforming to the specified shape. , circuit manufacturing method. The method of claim 1,
and heating the target location comprises directing a laser beam to irradiate the target location with sufficient energy to melt and reflow the deposited droplet. The method of claim 1,
printing a conductive pad at the target location on the acceptor substrate using a laser-induced forward transfer (LIFT) process, wherein directing the pulse sequence onto the printed conductive pad; and depositing the molten droplets of the solder material on. The method of claim 13,
wherein printing the conductive pad comprises forming a recess in the conductive pad for deposition of the molten droplet therein. In the circuit manufacturing system,
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 the acceptor substrate;
As a printing station,
Transparent donor substrate - the transparent donor substrate having a donor film comprising the designated solder material disposed on the second surface and having opposing first and second surfaces, the donor film having a donor film on the acceptor substrate; positioned proximate to the target location; and
laser - the laser directs a sequence of pulses of laser radiation to the first surface of the donor substrate to direct ejection of molten droplets of the solder material from the donor film onto the target location on the acceptor substrate. configured to pass through and direct to impinge on the donor film -
wherein the controller is configured to drive the printing station to eject a plurality of droplets toward the target location so that the droplets deposited at the target location cumulatively reach the designated bump volume. ; and
A reflow station configured to heat the target location so that the deposited droplets are melted and reflowed to form solder bumps.
Including, circuit manufacturing system. The method of claim 15
The droplet has a respective droplet volume that depends on the intensity of the pulse of the laser radiation and depends on a set of pulse parameters consisting of the spot size and duration of the pulse of the laser radiation, the controller controlling the specified bump volume. and responsively set the intensity of the pulses of the laser radiation and the number of pulses in the sequence, wherein the controller is configured to adjust the droplet volume by changing one or more of the pulse parameters. The method of claim 15
wherein 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;
The controller directs different first and second pulse sequences to different points 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. configured to drive the laser to direct it through
The definition of the first and second solder bumps specifies different respective first and second compositions of the first and second solder bumps, wherein one or more donor films comprising a plurality of different solder materials are formed on the donor substrate. and wherein the solder material is selected to produce the first and second compositions. The method of claim 15
The controller is configured to receive definitions of first and second solder bumps having different respective first and second compositions, wherein at least one donor film comprising a plurality of different solder materials is disposed on the second surface of the donor substrate. wherein the solder material is selected to produce the first and second compositions. The method of claim 15
the definition specifies a composition of the solder bump comprising different first and second materials;
The transparent donor substrate includes first and second donor films each comprising the first and second materials;
The controller drives the print station to direct first and second pulse sequences impinging on the first and second donor films, respectively, so that droplets deposited at the target location cumulatively reach the specified composition. configured to
The definition specifies a gradient of the material in the composition of the solder bump, and the controller directs the printing station to deposit droplets of the first and second materials in multiple layers on the target location according to the specified gradient. A circuit manufacturing system configured to drive. The method of claim 15
wherein the controller is configured to drive the printing station to deposit the droplet in multiple layers on the target location to reach the designated bump volume. The method of claim 20,
wherein the controller is configured to drive the print station and the reflow station to alternately deposit layers of droplets multiple times until the specified bump volume is reached and heat the layers to melt the droplets. manufacturing system. The method of claim 15
wherein the definition specifies a shape of the solder bump, and the controller is configured to drive the printing station to direct the pulse sequence to deposit the molten droplet in a pattern conforming to the specified shape. manufacturing system. The method of claim 15
wherein the printing station is configured to print a conductive pad at the target location on the acceptor substrate using a laser induced forward transfer (LIFT) process and deposit a molten droplet of the solder material onto the printed conductive pad. which is, a circuit manufacturing system. The method of claim 23
wherein the printing station is configured to print the conductive pad having a recess therein for deposition of the molten droplet. In the circuit manufacturing method,
depositing solder material at one or more target locations on the circuit board; and
focusing one or more pulses of a laser beam on each of the target locations with sufficient energy to melt and reflow the deposited droplets to form solder bumps.
Including, circuit manufacturing method. The method of claim 25
and wherein depositing the solder material includes ejecting a molten droplet of the solder material toward the one or more target locations. The method of claim 25
wherein the pulse has a pulse duration of 1 ms or less. The method of claim 27
wherein the pulse has a pulse duration of less than 100 μs. The method of claim 25
wherein the pulse has a pulse energy of 3 mJ or less. The method of claim 25
wherein focusing the one or more pulses comprises focusing a respective single pulse of the laser beam onto each of the target locations.

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