U.S. patent application number 10/385581 was filed with the patent office on 2003-09-25 for process of producing metal powders.
Invention is credited to Iwabuchi, Mitsuru, Ohashi, Yuji, Ono, Takao.
Application Number | 20030177865 10/385581 |
Document ID | / |
Family ID | 28035371 |
Filed Date | 2003-09-25 |
United States Patent
Application |
20030177865 |
Kind Code |
A1 |
Ono, Takao ; et al. |
September 25, 2003 |
Process of producing metal powders
Abstract
The invention provides an industrially efficient, low-cost,
mass-production system for the process of producing fine solder
powders using an in-oil atomization method wherein solder is melted
in a heated dispersion medium for fine granulation. Molten solder
melted in a solder melting tank and a mixture of a particle
dispersion medium and a particle coalescence-preventing agent,
prepared in a dispersion medium heating tank, are fed to a
fine-granulation machine, in which dispersion energy is applied to
obtain a dispersion of molten solder particles. The dispersion is
processed in a solidifier-by-cooling to obtain a dispersion of
solid solder particles, which is processed in a solid-liquid
separator to separate the solid solder particles. The solid solder
particles are washed and dried to obtain fine powders. The
respective devices at these steps are connected together by way of
piping, so that fine solder powders can continuously be
produced.
Inventors: |
Ono, Takao; (Saitama,
JP) ; Iwabuchi, Mitsuru; (Saitama, JP) ;
Ohashi, Yuji; (Saitama, JP) |
Correspondence
Address: |
KING & SCHICKLI, PLLC
247 NORTH BROADWAY
LEXINGTON
KY
40507
US
|
Family ID: |
28035371 |
Appl. No.: |
10/385581 |
Filed: |
March 11, 2003 |
Current U.S.
Class: |
75/331 |
Current CPC
Class: |
B22F 9/06 20130101 |
Class at
Publication: |
75/331 |
International
Class: |
B22F 009/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 19, 2002 |
JP |
2002-075487 |
Claims
What is claimed is:
1. A process of producing metal powders, comprising steps of: (a)
melting a raw low-melting metal to obtain a metal melt, (b) mixing
a particle dispersion medium and a particle coalescence-preventing
agent to obtain a dispersion medium that may or may not be heated,
(c) supplying the melt of said low-melting metal from said step (a)
and supplying said dispersion medium from said step (b) with
application of dispersion energy that disperses the melt of said
low-melting metal in the form of fine particles, thereby obtaining
a molten metal particle dispersion wherein molten metal particles
are dispersed in said dispersion medium, (d) cooling said molten
metal particle dispersion to solidify said molten metal particles
into solid metal particles, (e) separating said solid metal
particles from a liquid residue, (f) washing said separated solid
metal particles with a detergent to remove depositions to said
solid metal particles, and (g) drying said washed metal particles,
wherein: in said step (c), said particle coalescence-preventing
agent adsorbs to and/or reacts with said molten metal particles to
prevent coalescence of at least said molten metal particles so that
said solid metal particles can be finely granulated in said steps
(c) to (g), and said steps (a) to (g) are controlled in the form of
a series of mutually correlative steps.
2. The process of claim 1, wherein there is provided a dispersion
medium recycle step in which the liquid residue separated in said
step (e) is directly used as a part or the whole of the dispersion
medium in said step (b) or a dispersion medium regenerated from
said liquid residue in a dispersion medium regeneration step (h) is
recycled as a part or the whole of the dispersion in said step (b),
wherein said dispersion medium recycle step is continuously
controlled in correlation to said step (b).
3. The process of claim 1, wherein there is provided a detergent
recycle step in which a spent detergent that may contain the
depositions removed in said step (f) is directly used as a part or
the whole of the detergent used in said step (f) or a detergent
regenerated from said spent detergent in a detergent regeneration
step (i) is recycled as a part or the whole of the detergent used
in said step (f), wherein said detergent recycle step is
continuously controlled in correlation to said step (f).
4. The process of claim 2, wherein there is provided a detergent
recycle step in which a spent detergent that may contain the
depositions removed in said step (f) is directly used as a part or
the whole of the detergent used in said step (f) or a detergent
regenerated from said spent detergent in a detergent regeneration
step (i) is recycled as a part or the whole of the detergent used
in said step (f), wherein said detergent recycle step is
continuously controlled in correlation to said step (f).
5. The process of claim 1, wherein the detergent used in said step
(f) has a vapor pressure of at least 15 kPa at 40.degree. C. and a
latent heat of vaporization of at most 100 kJ/kg.
6. The process of claim 2, wherein the detergent used in said step
(f) has a vapor pressure of at least 15 kPa at 40.degree. C. and a
latent heat of vaporization of at most 100 kJ/kg.
7. The process of claim 3, wherein the detergent used in said step
(f) has a vapor pressure of at least 15 kPa at 40.degree. C. and a
latent heat of vaporization of at most 100 kJ/kg.
8. The process of claim 4, wherein the detergent used in said step
(f) has a vapor pressure of at least 15 kPa at 40.degree. C. and a
latent heat of vaporization of at most 100 kJ/kg.
9. The process of claim 1, wherein in said step (g), said washed
solid metal particles are dried such that the liquid residue
deposited to said solid metal particles accounts for 0.01 to 1% of
said solid metal particles, thereby reducing oxidization and
dusting of powders of said solid metal particles.
10. The process of claim 2, wherein in said step (g), said washed
solid metal particles are dried such that the liquid residue
deposited to said solid metal particles accounts for 0.01 to 1% of
said solid metal particles, thereby reducing oxidization and
dusting of powders of said solid metal particles.
11. The process of claim 3, wherein in said step (g), said washed
solid metal particles are dried such that the liquid residue
deposited to said solid metal particles accounts for 0.01 to 1% of
said solid metal particles, thereby reducing oxidization and
dusting of powders of said solid metal particles.
12. The process of claim 4, wherein in said step (g), said washed
solid metal particles are dried such that the liquid residue
deposited to said solid metal particles accounts for 0.01 to 1% of
said solid metal particles, thereby reducing oxidization and
dusting of powders of said solid metal particles.
13. The process of claim 1, wherein said step (d) of cooling the
molten metal particle dispersion to solidify said molten metal
particles into solid metal particles is carried out while said
molten metal particles dispersion is passed through an inner pipe
of a double pipe structure and a coolant is passed through an outer
pipe of the double pipe structure.
14. The process of claim 2, wherein said step (d) of cooling the
molten metal particle dispersion to solidify said molten metal
particles into solid metal particles is carried out while said
molten metal particles dispersion is passed through an inner pipe
of a double pipe structure and a coolant is passed through an outer
pipe of the double pipe structure.
15. The process claim 3, wherein said step (d) of cooling the
molten metal particle dispersion to solidify said molten metal
particles into solid metal particles is carried out while said
molten metal particles dispersion is passed through an inner pipe
of a double pipe structure and a coolant is passed through an outer
pipe of the double pipe structure.
16. The process of claim 1, wherein said double pipe is located at
an angle of 45 to 90 degrees with respect to horizontal.
17. The process of claim 2, wherein said double pipe is located at
an angle of 45 to 90 degrees with respect to horizontal.
18. The process of claim 3, wherein said double pipe is located at
an angle of 45 to 90 degrees with respect to horizontal.
19. The process of claim 1, wherein in said step (b), the
dispersion medium that is a mixture of the particle dispersion
medium with the particle coalescence-preventing agent is heated in
such a way that said dispersion medium is preheated in a preheating
tank, and then passed through a heated delivery pipe for a
residence time that does not exceed 10 minutes at most.
20. The process of claim 2, wherein in said step (b), the
dispersion medium that is a mixture of the particle dispersion
medium with the particle coalescence-preventing agent is heated in
such a way that said dispersion medium is preheated in a preheating
tank, and then passed through a heated delivery pipe for a
residence time that does not exceed 10 minutes at most.
Description
[0001] This application claims priority to Japanese Patent
Application No. 2002-075487, filed Mar. 19, 2002.
TECHNICAL FIELD
[0002] The present invention relates generally to a process of
producing metal powders such as powders of low-melting metals or
alloys represented by solders, and more particularly to a
production process that enables spherical solder powders used for
solder pastes and having an average particle diameter in the range
of 0.1 to 100 .mu.m, inter alia, an average particle diameter of up
to 10 .mu.m to be mass-produced in an industrially efficient
fashion.
BACKGROUND OF THE INVENTION
[0003] In recent years, surface mounting techniques have grown
rapidly with the advent of multifunctional, miniaturized wiring
boards. For high-density packaging such as surface mounting of
electronic parts, solder pastes, other soldering materials and
soldering processes which do not only make metal mask printing of
fine patterns feasible but also ensure satisfactory solder-ability
are now in increasing demand.
[0004] For solder pastes it is preferable to use spherical solder
particles rather than aspheric solder particles in view of
fine-patterned metal mask printing. On the other hand, solders are
now required to be finely granulated so as to meet current demands
for accommodation to minuscule wiring areas that enable LSI chips
to be directly on wiring boards.
