TWI862818B - Methods and systems for circuit fabrication - Google Patents

Abstract

A method for circuit fabrication includes defining a solder bump, including 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 including the specified solder material, is positioned such that the donor film is in proximity to the target location on the acceptor substrate. A sequence of pulses of laser radiation is directed to pass through the first surface of the donor substrate and impinge on the donor film so as to induce ejection from the donor film onto the target location on the acceptor substrate of a number of molten droplets of the solder material such that the droplets deposited at the target location cumulatively reach the specified bump volume. The target location is heated so the deposited droplets melt and reflow to form the solder bump.

Description

Methods and systems for circuit manufacturing

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

In laser direct writing (LDW), a laser beam is used to produce a patterned surface with a spatially resolved three-dimensional structure by controlled material ablation or deposition. Laser-induced forward transfer (LIFT) is a type of LDW technique that can be used to deposit micropatterns on a surface.

In LIFT, laser photons provide the driving force to eject small volumes of material from a donor film toward a receiver substrate. Typically, the laser beam interacts with the inside of the donor film, which is coated onto a non-absorbing carrier substrate. In other words, the incident laser beam propagates through the transparent carrier substrate before the photons are absorbed by the inner surface of the film. Beyond a certain energy threshold, the material is ejected from the donor film toward the surface of the receiver substrate. After appropriate selection of the 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 and harden onto the receiver substrate.

LIFT systems are particularly, though not exclusively, useful for printing conductive metal drops and traces for electronic circuit fabrication. Such a LIFT system is described, for example, in U.S. Patent 9,925,797, the disclosure of which is incorporated herein by reference. This patent describes a printing apparatus comprising a donor supply assembly configured to provide a transparent donor substrate having opposing first and second surfaces and a donor film formed on the second surface to position the donor film proximate a target area on a receptor substrate. An optical assembly is configured to simultaneously direct multiple output beams of laser radiation in a predetermined spatial pattern through the first surface of the donor substrate and onto the donor film to induce material to be ejected from the donor film onto the receptor substrate, thereby writing the predetermined pattern onto the target area of the receptor substrate.

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

Therefore, according to one embodiment of the present invention, a method for circuit manufacturing is provided, which includes: defining a solder bump to be formed at a target location on a receptor substrate, the solder bump comprising a specified solder material and having a specified bump volume. Positioning a transparent donor substrate having first and second surfaces opposite to each other and a donor film comprising the specified solder material on the second surface such that the donor film is close to the target location on the receptor substrate. Directing a pulse sequence of laser radiation through the first surface of the donor substrate and irradiating the donor film so as to induce a plurality of molten droplets of the solder material to be ejected from the donor film onto the target location on the receptor substrate, such that the droplets deposited at the target location accumulate to achieve the specified bump volume. The target location is heated so that the deposited droplets melt and reflow to form the solder bump.

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

In some embodiments, defining the 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 receptor substrate, and directing the pulse sequence includes directing different, first and second sequences of pulses through different points on the donor substrate such that the droplets cumulatively reach each of the different first and second bump volumes at the respective first and second target locations. In one embodiment, defining the first solder bump and the second solder bump includes specifying different, respective first and second compositions of the first solder bump and the second solder bump, and positioning the transparent donor substrate includes providing one or more donor films including a plurality of different solder materials selected to produce the first composition and the second composition.

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

Further additionally or alternatively, defining the solder bump includes specifying a composition of the solder bump, the composition including different, first and second materials, and positioning the transparent donor substrate includes providing a first donor film and a second donor film including the first material and the second material, respectively, and directing the sequence of pulses includes directing the first sequence and the second sequence of pulses to irradiate the first donor film and the second donor film, respectively, so that the droplets deposited at the target location cumulatively reach the specified composition. In one embodiment, specifying the composition includes specifying a gradient of the materials in the composition of the solder bump, and directing the first sequence and the second sequence of pulses includes depositing the droplets of the first material and the second material in multiple layers on the target location according to the specified gradient.

