KR102668012B1 - High resolution soldering - Google Patents

Abstract

A circuit fabrication method includes defining solder bumps containing a specified solder material and having a specified bump volume to be formed at a target location on an acceptor substrate. A transparent donor substrate having a donor film containing the designated solder material is positioned such that the donor film is positioned proximate to a target location on the acceptor substrate. A pulse sequence of laser radiation is directed to pass through the 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 the target location on the acceptor substrate, This ensures that the droplets deposited at the target location cumulatively reach the specified bump volume. The target location is heated and the deposited droplets melt and reflow to form a solder bump.

Description

High resolution soldering

The present invention relates generally to methods and systems for manufacturing electronic devices, and in particular to 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. Laser-Induced Forward Transfer (LIFT) is an LDW technology 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 toward an acceptor substrate. Typically the laser beam interacts with the inner side of the donor film, which is coated on a non-absorbent carrier substrate. Put differently, the incident laser beam propagates through a 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 toward the surface of the acceptor substrate. Given appropriate selection of donor film and laser beam pulse parameters, the laser pulse causes molten droplets of donor material to eject from the film and then land on the acceptor substrate and cure.

The LIFT system is particularly (but not exclusively) useful for printing conductive metal droplets and traces for the purpose of electronic circuit manufacturing. A LIFT system of this type 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, the donor supply assembly having opposing first and second surfaces, the second surface 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 for simultaneously directing a plurality of output beams of laser radiation in a predefined spatial pattern to pass through a first surface of the donor substrate and impinge on the donor film to induce ejection of material from the donor film on the acceptor substrate. is configured, thereby recording a predefined pattern on the target area of the acceptor substrate.

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

Accordingly, in accordance with an embodiment of the present invention, a circuit manufacturing method is provided, the circuit manufacturing method comprising defining a solder bump containing a specified solder material and having a specified bump volume to be formed at a target location on an acceptor substrate. It includes A transparent donor substrate having a donor film containing the designated solder material is positioned such that the donor film is positioned proximate to a target location on the acceptor substrate. A pulse sequence of laser radiation is directed to pass through the 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 the target location on the acceptor substrate, This ensures that droplets deposited at the target location cumulatively reach the designated bump volume. The target location is heated so that the deposited droplets melt and reflow to form solder bumps.

Typically, the droplets have a respective droplet volume that varies depending on the intensity of the pulse of laser radiation, and directing the pulse sequence involves setting the intensity of the pulse of laser radiation and the number of pulses in the sequence in response to the specified bump volume. . In the disclosed embodiment, 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 involves adjusting the droplet volume by changing one or more of the pulse parameters. Includes more.

In some embodiments, defining a solder bump 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. and directing the pulse sequences, directing the different first and second pulse sequences on the donor substrate such that the droplets cumulatively reach different first and second bump volumes, respectively, at their respective first and second target locations. It involves directing it to pass 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 further comprises specifying the first and second different compositions of the first and second solder bumps. and providing one or more donor films comprising a plurality of different solder materials selected to produce a 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 produce the first and second compositions. and providing one or more donor films comprising a plurality of different solder materials.

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

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

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

Additionally or alternatively, defining a 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 consistent with the specified shape.

In a further embodiment, heating the target location includes directing a laser beam to illuminate the target location with sufficient energy to melt and reflow the deposited droplets. Typically, directing a laser beam involves focusing one or more laser pulses onto 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, wherein directing the pulse sequence melts the solder material onto the printed conductive pad. It includes depositing the liquid droplets. In disclosed embodiments, printing a conductive pad includes forming depressions in the conductive pad for deposition of molten droplets therein.

Also provided in accordance with embodiments of the present invention is a circuit manufacturing system, the circuit manufacturing system comprising: defining a solder bump containing a specified solder material and having a specified bump volume to be formed at a target location on an acceptor substrate; Contains a controller configured to receive. The printing station includes a transparent donor substrate, the transparent donor substrate having opposing first and second surfaces and a donor film comprising a designated solder material disposed on the second surface, the donor film being positioned on an acceptor substrate. It is located 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 the target location on the acceptor substrate. It 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 designated bump volume. The reflow station is configured to heat the target location so that the deposited droplets melt and reflow to form solder bumps.

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

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

In some embodiments, the pulse has a pulse duration of less than 1 ms, 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 disclosed embodiments, focusing 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 together with the drawings.

