Tin-based solder alloy powder

Abstract

A tin-based solder alloy powder in the form of 2 µm to 38 µm spherically shaped tin-based solder alloy particles exhibiting an O / C weight ratio in the range of 1.0 to 2.2, wherein the spherically shaped tin-based solder alloy particles have a thin vegetable oil outer surface layer and a method of manufacturing the same.

Description

[0001] This invention relates to a tin-based solder alloy powder and a method for manufacturing the same, wherein the powder particles have a thin outer surface layer of vegetable oil. The invention also relates to a solder paste comprising the tin-based solder alloy powder.

[0002] Both US 6,290,745 B1 and US 2006 / 0208042 A1 disclose a method for manufacturing solder alloy balls. In these methods, a metallic solder alloy is melted in a heat-resistant vegetable or animal oil, and then stirred and dispersed by a rotor and stator in multiple shearing processes to form solder alloy balls having a defined diameter in the range of 1 µm to 100 µm. US 6,290,745 B1 discloses a final washing step of rinsing the solder alloy balls thus manufactured with a grease-dissolving solvent such as acetone.

[0003] The object of this invention is to improve the storage stability of tin-based solder alloy powders, which can be evaluated from the solderability properties of solder pastes made from tin-based solder alloy powders and solder flux compositions, as is conventional in the art. Solderability properties can be determined by so-called solderability tests (solder ball tests) on ceramic substrates or by wetting tests on copper sheets. Such tests can be performed using solder pastes prepared from fresh and aged tin-based solder alloy powder samples. Aging means holding the freshly prepared tin-based solder alloy powder in question under defined conditions (e.g., under environmental exposure, i.e., in the presence of air and stored at 55°C with 50% relative humidity) and sampling after 16 days, 31 days, and 63 days; solder pastes can then be prepared from these powder samples.

[0004] During solderability testing, solderability performance is determined, and an indication is given of the effect of inorganic oxygen-containing substances present on the surface of tin-based solder alloy powder on the formation of undesirable solder clusters during the soldering process. The solder paste to be tested is printed onto a ceramic substrate, and then the test is performed. For this purpose, the ceramic substrate is placed in a reflow oven at an object temperature of 250°C in a nitrogen atmosphere until the tin-based solder alloy powder melts; then the formation of residual tin-based solder alloy powder (unsoldered tin-based solder alloy powder) and the percentage of area covered by inorganic oxygen-containing substances such as tin oxide residues are evaluated, with both being smaller, the better. For details of the solderability testing, refer to the following examples.

[0005] During the wetting test, wetting performance is determined, and an indication is given of the effect of inorganic oxygen-containing substances present on the surface of tin-based solder alloy powder on undesirable dewetting behavior. The solder paste to be tested is printed onto a copper sheet, and then the test is performed. For this purpose, the copper sheet is placed in a reflow oven at an object temperature of 250°C in a nitrogen atmosphere until the solder paste melts and wets the copper surface; then the percentage of dewetting area relative to the initially printed area is evaluated, with a smaller percentage indicating better results. For details of the wetting test, refer to the following example.

[0006] It has been found that the object of the present invention can be achieved by providing a tin-based solder alloy powder in the form of small spherical tin-based solder alloy particles of 2µm to 38µm, preferably 2µm to 25µm, exhibiting an O / C (oxygen / carbon) weight ratio in the range of 1.0 to 2.2, preferably in the range of 1.0 to 2.0, wherein the spherical tin-based solder alloy particles have a thin outer surface layer of vegetable oil (e.g., castor oil, especially fortified castor oil). Surprisingly, the O / C weight ratio has proven to be an essential feature. If the spherical tin-based solder alloy particles have an O / C weight ratio below 1.0, or if they exceed an O / C weight ratio of 2.2, solder pastes based on such tin-based solder alloy particles exhibit poor solderability and wetting properties.