In some embodiments, directing the sequence of pulses includes depositing the droplets in multiple layers on the target location to achieve the specified bump volume. In one disclosed embodiment, heating the target location includes alternately depositing a layer of droplets multiple times and heating the layer to melt the droplets until the specified bump volume is achieved.

Additionally or alternatively, defining the solder bump includes specifying a shape of the solder bump, and directing the sequence of pulses includes depositing the molten droplets in a pattern conforming to the 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 includes focusing one or more laser pulses onto the target location.

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

According to an embodiment of the present invention, a system for circuit manufacturing is also provided, which includes a controller configured to receive a definition of a solder bump to be formed at a target location on a receptor substrate, the solder bump including a specified solder material and having a specified bump volume. A printing station includes a transparent donor substrate having first and second opposing surfaces and having a donor film including the specified solder material disposed on the second surface, and the transparent donor substrate is positioned so that the donor film is proximate to the target location on the receptor substrate. A laser is configured to direct a pulse sequence of laser radiation through the first surface of the donor substrate and onto the donor film to induce molten droplets of the solder material to be ejected from the donor film onto the target location on the receptor substrate. The controller is configured to drive the printing station to spray a plurality of droplets toward the target position, so that the droplets deposited at the target position cumulatively achieve the specified bump volume. A reflow station is configured to heat the target position so that the deposited droplets melt and reflow to form the solder bump.

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

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

In some embodiments, the pulses have a pulse duration of no more than 1 ms, and preferably no less than 100 μs.

Additionally or alternatively, the pulses have a pulse energy of no more than 2 mJ.

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

The present invention will be more fully understood from the following [implementation method] of the present invention together with the attached drawings, wherein:

20: System

22: Printing station

24: Reflow station

26: Placement Station

28: Final return station

30:Optical assembly

32: Droplets

34: Receptor substrate

36: X-Y platform

38: Laser

40: Beam deflector

42: Focusing optical devices

44: Donor foil

46: Donor substrate

48: Donor membrane

50: Droplets

51: Controller

52: Optical assembly

54:Laser

56: Beam deflector

58: Focusing optical devices

60: Solder bump

62: Pick and place machine

64:Components

66: Heat source

68:Joining

70: Substrate

72: Contact pad

73:Traces

74: Droplet collection

75: Contact pad

76: Gathering

77: Contact pad

82: Solder bump

84: Solder bump

86: Solder bump

92: Reflow layer

94: Solder bump

96: Droplets

98: Droplets

100: Solder bump

110: Contact pad

112: Substrate

114: Concave

116: Solder bump

FIG. 1 schematically shows a block diagram of a system for manufacturing electronic circuits according to an embodiment of the present invention; FIG. 2A is a schematic front view of a printed circuit substrate according to an embodiment of the present invention, on which solder droplets have been deposited by a LIFT process; FIG. 2B is a schematic front view of the printed circuit substrate of FIG. 2A after solder reflow according to an embodiment of the present invention; FIG. 3A, FIG. 3B, FIG. 3C and FIG. 3D are schematic cross-sectional views of a circuit substrate according to an embodiment of the present invention, showing a deposition and reflow of a solder bump. 4A is a schematic cross-sectional view of a circuit substrate according to an embodiment of the present invention, on which droplets of two different solder materials have been deposited by a LIFT process; FIG. 4B is a schematic front view of the circuit substrate of FIG. 4A after reflow of the solder material according to an embodiment of the present invention; and FIG. 5A is a micrograph of a contact pad formed by a LIFT process according to an embodiment of the present invention; and 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.

Overview

In a method of manufacturing electronic circuits known in the art, electrical traces and contact pads are printed on a circuit substrate, and a layer of solder is printed onto the contact pads by photolithography. The circuit components are then placed on the solder-covered pads, and the circuit is heated to melt and reflow the solder, thereby creating a conductive joint between the component and the pad. In this known method, the pad position and size on each contact pad and the volume of solder material are fixed by photolithography masking and solder deposition procedures.

