US20030136222A1 - High-speed fabrication of highly uniform ultra-small metallic microspheres - Google Patents
High-speed fabrication of highly uniform ultra-small metallic microspheres Download PDFInfo
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- US20030136222A1 US20030136222A1 US10/292,998 US29299802A US2003136222A1 US 20030136222 A1 US20030136222 A1 US 20030136222A1 US 29299802 A US29299802 A US 29299802A US 2003136222 A1 US2003136222 A1 US 2003136222A1
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F9/00—Making metallic powder or suspensions thereof
- B22F9/02—Making metallic powder or suspensions thereof using physical processes
- B22F9/06—Making metallic powder or suspensions thereof using physical processes starting from liquid material
- B22F9/08—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
- B22F1/06—Metallic powder characterised by the shape of the particles
- B22F1/065—Spherical particles
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F9/00—Making metallic powder or suspensions thereof
- B22F9/02—Making metallic powder or suspensions thereof using physical processes
- B22F9/06—Making metallic powder or suspensions thereof using physical processes starting from liquid material
- B22F9/08—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying
- B22F9/082—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying atomising using a fluid
- B22F2009/0836—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying atomising using a fluid with electric or magnetic field or induction
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F9/00—Making metallic powder or suspensions thereof
- B22F9/02—Making metallic powder or suspensions thereof using physical processes
- B22F9/06—Making metallic powder or suspensions thereof using physical processes starting from liquid material
- B22F9/08—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying
- B22F9/082—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying atomising using a fluid
- B22F2009/084—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying atomising using a fluid combination of methods
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
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- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K3/00—Apparatus or processes for manufacturing printed circuits
- H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
- H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
- H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
- H05K3/3457—Solder materials or compositions; Methods of application thereof
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- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K3/00—Apparatus or processes for manufacturing printed circuits
- H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
- H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
- H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
- H05K3/3457—Solder materials or compositions; Methods of application thereof
- H05K3/3478—Applying solder preforms; Transferring prefabricated solder patterns
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12014—All metal or with adjacent metals having metal particles
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Abstract
In a high-speed fabrication process for producing highly uniform ultra-small metallic micro-spheres, a molten metal is passed through a small orifice, producing a stream of molten metal therefrom. A series of molten metal droplets forms from the break up of the capillary stream. Applied harmonic disturbances are used to control and generate satellite and parent droplets. Significantly, the satellite droplets formed are smaller than the orifice, allowing for the production of smaller metal balls with larger orifices. The satellite droplets are separated from the parent droplets by electrostatic charging and deflection or by aerodynamic or acoustic sorting. Preferably, the satellite droplets are cooled before being collected to avoid defects and achieve high uniformity of the resulting metal balls.
Description
- This application is a divisional of U.S. application Ser. No. 09/860,803 filed May 18, 2001, the contents of which are hereby fully incorporated by reference. This application is related to provisional U.S. application serial No. 60/206,507, filed May 22, 2000, the contents of which are hereby fully incorporated by reference.
- [0002] This invention was made with Government support under Grant No. DMI-9457205, awarded by NSF. The Government has certain rights in this invention.
- The invention relates to methods of fabricating highly uniform, ultra-small metallic micro-spheres or balls, and to the balls themselves from capillary stream break-up at high rates.
- The generation of droplets from capillary stream break-up has been studied at least as early as Lord Rayleigh in the 1800s. More recently, the formation of metallic micro-spheres, or balls, from the break-up of a molten metal capillary stream has been studied. Such balls are commonly used in the electronics industry for various applications, including interconnects for small electronics packages and in the manufacture of conductive pastes. Using the process of capillary stream break-up, the balls can be produced at very high rates—typically tens of thousands of droplets per second. Further, the nature of droplet formation due to capillary stream break-up results in highly uniform balls. The highly uniform size of the metal balls formed from capillary stream break-up is a significant improvement over other methods of forming conductive powders—such as spray atomization or melt spinning—which require the extra step of sieving or sorting the differently sized balls. This extra step is labor intensive, significantly increasing the time and cost of the manufacturing process; however, with such technologies, sorting or sieving is necessary to achieve tight ball diameter tolerances (on the order of five percent).
- In the production of metal balls from capillary stream break-up, it is advantageous to effectively cool the balls so that they solidify before landing or bonding with each other. Effective solidification reduces or eliminates (1) irregularly shaped balls that have dented when they impinge and (2) irregularly sized balls that have bonded together because they were insufficiently cooled. Without effective solidification, removal of these defects requires that the balls be sieved or sorted.
