FR2827793A1 - Device for the production of spherical balls by passing droplets through a cooling tower incorporating an outlet chamber and an ultrasound generator to agitate the molten material prior to granulation - Google Patents

Abstract

A device for the production of spherical balls by passing a jet of molten material through vibrating orifices (8) of a granulation crucible (4) to form droplets that solidify during their fall through a cooling tower (16) filled with an inert gas, incorporates an outlet chamber (20) in the fusion vessel (1) and an ultrasound generator (21, 22) to agitate the molten material contained in the outlet chamber before its transfer into the granulation crucible. The inert gas in the cooling tower contains a predetermined quantity of oxygen. The quantity of oxygen in the inert gas is less than about 15 to 150 ppm. The volume of the outlet chamber is less than about 20% of the volume of the fusion vessel.

Description

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Technical device of the invention The invention relates to a device for producing spherical balls comprising: means for introducing, into a melting vessel, material intended to constitute the balls, a second container communicating with the melting container so as to receive the molten material, means for forming, from the molten material contained in the second container, a jet through at least one orifice, vibration means for transmitting vibrations to the orifice, so as to transform the jet into droplets, a cooling tower, disposed at the outlet of the orifice and filled with an inert gas, in which the droplets, falling by gravity, solidify to form the balls, and means for receiving the balls at a lower end of the cooling tower

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 STATE OF THE ART There are many granulation processes and devices, applied in many industrial fields, whether in metallurgy, fertilizers, the food or pharmaceutical industry, etc. They all aim to transform into balls of materials which, melted, have, in air or under neutral gas, low viscosity, good surface tension, good ease of flow through orifices and are liable to be solidified by cooling.

By way of example, the document WO-A-8101811, corresponding to US Pat. No. 4,428,894, describes a method and a device for manufacturing metal granules, 0.1 to 5 mm in diameter, from a metal bath in fusion.

According to this known method, a jet of molten metal is formed, it is passed through a vibrating orifice to divide the jet into individual drops, the drops of the jet are dropped by gravity through an atmosphere of inert gas, so as to by cooling, the solidification of the granular drops.

However, in practice, there are few calibrated granulation methods and devices, that is to say capable of industrially producing, with good yield, balls of uniform dimensions having a very good surface condition.

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 However, it is essential, for certain applications, to produce beads, more specifically microbeads, without surface defect and with a very precise gralunometry. This is the case, in particular, for solder alloy balls intended to be used in electronics to form connections, for example for boxes of the BGA (Ball Grid Array) type, that is to say with a network. of marbles. However, the metals used in the welding alloys are soft metals and the surface condition of a ball made up of such metals can be very easily altered.

OBJECT OF THE INVENTION The object of the invention is to obtain balls without surface defect. More particularly, the balls must be perfectly spherical, with an excellent surface condition, not oxidized. The composition of the balls must be very stable within each manufacturing batch and the tolerances very tight with regard to the gralunometry.

According to the invention, this object is achieved by the fact that the device comprises an outlet chamber in the melting vessel and means for ultrasonic agitation of the molten material contained in the outlet chamber before its

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 transfer to the second container, the inert gas contained in the cooling tower containing a predetermined amount of oxygen.

According to a development of the invention, the means for receiving the balls comprise damping means. These preferably include brushes formed by wires based on polyamide, making an angle of about 450 with the trajectory of the balls in the cooling tower. Canvas rollers can also be arranged on the lower periphery of the internal wall of the cooling tower, above the brushes. According to a preferred embodiment, the device comprises, at the outlet of the cooling tower, means for periodically extracting the balls and means for calibrating comprising means for sorting the balls into three categories, according to their dimensions. It may also include means for weighing all of the balls of each category obtained at each extraction sequence, means for determining, from information provided by the weighing means, of a percentage of balls in predetermined standards and means for adjusting the frequency of the vibration means as a function of said percentage.

Brief description of the drawings

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 Other advantages and characteristics will emerge more clearly from the description which follows of particular embodiments of the invention given by way of nonlimiting examples, and represented in the appended drawings, in which: FIG. 1 schematically represents, in section , a particular embodiment of a device according to the invention.

FIG. 2 shows in more detail the lower part of the granulation crucible of the device according to FIG. 1.

FIG. 3 illustrates in more detail a particular embodiment of an outlet orifice of the granulation crucible according to FIG. 2.

FIG. 4 is a sectional view, along A-A, of a melting crucible according to FIG. 1.

FIG. 5 represents a particular embodiment of a damping device arranged at the bottom of the cooling tower of a device according to FIG. 1.