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

In the embodiments described below, a solder bump is defined in view of a specified solder material and bump volume and a target location where the bump is to be formed on a receptor substrate. A transparent donor substrate is positioned, the donor substrate having a donor film including the specified solder material on one of its surfaces, wherein the donor film is located near the target location on the receptor substrate. (For convenience, the surface of the donor substrate close to the receptor substrate is referred to herein as the lower surface, and the opposite surface of the donor substrate is referred to as the upper surface.) A sequence of pulses of a laser-guided laser radiation passes through the upper surface of the donor substrate and impinges on the donor film so as to induce a plurality of molten droplets of solder material to be ejected from the donor film onto the target location on the receptor substrate.

The laser pulse parameters and the number of pulses in the sequence are selected so that the droplet accumulation at the target location achieves a specified bump volume. Pulse parameters that can be varied to control the droplet volume include pulse intensity, i.e., the optical power per unit area incident on the donor film, as well as spot size and pulse duration. These parameters can be adjusted to give consistent droplet volumes on the order of 0.1 pl (picoliter), i.e., 100 μm ³ or less, depending on the type and thickness of solder material in the donor film, and ensure that the ejection of droplets from the donor film is precisely positioned toward the target location at high speed. Thus, by properly selecting the pulse parameters and the number of droplets, precisely sized solder bumps with diameters as small as about 20 μm can be printed. The process according to embodiments of the present invention can be used to print a variety of solder materials (including low-temperature, medium-temperature, and high-temperature solders) onto a variety of substrates, and also facilitates flux-free soldering.

After depositing one or more layers of droplets on the receptor substrate, the target location is heated so that the deposited droplets melt and reflow to form solder bumps. Advantageously, the heating is performed locally, for example by laser irradiation, to drive rapid reflow and minimize damage to the substrate. The laser pulse used in this stage can be narrowly focused on the solder bump, and the duration of the laser pulse needs to be no more than about one millisecond and in most cases, less than 100 microseconds, such as tens of microseconds (for small solder bumps or even less). Therefore, this laser-driven reflow technique can be performed in ambient air and is applicable to heat-sensitive substrates. It is particularly suitable for use in conjunction with the above-mentioned LIFT-based solder printing technique; however, it can also be applied to reflow solder material that has been deposited by other methods such as inkjet solder printing and photolithography. Alternatively, however, the reflow stage can be performed by heating the entire receptor substrate, for example in a high temperature furnace. After the solder bumps are formed, the circuit components can be placed on the bumps and soldered in place by known methods.

System Description

FIG. 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 that receives a definition of a solder bump 60 to be formed at a target location on a receptor substrate 34, the solder bump 60 comprising a specified solder material and having a specified bump volume. The printing station deposits a plurality of droplets 32 of the desired solder material at each target location so that the droplets accumulate to achieve the specified bump volume. A reflow station 24 heats the target location so that the deposited droplets 32 melt and reflow to form the solder bump 60. As shown in FIG. 1, this heating process can be locally focused on the bump location, or can be performed globally on the entire receptor substrate 34 depending on the properties of the solder material and the substrate and other application requirements.

Typically, after forming the solder bump 60, a placement station 26 places the component 64 on the solder bump, for example using a pick and place machine 62, as is known in the art. Then, a heat source 66 in a final reflow station 28 heats the solder bump to form a permanent bond 68 between the component and the receptor substrate 34. The heat source 66 may apply localized heating using, for example, a laser, or it may include a reflow oven or any other suitable type of heater known in the art. The bond 68 typically forms both an electrical connection and a mechanical connection between the component 64 and the conductive traces on the receptor substrate 34. Alternatively or in addition, the solder bump 60 may be configured to form a frame having a rectangular, circular, or other shape to create a mechanical seal around the edges of the component 64. Such seals 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, an optical assembly 30 in the printing station includes a laser 38 that directs short pulses of optical radiation with pulse durations on the order of 1 ns toward a donor foil 44 under the control of a controller 51. (As used in the context of this specification and the claims, the term "optical radiation" refers to electromagnetic radiation in any of the visible, ultraviolet, and infrared ranges; and "laser radiation" refers to optical radiation emitted by a laser.) The controller 51 typically comprises a general purpose computer or dedicated microcontroller having appropriate interfaces to the other components of the system 20 and driven by software to perform the functions described herein. The donor foil 44 typically comprises a thin, flexible sheet of a transparent donor substrate 46 coated on one side adjacent to the receptor substrate 34 with a donor film 48 comprising one or more specified solder materials. Alternatively, the donor substrate may comprise a rigid or semi-rigid material. The receptor substrate 34 may comprise any suitable material, such as glass, ceramic or polymer, as well as other dielectric, semiconductor or even conductive materials.