- Conventional methods of formation of metal balls due to capillary stream break-up tend to be limited to metal balls having diameters in excess of 50 microns. A significant limitation on the size of metal balls produced from capillary stream break-up is the size of the orifice from which the capillary stream emerges. Typically, droplets generated from capillary stream break-up have diameters that are roughly twice as large as the diameter of the capillary stream orifice. The production of smaller balls, therefore, typically requires smaller orifices. As the orifice becomes very small, it tends to be more easily clogged by, e.g., impurities in the molten metal. Further, obtaining smaller orifices that are also uniform tends to be difficult and expensive. Current state-of-the-art provides a lower limit of orifice diameter available off-the-shelf and suitable for use with molten metals of 25 microns.
- Accordingly, the present invention enables the formation of metallic micro-spheres due to capillary stream break-up that are significantly smaller than metallic micro-spheres formed by conventional methods and, more particularly, to metallic micro-spheres that are significantly smaller than the capillary stream orifice from which they emerge, thereby overcoming many of the difficulties that plagued the prior art by advantageously enabling the formation of much smaller micro-spheres from larger orifices. The present invention further enables forming highly uniform metalic micro-spheres or balls, having diameters on the order of about 1 to 100 microns, and preferably less than 25 microns, without the defects and difficulties associated with conventional methods.
- A method of manufacturing ultra-small metallic spheres comprises directing a capillary stream of molten metal from an orifice by applying an excitation disturbance, wherein the excitation disturbance is determined so that parent droplets and satellite droplets form from the stream due to capillary stream break-up. In one innovative aspect of the present invention, the satellite droplets are separated from the parent droplets; cooled to form solid balls of substantially spherical shape; and collected as separate solid satellite balls. In another innovative aspect of the present invention, the satellite and parent drops are simultaneously cooled and collected as solid balls.
- In one embodiment, the separating step is accomplished by electrostatically charging the droplets and directing them through an electric field, whereby the satellite and parent droplets deflect differently due to the different charge-to-mass ratios. In another embodiment, the droplets may be directed through a second electric field, a rotating field, or both to further disperse the droplets. In either of these embodiments, the electrostatic charge may vary over time while the electric field remains constant or the electric field may vary over time while the electrostatic charge remains constant.
- In accordance with another embodiment, separation of the satellite and parent droplets is accomplished by acoustic forcing. In accordance with yet another embodiment, the satellite and parent droplets are separated with aerodynamic forces.
- In another innovative aspect, a solid metal ball of the present invention has a diameter that is preferably substantially less than the diameter of the capillary orifice. In a further innovative aspect, a solid metal ball of the present invention is substantially spherical and has a diameter in a range of about 1.0 to 100 microns, and preferably less than 25 microns. In yet a further innovative aspect, a metallic powder comprises a plurality of such balls, wherein the balls are highly uniform having a ball diameter tolerance of a mean ball diameter in the range of about 0.5 to 3.0 percent, and preferably less than 2.0 percent, without performing a mechanical sieving or sorting step.
- In another innovative aspect of the present invention, the metal balls, satellite or both satellite and parent, are produced at a rapid rate, wherein the balls are highly uniform, having highly uniform diameters. More particularly, the balls may be produced at a rate preferably in a range of about 1000 to 200,000 balls per second, and preferably at a rate greater than about 4000 balls per second while maintaining a ball diameter tolerance in the range of about 0.5 to 3.0 percent, and preferably a ball diameter tolerance of less than about 2.0 percent, without performing a mechanical sieving or sorting step.
- Other aspects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.
- FIG. 1 is a cross sectional view of the droplet generation system.
- FIG. 2 is a side view of the capillary stream and satellite droplet formation.
- FIG. 3 is a schematic view of an embodiment for generating satellite droplets.
- FIG. 4 is a graph of measured and theoretical droplet charge per mass versus charge electrode voltage.
- FIG. 5 is a graph of measured and theoretical droplet deflection given deflection plate biasing.
- FIGS. 6a and 6 b are is a schematic views of another embodiment for generating satellite droplets.
- FIG. 7 is a schematic view of another embodiment for generating satellite droplets.