1 is a block diagram schematically illustrating a system for electronic circuit manufacturing according to an embodiment of the present invention.
2A is a schematic front view of a printed circuit board on which solder droplets have been deposited in a LIFT process in accordance with an embodiment of the present invention.
FIG. 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 the deposition and reflow process of solder bumps in accordance with an embodiment of the invention.
4A is a schematic cross-sectional view of a circuit board on which droplets of two different solder materials have been deposited in a LIFT process in accordance with an embodiment of the present invention.
Figure 4B is a schematic front view of the circuit board of Figure 4A after reflow of solder material in accordance with an embodiment of the present invention.
Figure 5A is a micrograph of a contact pad formed by the LIFT process in accordance with an embodiment of the present invention.
FIG. 5B is a micrograph of a solder bump formed on the contact pad of FIG. 5A according to an embodiment of the present invention.

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In electronic circuit manufacturing methods 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 photolithography methods. The 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 the volume of solder material on each contact pad are fixed by a photolithography mask and solder deposition process.

Embodiments of the present invention provide a LIFT-based method of solder deposition that can optionally create solder bumps of substantially any desired size and shape and comprising substantially any suitable solder material or combination of materials. do. This LIFT-based method can create multiple bumps of different volumes, shapes and sizes and even containing different solder materials and solder material combinations, all in the same process step and on the same substrate. The volume and composition of the solder bump (even 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. Additionally, in contrast to conventional approaches, the present method can print solder bumps on non-uniform substrates, as well as on substrates on which components have already been placed. The disclosed embodiments thus provide much greater flexibility and precision in circuit fabrication than techniques known in the art.

In the embodiment described below, a solder bump is defined in terms of a specified solder material and bump volume and a target location where the bump is 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 the 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 is referred to as the top surface.) A plurality of molten droplets of solder material from the donor film, on the acceptor substrate. To direct the ejection to the target location, the laser directs a pulse sequence of laser radiation to pass through the upper surface of the donor substrate and impinge on the donor film.

The laser pulse parameters and number of pulses in the sequence are selected such that droplets deposited at the target location cumulatively reach the specified bump volume. Pulse parameters that can be changed to control droplet volume include pulse intensity, i.e. light power per unit area incident on the donor film, as well as spot size and pulse duration. These parameters are adjusted to provide a constant droplet volume of the order of 0.1 pl (picoliter), i.e. less than 100 μm ³ , and to ensure ejection of droplets from the donor film at high speed with precise directionality toward the target location. Can be adjusted depending on the type and thickness of the solder material. Therefore, by appropriate selection of pulse parameters and droplet number, it is possible to print precisely sized solder bumps with diameters as small as about 20 μm. Processes according to embodiments of the present invention can be used to print a wide range of solder materials, including low, medium, and high temperature solders, on 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 carried out locally, for example by laser irradiation, in order to drive fast reflow and minimize damage to the substrate. The laser pulses used in this step can be narrowly focused on the solder bump, and the duration of the laser pulse may be about 1 millisecond or less, in most cases less than 100 microseconds, for example tens of microseconds (or even the length of a small solder bump). cases, it should be less). Therefore, this laser-driven reflow technology can be performed in air and is suitable for heat-sensitive substrates. It is particularly suitable for use with the LIFT based solder printing technology described above, but can alternatively be applied to reflow solder material deposited by other methods such as inkjet type solder printing and photolithography techniques. However, alternatively 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 can be placed on the bumps and soldered in place by conventional methods.

System Description

1 is a schematic diagram of a system 20 for electronic circuit manufacturing according to an embodiment of the present invention. The system 20 includes a printing station 22, which prints a solder bump 60 comprising a specified solder material and having a specified bump volume to be formed at a target location on the acceptor substrate 34. Receives 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 designated bump volume. The reflow station 24 heats the target location so that the deposited droplets 32 melt and reflow to form solder bumps 60. This heating process may be concentrated locally at bump locations, as shown in FIG. 1, or may be performed globally over the entire substrate 34, depending on the solder material and substrate properties and other application requirements.