FIG. 6 is a sectional view, along B-B, of the damping device of FIG. 5.

FIG. 7 represents, in diagrammatic form, a particular embodiment of a calibration and weighing device disposed at the outlet of the cooling tower of a device according to FIG. 1.

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Description of particular embodiments The device according to FIG. 1 comprises a melting container or crucible 1, heated by any suitable means (not shown), into which the material intended to constitute the balls can be introduced, in the form of a solid alloy , via a feed airlock 2. The material, introduced in the form of billets, ingots or bars 3, is melted in the melting vessel. A second container or granulation crucible 4, also heated by any suitable means (not shown), is connected, at its base, to the base of the melting crucible 1 by at least one transfer tube 5. A connecting tube 6 connects the upper parts of crucibles 1 and 4, above the level 7 of the molten material, so that the pressure P of the gaseous atmosphere surmounting the molten material is identical in the two crucibles. In addition, a level detector (not shown) is connected to the ingot introduction system so as to keep the level of the molten material in the crucibles 1 and 4 constant.

The granulation crucible 4 conventionally comprises, at its lower part, at least one orifice 8 of predetermined diameter, through which the molten alloy flows in the form of a jet. A vibrator 9, fixed on the lid of the crucible

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 granulation 4, transmits vibrations to the orifices 8 via a vibrating pallet 10, which is driven in a vertical back and forth movement. In known manner, the vibrating pallet 10 is connected to the vibrator 9, for example of the electromagnetic type, by a metal rod 11 which passes through the cover 12 of the granulation crucible 4 by a sealed passage. The frequency of the vibrations is between 200Hz and 10,000Hz, preferably between 200Hz and 6,000Hz.

The material, for example a metallic welding alloy, is melted in the melting crucible 1, then transferred in liquid form, by the transfer tube 5, in the granulation crucible 4. The pressure P of the gaseous atmosphere (nitrogen for example) surmounting the molten material is continuously measured and regulated by a control circuit 13, so as to print a predetermined speed (preferably between 0.5 and 5 m / s depending on the diameter of the orifice) with a jet of molten material passing through each orifice 8 at the bottom of the granulation crucible. The vibrations of the vibrating pallet 10 are transmitted to the laminar jet 14, leaving the orifice 8. As shown in FIG. 2, the jet 14 is then divided into droplets 15 whose diameter is mainly determined by the diameter D of l orifice 8 (preferably between 80 and 800m). The droplets fall, by gravity, inside a cooling tower 16, filled with an inert gas in which they solidify to form spherical balls. The use of helium, under pressure included

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 between 30mbars and more than 100mbars, preferably of the order of 50mbar, perfectly regulated at 1 mbar, allows rapid cooling of the balls, in 2 to 3 seconds for example. The height of the cooling tower 16 is conventionally several meters, 7m for example. Another inert gas can of course be used. However, the use of nitrogen or argon would imply the use of a tower about twice as high.

One or more orifices 8, circular, of diameter D, are preferably formed from a material which is not wettable by the molten material passing through them and whose surface tension cancels the surface tension of the material to be granulated.

For example, the orifices 8 are made in sapphire or ruby. In a particular embodiment illustrated in FIGS. 2 and 3, the wall 17 of the granulation crucible 4 is made of stainless steel and comprises at its lower part an insert 18, made of sapphire or ruby, in which at least one orifice 8 is formed Each orifice 8 is delimited by a vertical wall, of height H less than or equal to the diameter D. In a preferred embodiment, D = 450 μm and H = 190) im. In the particular embodiment of FIGS. 2 and 3, the orifice 8 widens in its lower part where it is delimited by inclined or curved walls 19 of the insert 18.

According to the invention, the melting crucible 1 comprises an outlet chamber 20 in which the molten material is subjected to agitation by ultrasound. This

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 stirring can be achieved by means of a bar 21 immersed in the molten material contained in the outlet chamber 20 and driven in a vertical back and forth movement by a vibrating device 22, for example of the piezoelectric type.

This ultrasonic stirring, for example at a frequency between 20 kHz and 30 kHz, makes it possible to substantially improve the surface condition of the balls. This stirring cannot be carried out in the granulation crucible 4 because it would disturb the flow of the molten material through the orifices 8. This stirring essentially causes a homogenization of the molten material before its introduction into the granulation crucible 4. This stirring is carried out only in the outlet chamber 20, that is to say in a part of the melting crucible 1 situated near the outlet of the molten material in the direction of the granulation crucible 4 and in which all the material is already melted.