- In accordance with the present invention, ultra small metal balls or micro-spheres are produced at a high rate by capillary stream break-up, wherein highly uniform and predictable droplets break from a capillary stream of molten metal. The present invention enables the formation of metallic micro-spheres due to capillary stream break-up that are significantly smaller than metallic micro-spheres formed by conventional methods and, more particularly, to metallic micro-spheres that are significantly smaller than the capillary stream orifice from which they emerge, thereby overcoming many of the difficulties that plagued the prior art by advantageously enabling the formation of much smaller micro-spheres from larger orifices. The balls may be formed from one or a combination of various metals, including solder, copper, nickel, titanium, or any metal having physical properties (e.g., melting point) suitable for the process described herein.
- FIG. 1 shows a
system 10 for producing metal balls in accordance with one embodiment. To form a capillary stream, adroplet generator 12 is provided. One example of a droplet generator suitable for these purposes is described in U.S. Pat. No. 6,186,192 to Orme et al., hereby incorporated in full by reference. This patent describes a system for generating a capillary stream of molten metal, from which a continuous series of molten droplets form. Although the patent is directed to producing droplets of molten solder, any metal having a suitable melting point may be used therewith. - The
droplet generator 12 includes achamber 14 adapted to hold a reservoir ofmolten metal 16 therein. As explained, this molten metal comprises any metal having physical characteristics compatible with thesystem 10 and method described. The melting point of some metals, for example, may be too high to use with thesystem 10 shown in FIG. 1. A vibratingrod 18 is slidably disposed within thechamber 14, contacting themolten metal 16. Therod 18 is mechanically coupled to a piezoelectric crystal ortransducer 20 and, as described, is used to impart a disturbance in the molten metal. However, it should be appreciated that a disturbance may be imparted mechanically with a piezoelectric transducer with or without a rod or plunger—for example, the piezoelectric transducer may be placed under the orifice to eliminate the rod or plunger—or a disturbance may be imparted from magnetic, electric or acoustic forces. - As shown, the
piezoelectric crystal 20 is disposed outside thechamber 14 to protect it from the heat of themolten metal 16, as piezoelectric materials can be damaged if subjected to high temperatures. However, for metals with low melting points, such as solder, it may be possible to immerse the piezoelectric crystal in the molten fluid or position the piezoelectric crystal under the orifice where temperatures are high. To further protect thepiezoelectric crystal 20 from heat transferred from the vibratingrod 18, a coolingjacket 22 may be attached to the vibratingrod 18, or to a housing around therod 18, near thecrystal 20 to keep it at a cooler temperature. The coolingjacket 22 may be, for example, fluidly coupled to a circulating water supply that circulates room temperature water through the coolingjacket 22. Additionally, to maintain themolten metal 16 inside thechamber 14 above its melting point,heaters 24 may be coupled to the outer wall of thechamber 14 at spaced-apart locations. - A
controller 26, which may comprise one or more microprocessors and one or more power supplies, is electrically coupled to thepiezoelectric crystal 20 byelectrical connection 28. Thecontroller 26 delivers an alternating electrical signal to thepiezoelectric crystal 20, causing a corresponding mechanic response. The vibratingpiezoelectric crystal 20 causes the vibratingrod 18, to which thecrystal 20 is coupled, to oscillate. The vibratingrod 18 is preferably biased with a periodic waveform, typically with a magnitude of about 50 to 300 Volts, and a fundamental frequency ƒ, which corresponds to the frequency of perturbation applied to the capillary stream for uniform droplet production, determined by the following equation: - wherein V is the droplet or stream velocity, ro is the orifice radius, and k is a non-dimensional wavenumber constant, which depends on the fluid properties of viscosity, surface tension and density, and ambient gas density. See M. Orme, “On the Genesis of Droplet Stream Microspeed Dispersions,” Physics of Fluids, 3, (12) pp 2936-2947, 1991. The constant k tends to vary between 0.4 and 0.8; for inviscid fluids, k equals 0.697. It should be appreciated that uniform droplets may be produced at high rates and that the fundamental frequency ƒ varies according to orifice size and stream velocity. Preferably, the fundamental frequency ƒ, and thus the droplet production rate, is in a range of about 1000 Hz to 200 kHz.