Typically, after formation of solder bumps 60, placement station 26 places the solder bumps on the solder bumps, for example, using a pick-and-place machine 62, as is known in the art. Place the component 64 in . Next, a heat source 66 at the final reflow station 28 heats the solder bump to form a permanent bond 68 between the component and the substrate 34. Heat source 66 may apply localized heating, for example using 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 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 create a mechanical seal around the edges of component 64. This type of seal can be used, for example, for hermetic packaging of sensitive devices such as Micro-Electromechanical Systems (MEMS) devices.

Referring back to the printing station 22, the optical assembly 30 at the printing station includes a laser 38, which is directed toward the donor foil 44 under the control of the controller 51 on the order of 1 ns. Direct 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 of the visible, ultraviolet, and infrared ranges, and the term "laser radiation" refers to optical radiation emitted by a laser.) Controller 51 typically includes a general-purpose computer or special-purpose microcontroller, which has appropriate interfaces to other elements of system 20 and is driven by software to perform the functions described herein. The donor foil 44 comprises a thin, flexible sheet of a typically transparent donor substrate 46, which is coated on the side proximal to the acceptor substrate 34 with a donor film 48 containing the designated solder material or materials. Alternatively, the donor substrate may include a rigid or semi-rigid material. The acceptor substrate 34 may include any suitable material, such as glass, ceramic or polymer, as well as other dielectric, semiconductor or even conductive materials.

The optical assembly 30 includes a beam deflector 40 and focusing optics 42 which direct one or more output beams of radiation from the laser 38 to the donor substrate along a spatial pattern determined by the controller 51. It is directed to pass through the upper surface of 46 and thus impinge on the donor film 48 on the lower surface. In an exemplary embodiment, beam deflector 40 includes an acousto-optic modulator, as shown in FIG. 2A or FIG. 2B of the above-referenced U.S. Pat. No. 9,925,797 and described at columns 7-8 of the patent. The laser typically uses a series of pulses of suitable wavelength, duration and energy to induce ejection of molten droplets 50 of solder material from the donor film 48 onto a designated target location on the acceptor substrate 34. is controlled to output. Since the droplet 50 is 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 is at a small distance from the acceptor substrate 34. For example, the donor film 48 and the acceptor substrate 34 may be positioned with a gap of up to about 0.5 mm. Due to the high velocity ejection of the droplets 50 (typically greater than 10 m/sec), the droplet flight time is shorter than the time it takes for the droplets 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, medium, and high temperature solders. Low- and medium-temperature solders include, for example, tin-lead and tin-silver-copper (SAC) alloys. High-temperature solders, most commonly used to manufacture high-power electronic devices, contain alloys of silver (typically 45-90%) with other metals such as copper, zinc, tin, and cadmium, and typically have a temperature range of 700-950°C. melts at temperature. The thickness and composition of the film 48 as well as the pulse parameters of the optical assembly 30 are adjusted to provide a stable jet of molten droplets 50 of solder material toward the target location on the acceptor substrate 34. Adjusted according to your choice.

In some embodiments, multilayered and structured donor films 48 may be used to deposit droplets 32 of mixed composition. 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. This type of multi-composition LIFT approach is described, for example, in US Patent 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 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. This type of mixing composition scheme is described further 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 the appropriate number of droplets 32 of the desired volume on each target location where solder bumps are to be formed on the acceptor substrate 34. Adjust the scanning and focusing parameters in (30). As previously described, controller 51 sets the laser pulse parameters and the number of molten droplets of solder material such that the droplets deposited at each target location cumulatively reach the specified bump volume at that location. The droplet volume can be varied by adjusting the laser pulse parameters 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 volumes, it may be advantageous to rely on logarithmic statistics, especially when dealing with very small bumps, and it may be advantageous to use a larger number of smaller droplets rather than a smaller 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 an acceptor substrate 34 is mounted. Stage 36 positions the acceptor substrate 34 relative to the optical assembly 30 and donor foil 44 of the print station 22 to deposit droplets 32 at different target locations across the surface of the acceptor substrate. Move it. Additionally or alternatively, the positioning assembly may include a motion component (not shown) that moves the optical assembly 30 as well as, if appropriate, the donor film 44 over the surface of the acceptor substrate.

