Patent Number: 060834544
Section: summary

FIELD OF INVENTION The present invention is concerned with producing evenly sized and evenly shaped spheres or balls. In one aspect, the invention relates to making uniformly sized metallic spheres for use in solder compositions. BACKGROUND OF THE INVENTION Various methods have been proposed to form droplets of material having a known size. The present invention is an improvement over the method described in the Chun et al. U.S. Pat. No. 5,266,098 which is expressly incorporated herein by reference. The Chun et al. '098 describes placing a charge on a droplet in an attempt to produce uniformly sized droplets. The Chun et al. method produced a high percent of unacceptable and unusable pieces or bits of material having flat, oblong or irregular shapes. The Chun et al. method is limited to producing irregularly shaped droplets having undesirably rough and bumpy surfaces. Other previously considered methods for forming droplets by imparting a charge on the droplets include: Smith U.S. Pat. No. 5,560,543 describes a method for forming droplets and passing droplets through charged and grounded plates to selectively deflect droplets. Soviet Patent No. 541682039-A1 describes forming droplets which acquire a charge by passing through an electric field. Orme et al. U.S. Pat. Nos. 5,171,360; 5,226,948; 5,259,593 and 5,340,090 describe methods and apparatuses for forming a net product by directing a stream of uniformly sized droplets onto a collector of the shape of the desired product. C. H. Passow thesis, The Massachusetts Institute of Technology (MIT) May 1992, describes forming uniform droplets by using parallel plates positioned below a charging plate to selectively deflect some droplets off to one side where they are collected. Various other methods proposed to form droplets include: Hayes U.S. Pat. No. 5,411,602 describes ejecting solder drops from an ejection device into a flow of inert gas and catching the solidified solder balls; Hommel et al. U.S. Pat. No. 4,956,128 describes an aqueous calcium chloride hardening solution through which droplets are passed; Yabuki et al. U.S. Pat. No. 4,744,821 describes forming drops and passing the drops through layers of oil and water; Fulwyler et al. U.S. Pat. No. 4,302,166 describes a droplet forming apparatus where the droplets fall into an aqueous solution of a nonionic surfactant; Green et al. U.S. Pat. No. 4,628,040 describes forming droplets using a venturi process where the droplets pass through an oil to harden the droplets; Eylon et al. U.S. Pat. No. 4,787,935 describes a method for making powders using swirling, cooling fluids to harden the droplets; Anderson U.S. Pat. Nos. 4,216,718 and 4,419,303 describe forming sodium amalgam particles for high pressure discharge lamps where a vibrating discharge nozzle forms droplets which fall into a fluid; Rhim et al. U.S. Pat. No. 4,981,625 describes forming polymeric microspheres by ejecting droplets of monomers, charging the droplets, freezing the droplets in a cryogenic liquid, and thawing the droplets by irradiation to activate free radicals which polymerize the monomer. The prior art droplet formation methods have not been entirely satisfactory for a number of reasons. A major concern is the wide distribution of the particle size of the droplets. Inconsistent sizes in the droplets makes the use of the droplets more difficult in soldering applications. Another problem is that after the formation of the droplets, the droplets have to be cleaned to remove contaminants or oxides on the surface or to remove oils and solutions through which the droplets have been passed. The cleaning of the droplets adds to the manufacturing time and costs. Another major concern is that the droplets have irregular shapes and/or bumpy or rough surfaces. This lack of sphericity makes handling and use of the individual droplets more difficult. Therefore, it is an object of the present invention to develop an apparatus and method for manufacturing high quality uniformly sized and shaped droplets. The present invention further provides a process which does not involve the use of multiple formation steps and/or cleaning steps. BRIEF DESCRIPTION OF THE INVENTION The present invention provides a highly accurate method and apparatus for producing uniformly sized and shaped spheres of a desired material, or materials. The apparatus comprises a sphere generation means and a controlled temperature solidification environment. In preferred embodiments, the spheres are formed using a uniform sphere generation means where a low viscosity liquid material is supplied into a crucible or feed system. The crucible has a means for heating and melting the material. The low viscosity liquid material in the crucible is subjected to a certain periodic disturbance by a stimulation actuator means. The crucible has at least one orifice which permits passage of the material therethrough. The material is subjected to a pressure differential (preferably about 4-50 psi) which forces the material through the orifice as a stream. The periodic disturbance applied to the material causes a controlled breakup of the stream of material into uniformly sized spheres. As the spheres are formed, the spheres are subjected to a positive or negative charge by a charging means. When the charging means is held at a predetermined voltage with respect to the stream, the combination of the voltage and the capacitance between the charging means and the stream brings a charge to the leading point of the stream. Each sphere retains a charge that the sphere held before it broke free from the stream. The charge on the sphere causes each sphere to repel from adjacent spheres. The like charge on the individual spheres prevents the spheres from merging together in flight with neighboring or adjacent