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

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

In some embodiments, a multi-layer and structured donor film 48 may be used to deposit droplets 32 of mixed compositions. For example, the donor foil 44 may include multiple layers of donor films that include different, individual solder compositions to produce molten droplets 50 containing a large mixture of different materials. Such multi-combination LIFT schemes are described, for example, in U.S. Patent 10,629,442, which is incorporated herein by reference.

Alternatively or additionally, the donor foil 44 may include a donor film 48 that includes different materials at different locations on the donor foil. The optical assembly 30 guides the laser pulse sequence to irradiate different donor locations respectively, so that the droplets 32 of different materials deposited at a given target location accumulate to achieve a specified volume and composition. This hybrid combination scheme is further described below with reference to Figure 4A/Figure 4B.

The controller 51 adjusts the pulse parameters of the laser 38 and the scanning and focusing parameters of the optical assembly 30 so as to deposit an appropriate number of droplets 32 of a desired volume at each target location where a solder bump will be formed on the receptor substrate 34. As explained earlier, the controller 51 sets the laser pulse parameters and the number of molten droplets of solder material so that the accumulation of droplets deposited at each target location achieves a specified bump volume at that location. Since the droplet volume can be varied by adjusting the laser pulse parameters, a given bump volume can be produced by depositing a smaller number of droplets of larger volume or a larger number of droplets of smaller volume. The thickness 38 of the donor film also contributes to the droplet size. However, given the inherent tolerances in actual droplet volume, it is dependent on a large amount of statistical data, and using a larger number of smaller droplets rather than a smaller number of larger droplets may be advantageous, particularly when processing very small bumps.

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

The reflow station 24 includes an optical assembly 52 that directs a radiation beam to locally melt the droplets 32, thereby causing the droplets to coalesce into solder bumps 60. Such localized heating is particularly advantageous in avoiding damage to the sensitive receptor substrate 34. The optical assembly 52 in the illustrated example includes a laser 54 together with a beam deflector 56 and focusing optics 58 that direct the laser radiation to the target location with sufficient energy to cause the deposited droplets to melt and reflow into the solder bumps 60. The reflow station 24 also includes a positioning assembly that can be based on the same stage 36 as in the printing station 22, or a different stage or other motion device.

The controller 51 adjusts the pulse parameters of the laser 54 and the scanning and focusing parameters of the optical assembly 52 so as to apply sufficient energy to melt and reflow each solder bump 60 while avoiding damage to the receptor substrate 34. The pulse duration and energy are selected so that the solder material at the bottom of each bump is completely melted without evaporating the solder material at the top of the bump. The actual power and pulse duration required depends on the melting temperature and thermal conductivity of the solder material. For this reason, short laser pulses are generally preferred because they minimize the time the solder material is melted and therefore minimize oxidation and avoid damage to the receptor substrate 34. Thus, the reflow station 24 is able to operate under ambient atmospheric conditions. Short, high-power laser pulses are particularly advantageous in achieving unassisted reflow and supporting the use of high-temperature solder materials. Such a single laser pulse focused on the location of each solder bump is generally sufficient to achieve complete reflow of small solder bumps, although multiple pulses may be used instead, particularly for larger solder bumps. The resulting rapid, localized reflow process also helps reduce the formation of intermetallic compounds between the solder bump and the contact pad, and thus produces a stronger solder joint, compared to thermal reflow methods known in the art.