Reflow station 24 includes an optical assembly 52 that directs a radiation beam to locally melt droplets 32, thereby causing them to coalesce into solder bumps 60. This kind of local heating is particularly advantageous to avoid damage to the sensitive acceptor substrate 34. In the example shown, optical assembly 52 includes a laser 54 along with a beam deflector 56 and focusing optics 58, which melt the deposited droplets and reflow them into solder bumps 60. Direct the laser radiation to illuminate the target location with sufficient energy. Reflow station 24 also includes a positioning assembly that may be based on the same stage 36 as in print station 22, or on a different stage or other motion device.

The controller 51 controls the pulse parameters of the laser 54 and the scanning of the optical assembly 52 to apply sufficient energy to melt and reflow each solder bump 60 while avoiding damage to the substrate 34. and adjust the focusing parameters. The pulse duration and energy are selected to completely melt the solder material at the bottom of each bump without vaporizing the solder material at the top of the bump. The actual power and pulse duration required will depend on the melt temperature and thermal conductivity of the solder material. Short laser pulses are generally preferred for this purpose as they minimize the time it takes for the solder material to melt, thus minimizing oxidation and avoiding damage to the substrate 34. Accordingly, reflow station 24 can operate in ambient atmospheric conditions. Short, high-power laser pulses are particularly advantageous in that they enable flux-free reflow and support the use of high-temperature solder materials. A single laser pulse of this type, focused at the location of each solder bump, is typically sufficient to achieve complete reflow of small solder bumps, but multiple pulses can be used as an alternative, especially for larger solder bumps. do. The resulting fast local reflow process is also advantageous in reducing the formation of intermetallic compounds between the solder bumps and contact pads, thus producing stronger solder bonds compared to thermal reflow methods known in the art. It is advantageous to

For a small solder bump, for example, made from a pile of droplets 32 of tin-based solder with a thickness of 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 complete reflow while avoiding substantial heat diffusion into the The optimal laser wavelength, pulse power, duration and focus size in each case can be selected to match the absorption spectrum, volume and thermal properties 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.

In the following example, 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 a Ytterbium Continuous Wave Fiber Laser in the 976 nm to 1075 nm range and tens of watts of power (e.g. available from IPG). You can. 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 after reading this specification.

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

In another example, to reflow a bump with a volume of about 85 pl (corresponding to a bump diameter of about 100 μm), 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 with . Optical assembly 52 focuses the beam to a spot size of approximately 50 to 100 μm on the solder bump.

Alternative choices 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 operating unit with an optical assembly 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 the process of forming solder bumps on a printed circuit board 70 in accordance with an embodiment of the present invention. FIG. 2A is a front view of a substrate 70 on which droplets 32 of solder have been deposited, for example, by the LIFT process at print station 22 (FIG. 1), and FIG. 2B is a front view of the substrate 70 after reflow of solder. This is a schematic front view. This embodiment is described herein in defining and creating solder bumps with 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). Illustrates the use of the techniques described.

As shown in Figure 2A, prior to depositing solder bumps on the substrate 70, electronic traces 73 and various contact pads 72, 75, and 77 of different sizes and shapes are formed on the substrate. These pads and traces may be printed on substrate 70 using photolithography processes as known in the art, or alternatively may be written directly on substrate 70 using, for example, the LIFT process. there is. LIFT printing of contact pads can be advantageous to improve adhesion of solder material to the contact pad, 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 to pass through different points on the donor substrate such 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, with larger collections of droplets 74 deposited on pad 75. When very fine contact is required, as in the case of pad 72, it is also possible to solder the component directly to the trace by depositing a droplet directly at a target location on trace 73 without a dedicated contact pad.

As previously mentioned, by proper selection and configuration of the donor film 48, the print station 22 can be controlled to print different respective compositions of solder material on different contact pads. For example, the print station 22 prints a low temperature solder suitable for delicate contacts on the pads 72 while the pads 75 are designed to carry higher operating currents during operation of the circuitry on the substrate 70. High temperature solder can be printed on this. Optical assembly 30 directs the laser pulses to pass through appropriate points on donor substrate 46 to deposit solder material of appropriate composition on each of the contact pads or locations.

Controller 51 may also be programmed to specify different shapes of solder bumps, including non-circular shapes, such as the elongated shape defined by contact pad 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 matching the specified shape. Accordingly, an elongated collection of droplets 76 is deposited on the contact pad 77 . Solder contacts can be printed in this manner in virtually any desired shape, including, for example, circular and angular shapes.