[0005] To obtain solder powders in a spherical particle form, it is
common to use an atomization method wherein molten solder is
atomized and solidified in an inert gas atmosphere having a low
oxygen concentration. Depending on how to atomize molten solder,
the atomization method is generally broken down into a centrifugal
atomization method of the type that uses the centrifugal force of a
rotary disk and a gas atomization method of the type wherein gas is
jetted onto the molten solder to scatter the molten solder for
atomization. There is also known an ultrasonic atomization method
of the type wherein ultrasonic vibrations are applied to molten
solder for fine granulation.
[0006] In the centrifugal atomization method, molten solder is cast
onto a rotary disk while it is formed by centrifugal force into a
thin film. Then, this film is released out of the edge of the disk
in the form of droplets, which are solidified by cooling in an
inert gas atmosphere having a low oxygen concentration for fine
granulation. With this method, the average particle diameter of the
resulting solder powders may be reduced by increasing the rpm of
the disk. However, there are practically some restrictions on the
rpm of motors for driving the disk, and so it is industrially
difficult to retain spherical solder particles while the average
particle diameter of solder powders is reduced down to 10 .mu.m or
less.
[0007] In the gas atomization method, on the other hand, an inert
gas having a low oxygen concentration is jetted to molten solder
for scattering and atomization. The resulting solder powders have a
broad particle size distribution or, in some cases, they include a
large proportion of so-called satellite particles that are large
particles with small particles deposited to them. Thus, the gas
atomization method is just only less than satisfactory in terms of
fine granulation efficiency but also renders it difficult to obtain
spherical particles. With this method, the average particle
diameter of the resulting powders may be reduced by increasing the
pressure of the gas to be jetted. In that case, however, the
"satellite particle" problem becomes more noticeable and graver,
and so makes it more difficult to obtain spherical particles.
[0008] In the ultrasonic atomization method, the higher the
frequency of the ultrasonic oscillator, the smaller the average
particle diameter of the resulting powders becomes. To make that
frequency high, however, the size of the ultrasonic oscillator must
be decreased and, accordingly, fine granulation efficiency becomes
worse. It is thus very difficult to industrially produce solder
powders having an average particle diameter of up to 10 .mu.m.
[0009] Moreover, there is available an in-oil atomization method
that does not rely on jetting-in-gas, wherein a solder lump is
melted by heating at a temperature higher than the melting point of
the solder in a high-boiling dispersion medium, the melt is then
agitated into droplets, and the droplets are finally solidified by
cooling for fine granulation.
[0010] With this in-oil atomization method, substantially
spherical, fine solder particles can be obtained, because solder is
melted in a heated dispersion medium in the form of an oily liquid,
the melt is agitated into finely particulate droplets, and the
droplets are solidified by cooling. The fine particles are
substantially free from the aforesaid satellite particles or
deformed particles, and fine particles having an average particle
diameter of up to 10 .mu.m can be obtained, with relative ease, by
increasing the number of agitations. This in-oil atomization method
is a wet process wherein the fine granulation of solder is carried
out in the dispersion medium such as an oily liquid, and so is
advantageous in terms of handling during production operation,
because it does not have most of problems unique to the aforesaid
centrifugal atomization or other dry methods, for instance,
deposition of solder powders to the machine used, oxidization of
solder, deterioration in the flowability of solder powders,
dusting, and inconvenience caused by decreases in powder particle
diameters.
[0011] Thus, the centrifugal, gas and ultrasonic atomization
methods may address the production of solder powders having an
average particle diameter of greater than 10 .mu.m; however, they
have problems with the production of solder powders having an
average particle diameter of 10 .mu.m or less, and the production
of spherical solder powders as well. On the other hand, the in-oil
atomization method has a merit of providing a solution to those
problems.
[0012] A problem with the in-oil atomization method is, however,
that as the concentration of dispersed droplets (i.e., the
volumetric ratio of molten solder droplets to the dispersion
medium) increases, the so-called coalescence occurs, wherein upon
scission and division of the molten solder droplet by an agitator
into small droplet pieces, those droplet pieces immediately go back
to the initial droplet, and so fine granulation of the molten
solder droplets does not proceed any longer. Another problem is
that even when the fine granulation proceeds somehow, the droplets
contact one another due to settling or the like, and become coarse
due to their coalescence. Especially when vegetable oils having a
high acid number, oily materials having high viscosity, etc. are
used for the dispersion medium so as to obtain solder particles
with less oxidized surfaces, that problem becomes more
perceptible.
[0013] To avoid such problems, it has been required to reduces the
concentration of dispersed droplets as much as possible so that the
molten solder droplets that are finely granulated by agitation are
unlikely to contact one another. To this end, however, a large
amount of dispersion medium is needed, and to obtain the end solder
powder product, that dispersion medium must be removed. Disposal of
much dispersion medium results in an increased consumption of
dispersion medium, ending up with added production costs.
[0014] These problems could be solved by use of a specific
coalescence-preventing agent or the like, as set forth in
applicant's co-pending Japanese Patent Application No. 2001-395566;
never until now, however, is any systematization for industrially
efficient mass production figured out. In the conventional in-oil
atomization method, a solder lump is melted by heating at a
temperature higher than the melting point of the solder in a
high-boiling dispersion medium, the molten solder is agitated into
droplets, and the droplets are solidified by cooling for fine
granulation. Even so, carrying out these steps in one batch tank is
not well fit for mass production, and is less efficient as well. To
develop a mass-production process that can be carried out with high
efficiency yet at low costs, nothing is suggested at all about how
a series of steps from the step of feeding raw materials to the
step of obtaining the end dry metal powder product are separated
and how the individual steps are set up. Thus, such a
mass-production process is an unheard-of challenge.
[0015] One object of the invention is to provide a metal powder
production process for mass-producing fine metal particles in
industrially efficient fashions.
[0016] Another object of the invention is to provide a metal powder
production process for mass-producing spherical fine metal
particles in industrially efficient fashions.
[0017] Yet another object of the invention is to provide a metal
powder production process for mass-producing fine metal particles
in industrially efficient fashions while components consumed during
the production process are reduced.
[0018] A further object of the invention is to provide a metal
powder production process for mass-producing fine metal particles
in industrially efficient fashions yet at reduced production
costs.
[0019] A further object of the invention is to provide a metal
powder production process for mass-producing fine metal particles
applicable even to fine soldering areas on wiring boards in
industrially efficient fashions.
SUMMARY OF THE INVENTION
[0020] The inventors have made intensive studies for the purpose of
accomplishing the aforesaid objects and consequently found that the
step for fine granulation of a molten metal and the
solidification-by-cooling step, solid-liquid separation step,
washing step, drying step or the like added thereto can be
controlled in the form of a series of mutually correlative steps,
thereby mass-producing spherical fine metal particles. On the basis
of this finding, the inventors have figured out the present
invention according to which there can be provided a production
system that enables spherical, fine metal particles to be
mass-produced.
[0021] The present invention is embodied as follows.
[0022] (1) A process of producing metal powders, comprising steps
of:
[0023] (a) melting a raw low-melting metal to obtain a metal
melt,
[0024] (b) mixing a particle dispersion medium and a particle
coalescence-preventing agent to obtain a dispersion medium that may
or may not be heated,
[0025] (c) supplying the melt of said low-melting metal from said
step (a) and supplying said dispersion medium from said step (b)
with application of dispersion energy that disperses the melt of
said low-melting metal in the form of fine particles, thereby
obtaining a molten metal particle dispersion wherein molten metal
particles are dispersed in said dispersion medium,
[0026] (d) cooling said molten metal particle dispersion to
solidify said molten metal particles into solid metal
particles,
[0027] (e) separating said solid metal particles from a liquid
residue,
[0028] (f) washing said separated solid metal particles with a
detergent to remove depositions onto said solid metal particles,
and
[0029] (g) drying said washed metal particles, wherein:
[0030] in said step (c), said particle coalescence-preventing agent
adsorbs to and/or reacts with said molten metal particles to
prevent coalescence of at least said molten metal particles so that
said solid metal particles can be finely granulated in said steps
(c) to (g), and said steps (a) to (g) are controlled in the form of
a series of mutually correlative steps.
[0031] (2) The process of (1) above, wherein there is provided a
dispersion medium recycle step in which the liquid residue
separated in said step (e) is directly used as a part or the whole
of the dispersion medium in said step (b) or a dispersion medium
regenerated from said liquid residue in a dispersion medium
regeneration step (h) is recycled as a part or the whole of the
dispersion in said step (b), wherein said dispersion medium
recycling step is continuously controlled in correlation to said
step (b).
[0032] (3) The process of (1) or (2) above, wherein there is
provided a detergent recycle step in which a spent detergent that
may contain the depositions removed in said step (f) is directly
used as a part or the whole of the detergent used in said step (f)
or a detergent regenerated from said spent detergent in a detergent
regeneration step (i) is recycled as a part or the whole of the
detergent used in said step (f), wherein said detergent recycle
step is continuously controlled in correlation to said step
(f).
[0033] (4) The process of any one of (1) to (3) above, wherein the
detergent used in said step (f) has a vapor pressure of at least 15
kPa at 40.degree. C. and a latent heat of vaporization of at most
100 kJ/kg.
[0034] (5) The process of any one of (1) to (3), wherein in said
step (g), said washed solid metal particles are dried such that the
liquid residue deposited to said solid metal particles accounts for
0.01 to 1% of said solid metal particles, thereby reducing
oxidization and dusting of powders of said solid metal
particles.