[0007] Those skilled in the art will understand 2µm to 38µm, preferably 2µm to 25µm, small spherical tin-based solder alloy particles as 2µm to 38µm, preferably 2µm to 25µm small tin-based solder alloy spheres, or as tin-based solder alloy spheres with a diameter in the range of 2µm to 38µm, preferably 2µm to 25µm. The synonyms “spherical tin-based solder alloy particles” and “tin-based solder alloy spheres” are used herein. In other words, spherical tin-based solder alloy particles or tin-based solder alloy spheres are spherical or very close to spherical in shape (i.e., exhibiting a cross-sectional area value of 0.95 to 1.0, where “cross-sectional area value” refers to the quotient of the maximum and minimum linear dimensions of the particle; i.e., a cross-sectional area value of 1.0 indicates a perfect sphere), wherein the absolute spherical size is in the range of 2µm to 38µm, preferably 2µm to 25µm. In other words, the discussion here concerns solder alloy balls of types T4 to T8 according to industry standard IPC J-STD-005A (T4, T5, T6, T7, or T8 correspond to particle sizes or ball sizes of 20µm to 38µm, 15µm to 25µm, 5µm to 15µm, 2µm to 11µm, or 2µm to 8µm). The particle size or ball size or diameter and the particle shape can be determined by SEM (scanning electron microscopy) analysis of a statistically significant number of particles or balls, for example, in the range of up to 400 to even up to 2000.

[0008] The term "tin-based solder alloy" is used herein. It should be understood as a tin-rich solder alloy or a solder alloy containing at least a substantial and non-negligible proportion of tin (e.g., at least 42 wt% tin). Examples of tin-rich solder alloys are those with a tin content, for example, ranging from 90 wt% to 99.5 wt%. Examples of alloy metals are copper, silver, indium, germanium, nickel, lead, bismuth, and antimony. Tin-based solder alloys may contain lead or are preferably lead-free. Lead-free tin-based solder alloys may be selected, for example, from the group consisting of SnAg, SnBi, SnSb, SnAgCu, SnCu, SnSb, InSnCd, InBiSn, InSn, BiSnAg, or SnAgCuBiSbNi alloy types. Lead-containing tin-based solder alloys may be selected, for example, from the group consisting of SnPb and SnPbAg alloy types. The liquidus temperature of tin-based solder alloys can be, for example, in the range of 140°C to 230°C. Specific examples of tin-based solder alloys in the sense of this invention include common alloys such as SAC305, SAC405, SnCu0.7, and SnBi.

[0009] The term “fortified castor oil” is used in this document; those skilled in the art will find further explanation of the term below.

[0010] To avoid misunderstanding, the thin outer surface layer of vegetable oil (e.g., castor oil, especially fortified castor oil) is integral part of the spherical tin-based solder alloy particles of the present invention. In a preferred embodiment, the vegetable oil (e.g., castor oil, especially fortified castor oil) contains no additives or auxiliary substances, such as corrosion inhibitors or emulsifying agents; in other words, preferably, the vegetable oil (e.g., castor oil, especially fortified castor oil) consists only of itself. The outer surface layer of vegetable oil (e.g., castor oil, especially fortified castor oil) may have a layer thickness in the nanoscale range (e.g., in the range of 0.5 nm to 5.2 nm); the thickness of this thin outer surface layer can be determined by Auger electron spectroscopy using an argon sputtering ion beam of 1 keV. The inorganic surface of the spherical tin-based solder alloy particles is located near and below this thin outer surface layer; this inorganic surface comprises or is composed of inorganic oxygen-containing substances of tin, and, where applicable, is also the inorganic oxygen-containing substance of the alloy metal of the tin-based solder alloy. The inorganic surface can also have a thickness in the nanoscale range (e.g., in the range of 0.3 nm to 2.1 nm); the thickness of the inorganic surface can also be determined by Auger electron spectroscopy using an argon sputtering ion beam of 1 keV. Beneath the inorganic surface, only the actual metallic tin-based solder alloy exists. The thin vegetable oil outer surface layer, together with the inorganic surface, forms the surface region of the spherical tin-based solder alloy particles of the present invention; within this range, the surface region can have a thickness in the nanoscale range, for example, from 0.8 nm to 7.3 nm, as the sum of the outer surface layer thickness and the inorganic surface thickness within the nanoscale range.

[0011] As disclosed above, the tin-based solder alloy particles of the present invention exhibit an O / C weight ratio in the range of 1.0 to 2.2, preferably 1.0 to 2.0. This can be calculated by dividing the weight content of oxygen (“oxygen content”) of the tin-based solder alloy particles of the present invention by their weight content of carbon (“carbon content”). Both oxygen and carbon are present in the surface region of the spherical tin-based solder alloy particles and not in the interior of the particles; in other words, and to avoid misunderstanding, the O / C weight ratio is the O / C weight ratio of the surface region of the spherical tin-based solder alloy particles of the present invention. Oxygen or oxygen content originates from (i) inorganically bonded oxygen (inorganic oxygen-containing substances, such as, in particular, oxides and / or hydroxides of tin, and, where applicable, oxides and / or hydroxides of the alloy metal of the tin-based solder alloy) and (ii) organically bonded oxygen from vegetable oils. Some inorganically bonded oxygen may also originate from tin carbonates (and, where applicable, carbonates of the alloy metal of the tin-based solder alloy), however to a minimum. Those skilled in the art will understand that the term "organically bonded oxygen" refers to oxygen in the form of, for example, ester groups, hydroxyl groups, carboxyl groups, epoxy groups, peroxy groups, hydrogen peroxide groups, aldehyde groups, ketone groups, etc. The carbon or carbon content is primarily derived from organically bonded carbon in vegetable oils, and at most minimally from inorganically bonded carbon, such as carbonates of tin (and, where applicable, carbonates of alloy metals in tin-based solder alloys).