spheres. In an especially preferred embodiment, the stream is broken by introducing minute periodic disturbances (preferably from about 1 to 30 kHz) by the stimulation actuator means. In certain embodiments the stimulation actuator means comprises either an electromechanical or piezoelectrical transducer. In one especially preferred embodiment, the transducer comprises a stack of five piezoelectric crystals mounted on a top portion of the crucible. The bottom four crystals are mechanically connected in serial and are electrically connected in parallel to a high sinusoidal voltage source. The top piezoelectric crystal serves as a motion sensor. The output voltage is an indication of the amplitude of the stimulation actuator means. In an alternative embodiment, the periodic disturbance or stimulation of the material can be obtained from a monolithic multilayer piezoelectric stimulation actuator. One preferred type of multilayer actuator has the following dimensions: length 5 mm, height 5 mm and width 5 mm. The multilayer actuator contains about 29 piezoelectric ceramic layers cofired together. The multilayer actuator is capable of over 3 micron expansion. In certain embodiments, an extender means, such as a rod, is attached to the bottom of the stimulation actuator means and extends into the supply of the material. The stimulation actuator means transfers the minute periodic disturbances to the material through the rod. In a different embodiment, the stimulation actuator means can comprise a piezoelectric ceramic material having a nozzle which is connected to the voltage source. The nozzle has a fixed aspect ratio defining the orifice in the crucible. The sinusoidal voltage is directly applied on the nozzle causing a minute periodic disturbance which radiates into the stream of material through the nozzle wall. It is contemplated that in certain very high temperature applications (for example, where the temperature of the low viscosity liquid material is over about 300.degree. C.), a lithium niobate (LiNoO.sub.3) material can be used instead of a piezoelectric ceramic material. At least one pressure regulator means supplies a constant hydrostatic pressure in the crucible. In a preferred embodiment, the pressure regulator means keeps the crucible at a negative pressure before the operation of the sphere generation means. The negative pressure prevents the material from dripping out of the nozzle. In a preferred embodiment, the pressure regulator means keeps the crucible at a positive pressure during the operation of the sphere generation means by maintaining the low viscosity liquid material at a desired level in the crucible. In a preferred embodiment, the pressure regulator means supplies a dry and inert gas, such as nitrogen, onto the low viscosity liquid material in the crucible. The positive pressure forces the material out through the orifice in the crucible. The amount of pressure on the material controls the flow of the material through the orifice. During operation of the sphere generation means, the applied constant positive pressure forces the material out of the nozzle and forms the jet or stream. The sinusoidal frequency from the stimulation actuator causes a minute periodic disturbance to the stream. Due to the Rayleigh instability effect, the disturbance builds due to energy of momentum in the stream, breaking the stream into uniform sized and uniform spaced spheres. The space between two neighboring spheres .lambda. is a function of the jet speed v.sub.j and the stimulation frequency, .lambda.=v.sub.j /f. In certain embodiments, the present invention provides a deflection means which is in a spaced apart relationship to the charging means. The deflection means comprises at least one set of deflection surfaces which are spatially separated from each other. A high voltage is applied across the set of deflection surfaces to generate an electric field between the surfaces. The deflection means creates an electrical force field through which the spheres pass. Since the spheres are charged, the electric field deflects on the spheres, depending upon their polarity. The deflection means spatially separates the spheres in the plane perpendicular to a center or vertical axis, further preventing the spheres from merging, thus further maintaining size consistency. The deflection distance of the spheres is a function of the size and speed of the spheres, the charge of the spheres and the force of the deflection field. The bigger spheres remain close to the center axis through the deflection means while the smaller spheres are deflected further away from the center axis. A visual observation system is positioned in a spaced apart relationship to the orifice in the crucible to monitor the formation of the spheres and to measure the size of the spheres. The visual observation system, in preferred embodiments, is operatively connected to the stimulation actuator means to increase or decrease the periodic disturbance being supplied to the low viscosity liquid material. The visual observation system is also operatively attached to the deflection means which increases, maintains or decreases the charge on the spheres in response to the information being collected by the visual observation system. The present invention is an improvement over the Chun et al. '098 technology where the droplets are not formed in a temperature controlled environment. According to another aspect of the present invention, the sphere generation means is operatively positioned in a controlled temperature solidification environment. The controlled temperature solidification environment allows the spheres to be formed in a short amount of time and distance. Also, the controlled temperature solidification environment provides less risk of contamination, such as oxidation, from occurring