For a small solder bump made from a pile of droplets 32 of 20μm to 30μm thick tin-based solder, for example, a laser pulse with an optical power of approximately 10W and a duration of 50μs to 100μs is typically sufficient to achieve complete reflow while avoiding significant heat diffusion to the substrate. The optimum laser wavelength, pulse power, duration and focal length may be selected in each case to match the absorption spectrum, volume and thermal properties of the solder material. For small to medium sized solder bumps, the pulse energy needs to be no more than about 2mJ. The optimum laser parameters may 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, the laser 54 in the reflow station 24 may be a high power CW Nd:YAG laser operating at 1064nm. Alternatively, the laser 54 may be a diode pumped fiber laser, such as a continuous wave fiber laser in the 976nm to 1075nm range and tens of watts of power (e.g., available from IPG). Alternatively, the laser 54 may be a high power diode laser module, such as a diode laser module manufactured by BTW. Those skilled in the art will understand other types of lasers after reading this specification.

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

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

Those familiar with this technology will understand the alternatives for laser-driven reflow parameters after reading this manual.

In one embodiment, the printing station 22 and the reflow station 24 are combined into a single operating unit with an optical assembly that provides 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 purposes. Alternatively, the combined station may include two or more different laser sources with a common positioning assembly and possible common optics.

Formation of solder bumps

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

As shown in FIG. 2A , electronic traces 73 and various contact pads 72, 75, 77 of different sizes and shapes are formed on the substrate 70 prior to depositing solder bumps on the substrate. As is known in the art, these pads and traces may be printed on the substrate 70 using a photolithography process, or they may alternatively be written directly onto the substrate 70, for example using a LIFT process. LIFT printing of the contact pads may be beneficial in enhancing the adhesion of the solder material to the contact pads, as further explained below with reference to FIG. 5A / FIG. 5B . The controller 51 is programmed to specify the different solder bump volumes to be produced on the different solder pads. The controller drives the optical assembly 30 to direct different sequences of laser pulses across different points on the donor substrate so that the deposited droplets 32 cumulatively reach a specified bump volume on each pad. Thus, for example, only a single droplet 32 or a small number of droplets is deposited on each of the pads 72, while a larger collection of droplets 74 is deposited on pad 75. When very fine contacts are required, as in the case of pad 72, droplets can also be deposited directly onto target locations on traces 73 without the need for dedicated contact pads, and thus the component is welded directly to the trace.

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

Controller 51 may additionally 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 across the donor substrate so that droplets 32 are deposited on each contact pad in a pattern conforming to the specified shape. Thus, an elongated collection of droplets 76 is deposited on contact pads 77. Solder contacts may be printed in this manner in virtually any desired shape, including, for example, circular and angled shapes.

After the droplets 32 are deposited, the substrate 70 is heated to melt the droplets and reflow, thereby coalescing into solder bumps 82, 84, 86, as shown in FIG. 2B. At this stage, the tendency of the droplets is to coalesce into a spherical shape, which minimizes the surface energy. To minimize this tendency, especially when producing non-circular solder bumps, the reflow station 24 can use short and intense laser pulses to locally melt the solder bumps. As explained above. The laser pulse parameters and the irradiation pattern in the reflow station 24 can be adjusted to achieve the desired shape characteristics.

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

To address this problem, droplets 32 are deposited in multiple layers at a target location to achieve a specified bump volume. Substrate 70 is shuttled between printing station 22 and reflow station 24 multiple times to alternately deposit a layer of droplets and then heat the layer to melt the droplets until the specified bump volume is achieved. Alternatively, LIFT printing and melting of droplets can be performed in a single station, where the optical assembly has the required capabilities for LIFT printing and reflow. In either case, the energy that must be applied to melt each successive layer of droplets is relatively small, and the risk of damage is therefore reduced.

Thus, in the example depicted, an initial layer of droplets 32 is deposited on substrate 70, as shown in FIG3A (or more precisely, on a contact pad on the substrate). This layer is heated and thus melted to form a reflow layer 92, as shown in FIG3B. A further layer of droplets is deposited over reflow layer 92, as shown in FIG3C, and then heated to reflow again, as shown in FIG3D. This process is repeated for as many cycles as are necessary to produce solder bumps 94.