In a preferred embodiment, illustrated in FIG. 4, the outlet chamber 20 has a volume of less than about 20% of the volume of the melting crucible 1. A wall 23, which can be curved, delimits the outlet chamber 20 at the interior of the melting crucible 1. The wall 23 is provided with a passage 24 of a few millimeters (for example 5 mm) at its lower part to allow the molten material to enter the outlet chamber.

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The homogenization of the molten material before its introduction into the granulation crucible 4 has, at the same time, the effect of promoting the dispersion of the germs of crystallization and the suspension in the previously molten material of micro-impurities, which would risk disturbing the flow of molten material through the orifices 8, or even to obstruct them.

It is observed that a device not comprising this ultrasonic agitation produces beads which may have, on their surface, craters of various sizes and depths, which make them unsuitable for use in electronics. The ultrasonic agitation according to the invention makes it possible to considerably attenuate this type of defects and generally even to make them disappear completely. Ultrasonic agitation can be carried out permanently. It can also be performed only intermittently under the control of the control circuit 13. It should be noted that the molten material can be transferred continuously from the outlet chamber 20 to the granulation crucible 4, the effect of homogenization being felt on the surface state of the balls even if the molten material must remain on standby for a certain time, for example up to 30 minutes, in the granulation crucible 4 before it passes through the orifices 8.

Another measure, making it possible to improve the surface condition of the beads obtained, consists in acting on the nature of the gas contained in the cooling tower 16.

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 Indeed, this has a primary importance on the sphericity of the balls, their speed of solidification and their surface condition. According to another aspect of the invention, there is introduced into the inert gas of the cooling tower 16 a predetermined amount of oxygen, preferably a few ppm, for example of the order of 15 to 150 ppm. In the absence of oxygen in the inert gas of the cooling tower, the beads may have microfacets on their surface, which disappear if oxygen is introduced into the inert gas. However, if the amount of oxygen is too large, the beads do not always have the required sphericity.

The combination of the homogenization, by ultrasound in the outlet chamber, of the molten material and the introduction of a predetermined quantity of oxygen into the cooling tower makes it possible to optimize the surface condition of spherical balls, in particular microbeads formed from a metallic alloy of solder.

The reception of the balls at the lower end of the cooling tower 16 is preferably damped to avoid any deterioration in the surface condition of the balls. In many known devices, the balls are received in a liquid. This type of reception has the disadvantage of requiring subsequent drying of the balls.

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In the preferred embodiment represented in FIGS. 5 and 6, the damping is carried out essentially by brushes 25. This makes it possible to eliminate any liquid in the reception zone, all the phases of the process then being carried out dry. Each brush 25 is made up of polyamide-based wires, one end of which is fixed close to the junction of the vertical lower part of the internal wall of the cooling tower and of a receiving cone 26. The wires form an angle approximately 450 with the trajectory of the balls in the cooling tower, that is to say substantially an angle of 45 with the vertical. In a particular embodiment, four brushes approximately 10 cm wide, opposite in pairs, are distributed along the periphery of the cooling tower, the free ends of the wires of two opposite brushes overlapping. The diameter of the wires is less than the diameter of the balls. The flexibility of the brushes allows the balls to pass after being damped, without altering their surface condition.

The damping of the balls by the brushes 25 before their passage in the reception cone 26 can be supplemented by the use of rollers 27, preferably in polyamide-based fabric, arranged on the lower periphery of the internal wall of the tower cooling, above the brushes. The rollers 27, which can be arranged vertically (Figures 5 and 6) line the area immediately above the brushes. Any impact of a ball against the internal wall of the cooling tower, which can possibly be

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 caused by a rebound of the ball on a brush 25, is thus also damped.

The receiving cone 26 of the cooling tower is extended by an airlock 28 provided with an inlet valve 29 and an outlet valve 30, both controlled by the control circuit 13. The airlock makes it possible to extract periodically the balls accumulated in the reception cone 26. These then pass through a calibration system.

In the particular embodiment of FIG. 7, the extracted balls (typically between 100 and 300 g / min) fall by gravity onto an upper sieve 31 which retains the balls whose dimensions are greater than predetermined standards. The balls which have passed through the upper screen 31 fall, through a first funnel 32, onto a lower screen 33, which allows the top fine balls to pass. These fall, through a second funnel 34, into a first tank 35. The balls corresponding to the standards are retained in the lower sieve 33. By tilting the sieves 31 and 33, the excessively large balls and the balls are dropped corresponding to standards, respectively through third and fourth funnels 36 and 37, respectively in second and third trays 38 and 39. The balls are thus sorted into three categories, according to their dimensions: the conforming balls in the third tray 39 ,

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 the balls that are too fine in the first container 35 and the balls that are too large in the second container 38.