on the surface of the spheres. The controlled temperature solidification environment contains at least one heat transfer medium, such as a cold or liquified gas. The heat transfer medium provides a solidification environment where the spheres are cooled and solidified in a controlled manner. In certain embodiments the controlled temperature solidification environment provides a thermal gradient which allows the spheres to be quickly cooled in a controlled manner so that the spheres being formed have a consistently round or spherical shape in addition to a uniform size. In an embodiment where a tin-lead alloy is being used to form spheres for solder, the thermal gradient in the controlled temperature solidification environment ranges from about room temperature to about -90.degree. C. (and in certain embodiments about 0.degree. C.) in an upper chamber adjacent the crucible and ranges from about -110 to about -170.degree. C. in a lower chamber adjacent a bottom of the controlled temperature solidification environment. In other embodiments where different low viscosity liquid materials are being formed in spheres, the temperatures in the solidification environment are regulated so that the spheres form both quickly and uniformly. The temperatures in the controlled temperature solidification environment are affected by the type of heat transfer medium being used. It is to be understood that various heat transfer media, including liquified gases, halo-carbon fluids, ammonia, water and steam are within the contemplated scope of the present invention and that the temperatures of the heat transfer media can range from about room temperature to about -200.degree. C., depending upon the type of low viscosity liquid material being used to form the spheres. In embodiments where the low viscosity liquid material comprises a material which solidifies by radiating heat, for example, metals such as copper and steel, different types of heat transfer media may be used and the heat transfer media may be supplied at different temperatures. For example, in embodiments where titanium spheres are being formed the heat transfer medium can comprise a heated gas or vapor. In certain embodiments, it has been found that the contact of the spheres by rapidly moving or flowing heat transfer medium tends to cause the spheres to have uneven shapes. Therefore, in certain preferred embodiments, it is desired that there be as little movement or flow of the heat transfer medium. The heat transfer medium present in the controlled temperature solidification environment is substantially at rest or still such that there are generally no currents or flows of medium to contact or misshape the spheres being formed. In certain embodiments, the controlled temperature solidification environment comprises a first or gaseous environment containing a first heat transfer medium and a second or liquid environment containing a second heat transfer medium. In other embodiments the controlled temperature solidification environment comprises the first or gaseous environment without the use of the second or liquid environment. However, the description herein will describe in detail the gaseous/liquid controlled temperature solidification environment in order to provide a full understanding of all embodiments of the present invention. It should be further understood that all the embodiments are within the contemplated scope of the present invention. The use of the gaseous/liquid controlled temperature solidification environment is especially useful in the formation of spheres from relatively soft or malleable materials and in the formation of spheres which have large diameters or have a high latent heat of fusion (i.e., the quantity of heat evolved in the transformation of the material from the liquid phase to the solid phase). As the spheres exit the deflection means the spheres begin to solidify by first forming a skin or shell on the outer surface of the sphere. Before the spheres exit the gaseous environment zone, the spheres have substantially solidified, i.e., the heat of fusion has been transferred from the spheres to the gaseous heat transfer medium, or has been radiated. The spheres may still be at a high temperature, rending the spheres malleable. The spheres then pass into the liquid environment which preferably comprises a low temperature inert liquid material such as a supply of liquid nitrogen. The liquid environment further cools the spheres to remove the specific heat and to harden the spheres. The liquid environment also acts as a cushioning medium to prevent the spheres from mechanically deforming by colliding with each other or the walls and the bottom of the upper and lower chambers. In one preferred embodiment a low temperature liquid material is dispensed both into a top portion and into a bottom portion of the controlled temperature solidification environment. The low temperature liquid dispensed from the top portion is dispensed into the gaseous environment and at least partially vaporizes thus keeping the gaseous environment at a preferred low temperature. In certain embodiments it is contemplated that the top dispensed liquid can contact the descending spheres to quicken their cooling. In a preferred embodiment, the bottom portion of the controlled temperature solidification environment comprises a funnel which contains the second or liquid environment. The liquid environment cushions the spheres prior to the spheres hitting the bottom of the funnel. No further processing steps need to be carried out on the finished spheres once the spheres have passed with the liquid environment pool. There is no need to remove any oil or other materials from the surface of the spheres. It is within the contemplated scope of the present invention to monitor and respond to any changes in the