- The
molten metal 16 is ejected from thechamber 14 through anorifice 30 in the bottom of thechamber 14, from which astream 32 of the molten metal forms. The oscillation of the vibratingrod 18 produces a standing wave in themolten metal 16 and in thestream 32 as it leaves theorifice 32. Due to capillary stream break-up,molten metal droplets 34 form by detaching from thestream 32. Adroplet 34 formed from capillary stream break-up has a diameter typically about twice the diameter of theorifice 30. With the current state-of-the-art of off-the-shelf orifices having diameters limited to 25 microns or greater, the droplets formed from streams emerging from such orifices tend to be in excess of 50 microns. To control the formation ofmolten metal droplets 34 leaving thedroplet generator 12, asupply 36 delivers nitrogen gas (or other inert gas, such as argon) along agas line 38 to pressurize thechamber 14, thereby affecting the tendency ofmolten metal 16 to leave thechamber 14 through theorifice 30. Nitrogen (or other inert gas, such as argon) may also be supplied through a gas line to a detachable end assembly to further control solder droplets. Preferably, the inert gas is a high purity gas, such as research grade or better. - FIG. 2 illustrates the process of generating droplets from capillary stream break-up. An axisymmetric excitation disturbance is imparted to the
stream 32 whose fundamental wavelength is in the region of Rayleigh growth. As described above, the disturbance is imparted, in this embodiment, by driving thepiezoelectric crystal 20, to which the vibratingrod 18 is mechanically coupled, with an electrical signal representing the disturbance vialine 28. Alternatively, as described above, the disturbance may be imparted with a piezoelectric transducer with or without a rod or plunger, or from magnetic, electric or acoustic forces. As illustrated, the disturbance grows, resulting in the standing wave on thestream 32 and causing the series ofdroplets larger parent droplets 37 are typically on the order of twice the diameter of theorifice 30, whereas thesmaller satellite droplets 35 have diameters much smaller than theorifice 30. - Depending on the characteristics of the excitation disturbance, a
satellite droplet 35 will merge with the forward orrearward parent droplet 37 to form amerged droplet 34, or can be forced to maintain its position between the forward andrearward parent droplets 37 using an appropriate application of harmonics on the axisymmetric disturbance. An example of such a disturbance is one having second and third order harmonics, although many other disturbances are possible. In the example of FIG. 2, the satellite droplets merge with a parent droplet within one wavelength, λ, of the excitation disturbance. The merging time and the diameter of the satellite droplets can be manipulated by the waveform conditions. For example, waveforms composed of added harmonics, or waveforms with very high driving amplitudes, which effectively distort the linearity of the disturbance will affect the properties of the stellite droplets. Accordingly, the present invention uses harmonic disturbances to prevent instantaneous merging so that the satellite droplets can be deflected out of the main stream to separate the satellite droplets from the parent droplets. The diameter of the satellite droplet tends to be a function of the characteristics of the excitation disturbance, while the diameter of the parent droplet tends to be a function of the excitation disturbance and the nozzle orifice as shown by the following: - r d =[r o 3(8π)/(3ko)−r s 3]1/3 (2.0)
- where ro is the orifice radius and rs is the satellite droplet radius.
- Once the
satellite droplets 35 andparent droplets 37 are formed, they are separated, and then thesatellite droplets 35 or satellite andparent droplets charge electrode 40 is located near theorifice 30 wheredroplets capillary stream 32. Thecharge electrode 40 allows for an electrostatic charge to be selectively applied to any of thedroplets charge electrode 40 is coupled to thecontroller 26 byelectrical connection 42. Because of the highly predictable nature of droplet formation from capillary stream break-up, the time at whichdroplets capillary stream 32 is known to a precise degree, given the function at which thepiezoelectric crystal 20 is driven and other system parameters. More particularly, the perturbation on the stream's radius grows exponentially in time, t, as r(t)=ro±roκeβt, where κ and β are the amplitude of the initial perturbation and the disturbance growth rate, respectively. The time at which droplets break from the capillary stream is the time when r(t)=0, i.e., when t=(1/β)ln(1/κ). See M. Orme, “On the Genesis of Droplet Stream Microspeed Dispersions,” Physics of Fluids, 3, (12) pp 2936-2947, 1991. - It can be appreciated that an electrostatic charge on the
charge electrode 40 causes a corresponding electrostatic charge on theconductive capillary stream 32. When adroplet stream 32, thedroplet droplet droplet controller 26, by charging thecharge electrode 40 to a predetermined value at the time that eachdroplet stream 32. The electrostatic charge, Q, per mass, m, of each droplet is given theoretically by Schneider's Equation: - where εo is the permitivity of free space, Vc is the charge potential, ρ is the fluid density and b is the electrode radius. See J. M. Schneider, N. R. Lindblad, & C. D. Hendricks, “Stability of an Electrified Liquid Jet,” J. Applied Physics. 38, 6, 2599, 1967. The graph of FIG. 4 compares measured and predicted results for the charge per unit mass of the droplets, using the apparatus and method described herein. As FIG. 4 shows, Schneider's Equation is useful to predict the charge of the droplets.