[0035] (6) The process of any one of (1) to (5) above, wherein said
step (d) of cooling the molten metal particle dispersion to
solidify said molten metal particles into solid metal particles is
carried out while said molten metal particles dispersion is passed
through an inner pipe of a double pipe structure and a coolant is
passed through an outer pipe of the double pipe structure.
[0036] (7) The process of any one of (1) to (6) above, wherein said
double pipe structure is set at an angle of 45 to 90 degrees with
respect to horizontal.
[0037] (8) The process of any one of (1) to (7) above, wherein in
said step (b), the dispersion medium that is a mixture of the
particle dispersion medium with the particle coalescence-preventing
agent is heated in such a way that said dispersion medium is
preheated in a preheating tank, and then passed through a heated
delivery pipe for a residence time that does not exceed 10 minutes
at most.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is a flowchart illustrative of one embodiment of the
metal powder production process according to the invention.
[0039] FIG. 2 is a flowchart illustrative of another embodiment
according to the invention.
[0040] FIG. 3 is illustrative of the front and bottom of the
generator used in one embodiment of the invention.
[0041] FIG. 4 is illustrative in longitudinal section of that
front.
[0042] FIG. 5 is a sectional schematic illustrative of the system
using that generator.
DETAILED EXPLANATION OF THE PREFERRED EMBODIMENTS
[0043] By way of example but not exclusively, details of the
invention are now explained specifically with reference to the
in-oil atomization method wherein molten solder is dispersed by
means of an agitator comprising a stator and a rotator. In
particular, the fine granulation method used herein includes other
method for dispersing molten metals in liquids.
[0044] As shown in the flowchart of FIG. 1, fine powders of a metal
having a low melting point are produced through the following
steps. For instance, "solder" (the starting solder such as solder
metal) is fed into a temperature-controllable "solder melting tank"
wherein molten solder is prepared. Apart from this, a "particle
dispersion medium" and a "particle coalescence-preventing agent"
are heated-and mixed together in a "dispersion medium heating tank"
to prepare a dispersion medium. While the present invention is
explained specifically with reference to solder working as the
low-melting metal, it is understood that the invention is
applicable to other low-melting metals.
[0045] The molten solder in the solder melting tank and the
dispersion medium heated in the dispersion medium heating tank are
passed by a constant-feeding gear pump or the like for the former
and a constant-feeding pump or the like for the latter through the
respective lines, and preferably a heating controllable line
(designed to prevent coagulation of solder by temperature drops and
to melt previously solidified solder on restart-up thereby creating
a melt flow) for the former, to a "fine-granulation machine".
Preferably in the fine granulation machine, the mixture is heated,
so that, while cooled if whenever required, dispersion energy for
dispersing molten solder particles is applied to disperse the
molten metal particles in the dispersion medium, thereby obtaining
a molten metal particle dispersion.
[0046] Then, the molten metal particle dispersion is passed through
a line to a "solidifier-by-cooling", wherein the molten metal
particles are cooled by a coolant passed between a "coolant-in
(introduction of coolant)" and a "coolant-out (discharging of
coolant)", whereby the molten metal particles are solidified into
solid metal particles. Subsequently, the liquid material that
contains the solid metal particles is fed to a "solid-liquid
separator" wherein the solid metal particles are separated from the
liquid reside ("spent dispersion medium"). The thus separated solid
metal particles are fed to a "washer" wherein they are washed with
a "detergent" to remove depositions to the solid metal particles
and separate the solid metal particles from the detergent
containing such depositions ("spent detergent"). The thus separated
solid metal particles are dried in a "drier" to obtain a "solder
powder product" (fine solder powders).
[0047] As shown in FIG. 2, it is acceptable to add two circulation
circuits to the flowchart of FIG. 1. One circuit is used as a
dispersion medium recycle step wherein the remaining liquid, from
which the solid metal particles have been removed, is regenerated
by a "dispersion medium regenerator" as the spent dispersion medium
to remove solid matters entrained therein, so that the spent
dispersion medium can be used as a part or the whole of the
dispersion medium in the dispersion medium heating tank (a
deficiency, if any, is made up by a fresh dispersion medium).
[0048] Another circuit is used as a detergent recycle step wherein
the spent detergent in the washer is regenerated by distillation or
the like in a "detergent regenerator", so that the spent detergent
is used as a part or the whole of the detergent used in the washer
(a deficiency, if any, is made up by a fresh detergent).
[0049] Otherwise, FIG. 2 is the same as FIG. 1.
[0050] Whether in the case of FIG. 1 or in the case of FIG. 2, a
series of steps using the respective devices can be mutually
connected together by lines so that they can continuously be
carried out under control. For instance, the heating temperature in
the solder melting tank, the mixing ratio and heating temperature
of the particle dispersion medium and particle
coalescence-preventing agent in the dispersion medium heating tank,
the ratio and heating temperature of the molten solder and
dispersion medium in the fine-granulation machine, the magnitude of
dispersion energy, the traveling speed and temperature of the
coolant in the solidifier-by-cooling, regulation of the
solid-liquid separation rate in the solid-liquid separator, the
degree of washing in the washer, the degree of drying in the drier,
the degree of regeneration in the dispersion medium regenerator,
the degree of regeneration in the detergent regenerator, the
interior temperature of the lines, etc. may be controlled by use of
computer processing. In this case, some or all of such factors may
be provided as numerical data, and those data may be checked
against actual data obtained from sensors attached as accessories
to the respective devices and lines. Thus, control can be
implemented by computer processing or the like.
[0051] That is, the term "control" used herein is understood to
include "automatic control" and "control including automatic
control".
[0052] The aforesaid solder melting tank should preferably be
formed of materials less susceptible to erosion by molten solder,
for example, ceramic materials and carbonaceous materials. Although
it is acceptable to use metals relatively less susceptible to
erosion by molten solder, e.g., SUS 316 and titanium, it is
preferable that these metals should be coated with an oxide film, a
nitride film, a titanium nitride film, etc., thereby making erosion
resistance much higher. The solder melting tank includes therein a
heater for melting solder, for which a graphite heater, a ceramic
heater, a quartz heater, a heater comprising a heat generator
covered with a metal or the like may be used. For the heater
comprising a heat generator covered with a metal, that metal should
preferably be coated with an oxide film, a nitride film, a titanium
nitride film, etc., thereby enhancing the resistance to erosion as
is the case where the metal is selected for the material of the
solder melting tank. To feed the molten solder to the
fine-granulation machine through the line, for instance, a
constant-feeding gear pump may be used. In this case, however, it
is acceptable to immerse the constant-feeding gear pump in the
molten solder in its entirety. The amount of the molten solder to
be fed to the fine-granulation machine may be determined in
proportion to the amount of the dispersion medium to be fed to the
fine-granulation machine. For instance, the former-to-latter ratio
should be in the range of 10 to 1,000 (by volume).
[0053] While the production of fine solder powders (fine powders of
the metal having a low-melting point) has been described with
reference to FIGS. 1 and 2, it is understood that the term
"low-melting metal" used herein refers to at least one of
low-melting metals and low-melting alloys and, in some cases, a
low-melting metal alone or a low-melting alloy alone or both. These
fine metal powders, too, may be produced according to FIGS. 1 and
2.
[0054] Set out below are examples of the low-melting pure metal
together with their melting points.
[0055] Ga (29.8.degree. C.), In (156.degree. C.), Li (186.degree.
C.), Se (217.degree. C.), Sn (232.degree. C.), Bi (271.degree. C.),
Tl (302.degree. C.), Pb (327.degree. C.), Zn (419.degree. C.), and
Te (452.degree. C.).
[0056] Besides, Cd, Cs, Rb, K and Na may be used.
[0057] Exemplary low-melting alloys include 67Ag/33Te (351.degree.
C.), 97.2Ag/2.8Tl (291.degree. C.), 45.6Ag/54.4Zn (258.degree. C.),
95.3Ag/4.7Bi (262.degree. C.), 52.7Bi/47.3In (110.degree. C.),
47.2In/52.8Sn (117.degree. C.), 95.3Ag/4.7Pb (304.degree. C.),
86.6Ag/3.4Li (154.degree. C.), and 8.1Bi/91.9Zn (254.5.degree.
C.).
[0058] Solder is well known as a low-melting metal, and Pb/Sn
eutectic solder in particular is used as joining materials or the
like in the electronic and other industries. Specific mention is
made of not only 100% Sn (232.degree. C.), but also Pb--Sn base
solders such as 37Pb/63Sn (183.degree. C.), 40Pb/60Sn(183.degree.
C.), 50Pb/50Sn (212.degree. C.) and 44Pb/56Sn (125.degree. C.);
Pb--In base solders such as 50Pb/50In (198.degree. C.); Sn--In base
solders such as 49Sn/51In (120.degree. C.), 48Sn/52In
(117-120.degree. C.) and 65Sn/35In (162.degree. C.); Sn--Bi base
solders such as 43Sn/57Bi (139.degree. C.) and
42Sn/58Bi(138.degree. C.); Sn--Ag base solders such as 98Sn/2Ag
(221-226.degree. C.), 96.5Sn/3.5Ag (221.degree. C.), 96Sn/4Ag
(232.degree. C.) and 95Sn/5Ag (232.degree. C.); Sn--Zn base solders
such as 91Sn/9Zn (199-203.degree. C.) and 30Sn/70Zn; Sn--Cu base
solders such as 99.3Sn/0.7Cu (227.degree. C.): Cd--Zn base solder
such as 60Cd/30Zn; Sn--Sb base solder such as 95Sn/5Sb (238.degree.