After deposition of droplets 32, substrate 70 is heated to melt and reflow the droplets, thereby coalescing into solder bumps 82, 84, and 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, especially when creating solder bumps of non-circular shapes, reflow station 24 can apply short, intense laser pulses to locally melt the solder bumps as described above. 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 steps in the deposition and reflow process of solder bumps 94 on board 70 in accordance with another embodiment of the present invention. This embodiment specifically addresses reflow issues that can occur with large solder bumps. When the deposition process is performed in a single step, high energy laser pulses may be required to melt the droplet 32 at the bottom of the solder bump. High pulse energy increases the risk of damage to the substrate around the solder bump. On the other hand, if the laser pulse energy is insufficient, the droplet at the bottom of the bump may not completely melt, resulting in poor contact integrity and increased electrical resistance.

To solve this problem, droplets 32 are deposited in multiple layers on the 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 deposit alternating layers of droplets until a designated 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 has the necessary capabilities for 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.

Accordingly, in the example shown, an initial layer of droplets 32 is deposited on substrate 70 (or more precisely, on a contact pad on the substrate), as shown in Figure 3A. As shown in Figure 3b, this layer is heated and thus melts to form reflowed layer 92. An additional layer 32 of droplets is deposited over the reflowed layer 92 as shown in Figure 3C and then heated to reflow again as shown in Figure 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 mixed composition solder bumps 100 on substrate 70 in accordance with an embodiment of the present invention. FIG. 4A is a cross-sectional view showing two different respective droplets 96 and 98 of solder material being deposited by the LIFT process at print station 22. 4B is a front view of solder bump 100 after reflow of solder material at reflow station 24.

Controller 51 receives specifications for solder bump 100 indicating that the solder bump will contain two (or more) different materials in specific proportions. For example, for improved mechanical strength and/or conductivity, the solder bumps may include copper grain boundaries mixed with tin solder or palladium mixed with SAC solder. In some cases, it may be advantageous to have the different materials distributed non-uniformly within the solder bump with a defined gradient of material. 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 while decreasing in concentration relative to the material in droplet 98 toward the top of the solder bump. This type of gradient composition of palladium and copper with a higher concentration of palladium at the bottom of the bump, as described for example in U.S. Pat. No. 9,607,936, is believed to improve the strength of the solder joint.

In the example shown in Figure 4A, the donor foil 44 comprises two donor films 48, which comprise two different donor materials, such as the different types of materials mentioned above. Optical assembly 30 directs the laser pulses toward one of the donor films to deposit droplets 96 on substrate 70 and toward the other donor film to deposit droplets 98. The number of pulses directed toward each of the donor films is selected so that droplets 96 and 98 are deposited at the appropriate 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 Figure 4A. do. Rapid heating of the collection of droplets 96 and 98 in reflow station 24 will cause the droplets to coalesce into solder bump 100 with minimal mixing, thereby maintaining the specified gradient as schematically illustrated in FIG. 4B. We will do our best.

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

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

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

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

Claims (30)