Referring now to FIG. 4A/FIG. 4B, a process of producing a solder bump 100 of a mixed composition on a substrate 70 according to an embodiment of the present invention is schematically illustrated. FIG. 4A is a cross-sectional view showing droplets 96 and 98 of two different, respective solder materials deposited in a printing station 22 by a LIFT process. FIG. 4B is a front view of the solder bump 100 after the solder material has been reflowed in a reflow station 24.

Controller 51 receives a specification for solder bump 100 indicating that the solder bump will include two (or more) different materials in a certain ratio. For example, to improve mechanical strength and/or conductivity, the solder bump may include copper particles mixed with a tin solder, or palladium particles mixed with a SAC solder. In some cases, it may also be advantageous to distribute the different materials unevenly within the solder bump with a specified material gradient. 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 the concentration decreasing toward the top of the solder bump relative to the material in droplet 98. Such a gradient composition of palladium and copper, with a higher palladium concentration at the bottom of the bump, is believed to improve the strength of the solder joint, as disclosed, for example, in U.S. Patent 9,607,936.

In the example shown in FIG. 4A , the donor foil 44 includes two donor films 48 that include two different donor materials, such as the different types of materials mentioned above. The optical assembly 30 directs laser pulses toward one of the donor films to deposit droplets 96 on the substrate 70, and toward the other donor film to deposit droplets 98. The number of pulses directed toward each of the donor films is selected so that droplets 96 and 98 are deposited and accumulated in the appropriate proportion to achieve a specified composition and total solder bump volume. In order to produce a gradient composition, the ratio of pulses directed toward the two donor films, and therefore the ratio of droplets 96 to droplets 98, varies layer by layer from the bottom to the top of the droplet, as shown in FIG. 4A . Rapid heating of the collection of droplets 96 and 98 in the reflow station 24 causes the droplets to coalesce into solder bump 100 with minimal mixing, such that the specified gradient is maintained, as schematically illustrated in FIG. 4B .

This multi-material solder bump deposition can also be used in other applications. For example, the base layer of a solder bump can be made by printing droplets of one material that improves or alternatively confines the solder wetting. As another example, the base layer can be selected to improve the matching of the coefficients of thermal expansion between the substrate and the solder material. This property can be fine-tuned by mixing two materials with different coefficients of thermal expansion in order to match the coefficient of thermal expansion of the substrate.

FIG. 5A is a micrograph 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, for example, a suitable copper donor film 48 is used to write the contact pad directly onto the substrate 112 by LIFT, rather than producing the contact pad by conventional photolithography. LIFT printing of the contact pad 110 can be used to control the shape and texture of the contact pad in order to improve the adhesion of the solder bump to the pad. Thus, as shown in the figure, the contact pad 110 has a rough surface with a concave surface 114 in the center of the pad.

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

It will be understood that the above embodiments are cited by way of example, and the present invention is not limited to what has been specifically shown and described above. On the contrary, the scope of the present invention includes combinations and sub-combinations of the various features described above, as well as variations and modifications thereof that will occur to a person skilled in the art after reading the foregoing description and which are not disclosed in the prior art.

20: System

22: Printing station

24: Reflow station

26: Placement station

28: Final return station

30:Optical assembly

32: Droplets

34: Receptor substrate

36: X-Y platform

38: Laser

40: Beam deflector

42: Focusing optical devices

44: Donor foil

46: Donor substrate

48: Donor membrane

50: Droplets

51: Controller

52: Optical assembly

54:Laser

56: Beam deflector

58: Focusing optical devices

60: Solder bump

62: Pick and place machine

64:Components

66: Heat source

68:Joining

Claims (29)