C.) Ag--In base solders such as 3Ag/97In (141.degree. C.); Au--Sn
base solders such as 80Au/20Sn (283.degree. C.); Sn--Cd--Ag base
solders such as 10Sn/85Cd/5Ag; Sn--Ag--In base solders such as
95.5Sn/3.5Ag/1In; Sn--Zn--In base solders such as 86Sn/9Zn/5In
(192.degree. C.) and 81Sn/9Zn/10In (178.degree. C.); Sn--Ag--Cu
base solders such as 95.5Sn/0.5Ag/4Cu (216.degree. C.) and
96.5Sn/3.0Ag/0.5Cu; Sn--Pb--Bi base solders such as 16Sn/32Pb/52Bi
(99.5.degree. C.), 19Sn/31Pb/50Bi (96.degree. C.), 34Sn/20Pb/46Bi
(100.degree. C.) and 43Sn/43Pb/14Bi (136-166.degree. C.);
Sn--Pb--Sb base solders such as 35Sn/64.5Pb/0.5Sb and
32Sn/66Pb/2Sb; Sn--Bi--In base solders such as 17Sn/57Bi/26In;
Pb--Ag base solders such as 97.5Pb/2.5Ag; Sn--Bi--Ag base solders
such as 90.5Sn/7.5Bi/2Ag (207-212.degree. C.), 41.0Sn/58Bi/1.0Ag;
and Sn--Zn--Bi base solders such as 89.0Sn/8.0Zn/3.0Bi.
[0059] As shown in FIGS. 1 and 2, the particle dispersion medium
and the particle coalescence-preventing agent are mixed together in
the dispersion medium heating tank, so that the latter is dissolved
in the former, resulting in the dispersion medium. The term
"particle dispersion medium" used herein means an oily liquid that
provides a base for the dispersion of particles. Although not
shown, it is acceptable to store this oily liquid in the required
amount in a separately provided tank, from which the required
amount of the oily liquid is fed to the dispersion medium heating
tank. For the particle dispersion medium in the invention, use may
be made of an organic compound having a boiling point that is
greater than (or not lower than) the melting point (temperature) of
the low-melting metal or the highest possible decomposition
temperature, and in which the particle coalescence-preventing agent
can be dissolved.
[0060] Exemplary organic compounds include silicone oil; mineral
oils obtained by petroleum refining; industrial lubricating oils
such as engine oil, spindle oil, machine oil, cylinder oil and gear
oil; or synthetic lubricating oils prepared by chemical synthesis
wherein the chemical components used are hydrocarbon components
such as polyolefin, e.g., polybutene and alkyl aromatics, e.g.,
alkylbenzene, and non-hydrocarbon components such as polyethers
such as polyglycol and phenyl ethers, e.g., polyphenyl ether and
alkyl diphenyl ether, diesters, polyol esters, complex polyol
esters, esters such as natural fats and oils (tryglyceride),
phosphoric compounds such as phosphate esters, and fluorinated
polyethers of the aforesaid compounds. Vegetable oils such as
coconut oil, palm oil, olive oil, sunflower oil, castor oil,
soybean oil, linseed oil, tung oil and cottonseed oil and animal
oils such as whale oil and beef tallow may be used. Further, use
may be made of liquid paraffin; higher hydrocarbon compounds such
as decane, dodecane, tetradecane, hexadecane, octadecane and
undecane; glycols such as glycerin, ethylene glycol, diethylene
glycol, triethylene glycol, polyethylene glycol and polypropylene
glycol (which may be called polyalkylene glycols of the aforesaid
triol and diol types; the glycols of the monol type may be used
such as MB-7, MB-11 and MB-22, all being Nissan Uniloop MB series
(of the non-water soluble type) as well as Nissan Uniloop 50MB
series (of the water soluble type); derivatives of these glycols;
phosphates such as trimethyl phosphate, triethyl phosphate and
tributyl phosphate; substituted phenols such as octylphenol,
trichlorophenol and nonylphenol; trichloroaniline; organic heat
media such as those based on diphenyls and triphenyls;
phenylimidazoles; undecylimidazoles; and heptadecylimidazoles. It
is then preferable to use the organic compound having no
flammability because there is no risk of fires. It is here noted
that the particle dispersion medium may be composed of two or more
of the aforesaid compounds. In this case, two or more reservoirs
may separately be provided for storing separate dispersion medium
compounds, so that on mixing, they are separately fed
therefrom.
[0061] The aforesaid particle dispersion medium should preferably
be used at a temperature that is not higher than the highest
possible temperature and is higher than the melting temperature of
the low-melting metal, and heated in an inert gas atmosphere. The
threshold temperature at which the particle dispersion medium can
be used is usually selected from the range of 120 to 470.degree.
C., and that threshold temperature is usually set at a temperature
that is lower than the decomposition temperature of the organic
compound.
[0062] It is preferable to add an antioxidant to the particle
dispersion medium for the purpose of preventing its oxidization on
heating. With such an antioxidant, it is possible to prevent
oxidization by oxygen that may possibly be contained in a slight
amount even in the inert gas atmosphere. The antioxidant used
herein may be selected from those used with oils and fats, rubbers,
synthetic resins or the like. For instance, phenol antioxidants,
bisphenol antioxidants, polymer type phenol antioxidants, sulfur
antioxidants and phosphoric acid antioxidants may be used. The
antioxidant may be used alone or in combination with imidazoles
having an oxidization inhibition effect. Exemplary compounds are
set forth in Japanese Patent Unexamined Publication No.
09-49007.
[0063] In the present invention, the particle
coalescence-preventing agent is used to prevent coalescence of
molten metal particles due to fusion. This preventing agent may
also prevent coalescence of metal particles resulting from
solidification of molten metal particles and coalescence of the
molten metal particles with the solidified metal particles.
[0064] Generally, when a dispersoid (particles) is dispersed in a
dispersion medium such as an emulsion, the process in which the
dispersion system becomes instable proceeds from creaming (a
phenomenon that the particles float up or settle down due to a
specific gravity difference between the particles and the
dispersion medium) via agglomeration (a phenomenon that the
particles come close to one another and agglomerate together due to
attractive force) to coalescence (a phenomenon that the particles
come together). As surfactants or polymers are adsorbed onto the
surfaces of the particles, the particles come into contact with one
another through the resulting adsorbed films at the agglomeration
stage. It is then of importance to create on the surfaces of the
particles layers having adsorbability strong enough to prevent the
adsorbed films from peeling off or being forced out due to the
surface slip stresses of the particles. Otherwise, the particles
may come into direct contact with one another and come together.
For this purpose, it is considered important that a substance
having a high affinity for both the dispersoid particles and the
dispersion medium is used and, at the same time, the adsorbed
layers have large surface adhesion and surface elasticity (see
"CHMESITRY OF INTERFACIAL PHENOMENONS", pp. 16-18, Sankyo Shuppan,
Publishers).
[0065] As shown in FIGS. 1 and 2, the particle
coalescence-preventing agent is introduced together with the
particle dispersion medium in the dispersion medium heating tank.
Preferably in this case, the former is mixed under agitation with
the latter at a constant proportion (to be described later).
However, when the particle coalescence-preventing agent is solid,
it should be fed in a metered amount by a screw feeder or the like,
and when it is liquid, it should be fed in a constant amount by a
constant-feeding pump or the like.
[0066] The dispersion medium that is a mixture of the particle
coalescence-preventing agent and the particle dispersion medium is
then fed at high temperature to the next step "fine-granulation
machine". However, when this dispersion medium is thermally
instable, it is preferable to place the dispersion medium under
such precise temperature control that the dispersion medium is
preheated at a temperature that is 50-100.degree. C. lower than the
temperature at which it is fed to the fine-granulation machine, and
then heated by a separately provided line heater (a heated delivery
pipe) having a short residence time to the temperature at which it
is fed to the fine-granulation machine. This is because when the
thermally instable dispersion medium is retained at that high
temperature for a long period of time, the dispersion medium is
often modified by heat, reducing the effect on prevention of
coalescence of the particles. The residence time of the dispersion
medium in this case is found by the following formula; however, the
residence time should preferably be 10 minutes at the longest (or
10 minutes or shorter).
Residence Time (min.)=the volume of feed in the line heating
furnace (L).div.the flow rate of feed (L/min.)
[0067] For the particle coalescence-preventing agent, compounds
adsorbing onto and/or reacting with the surfaces of metal
particles, especially molten metal particles are used. Exemplary
such compounds are given just below.
[0068] (i) Rosin and/or Its Derivatives (Rosins)
[0069] (a) The rosin is exemplified by tall oil rosin, gum rosin,
and wood rosin.
[0070] (b) The rosin derivatives are exemplified by hydrogenated
rosin, polymerized rosin, unhomogenized rosin, acrylic
acid-modified rosin, maleic acid-modified rosin, rosin alcohol,
rosin amine, and rosined soap.
[0071] Tall oil rosin, gum rosin and wood rosin are composed
primarily of abietic acids (abietic acid, dehydroabietic acid and
neoabietic acid) and contain as subordinate components pimaric
acid, Palustric acid, isopimaric acid, and other resin acids, with
the components contained at different ratios.