In the circuit manufacturing method,
defining a solder bump containing a specified solder material and having a specified bump volume to be formed at a target location on an 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. ordering step;
A pulse sequence of laser radiation is used to induce ejection of multiple molten droplets of solder material from the donor film onto the target location on the acceptor substrate. directing droplets deposited at the target location to pass through a surface and impact the donor film, thereby cumulatively reaching the designated bump volume; and
Heating the target location so that the deposited droplets melt and reflow to form solder bumps.
Including, a circuit manufacturing method. In claim 1,
The droplet has a respective droplet volume that varies depending on the intensity of the pulse of laser radiation, and directing the pulse sequence sets the intensity of the pulse of 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: In claim 2,
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 adjusts the droplet volume by changing one or more of the pulse parameters. A method of manufacturing a circuit further comprising steps. In 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 sequences may include directing different first and second pulse sequences such that the droplets cumulatively reach the different first and second bump volumes, respectively, at the respective first and second target locations. A method of manufacturing a circuit, comprising directing the donor substrate through different points. In claim 4,
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 includes specifying different respective first and second compositions of the first and second solder bumps. and providing one or more donor films comprising a plurality of different solder materials selected to produce a second composition. In claim 1,
wherein defining the solder bumps includes defining first and second solder bumps having different respective first and second compositions,
wherein positioning the transparent donor substrate includes providing one or more donor films comprising a plurality of different solder materials to produce the first and second compositions. In claim 1,
defining the solder bump includes specifying a composition of the solder bump comprising 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 includes directing first and second pulse sequences to impact 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: In 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. A method of manufacturing a circuit comprising depositing droplets of said first and second materials in multiple layers. In claim 1,
and wherein directing the pulse sequence includes depositing the droplets in multiple layers on the target location to reach the designated bump volume. In claim 9,
wherein heating the target location includes alternately depositing layers of droplets multiple times until the specified bump volume is reached and heating the layers to melt the droplets. In 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 consistent with the specified shape. , circuit manufacturing method. In claim 1,
wherein heating the target location includes directing a laser beam to illuminate the target location with sufficient energy to melt and reflow the deposited droplet. In 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. A method of manufacturing a circuit comprising depositing a molten droplet of said solder material. In claim 13,
and wherein printing the conductive pad includes forming a depression 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 opposing first and second surfaces and having a donor film comprising the designated solder material disposed on the second surface, the donor film being positioned on the acceptor substrate. Located close to the target location - and,
Laser - The laser applies a sequence of pulses of laser radiation to the first surface of the donor substrate to induce ejection of molten droplets of the solder material from the donor film onto the target location on the acceptor substrate. configured to be oriented so as to pass through and impinge on said donor film -
wherein the controller is configured to drive the printing station to eject a plurality of droplets toward the target location such 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 melt and reflow to form solder bumps.
A circuit manufacturing system comprising: In claim 15,
The droplet has a respective droplet volume that varies depending on the intensity of the pulse of laser radiation and a set of pulse parameters consisting of a spot size and duration of the pulse of laser radiation, and the controller controls the specified bump volume. and in response to set the intensity of the pulse of 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. In 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;
The controller directs different first and second pulse sequences to different points on the donor substrate such that the droplets cumulatively reach each of the different first and second bump volumes at the respective first and second target locations. configured to drive the laser to direct it to pass through,
The definitions of the first and second solder bumps specify different respective first and second compositions of the first and second solder bumps, and wherein one or more donor films comprising a plurality of different solder materials are disposed on the donor substrate. A circuit manufacturing system disposed on a second surface, wherein the solder material is selected to produce the first and second compositions. In 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 on the second surface of the donor substrate. and wherein the solder material is selected to produce the first and second compositions. In claim 15,
The definition specifies a composition of the solder bump comprising different first and second materials,
wherein the transparent donor substrate includes first and second donor films comprising the first and second materials, respectively;
The controller drives the printing station to direct first and second pulse sequences to impinge on the first and second donor films, respectively, such that droplets deposited at the target location cumulatively reach the specified composition. It is configured to
The definition specifies a gradient of the material in the composition of the solder bump, and the controller directs the print 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 operate. In claim 15,
wherein 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. In claim 20,
wherein the controller is configured to drive the print station and the reflow station to alternately deposit layers of droplets multiple times and heat the layers to melt the droplets until the specified bump volume is reached. manufacturing system. In claim 15,
wherein the definition specifies the shape of the solder bump, and the controller is configured to drive the print station to direct the pulse sequence to deposit the molten droplet in a pattern consistent with the specified shape. manufacturing system. In 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 solder material on the printed conductive pad. A circuit manufacturing system. In 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,
positioning a transparent donor substrate having opposing first and second surfaces and a donor film comprising a designated solder material on the second surface such that the donor film is proximate to a target location on the acceptor substrate;
A pulse sequence of laser radiation passes through the first surface of the donor substrate and impinges on the donor film to induce ejection of a molten droplet of solder material from the donor film onto the target location on the acceptor substrate. ejecting molten droplets of solder material toward a target location by directing them so that the droplets deposited at the target location cumulatively reach a designated bump volume; and
Focusing one or more pulses of a laser beam onto the target location with sufficient energy to melt and reflow the deposited droplets to form a solder bump.
Including, circuit manufacturing method. In claim 25,
The method of manufacturing a circuit, wherein the pulse has a pulse duration of 1 ms or less. In claim 26,
The method of claim 1 , wherein the pulse has a pulse duration of less than 100 μs. In claim 25,
The method of manufacturing a circuit, wherein the pulse has a pulse energy of 3 mJ or less. In claim 25,
and wherein focusing the one or more pulses includes focusing a respective single pulse of the laser beam on each of the target locations. delete

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