- After being electrostatically charged, the
droplets deflection plates 44. The bias voltage across thedeflection plates 44 is controlled by thecontroller 26. When a bias voltage is applied across thedeflection plates 44 byelectrical connections 46, it can be appreciated that an electric field is formed between theplates 44. As chargeddroplets plates 44, and thus through this electric field, thedroplets - The magnitude of the electrostatic force acting on the
droplet droplet capillary stream 32—and thus the path thedroplet -
- where D is the aerodynamic drag force, g is the gravitational constant, θ is the deflection angle measured from the undeflected stream, A is the frontal surface area of the sphere, Cd is the dimensionless drag coefficient, and Re is the dimensionless Reynolds number. See Q. Liu, C. Huang, and M. Orme, “Mutual Electrostatic Charge Interactions Between Closely Spaced Charged Solder Droplets.” J. of Atomization and Sprays, Vol. 10 no. 6, pp 565-585, 2000.
- As FIG. 5 shows, this model (Equations 5.0-5.3), which incorporates drag, very accurately predicts measured deflection values. Fillmore's Approximation (Equation 4.0) also tends to indicate a droplet's deflection, although it tends to underestimate the actual deflection somewhat.
- For the embodiment shown in FIG. 3, the
satellite droplets 35 will have higher charge to mass ratios than theparent droplets 37, so the electrostatic deflection of thesatellite droplets 35 will be greater. Accordingly, acollector 48 is provided to catch at least thesatellite droplets 35, preferably after they have solidified to avoid defects. In one aspect of a preferred embodiment, the collector has afirst section 50 and asecond section 52, wherein the first andsecond sections parent droplets - Another method of separating the satellite droplets from the parent droplets is by acoustic forcing. As shown in FIGS. 6a and 6 b, acoustic forcing is used to exploit the rotation imparted onto the
capillary stream 32 as it exists from theorifice 30. The direction of rotation is shown by arrow A. Due to conservation of angular momentum, increasing the amplitude of the excitation disturbance (as shown in FIG. 6a) causes thesatellite droplets 35 to be deflected out of the main stream and away from theparent droplets 37. When the excitation amplitude is reduced (as shown in FIG. 6a), the effects of the rotation are less pronounced, and the satellite droplets do not separate from the main stream. As with the embodiment shown in FIG. 3, acollector 48 is provided to catch at least thesatellite droplets 35, preferably after they have solidified to avoid defects. The collector preferably has afirst section 50 and asecond section 52, wherein the first andsecond sections parent droplets - Another method of separating the satellite droplets from the parent droplets uses aerodynamic forces, as shown in FIG. 7. A transverse aerodynamic force is a applied to the
satellite droplets 35 andparent droplets 37 by, e.g.,fans 54, air jets or the like. Because of the mass difference between the satellite and parent droplets, the transverse aerodynamic force is large enough to propel thesatellite droplets 35 out of the main stream, but it is insufficient to significantly affect thelarger parent droplets 37. Acollector 48 is provided to catch at least thesatellite droplets 35, preferably after they have solidified to avoid defects. - With respect to any of the embodiments described, the parent droplets can be recycled back into the
chamber 14 after they are collected. To avoid impurities, the recycled metal is preferably filtered. - To produce highly uniform and substantially spherical metal balls with little or no defects, it is important that the droplets solidify in flight. Cooling the spheres in flight avoids the problem of bonding between the molten droplets, either in flight or during their collection. Further, solidifying the droplets before they are collected avoids defects of their spherical shape that would result from a molten, or partially molten, droplet hitting a hard surface. Therefore, the balls formed in accordance with the present invention are preferably solidified before being collected. More effective cooling of the droplets can be accomplished in various ways. For example, lengthening the flight path gives the droplets more time to solidify. To further facilitate the cooling of the droplets in flight, the droplets may be actively cooled by directing them through a chamber filled with cryogenic (or otherwise cooled) inert gas.