[0072] The rosined soap is a metal salt of rosin derivatives
containing rosin or carboxyl groups. The metal in this case is
exemplified by Na, K, Li, Ca, Mg, Al, Zn, Sn, Pb, Ni, Cu, Co, Mn,
Fe, In, Bi and Ag. In view of solder material, however, salts of Sn
are preferred. Preferably in view of the action of the particle
coalescence-preventing agent on prevention of coalescence of
particles, on the other hand, the number of carboxyl groups in the
rosin derivatives having rosin or carboxyl groups should be as
large as possible. More preferably, use should be made of metal
salts of monobasic acid-modified rosin, and especially metal salts
of dibasic acid-modified rosin.
[0073] (c) Colorless rosin derivatives (see Japanese Patent
Unexamined Publication No. 05-86334) may also be used, which are
obtained by hydrogenation of addition reaction products of
.alpha.,.beta.-unsaturated monocarboxylic acids and/or
.alpha.,.beta.-unsaturated dicarboxylic acids to refined rosin.
[0074] At least one of (a) rosin or rosin derivatives, (b) rosined
soaps, and (c) colorless rosin derivatives is used. However,
particular preference is given to those having affinity
(adsorbability and/or reactivity) for the surfaces of metal
particles, especially molten metal particles. Among others, rosins
modified by monobasic acids (e.g., acrylic acid, methacrylic acid
and crotonic acid), rosins modified by glycol, and rosins modified
by dibasic acids (e.g., maleic acid, maleic anhydride and fumaric
acid).
[0075] (ii) Triazoles
[0076] Benzotriazole and/or its derivatives may be used to this
end.
[0077] (iii) Imidazole and/or Its Derivatives
[0078] (iv) Amine Compounds
[0079] Aromatic amines (aniline, o-toluidine, m-toluidine and
p-toluidine), aliphatic amines and cyclic ketoamines may be used to
this end.
[0080] (v) Organic Acids such as Fatty Acids Having Carboxyl Groups
and/or Their Metal Salts
[0081] To this end, use may be made of dicarboxylic acids,
polycarboxylic acids, hydroxycarboxylic acids (e.g.,
12-hydroxystearic acid and ricinolic acid), aromatic carboxylic
acids, aminocarboxylic acids, fatty acids having at least 8 carbon
atoms such as higher fatty acids (e.g., oleic acid and stearic
acid), acrylic acid and polyacrylic acid as well as their metal
salts.
[0082] The metal salt of each of the aforesaid organic acids is
generally called a metal soap wherein the metal may be Na, K, Li,
Ca, Mg, Al, Zn, Sn, Pb, Ni, Cu, Co, Mn, Fe, In, Bi, Ag or the like.
In view of solder material, however, metal salts of Sn are
preferred. In view of the action of the particle
coalescence-preventing agent on prevention of coalescence of
particles, on the other hand, it is preferable to use as metal
salts of organic acids (carboxylic acids) having carboxyl groups
those of straight-chain or hydroxy fatty acids having at least 8
carbon atoms, especially metal salts of stearic acid, metal salts
of 12-hydroxstearic acid and metal salts of ricinolic acid. It is
here noted that the metal salts of derivatives of fatty acids or
12-hydroxystearic acid may be called fatty acid soaps.
[0083] (vi) Hydrazines
[0084] To this end, use may be made of hydrated hydrazine, and
alkylhydrazine compounds (benzylhydrazine, tert-butylhydrazine
hydrochlorate, isopropylhydrazine sulfate, and hydrazinomethyl
acetate hydrochlorate).
[0085] (vii) Pyrazoles
[0086] (viii) Azo Compounds
[0087] (ix) Thermoplastic Resins such as Acrylic Resin and Phenol
Resin
[0088] (x) Alcohols such as Propargyl Alcohol, Butinediol, Hexynol,
Ethylaxynol
[0089] (xi) Isocynates
[0090] (xii) Sulfur-Containing Compounds
[0091] To this end, use may be made of thiourea, thioureas such as
N-substituted alkylthiourea, and heterocyclic compounds containing
--SH groups per molecule (e.g., 2-mercaptobenzothiazole, and
2-mercaptobenzoimidazole).
[0092] (xiii) Polyamine Compounds
[0093] Poly 4-vinylpyridine or the like may be used to this
end.
[0094] The particle coalescence-preventing agent may be composed of
not only the compound belonging to each of the classes (i) to
(xiii) but also the compounds belonging to two or more of these
classes.
[0095] Referring to the action of the carboxyl group (--COOH) on a
metal, there are chemical adsorption onto the surface of the metal
as represented by --COO--Me (metal) --OOC-- and physical adsorption
exerted by the attractive forces of charges as expressed by
--O.sup.---H.sup.+/Me.sup.-+/--O.sup.---H.sup.+ as in the case of
--OH. The chemical adsorption, because of involving chemical
reactions, generates much heat of adsorption, and so requires high
activation energy. However, the chemical adsorption has energy
higher than the physical adsorption, making particles less
susceptible to disengagement at high temperature.
[0096] The molten solder (the molten metal having a low-melting
point) prepared in the solder melting tank and the dispersion
medium prepared in the dispersion medium heating tank, etc. are fed
to the fine-granulation machine, wherein they are dispersed. The
wording "dispersing energy for dispersing particles is applied to
the dispersion medium" used in the present disclosure means that
when a lump or a coarse particle is finely divided into fine
particles, mechanical energy is applied to those divided articles
so as to prevent agglomeration or coalescence thereof. In this
sense, it is preferable to divide the low-melting molten metal into
particles and then disperse those particles in the aforesaid
dispersion medium. However, it is acceptable to disperse a lump
form of low-melting metal or a powder form of low-melting metal or
both in the dispersion medium. If heated during or after
dispersion, the latter can then be dispersed in the form of molten
metal particles.
[0097] The fine-granulation machine may be of the batch type that
is used with a buffer tank provided on its upstream side, from
which a constant amount of feed is supplied to the fine-granulation
machine. Thereafter, the same operation is repeated as often as
necessary. If, in this case, the fine-granulation machine is
operated in operative association with the buffer tank for
automatic running, then some mass-production can be achieved. When
mass-production is carried out on a full scale with improved
productivity or when the machine used is simplified, however, it is
preferable to use a continuous type fine-granulation machine to
which the molten solder and the dispersion medium are continuously
supplied for continuous running.
[0098] For such a continuous type fine-granulation machine, for
instance, use may be made of a combined agitation and dispersion
machine including a generator comprising a rotator and a stator, an
ultrasonic machine, a high-pressure homogenizer, and a high-speed
agitator as set forth in Japanese Patent Unexamined Publications
9-75698, 10-161667 and 11-347388.
[0099] One exemplary combined agitation and dispersion machine
comprising a rotator and a stator is shown in FIGS. 3 and 4. As
shown in FIGS. 3 and 4, a stator 1 is built up of a concave (a deep
dish form of) member and slots 4, 4, . . . , provided radially on a
peripheral wall thereof and open at their ends, and a rotator 2 is
made up of a two-wing blade rotatable around the axis of rotation.
By rotating the rotator 2 at high speed relative to the stator 1, a
liquid mixture to which a low-melting metal melt is added is sucked
in the dispersion medium obtained by mixing and dissolving the
particle coalescence-preventing agent in the particle dispersion
medium, so that the low-melting metal melt in the liquid mixture is
divided by high shear action exerting between the stator 1 and the
rotator 2 into particles, and a dispersion containing the metal
melt particles is discharged from the slots 4, 4, . . . . Reference
numeral 5 is the axis of rotation.
[0100] The machine of FIG. 3 is spaced away from the bottom of a
processing tank 6, as shown in FIG. 5, and a shaft of rotation 5
(that is not seen in FIG. 5) is inserted through a cylindrical
member extending hermetically through an upper end lid sheet 7, so
that the rotating force of a motor 8 (the number of rotation of
which is controlled by a rpm controller 8a) can be transmitted to
the shaft 5. Through an inlet hole and an outlet hole (shown by
arrows) formed in the closeably and detachably provided lid sheet
7, an inert gas can be circulated in the interior of the processing
tank 6 so that the interior of the processing tank 6 can be placed
in an inert gas atmosphere. Heaters 10, 10 and 10 are located on
the outside of the bottom and both sides of the processing tank 6.
As the bottom and both sides of the processing tank 6 are heated by
the heaters 10, 10 and 10, a liquid mixture 6a that is a liquid
mixture of the aforesaid particle dispersion medium and the
particle coalescence-preventing agent with the low-melting metal
melt is agitated at high speed by the high-shear machine immersed
therein, while that liquid mixture 6a is heated to a proper
processing temperature, thereby obtaining a slurry. In this case,
the temperature of the slurry is sensed by a thermocouple 11 to
control the amount of heat generated from the heaters 10, 10 and 10
by a temperature controller 12, so that the temperature of the
slurry can properly be controlled. Between the processing tank 6
and the heaters 10, 10 and 10 a copper pipe is provided in such a
way as to surround the processing tank 6 so that, by passing
cooling water through the copper pipe, the temperature of the
slurry under agitation can be prevented from becoming higher than a
certain threshold. A turning blade 9 is provided to prevent
entrainment of gas at the time the middle of the liquid surface is
lowered by generation of vortexes. Reference numeral 13 is a
processing tank-supporting base lined with a heat-resistant
material with the heaters 10, 10 and 10 embedded therein.