A method for circuit manufacturing includes: defining a solder bump to be formed at a target location on a receptor substrate, the solder bump including a specified solder material and having a specified bump volume; positioning a transparent donor substrate having first and second surfaces opposite to each other and a donor film including the specified solder material on the second surface so that the donor film is close to the target location on the receptor substrate; introducing A pulse sequence of laser radiation is directed through the first surface of the donor substrate and irradiated onto the donor film to induce a plurality of molten droplets of the solder material to be ejected from the donor film onto the target position on the receptor substrate, so that the droplets deposited at the target position accumulate to achieve the designated bump volume; and the target position is heated so that the deposited droplets melt and reflow to form the solder bump. The method of claim 1, wherein the droplets have respective droplet volumes that depend on an intensity of the pulses of the laser radiation, and wherein directing the sequence of the pulses includes setting the intensity of the pulses of the laser radiation and the number of the pulses in the sequence in response to the specified bump volume. The method of claim 2, wherein the droplet volumes are further dependent on a set of pulse parameters, the set of pulse parameters comprising a spot size and duration of the pulses of the laser radiation, and wherein directing the sequence of pulses further comprises adjusting the droplet volumes by varying one or more of the pulse parameters. The method of claim 1, wherein defining the solder bump comprises: defining first and second solder bumps having different, respective first and second bump volumes at different, respective first and second target locations on the same receptor substrate, and wherein directing the pulse sequence comprises directing different, first and second pulse sequences of the pulses through different points on the donor substrate such that the droplets cumulatively achieve each of the different first and second bump volumes at the respective first and second target locations. The method of claim 4, wherein defining the first solder bump and the second solder bump comprises: specifying different, respective first and second compositions of the first solder bump and the second solder bump, and wherein positioning the transparent donor substrate comprises providing one or more donor films comprising a plurality of different solder materials selected to produce the first composition and the second composition. The method of claim 1, wherein defining the solder bump comprises: defining a first solder bump and a second solder bump having different, respective first and second compositions, and wherein positioning the transparent donor substrate comprises providing one or more donor films comprising a plurality of different solder materials to produce the first composition and the second composition. The method of claim 1, wherein defining the solder bump comprises: specifying a composition of the solder bump, the composition comprising different, first and second materials, and wherein positioning the transparent donor substrate comprises providing a first donor film and a second donor film comprising the first material and the second material, respectively, and wherein directing the pulse sequence comprises directing the first sequence and the second sequence of the pulses to irradiate the first donor film and the second donor film, respectively, so that the droplets deposited at the target location cumulatively achieve the specified composition. The method of claim 7, wherein specifying the composition comprises: specifying a gradient of the materials in the composition of the solder bump, and wherein directing the first sequence and the second sequence of pulses comprises depositing the droplets of the first material and the second material in multiple layers at the target location according to the specified gradient. The method of claim 1, wherein directing the sequence of pulses comprises depositing the droplets in multiple layers at the target location to achieve the specified bump volume. The method of claim 9, wherein heating the target position comprises: alternately depositing a layer of droplets multiple times and heating the layer to melt the droplets until the specified bump volume is achieved. The method of claim 1, wherein defining the solder bump comprises: specifying a shape of the solder bump, and wherein directing the sequence of pulses comprises depositing the molten droplets in a pattern conforming to the specified shape. The method of claim 1, wherein heating the target location includes: directing a laser beam to irradiate the target location with sufficient energy to melt and reflow the deposited droplets. The method of claim 1, comprising printing a conductive pad at the target location on the receptor substrate using a laser induced forward transfer (LIFT) process, wherein the sequence of directing the pulses includes depositing the molten droplets of the solder material on the printed conductive pad. The method of claim 13, wherein printing the conductive pad includes: forming a concave surface in the conductive pad for depositing the molten droplets therein. A system for circuit manufacturing, comprising: a controller configured to receive a definition of a solder bump to be formed at a target location on a receptor substrate, the solder bump comprising a specified solder material and having a specified bump volume; a printing station comprising: a transparent donor substrate having first and second opposing surfaces and having a donor film comprising the specified solder material disposed on the second surface and positioned so that the donor film is proximate to the target location on the receptor substrate; and a laser configured to A pulse sequence of laser radiation is directed to pass through the