[0012] The oxygen content of spherical tin-based solder alloy particles can be measured using inert gas fusion. For this purpose, a pre-weighed sample of spherical tin-based solder alloy particles is placed in a graphite crucible and inserted into a pulse furnace, where the spherical particles are held between electrodes. After purging with an inert gas (He or Ar), a high current is passed through the crucible, generating a high temperature rise to release the analyte gas. Oxygen present in the sample reacts with carbon in the graphite crucible and / or with carbon in the sample to form CO and CO2. Any CO and CO2 generated in the furnace is released into a flowing inert gas stream, which is directed to an infrared detector for measurement as CO and CO2. The inert gas carrier sweeps the released gas out of the furnace and through a mass flow controller. The gas then flows through a heated reagent, where any CO is oxidized to form CO2, and hydrogen is oxidized to form H2O. Oxygen is detected as CO2 using a non-dispersive infrared (NDIR) cell. CO2 and H2O are then washed out from the carrier gas stream. Instrument calibration is performed using known reference materials to define the concentration range of the material being tested. This oxygen analysis can be performed, for example, using a Leco O736 instrument (see https: / / www.leco.com / product / 736-series).

[0013] Carbon content can be measured based on combustion and infrared detection during instrumental gas analysis. The analytical method is based on the complete oxidation of a solid sample by combustion in an oxygen plasma. A pre-weighed sample of spherical tin-based solder alloy particles is placed in a ceramic crucible in a high-frequency induction furnace, which is heated using RF (radio frequency) induction and combusted in an oxygen feed stream. The combustion of the sample releases various gases, which are measured by four infrared detectors. Analysis of CO and CO2 determines the carbon content. The gas stream continues through a heated catalyst, where any CO is converted to CO2. Carbon is detected as CO2 using a non-dispersive infrared (NDIR) cell. This carbon analysis can be performed, for example, using a Leco C744 instrument (see https: / / www.leco.com / product / 744-series).

[0014] This disclosure uses the term "fortified castor oil" to distinguish it from other types of castor oil, such as fresh castor oil. Fortified castor oil contains castor oil derivatives, meaning it consists of castor oil containing contaminants in the form of castor oil derivatives. Examples of castor oil derivatives include oxidized, oligomeric, or polymerized castor oil derivatives, wherein the oligomeric or polymerized castor oil derivatives may also be oxidized (or subjected to oxidation). Fortified castor oil is castor oil that has undergone oxidative stress and usually also thermal stress (heat stress), for example, castor oil subjected to heat stress for a total of 10 to 120 minutes in the presence of air at a temperature range of 200°C to 250°C. On the other hand, fresh castor oil means castor oil that has not undergone oxidation and usually has not undergone thermal stress, such as castor oil freshly obtained from castor seeds, and stored in a cool and dark place where no air or oxygen enters it if it is not used immediately or soon. In the sense of this disclosure, fresh castor oil can have an acid value in, for example, the range of 0.14 mg KOH / g to 1.97 mg KOH / g, while fortified castor oil has a higher acid value in, for example, the range of 3 mg KOH / g to 4 mg KOH / g. The term "acid value" as used herein refers to an acid value that can be determined in mg KOH / g (milligrams of KOH per gram) according to the industrial standard DIN EN ISO 2114. Those skilled in the art will understand that, in the sense of this disclosure, fortified castor oil can be obtained by subjecting fresh castor oil to the aforementioned oxidative stress and generally also to thermal stress until the aforementioned acid value in the range of 3 mg KOH / g to 4 mg KOH / g is achieved. Needless to say, this type of fortified castor oil can be obtained by blending fresh castor oil with highly fortified castor oil exhibiting an acid value at the upper limit of the aforementioned acid value range of 3 mg KOH / g to 4 mg KOH / g or even higher.