[0101] Exemplary agitators comprising a stator and a rotator are an
agitator manufactured by Kinematica (Switzerland), an agitator
manufactured by IKA (Germany), an agitator manufactured by
Silverson (Great Britain), Cavitron manufactured by Pacific
Machinery & Engineering Co., Ltd., and Clearmix manufactured by
M Technics. Preferably in this respect, the rotator should have a
diameter of at least 80 mm with a peripheral speed of at least 15
m/sec., and the clearance between the rotator and the stator is 1
mm at most. The aforesaid slurry should preferably be supplied to
the vicinity of the suction side on which the flow created by the
rotator and stator is sucked.
[0102] The ultrasonic dispersion machine is designed to apply
ultrasonic energy to the slurry while agitated by means of a
homogenizer or other agitator. Even with this, the slurry is
processed while heated, thereby dividing the low-melting metal melt
into fine particles. To generate ultrasonic vibrations, for
instance, use may be made of a separate type ultrasonic generator
wherein the generator is separate from a vibrator, a one-piece type
ultrasonic generator wherein the generator is integrated with a
vibrator, an immersion type ultrasonic generator, and a horizontal
type ultrasonic generator. Alternatively, a combined type
ultrasonic generator designed to use ultrasonic vibrations having
varying wavelengths at the same time may be used. For instance, a
multiple ultrasonic dispersion machine manufactured by Nippon Seiki
Seisakusho and a triple ultrasonic dispersion machine may be
used.
[0103] The feature of the high-pressure homogenizer is that the
slurry is passed through a narrow gap under pressure so that fine
granulation takes place by way of cavitation occurring on
instantaneous change from high to low pressure. Exemplary such
homogenizers are Ultymizer manufactured by Sugino Machine,
Micro-Fulldizer manufactured by Mizuho Industrial Co., Ltd., the H
series of high-pressure homogenizers manufactured by Nihon B. E.
E., Nanomizer System manufactured by Nakabishi Engineering, and a
high-pressure homogenizer manufactured by Niro Soavi (Italy).
[0104] Besides, Fillmix manufactured by Tokushu Kika Kogyo, and
other machines for dispersion, emulsification and fine granulation
in liquids may be used.
[0105] These dispersion machines may be used in parallel or series
arrangements of two or more for sharp particle size distributions,
high throughput capacities or depending on other requirements.
[0106] The aforesaid commercially available dispersion machines may
each be provided with devices for heating the aforesaid slurry or
cooling the resulting liquid, a temperature control indicator
(shown in FIG. 5), a washer and a bottom valve or, in some cases, a
pressure regulator for regulation of the pressure in the processing
tank.
[0107] The molten solder droplets (low-melting metal droplet)
finely granulated in the dispersion machine in the fine-granulation
system may then be passed through a mesh (sieve) or subjected to
gravitational settling, so that not fully divided or coarse
particles are removed to obtain fully divided particles alone. In
this case, it is acceptable to sort out coarse particles by a
liquid cyclone and allow them to go back to the dispersion
machine.
[0108] Alternatively, there may be provided another passage for
communicating the dispersion machine in the fine-granulation system
with the dispersion medium heating tank, so that on startup of
dispersion in the dispersion machine, only the dispersion medium is
fed as by a constant-feeding pump from the dispersion medium
heating tank to the dispersion machine until it is stabilized by
heating at a given temperature. After stabilization is reached, the
dispersion medium is fed back to the dispersion medium heating
tank.
[0109] By increasing the number of rotations and vibrations in the
dispersion machine thereby applying high dispersion energy to the
molten metal particles, they can be divided into fine particles
having an average particle diameter of 10 .mu.m+a few .mu.m at
most, preferably 10 .mu.m or less (10 .mu.m at most).
[0110] The low-melting metal is used in an amount of 0.1 to 100
grams, preferably 1 to 50 grams, more preferably 2 to 20 grams per
100 grams of the particle dispersion medium, and the particle
coalescence-preventing agent is used in an amount of 0.01 to 10
grams per 100 grams of the particle dispersion medium. When the
proportion of the low-melting metal is lower than that lower limit,
production efficiency drops, and when too much low-melting metal is
used, there is a decrease in the effect on prevention of
coalescence of the molten metal particles dispersed in the
dispersion medium as is the case where the amount of the
coalescence-preventing agent is less than its lower limit. When the
particle coalescence-preventing agent is used in an amount greater
than its upper limit, its effect remains unsaturated or is little
enhanced.
[0111] The slurry obtained by dispersing the molten metal particles
in the dispersion medium containing the particle
coalescence-preventing agent in this way is then fed to the
solidifier by cooling, if required, after coarse particles have
been removed therefrom, as shown in FIGS. 1 and 2, wherein the
slurry is cooled to a temperature lower than the solidifying point
of the molten metal, so that the molten metal particles are
solidified into solid metal particles. For the solidifier by
cooling, it is preferable to use a cooling double pipe of simple
construction that comprises an inner pipe through which the coarse
particle-free slurry is to pass and a jacket that surrounds the
inner pipe. Water or other coolant is circulated through the
jacket, and the flow rate of the coolant is regulated in such a way
that the temperature of the slurry is kept constant at a given
position in the inner pipe. It is preferable that the cooling
double pipe is set at an angle of 45 to 90.degree. with respect to
horizontal; at an angle smaller than 45.degree., the resulting
solid metal particles are likely to build up. It is then preferable
that the cooling temperature of the cooling double pipe is lower
than the temperature at which the droplets of the lower-melting
metal, e.g., solder are solidified and about 20 to 100.degree. C.
lower than the melting point of solder. This is required to make
the processing by the next solid-liquid separator easy, and save
the heat energy needed for reheating the spent dispersion medium
that is fed back to the dispersion medium heating tank after
regeneration in the dispersion medium regenerator.
[0112] The slurry may be cooled by water, as is the case with the
system of FIG. 5 or cooled off in a vessel in the system.
Alternatively, the slurry may rapidly be cooled by charging the
coolant throughout a pool of the slurry under agitation, or the
slurry may continuously be pored in the coolant. The coolant used
may be the aforesaid dispersion medium or other medium that may or
may not have volatility.
[0113] The thus obtained slurry free from any coarse particles and
containing the solid metal particles is fed to the solid-liquid
separator as shown in FIGS. 1 and 2, wherein the solid metal
particles are separated from liquid residue. On the upstream side
of the solid-liquid separator, there may be provided a slurry
buffer tank capable of storing a constant amount of slurry in such
a way that the slurry is fed from that buffer tank to the
solid-liquid separator as occasion may arise.
[0114] By way of example but not exclusively, the solid-liquid
separator used herein may appropriately be selected from a variety
of desired separators depending on the properties of the solid
metal particle-containing slurry, e.g., the particle size
distribution of the solid metal particles, the viscosity of the
liquid residue from which the solid metal particles have been
separated, and concentration of the solid matter. To this end, use
may be made of liquid cyclones, separators that harness spontaneous
settling, and filter devices (such as Oliver filter, horizontal
belt filter, rotary filter, ceramic filter, filter press and
centrifugal filter). However, when the solid metal particles have a
small particle diameter or the liquid residue has a high viscosity,
solid-liquid separation should preferably be carried out by means
of a centrifugal decanter.
[0115] In the solid-liquid separation process by the centrifugal
decanter, it is preferable that the solid metal particle-containing
slurry is continuously fed to the centrifugal decanter for
successively separating the solid metal particles that are the
solid matter from the liquid residue. In this case, it is
acceptable to leave too small particles of the solid metal
particles on the liquid residue side. The centrifugal decanter is
also preferable in that depending on the particle diameter of the
solid metal particles to be separated, the viscosity of the liquid
residue, a specific gravity difference between the solid metal
particles and the liquid residue, etc., the number of rotations can
be regulated for easy regulation of centrifugal force. It is here
noted that the solid metal particles are obtained in the form of a
slurry or cake because about 10% (on a weight basis) of liquid
residue deposit to the solid metal particles. The solid-liquid
separator should preferably be of the type that can tightly be
closed for solid-liquid separation processing in the presence of an
inert gas in consideration of prevention of oxidization of the
solid metal particles and liquid residue (the liquid residue is
recyclable after regeneration as will be described later), and
avoidance of risks of possible fires in the case where the liquid
residues contain flammable components. A solid-liquid separator
whose interior is easily cleanable is also preferred. For instance,
preference is given to the TRV series of upright centrifugal
decanters manufactured by Tomoe Kogyo.
[0116] Preferably, the solid metal particles with some liquid
residue deposited to them, separated by the solid-liquid separator,
should be washed in the washer as shown in FIGS. 1 and 2 so that
those depositions are washed away to some degrees. In some cases,
however, the solid metal particles may be used immediately or
without being washed. The washer used herein should preferably
comprise a tank in which the solid metal particles with the liquid
residue deposited onto them are immersed in a detergent, preferably
under agitation by an agitator or the like. The reasons why such
agitation is preferable are that after re-pulping (that is,
re-dispersion in the detergent of the solid metal particles with
the liquid residue deposited to them), the liquid residue
depositions to the solid metal particles are replaced by the
detergent, so that the solid material particles can easily be
separated from a liquid detergent residue in the next step. This
particularly holds true for the case where a volatile detergent is
used as the detergent, because the liquid detergent residue can
easily be removed from the solid metal particles by way of
volatilization. For the detergents used herein, a suitable
selection may be made from the types that can be more easily
separated from the solid metal particles than from the liquid
residue composed mainly of the aforesaid dispersion medium. The
separation means may also be selected depending on the selected
detergent. The more the number of repetition of the re-pulping and
subsequent solid-liquid separation process, the more the amount of
depositions is reduced and the higher the degree of washing
becomes, resulting in solid metal particles from which impurities
are reduced as much as possible.