- Additionally, to more effectively cool the molten droplets in flight, the droplets may be directed in paths that are different from the path of their adjacent downstream droplet. This ensures that the air through which a droplet passes has not been heated by the preceding droplet in the series and that each droplet is expelled from the protective aerodynamic wake of its neighboring or preceding downstream drop, thereby allowing each droplet to cool more effectively. When discussing droplets in a series of droplets, downstream droplets are understood to be droplets that are produced earlier in the series, whereas upstream droplets are produced later. A droplet thus follows downstream droplets and is followed by upstream droplets (the “stream” in this case referring to the downward flow of metal). An adjacent droplet is a droplet in a series of droplets that is immediately upstream or downstream in the series. For these definitional purposes, because the satellite and parent droplets are being separated, a droplet's adjacent downstream droplet in either the satellite or parent stream is the droplet produced two cycles of the excitation frequency earlier, not the droplet produced immediately before it.
- As described above, the
droplets charge electrode 40. In this example, thedroplets charge electrode 40—and thereby thedroplets controller 26, and it should be understood that any waveform that varies the charge on thedroplets droplets droplets - In the case of the embodiment of FIG. 3, the charge applied to the capillary stream and maintained by the
droplets deflection plates 44 functions to separate thesatellite droplets 35 from theparent droplets 37. If the charge applied to thecharge electrode 40—and thereby thedroplets deflection plates 44 functions to vary the deflection or path ofadjacent satellite droplets 35 andparent droplets 37 in their respective streams, in addition to separate thesatellite droplets 35 from theparent droplets 37. Alternatively, a second pair of deflection plates orthogonally oriented to the first pair ofdeflection plates 44 could be used to further disperse thedroplets deflection plates 44, in the case of a single pair of deflection plates, may rotate to radially deflect the droplets. In the case of two pairs of deflection plates, preferably the second pair may rotate to radially deflect the droplets. - Other alternatives may include applying electrostatic charges to the
droplets droplets satellite droplets 35 from theparent droplets 37, while the second pair of deflection plates is driven at a varying bias voltage to vary the deflection of thedroplets - A significant advantage of the present invention is that it enables the production of micro-metallic spheres that are significantly smaller than the diameter of the orifice from which they emerge, i.e., ball diameters preferably in a range of about 1.0 to 100 microns and preferably less than about 25 microns. Because much smaller spheres can be produced with larger diameter orifices, the difficulties plaguing smaller orifices, such as orifice clogging, tend to be avoided with the present invention. Additionally, an advantage of the present invention is that the micro-metallic balls, i.e., satellite or satellite and parent combined, can be produced at very high rates several orders of magnitude greater than conventional methods, i.e., preferably on the order of tens of thousands of balls per second, while still maintaining a high degree of uniformity without having to perform an additional step of mechanically sieving or sorting. More particularly, the micro-metallic balls may be produced in accordance with the present invention at a rate preferably in a range of about 1000 to 200,000 balls per second and preferably at a rate greater that 4000 balls per second, while the ball diameter may be maintained within a tolerance of a mean ball diameter preferably in the range of about 0.5 to 3.0 percent and preferably less than 2.0 percent.
- While the invention is susceptible to various modifications and alternative forms, a specific example thereof has been shown in the drawings and is herein described in detail. It should be understood, however, that the invention is not to be limited to the particular form disclosed, but to the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.
Claims (31)
1. A ball comprising:
a metal,
wherein the ball has a diameter in a range of about 1.0 micron to less than 50 microns.
2. The ball of claim 1 wherein the ball has a diameter of less than 25 microns.
3. The ball of claim 1 , wherein the ball is substantially spherical.
4. The ball of claim 1 , wherein the ball is formed by a process of capillary stream break-up wherein the diameter of the ball is less than two times the diameter of an orifice from which the ball was formed.
5. The ball of claim 1 , wherein the ball is formed by a process of capillary stream break-up wherein the diameter of the ball is less than the diameter of an orifice from which the ball was formed.
6. A metallic ball formed from a process of capillary stream break-up wherein the diameter is in the range of about 1.0 to 50.0 microns.
7. The ball of claim 6 wherein the diameter of the ball is less than 25 microns.
8. A metallic powder comprising:
a plurality of metal balls each having a diameter in the range of about 1.0 to 100 microns, and
wherein the diameters of each of the plurality of metal balls is within a range of about 0.5 to 3.0 percent of a mean ball diameter.
