Metallurgical apparatus – Means for melting or vaporizing metal or treating liquefied... – Spray refining means
Reexamination Certificate
2001-04-10
2002-11-26
Wyszomierski, George (Department: 1742)
Metallurgical apparatus
Means for melting or vaporizing metal or treating liquefied...
Spray refining means
C266S235000, C222S593000
Reexamination Certificate
active
06485674
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a device for the pressure-less production of soft solder powder, in particular exactly spherical fine metal particles in a grain size range of 1 to 100 &mgr;m and with a liquidus temperature <250° C. from a body of solder, with a heatable receiver for melting the solder in the oil stable at high temperatures.
BACKGROUND OF THE INVENTION
It is known to produce soft solder powder by subjecting a solder melt to flow dispersion in liquids in rapidly rotating agitators. So for instance D 237 575 A3 describes a method for producing solder paste, whereby a solder carrier consisting of colophonium, and organic solvent, a compound with reducing action and triethanolamine is mixed with solder metal. In a receiver which can be selectively cooled or heated with an agitator the solder carrier is produced at 50° C. by stirring. The solder metal is added in compact form to the receiver, while heating the latter to a temperature which exceeds the melting point of the solder metal by approximately 10° C. and the melted mass is dispersed by being agitated at a high speed of approximately 10000 rpm. After that it is cooled down to approximately 20° C. below the melting point of the solder metal and the agitator is operated at a lower speed until it is cooled to room temperature. This known process has the disadvantage that the obtained particle size of approximately 150 &mgr;m is not a fine metal powder. The dispersed solder particles have also different diameters, i.e. they have by far a too broad grain distribution range. Therefore the known method has not proven itself on a large industrial scale, especially because it does not work continuously.
It is also known to use shearing devices working according to the rotor/stator principle for the production of emulsions (liquid/liquid) and suspensions (solid/liquid) (see IKA Maschinenbau-Prospekt “Dispergieren”, pages 22-24, 1997). These devices are used for lacquers, dyes, pharmaceutical products, metal oxide suspensions and coatings. According to this known principle, as a rule, it has to be insured that in the case of highly viscous media the media flow has to be sustained by conveyor units.
Furthermore from DE 44 02 042 A1 a process is known for producing microparticulate reflow-solder agents, whose solder metal content is present in a small grain size range. The compact solder metal is melted into an organic liquid which can be heated to high temperature, such as castor oil, and by means of a flow dispersion process, brought to a spherical symmetrical grain size range of preferably 3 to 10 &mgr;m in diameter. The organic liquid is then removed to the extent that the metal particulate remains covered, so that it can be introduced in an emulsion and the individual particles of the suspension and emulsion are covered according to the method of complex coacervation with a melamine polymerisate within the layer thickness range of 50 to 250 nm. The microparticulate organic phase is then quantitatively separated from the microparticulated metal phase. This microparticulate metal powders are protected by a duroplastic polymer system, however they can be released again only through the action of a highly activated fluxing agent. These fluxing agents lead to the destruction of the microelectronic switching circuits and are therefore unsuitable. Besides this method has been used only in laboratories and is not capable of insuring a uniform sphere diameter from charge to charge.
Another known solution (U.S. Pat. No. 4 648 820) melts metal such as aluminum in a crucible, feeds the molten metal to a cooling chamber filled with cooling fluid, and disperses the liquid metal by means of spinning disks in drops, which again are drawn together with the cooling fluid into a recirculation cycle and in a separator are separated from the cooling fluid, whereby the latter is returned to the cooling chamber.
According to U.S. Pat. No. 5 411 602 the solder is melted and the molten solder is divided into drops by means of inert gas. This state of the art is also plagued by the drawback that the produced metal particles do not have uniform sphere diameters, so that in any case sorting processes are necessary in order to select metal particles of an approximately equal size having the same sphere diameter. That renders this known solution inefficient.
OBJECT OF THE INVENTION
It is the object of the invention to improve a device of the kind mentioned at the outset, so that the metal powders produced according to the flow dispersion principle have a narrow grain size range clearly below 100 &mgr;m, a precise spherical shape with an almost constant diameter, by avoiding any screening and at low cost in a quasi-continuous process.
SUMMARY OF THE INVENTION
This object is achieved with the following steps
a) gravity feeding the molten solder in a further oil receiver by setting a volume ratio of oil and solder melt of at least 10:1,
b) dispersion of the liquefied solder by agitating and subsequent shearing in successive shearing steps according to the rotor/stator principle at speeds of 1500 to 5000 rpm with the addition of oil from the receiver of step a),
c) circulating by at least 20 times of the solder/oil mixture of step b) in a counterflow over the oil receiver of step a) and the shearing steps, whereby through the control of the shearing speed the number of the steps and the geometry of the rotor, the particle size and particle distribution of the dispersed material in the dispersant are set,
d) discharging the solder/oil mixture from the circuit of step c) into a further oil receiver for the separation of the dispersed material through sedimentation and returning the oil to the receiver of step b) and/or a) and
e) extracting and feeding the dispersed material of step d) for subsequent cleaning.
According to a further preferred feature of the method of the invention, vegetable or animal oils, preferably castor oil, are used as oils.
It has surprisingly been found that solder melts with an extremely high viscosity at a density rate between the dispersed material and the dispersant of ≧2.5 can be separated with a shearing device without jamming the rotors in the stators and without requiring further conveying aggregates. Therefore in a further preferred embodiment of the method of the invention, the solder/oil mixture flows through the consecutive shearing steps in the direction of gravity. The solder/oil mixture is pressed by gravity into the inlet of the first shearing step, where it enters the inner space of the first rotor and reaches the crenelated shearing openings of the first rotor, flows through the slots of the stators surrounding the first rotor and is thereby radially forced into the inlet space of the second shearing step. Through the slots of the second rotor and stator, the solder/oil mixture reaches the third shearing step. By moving the rotor slots past the stator slots, due to the high peripheral speeds very high shearing forces are created, which separate the solder enveloped by the oil. Depending on the number and breadth of the slot openings in the individual shearing steps, the speed and the geometry of the rotors, it is possible to control the size of the solder particles at selected temperatures. Due to the high shearing forces the special advantage persists that the solder/oil mixture is conveyed to the circuit of step c). After a circulation of at least 20 times, and consequently repeated shearing, particles of clearly less than 100 &mgr;m are obtained.
In a further preferred embodiment of the method of the invention, the process temperature in steps b) and c) is set at approximately a maximum of 30° C. above the liquidus temperature of the solder and the oil temperature of step d) at approximately 90 to 130° C. through the heat-carrying medium.
In case the density ratio of the dispersed material to the dispersant is higher than ≧2.5, the solder/oil mixture is additionally agitated.
A further preferred embodiment of the method of the invention provides that the individual steps be supplied
Protsch Walter
Schulze Jürgen
Dubno Herbert
Wyszomierski George
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