[0117] To separate the solid metal particles from the liquid
residue such as detergent residue, a suitable selection should
preferably be made from various separation devices depending on the
particle size distribution of the solid metal particles, the
viscosity of the liquid residue, the concentration of the solid
matter, etc. To this end, use may be made of liquid cyclones,
separators that harness spontaneous settling, and filter devices
(such as Oliver filter, horizontal belt filter, rotary filter,
ceramic filter, filter press and centrifugal filter). However, when
it is desired to obtain solid metal particles having an average
particle diameter of 10 .mu.m or less, centrifugal machines,
especially a centrifugal decanter should preferably be used.
[0118] In the solid-liquid separation process by the centrifugal
decanter, it is preferable that the solid metal particle-containing
slurry is continuously fed to the centrifugal decanter for
successively separating the solid metal particles that are the
solid matter from the liquid residues. In this case, it is
acceptable to leave too small particles of the solid metal
particles on the liquid residue side. The centrifugal decanter is
also preferable in that depending on the particle diameter of the
solid metal particles to be separated, the viscosity of the liquid
residues, a specific gravity difference between the solid metal
particles and the liquid residues, etc., the number of rotations
can be regulated for easy regulation of centrifugal force. If a
double pipe or the like is used between the solid and the liquid
outlet of the centrifugal decanter to charge the detergent through
that double pipe, then washing due to counter-current contact is
achievable so that efficient washing can be carried out. The
solid-liquid separator should preferably be of the type that can
tightly be closed for solid-liquid separation processing in the
presence of an inert gas in consideration of prevention of
oxidization of the solid metal particles and liquid residues (the
liquid residues are recyclable after regeneration as will be
described later), and avoidance of risks of possible fires in the
case where the liquid residues contain flammable components. A
solid-liquid separator whose interior is easily cleanable is also
preferred. For instance, preference is given to the TRV series of
upright centrifugal decanters manufactured by Tomoe Kogyo.
[0119] The detergents used herein, for instance, include those of
the quasi-aqueous glycol ether type, water-soluble solvent type,
solvent type, terpene type and petroleum solvent type
("WELL-UNDERSTABLE EVERY ASPECT OF WASHING", pp. 45-55, September
1999, Nikkan Kogyo Shinbun); those of the non-aqueous hydrocarbon
type (normal paraffin, isoparaffin, naphthene, and aromatic
detergents); alcoholic detergents (based on isopropyl alcohol,
ethanol or other alcohols); silicone detergents; fluorine
detergents; chlorine detergents; bromine detergents (see
"WELL-UNDERSTABLE EVERY ASPECT OF WASHING"); and detergents based
on acetone, methyl ethyl ketone, benzene, toluene, xylene, ethyl
acetate, butyl acetate, cyclohexane, etc.
[0120] Among these detergents, it is preferable to use detergents
that have low water-solubility and low hygroscopicity for the
purpose of prevention of oxidization of the solid metal particles,
and further have non-flammability or low flammability. However, it
is acceptable to use detergents having high flammability if fire
prevention means are used in combination therewith. When the
detergent is distilled out of the liquid residue upon solid-liquid
separation for the purpose of recycling the spent detergent, it is
preferable to use a detergent that has a vapor pressure higher than
those of the particle dispersion medium and particle
coalescence-preventing agent contained in the dispersion medium and
has a low-boiling point as measured by itself and decreased latent
heat of vaporization. Even though this detergent remains in the
dispersion medium or the solid metal particles, there is no problem
or little or no influence.
[0121] The washing by the detergent should preferably be carried
out to such a degree that some portion of the liquid residue
deposited to the solid metal particles before washing remains. This
is because especially when a difficult-to-volatilize substance is
used for the particle dispersion medium contained in the liquid
residue, it is possible to reduce the degree of powder dusting of
the solid metal particles after the drying step by the drier or to
prevent oxidization of the solid metal particles. In this case, the
amount of the remaining hard-to-volatilize substance should
preferably be in the range of 0.01 to 1% with respect to the solid
metal particles. The "%" used herein is understood to mean "% by
mass".
[0122] The solid metal particles washed in the washer may be
discharged from the washer with the detergent remaining deposited
to them. For removal of such detergent depositions, however, it is
preferable to dry the solid metal particles in the drier, as shown
in FIGS. 1 and 2.
[0123] When a detergent of high volatility is used as the
detergent, the drier used should be designed to remove that
detergent by way of vaporization. By way of example but by way of
limitation, it is preferable to use a drier that relies upon drying
by heating and/or drying under reduced pressure. When the solid
metal particles are solder particles, however, it is preferable to
use a drier that relies on both indirect heating at about
50.degree. C. and drying under reduced pressure. When other solid
metal particles are obtained, one or both of these drying methods
may be used at different drying temperatures. If indirect heating
is combined with agitation of the solid metal particles, not only
is efficient drying achievable, but also the hard-to-volatilize
liquid residue (the liquid residue remaining upon solid-liquid
separation after the processing in the fine-granulation machine can
uniformly be deposited to the solid metal particles)(that liquid
residue contains the particle coalescence-preventing agent).
However, too strong agitation is not preferable because, for
instance, solder particles are susceptible to damage.
[0124] The drier used should preferably be of the continuous type
that enables the material to be dried to be continuously fed
thereto and the dried material to be successively discharged
therefrom. Since it is difficult to effect drying under reduced
pressure in a continuous manner, however, it is acceptable to use a
batch type drier. In this case, on the upstream side of the batch
type drier, there is provided a buffer tank for storing the
material to be dried. On demand the buffer tank feeds the material
to be dried to the drier, and the dried material is then discharged
therefrom. By repeating this operation, the buffer tank can be run
in automatically operative association with the drier so that large
amounts of the solid metal particles can be dried. To this end,
preference is given to a vibrating drier commercially obtainable
from Chuo Kakoki.
[0125] When the viscosity of the liquid residue on the solid-liquid
separation of the slurry discharged from the fine-granulation
machine is relatively low in the case where solder powders are
obtained, it is acceptable to use a combined filter and drier
manufactured by Tanabe Willtech or the like, so that the
solid-liquid separation of the slurry discharged out of the
fine-granulation machine, the washing of the solid matter of the
slurry, the solid-liquid separation of the washings and the drying
of the solid matter of the washings can be finished in one single
unit.
[0126] Through the aforesaid respective steps, solder products are
obtained from the starting solder material (metal) as shown in
FIGS. 1 and 2. To cut back on the production cost of solder
powders, however, it is preferable to recycle the particle
dispersion medium and the detergent.
[0127] Referring to how to recycle with reference to FIG. 2, the
spent dispersion medium is discharged out of the solid-liquid
separator. As already described, the spent dispersion medium is
composed of the liquid residue from which solid metal particles
having a given particle diameter have been removed (and which, in
some cases, contains solid metal particles having too small
particle diameters). Although depending on purposes, this spent
dispersion medium may be fed back to the dispersion medium heating
tank for immediate use. When regeneration is needed, however, the
spent dispersion medium is preferably fed to the dispersion medium
regenerator, from which the regenerated medium is sent to the
dispersion medium heating tank. For instance, for removal of solid
metal particles which, because of having too small diameters, are
accidentally or unavoidably entrained in the liquid residue or
other accidentally formed solid matter, it is preferable to use a
centrifugal decanter, Sharples centrifuge, De Laval centrifuge, or
filters such as a rotary filter and a ceramic filter. The thus
obtained clear liquid is allowed to go back to the dispersion
medium heating tank by means of a pump, because that liquid is a
dispersion medium that is a mixture of the particle dispersion
medium and the particle coalescence-preventing agent (often at a
ratio different from the initial ratio). Preferably, however, the
clear liquid should temporarily be stored in a separately provided
buffer tank from which, on demand, it is fed to such a tank. In
this way, the spent dispersion medium (dispersion medium) can be
recycled in a continuous manner.
[0128] As shown in FIG. 2, the spent detergent is discharged out of
the washer. As already stated, this spent detergent contains the
detergent that is used for wash processing in the washer and has
depositions (the liquid residue occurring upon solid-liquid
separation of the slurry discharged from the fine-granulation
machine) adhering to the solid metal particles discharged out of
the fine-granulation machine. The spent detergent may be allowed to
go back as such to the washer for subsequent use. When the spent
detergent must be regenerated, however, it is preferable to feed
the spent detergent to the detergent regenerator for removing the
liquid residue therefrom, and then return the regenerated detergent
back to the washer. When the detergent has high volatility and the
liquid residue has low volatility, the detergent regenerator should
be designed such that the detergent is distilled out. When there is
a large difference in vapor pressure between the detergent and the
liquid residue, it is preferable to carry out single-stage
distillation. When that vapor pressure difference is large,
however, it is preferable to carry out multistage distillation in a
continuous manner. In consideration of recycling, the detergent
should preferably have a vapor pressure of at least 15 kPa at
40.degree. C. and a latent heat of vaporization of up to 1,000
kJ/kg.