first surface of the donor substrate and irradiate the donor film to induce molten droplets of the solder material to be ejected from the donor film to the target position on the receptor substrate, wherein the controller is configured to drive the printing station to eject a plurality of droplets toward the target position so that the droplets deposited at the target position cumulatively achieve the specified bump volume; and a reflow station is configured to heat the target position so that the deposited droplets melt and reflow to form the solder bump. The system of claim 15, wherein the droplets have respective droplet volumes that depend on an intensity of the pulses of the laser radiation and a set of pulse parameters consisting of a spot size and duration of the pulses of the laser radiation, wherein the controller is configured to set the intensity of the pulses of the laser radiation and the number of pulses in the sequence in response to the specified bump volume, and wherein the controller is configured to adjust the droplet volumes by changing one or more of the pulse parameters. The system of claim 15, wherein the controller is configured to receive definitions of first and second solder bumps having different, respective first and second bump volumes at different, respective first and second target locations on the same receptor substrate, and wherein the controller is configured to drive the laser to direct different, first and second sequences of pulses through different points on the donor substrate such that the droplets accumulate to achieve the respective first and second bump volumes. Each of the different first and second bump volumes at a target location, and wherein the definitions of the first solder bump and the second solder bump specify different, respective first and second compositions of the first solder bump and the second solder bump, and wherein one or more donor films comprising a plurality of different solder materials are disposed on the second surface of the donor substrate, wherein the solder materials are selected to produce the first composition and the second composition. The system of claim 15, wherein the controller is configured to receive a definition of a first solder bump and a second solder bump having different, respective first and second compositions, and wherein one or more donor films of a plurality of different solder materials are disposed on the second surface of the donor substrate, wherein the solder materials are selected to produce the first composition and the second composition. The system of claim 15, wherein the definition specifies a composition of the solder bump comprising different, first and second materials, and wherein the transparent donor substrate comprises a first donor film and a second donor film comprising the first material and the second material, respectively, wherein the controller is configured to drive the printing station to direct the first sequence and the second sequence of pulses to irradiate the first donor film and the second donor film, respectively, so that the droplets deposited at the target location cumulatively achieve the specified composition, and wherein the definition specifies a gradient of the materials in the composition of the solder bump, and wherein the controller is configured to drive the printing station to deposit the droplets of the first material and the second material in multiple layers at the target location according to the specified gradient. A system as claimed in claim 15, wherein the controller is configured to drive the printing station to deposit the droplets in multiple layers at the target location to achieve the specified bump volume. A system as claimed in claim 20, wherein the controller is configured to drive the printing station and the reflow station to alternately deposit a droplet layer multiple times and heat the droplet layer to melt the droplets until the specified bump volume is achieved. The system of claim 15, wherein the definition specifies a shape of the solder bump, and wherein the controller is configured to drive the printing station to direct the sequence of pulses so as to deposit the molten droplets in a pattern conforming to the specified shape. The system of claim 15, wherein the printing station is configured to print a conductive pad at the target location on the receptor substrate using a laser induced forward transfer (LIFT) process and deposit the molten droplets of solder material on the printed conductive pad. A system as claimed in claim 23, wherein the printing station is configured to print the conductive pad having a concave surface for depositing the molten droplets therein. A method for circuit manufacturing, comprising: positioning a transparent donor substrate having first and second surfaces opposite to each other and having a donor film including a specified solder material on the second surface so that the donor film is close to the target location on the receptor substrate; spraying molten droplets of the solder material toward a target location by directing a pulse sequence of laser radiation through the first surface of the donor substrate and irradiating the donor film so as to induce the molten droplets of the solder material sprayed from the donor film to the target location on the receptor substrate so that the droplets deposited at the target location accumulate to achieve the specified bump volume; and focusing one or more pulses of a laser beam with sufficient energy to melt and reflow the deposited droplets to form solder bumps. A method as claimed in claim 25, wherein the pulses have a pulse duration of no more than 1 ms. A method as claimed in claim 26, wherein the pulses have a pulse duration of not less than 100 μs. A method as claimed in claim 25, wherein the pulses have a pulse energy not greater than 3 mJ. The method of claim 25, wherein focusing the one or more pulses comprises focusing a single, individual pulse of the laser beam onto each of the target locations.