operating parameters for forming the uniformly sized spheres. A first thermocouple measures the temperature at the top portion of the controlled temperature solidification environment and a second thermocouple measures the temperature at the bottom portion of the controlled temperature solidification environment. A differential pressure sensor monitors the pressure of the crucible. A data acquisition/control system is operatively connected to the pressure sensors, the thermocouples, the sphere generation means and the visual observation system. The data acquisition/control system collects pressure and temperature measurement data and controls the sphere generation means. The data acquisition/control system and the visual observation system provide the feasibility of actively controlling the size of the spheres being generated. The size of the sphere is measured by the visual observation system. The data acquisition/control system receives continuous and updated information on the crucible pressure and the frequency provided by the stimulation actuator means so that the sphere diameter is kept at a predetermined size. It is to be understood that the actual size of the spheres being generated depends upon the end use requirements. According to the present invention, the formation of spheres and the control over sphere shape and diameter is accurate to within microns. The spheres have a diameter that are precise to within about 1% of each other. The present invention is a further improvement over the Chun et al. '098 technology since the spheres produced according to the present invention have a substantially spherical and smooth surface. The spheres of the present invention have high degree of sphericity where the diameters through any section of a sphere vary less than about 1.0%. The method and apparatus of the present invention are useful for forming uniformly sized and shaped spheres having diameters that range from about 12 to about 1000 microns. The present invention is particularly useful for forming large spheres having a diameter greater than about 500 and in certain preferred embodiments about 760 microns (0.030 inches.+-.0.0003). The enclosed controlled temperature solidification environment controls the rate of transition from the liquid state to the solid state of the spheres. The volume and surface ratio of the spheres being formed affects the rate of cooling of the spheres. The larger diameter spheres are solidified at a controlled rate within the controlled temperature solidification environment so that the spheres maintain a substantially round and smooth surface and have a uniform shape. The operating parameters, including orifice diameter, frequency and amplitude of the periodic disturbances of the spheres, can be varied so that spheres having different diameters can be formed. It is to be understood that the optimum diameter of the spheres depends, in part, upon the type of sphere being formed. Other parameters, such as feed rate of the metal into the crucible, crucible pressure, temperature of the material being formed into spheres, and the amount of charge on the spheres also affect the size and rate of formation of the uniformly sized and shaped spheres. It is to be understood that the temperature of the low viscosity liquid material itself affects the thermal state of the spheres. In certain embodiments, the temperature of the material can vary from just above the melting point, and in other embodiments can be, for example, about 50.degree. C. above the melting point of the material. This difference in the temperature of the material affects the rate of solid sphere formation. In certain embodiments of the present invention, another parameter which can be varied is the "stand-off" distance between the sphere generation means and the point in time where the sphere solidifies. The present invention allows the spheres to solidify more quickly than in a conventional droplet formation apparatus. In preferred embodiments, the spheres descend through the enclosed controlled temperature solidification for about 0.5 to about 1.5 seconds prior to contacting the bottom of the enclosed environment. The enclosed controlled temperature solidification environment allows the spheres to be formed in a much shorter distance (about 1 to about 5 meters versus 10 to 20 meters found in conventional sphere forming apparatuses) and in a much shorter time (about 0.5 to about 1.5, preferably about 0.8 seconds versus 7 to 10 seconds). The formation of the spheres in the controlled temperature solidification environment allows the spheres to be formed substantially without any contamination or oxidation. In preferred embodiments, the spheres are formed from a low viscosity fluid, including, for example, glasses, ceramics and metals. In certain embodiments, it is within the contemplated scope of the present invention that the spheres can be formed of a wide variety of metals including tin/lead solder alloys, gold, aluminum, steel or copper alloys. In addition, the spheres can be plated with precious metals such as silver, gold or palladium or can be coated with organic coatings to prevent oxidation after their formation. The spheres are especially useful in such applications as solder for interconnection of integrated circuits to printed circuit boards, especially ball grid arrays, chip scale packages and flip chip packages. In certain embodiments, the spheres formed according to the present invention are especially useful in a solder composition and the spheres do not need any additional flux materials to prevent oxide formation on the surface of the spheres. While certain preferred embodiments have been shown and described herein, it is to be understood that the invention is not limited thereto, but may be embodied with the scope of the following claims.