[0129] As explained above, solder powder products are obtained from
the starting solder metal. In this case, the dispersion medium, and
the detergent may be recycled. Preferably for processing, each
device or unit should be run or operated while its interior is
filled with an inert gas. Likewise, if required, the dispersion
medium regenerator or the detergent regenerator should be run or
operated in the presence of an inert gas. This is favorable just
only for prevention of oxidization of the low-melting metal and its
metal as well as the solid metal particles, particle dispersion
medium and particle coalescence-preventing agent but also for
making the whole system inclusive of the detergent resistant to
flammability, thereby foreclosing the risk of possible fires. It is
also preferable to provide a washer for the necessary portion of
each device or unit, because it is easy to carry out washing upon
alteration of the type of the metal to be finely divided or upon
alteration of the particle dispersion medium, particle
coalescence-preventing agent and detergent, thereby preventing
contamination upon such alterations.
[0130] The shape and particle diameter of the solid metal particles
in the thus obtained fine metal powder products are determined
depending on the shape and particle diameter of the molten metal
particles dispersed in the heated dispersion medium in the
fine-granulation machine; the molten metal particles have
substantially true sphericity and so have the solid particles. To
reduce the particle diameter of the solid particles, on the other
hand, the diameter of the molten metal particles should be reduced
down to the average particle diameter of 2 to 30 .mu.m. This, for
instance, is achievable by increasing the number of rotations and
vibrations of each dispersion machine depending on the application
time of dispersion energy, operating conditions such as the heating
temperature and processing time of the slurry, the types and
proportions of the low-melting metal, particle dispersion medium
and particle coalescence-preventing agent. It is here understood
that the process for producing metal powders of the invention
includes a process for producing spherical metal powders, a process
for producing a spherical form of fine metal powders, and a process
for producing a spherical form of fine metal particles.
[0131] Thus, if fine metal particles, especially a spherical form
of fine metal particles are used in a reduced amount with respect
to the low-melting metal in the particle dispersion medium so that
the components of the particle dispersion, etc. consumed in
production process steps are reduced, it is then possible to mass
produce fine metal powder particles in an industrially efficient
manner.
[0132] The fine metal powders obtained by the aforesaid fine metal
powder production process, for instance, are used in an amount of
85 to 92% per solder paste (with the flux content of 8 to 15%).
Those powders are particularly suitable for reflow soldering for
recently developed printed circuit boards having soldering lands at
very narrow pitches.
EXAMPLE 1
[0133] According to the flowchart of FIG. 1, the starting solder
metal (Sn--Pb eutectic alloy (63Sn/37Pb) for solder is introduced
into the solder melting tank (that is filled therein with nitrogen
gas), where it is molten to prepare a solder melt. Meanwhile,
Uniloop MB-22 (manufactured by Nippon Oils & Fats) for the
particle dispersion medium and hydrogenated, acrylic acid-modified
rosin (KE-604 manufactured by Arakawa Kagaku Kogyo) for the
particle coalescence-preventing agent (2% with respect to the sum
of the particle dispersion medium and the particle
coalescence-preventing agent) are mixed together in the dispersion
medium heating tank (that is filled therein with a nitrogen gas).
The mixture is preheated in the dispersion medium heating tank as
long as it is at a temperature 50 to 100.degree. C. lower than the
temperature at which the mixture is to be fed to the
fine-granulation machine. Then, the mixture is heated to the
temperature at which it is to be fed to the fine-granulation
machine, using a separately provided line heater furnace (heated
delivery pipe) for a short residence time of up to 10 minutes in a
precisely temperature-controlled manner, thereby preparing a heated
dispersion medium.
[0134] Using separate constant-feeding gear pumps, the molten
solder (20 Kg/hour) and the heated dispersion medium (200 Kg/hour)
are continuously fed at the same flow rate through the respective
lines to the fine-granulation machine. In this case, the molten
solder in particular is passed through the line whose interior can
be controllably heated (to 190.degree. C.) and the mass ratio of
both is 1:10. In the fine-granulation machine, both are
continuously mixed together at a constant temperature of
190.degree. C., while dispersion energy for dispersion of particles
is applied to the mixture, thereby continuously obtaining a molten
solder particle dispersion having the molten solder particles
dispersed in the dispersion medium. To this end, a dispersion
machine comprising a stator and a rotator (the diameter of the
rotator is at 80 mm or longer, the peripheral speed is 15 m/second
or faster and the rotator-to-stator clearance is 1 mm or longer) is
used.
[0135] Then, the molten solder particle dispersion is continuously
passed through a line into the inner pipe of a cooling double pipe
set at an angle of 60.degree. for the solidifier by cooling, while
cooling water is circulated through the outer jacket (cooling by
coolant water passing between the coolant-in and the coolant-out in
FIG. 1). Thus, the molten solder particles are cooled and
solidified into solid particles.
[0136] Subsequently, the slurry that contains the solid solder
particles is continuously supplied to the solid-liquid separator
for which one of the TRV series of upright centrifugal decanters
manufactured by Tomoe Kogyo is used, wherein the solid solder
particles are continuously separated from the slurry to obtain a
throughput of 19 Kg/hour as calculated on a solder basis. The thus
separated solid solder particles are processed in the washer. This
washer is built up of a re-pulping tank and another decanter. In
the re-pulping tank, the solid solder particles are washed using
ethyl acetate as the detergent, and then subjected to solid-liquid
separation in the centrifugal decanter, threrby separating the
solder powders from the detergent. The solid solder particles are
rid of depositions while the spent detergent is recovered. The
detergent was used in an amount of 38 Kg/hour in the repulping tank
and in an amount of 19 Kg/hour in the centrifugal decanter. The
thus washed solid solder particles are dried in a vibration drier
manufactured by Chuo Kakoki (at a drying temperature of 50.degree.
C.) for the drier to obtain 19 Kg/hour of a solder powder product
(fine solder particles).
[0137] It is here noted that processing of the material by each
device and delivery of the material between the adjacent devices
according to the flowchart of FIG. 1 were automated to the greatest
extent practicable while numerical data are checked against those
from the sensor attached to each device, and as many devices as
possible were computer-controlled pursuant to the program for the
whole production process. It is also noted that the devices are
connected with one another by way of piping, and so the raw
materials can continuously be processed into the end product.
[0138] Observation under a scanning electron microscope (SEM) of
the obtained fine solder particles showed that they are in true
spherical forms having no satellite particle whatsoever, and laser
diffraction analysis indicated that the average particle diameter
is 9.5 .mu.m and the particle size distribution is 0.65 as
expressed by .epsilon.=(D.sub.90-D.sub.10)/D.sub.50 where D.sub.90,
D.sub.10 and D.sub.50 stand for the diameters of particles that
account for 90%, 10% and 50%, respectively, of all particles as
counted in ascending diameter order. The yield of the obtained fine
solder particles was 90%. The smaller the value of .epsilon., the
narrower and sharper the particle size distribution becomes.
EXAMPLE 2
[0139] As shown in the flowchart of FIG. 2, solder powders were
prepared as in Example 1 with the exception that two additional
steps were added to the flowchart of FIG. 1.
[0140] In one additional step, the solid-liquid separator was run
such that the liquid residue separated from the solid solder
particles was used as the spent dispersion medium and the
dispersion medium regenerator was run such that the solid matter
was removed by a centrifugal decanter and the resulting clear
liquid was used as the recovered dispersion medium in an amount of
90% relative to the original amount. The recovered dispersion
medium was used as a part of the dispersion medium in the aforesaid
dispersion medium heating tank (a 10% deficiency was made up by a
fresh dispersion medium).
[0141] In another additional step, the washer was run such that the
liquid residue composed mainly of ethyl acetate as the spent
detergent was distilled out and recovered in a single distillation
device working as the detergent regenerator. The washer was also
operated such that the rate of recovery of ethyl acetate (the
proportion of the recovered amount relative to the original amount)
was 90%. That is, the washer was operated such that the recovered
ethyl acetate could be used as a part of the detergent (the
deficient was made up by a fresh detergent).
[0142] It is understood that the aforesaid two steps added to the
flowchart of FIG. 2, too, can be automated in such a way as to be
computer-controlled. In this example, the respective devices are
again connected with one another by way of piping, so that the raw
materials can continuously be processed into the end product. The
obtained solder powders were the same as in Example 1.
[0143] According to the present invention, the step of fine
granulation of the molten metal and the solidification-by-cooling
step, solid-liquid separation step, washing step and drying step
added thereto are controlled in the form of a series of mutually
correlative steps, so that fine powder particles of the metal can
be mass-produced in an industrially efficient manner, thereby
achieving remarkable cost reductions. Especially if the dispersion
medium and detergents are regenerated and recovered for recycling,
then the production cost can be much more reduced.
[0144] Moreover, metal fine powders that can be applied even to
fine soldering portions on wiring substrates can be mass-produced
in an industrially efficient manner, and a solder paste composition
using such metal fine powders can be used for metal mask printing
of fine patterns, so that surface mounting of electronic parts,
etc. can be carried out at high densities, resulting in the
achievement of multifunctional, miniaturized wiring substrates for
electronic equipments.
* * * * *