9. The metallic powder of claim 8 , wherein each ball of the plurality of metal balls is substantially spherical in shape.
10. The metallic powder of claim 8 , wherein the diameter of each ball of the plurality of metal balls is less than 50 microns.
11. The metallic powder of claim 8 , wherein the diameter of each ball is less than 25 microns.
12. A method of manufacturing ultra-small metallic spheres comprising the steps of:
forming parent and satellite droplets from a capillary stream of molten metal;
separating the satellite droplets from the parent droplets;
cooling the satellite droplets to solidify the balls; and
collecting the satellite balls.
13. The method of claim 12 , wherein the forming step comprises the steps of
directing a capillary stream of molten metal from an orifice by applying an excitation disturbance, wherein the excitation disturbance is determined so that parent droplets and satellite droplets form from the stream due to capillary stream break-up;
14. The method of claim 12 , wherein the separating step comprises the steps of:
imparting an electrostatic charge to the satellite and parent droplets; and
deflecting the satellite and parent droplets by directing the droplets through an electric field.
15. The method of claim 14 , wherein the deflecting step includes the steps of varying the electric field.
16. The method of claim 14 , wherein the deflecting step includes the steps of applying a constant electric field and varying the electric charge on the droplets.
17. The method of claim 14 , wherein at least a portion of the electric field is created by applying a voltage across a pair of deflection plates, the satellite and parent droplets being directed through the pair of deflection plates.
18. The method of claim 12 , wherein the separating step comprises increasing the magnitude of the excitation disturbance to thereby increase rotation of the capillary stream as it exists the orifice, wherein the satellite droplets are deflected from the parent droplets.
19. The method of claim 12 , wherein the separating step comprises applying an aerodynamic force to the satellite and parent droplets, the aerodynamic force having at least a component in a direction orthogonal to the capillary stream.
20. The method of claim 12 , wherein the excitation disturbance comprises harmonic disturbances.
21. The method of claim 11 , wherein the collected satellite balls have a diameter in a range of about 1 to 100 microns.
22. The method of claim 21 , wherein the collected satellite balls have a diameter of less than 25 microns.
23. The method of claim 12 , wherein each diameters of the satellite balls is within a range of about 0.5 to 3.0% of a mean ball diameter.
24. The method of claim 12 , further comprising the step of recycling the parent droplets back into the molten metal.
25. The method of claim 24 , further comprising the step of filtering the molten metal.
26. The method of claim 12 , wherein the cooling step comprises actively cooling at least the satellite droplets in flight by directing the satellite droplets through a chamber filled with a cooled gas.
27. A plurality of metal balls, each having substantially the same diameter in a range of about 1.0 to 100 microns, wherein the plurality of balls is produced by a process comprising the steps of:
directing a capillary stream of molten metal from an orifice by applying an excitation disturbance, wherein the excitation disturbance is determined so that parent droplets and satellite droplets form from the stream due to capillary stream break-up;
separating the satellite droplets from the parent droplets; and
cooling the satellite droplets to form solid balls of substantially spherical shape.
28. The plurality of metal balls of claim 27 , wherein the diameters of the metal balls are within a range of about 0.5 to 3.0% of a mean ball diameter.
29. The plurality of metal balls of claim 28 , wherein the separating step is performed, at least in part, by electrostatic deflection.
30. The plurality of metal balls of claim 27 , wherein the separating step is performed, at least in part, by acoustic forcing.
31. The plurality of metal balls of claim 28 , wherein the separating step is performed, at least in part, with aerodynamic forces.
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US10/292,998 US20030136222A1 (en) | 2000-05-22 | 2002-11-12 | High-speed fabrication of highly uniform ultra-small metallic microspheres |
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US20650700P | 2000-05-22 | 2000-05-22 | |
US09/860,803 US6491737B2 (en) | 2000-05-22 | 2001-05-18 | High-speed fabrication of highly uniform ultra-small metallic microspheres |
US10/292,998 US20030136222A1 (en) | 2000-05-22 | 2002-11-12 | High-speed fabrication of highly uniform ultra-small metallic microspheres |
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US09/860,803 Division US6491737B2 (en) | 2000-05-22 | 2001-05-18 | High-speed fabrication of highly uniform ultra-small metallic microspheres |
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US6491737B2 (en) | 2002-12-10 |
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