First, second and third scales 40, 41 and 42, placed respectively under the first, second and third tanks 35, 38 and 39, supply the control circuit 13, for each extraction cycle, with signals M1, M2 and M3 , representative of the weight of the balls collected respectively in the first, second and third tanks. The control circuit 13 determines, from this weighing information, the percentage of balls extracted which are within the standards and acts on the granulation parameters to maintain a yield close to 100%. In a preferred embodiment, the control circuit 13 regulates and adjusts in particular the frequency of the vibrator 9. In fact, the increase in the frequency of the vibrator 9 tends to reduce the dimensions of the balls. For example, for a nominal vibrator frequency of 500Hz, the variation can relate to 5Hz.

The invention is not limited to the particular embodiments shown.

In particular, the number and the arrangement of the orifices 8 may be different from those shown in the figures. Similarly, the sorting and weighing of the balls at the outlet of the cooling tower can be carried out by any other suitable means capable of separating the balls meeting the standards from the others.

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 balls and provide the information sought. The number and arrangement of screens, funnels and bins may be different from what is shown.

The control circuit 13 preferably receives signals representative of the various parameters liable to have an influence on the balls produced by the device and controls the various members capable of influencing these parameters. It thus receives in particular, by any appropriate means, in addition to the signals M1, M2 and Mg, signals for measuring the pressure P and the temperature in the crucibles 1 and 4 as well as in the cooling tower 16. It controls in particular the entry of material 3 from the supply airlock 2, extraction of the balls through the airlock 28, the vibrating device 22, the vibrator 9, the heating of crucibles 1 and 4, the gas pressure inside the crucibles and the cooling tower and the amount of oxygen in the cooling tower.

Claims (11)

Claims 1. Device for producing spherical balls comprising: means for introducing, into a melting container (1), of material intended to constitute the balls, a second container (4) communicating with the melting container (1) of so as to receive the molten material, means for forming, from the molten material contained in the second container, a jet (14) through at least one orifice (8), vibration means (9, 10, 11) for transmitting vibrations to the orifice (8), so as to transform the jet (14) into droplets (15), a cooling tower (16), disposed at the outlet of the orifice (8) and filled with a inert gas, in which the droplets (15), falling by gravity, solidify to form the balls, and means for receiving the balls at a lower end of the cooling tower (16), device characterized in that it comprises an outlet chamber (20) in the melting container (1) and means (21, 22) for ultrasonic agitation of the molten material contained in the outlet chamber (20) before its transfer into the second container (4), the inert gas contained in the cooling tower (16) containing a quantity predetermined oxygen <Desc / Clms Page number 17>  2. Device according to claim 1, characterized in that the quantity of oxygen contained in the inert gas of the cooling tower (16) is of the order of 15 to 150 ppm. 3. Device according to one of claims 1 and 2, characterized in that the volume of the outlet chamber (20) is less than about 20% of the volume of the melting container (1). 4. Device according to any one of claims 1 to 3, characterized in that the melting container (1) has a wall (23) delimiting the outlet chamber (20), said wall (23) being provided with a passage (24) at its lower part to allow the molten material to enter the outlet chamber (20). 5. Device according to any one of claims 1 to 3, characterized in that the ultrasonic stirring means comprise a bar (21) plunging into the molten material of the outlet chamber and animated with a vertical movement of va -and-forth. 6. Device according to any one of claims 1 to 5, characterized in that the orifice (8) is made of a material not wettable by the molten material. <Desc / Clms Page number 18> 7. Device according to any one of claims 1 to 6, characterized in that the means for receiving the balls comprise damping means. 8. Device according to claim 7, characterized in that the damping means comprise brushes (25) constituted by wires based on polyamide, making an angle of about 450 with the trajectory of the balls in the cooling tower ( 16). 9. Device according to claim 8, characterized in that the damping means comprise rollers (27) in fabric arranged on the lower periphery of the internal wall of the cooling tower (16), above the brushes (25). 10. Device according to any one of claims 1 to 9, characterized in that it comprises, at the outlet of the cooling tower (16), means (28, 29.30) for periodic extraction of the balls and calibration means comprising means (31,33) for sorting the balls into three categories, according to their dimensions. 11. Device according to claim 10, characterized in that it comprises means (40,41, 42) for weighing all of the balls of each category <Desc / Clms Page number 19>  obtained at each extraction sequence, means (13) for determining, from information (M1, M2, M3) supplied by the weighing means, of a percentage of balls in predetermined standards and means of adjusting the frequency of the vibration means (9) as a function of said percentage.