[0015] Further disclosed below is a method for manufacturing the tin-based solder alloy powder of the present invention, the method comprising successive method steps (1) to (4); method step (1) may include sub-steps (1a) and (1b). Castor oil separated (and recycled) during sub-step (1b) and / or method step (3) may represent fortified castor oil in the sense of this disclosure.

[0016] As noted above, the present invention also relates to a method for manufacturing small spherical tin-based solder alloy particles of 2µm to 38µm, preferably 2µm to 25µm, which exhibit an O / C weight ratio in the range of 1.0 to 2.2, preferably 1.0 to 2.0, and have a thin outer surface layer of vegetable oil (e.g., castor oil, especially fortified castor oil). The method comprises the following sequential steps:

[0017] (1) The hot mixture of tin-based solder alloy melt and vegetable oil is converted into a dispersion of molten spherical tin-based solder alloy particles in vegetable oil by subjecting the hot mixture to a rotor-stator process until a particle size in the range of 2µm to 38µm is obtained, wherein the ratio of tin-based solder alloy melt to vegetable oil is in the range of 0.2 kg to 5 kg of tin-based solder alloy melt per liter of vegetable oil.

[0018] (2) The dispersed molten spherical tin-based solder alloy particles formed in step (1) are solidified in vegetable oil.

[0019] (3) Separate the cured spherical tin-based solder alloy particles from the vegetable oil to obtain solid spherical tin-based solder alloy particles covered with residual vegetable oil, and

[0020] (4) Repeat rinsing and finally drying the solid spherical tin-based solder alloy particles until they have a thin outer surface layer of vegetable oil to exhibit an O / C weight ratio in the range of 1.0 to 2.2, preferably 1.0 to 2.0.

[0021] The flushing fluid is a liquid composition consisting of 50% to 100% by weight of at least one halogenated hydrocarbon solvent and 0% to 50% by weight of at least one organic solvent other than the halogenated hydrocarbon solvent, wherein the total weight percentage is 100%.

[0022] In step (1) of the method of the present invention, the hot mixture of tin-based solder alloy melt and vegetable oil (e.g., castor oil, especially fortified castor oil) is converted into a dispersion of molten spherical tin-based solder alloy particles in vegetable oil by subjecting the hot mixture of tin-based solder alloy melt and vegetable oil (e.g., castor oil, especially fortified castor oil) to a rotor-stator process until a particle size of molten spherical tin-based solder alloy particles in the range of 2µm to 38µm, preferably 2µm to 25µm is obtained, wherein the ratio of tin-based solder alloy melt to vegetable oil is in the range of 0.2 kg to 5 kg of tin-based solder alloy melt per liter of vegetable oil.

[0023] In a preferred embodiment, the vegetable oil (e.g., castor oil, especially fortified castor oil) contains no additives or auxiliary substances, such as corrosion inhibitors or emulsifying agents; in other words, preferably, the vegetable oil (e.g., castor oil, especially fortified castor oil) consists only of itself.

[0024] "To subject a hot mixture to a rotor-stator process" means to treat a hot mixture with a colloid mill, i.e., to pass it through a rotor-stator assembly. Rotor-stator dispersion techniques, apparatus, and operating conditions are known to those skilled in the art; however, those skilled in the art are encouraged to seek useful information and details regarding rotor-stator assemblies and operating conditions in the aforementioned U.S. patent documents; within this scope, reference is expressly made herein to two U.S. patent documents, US 6,290,745 B1 and US 2006 / 0208042 A1.

[0025] "Hot mixture" refers to a mixture that has a temperature 20°C to 30°C higher than the liquidus temperature of a tin-based solder alloy.

[0026] A hot mixture of tin-based solder alloy melt and vegetable oil is characterized by a ratio of 0.2 kg to 5 kg of tin-based solder alloy melt per liter of vegetable oil. This hot mixture can be prepared directly by adding tin-based solder alloy and melting it in sufficiently heated vegetable oil (the temperature of the vegetable oil is, for example, 20°C to 30°C higher than the liquidus temperature of the tin-based solder alloy).

[0027] It is advantageous to use castor oil as the vegetable oil in step (1), particularly to use fortified castor oil. If fortified castor oil is used as the vegetable oil, the hot mixture can be prepared in two steps, wherein in the first sub-step (1a), the tin-based solder alloy is melted in fresh or fortified castor oil in a first container, and furthermore, in the subsequent sub-step (1b), only or substantially only the resulting solder alloy melt is then transferred to a second container. This second container contains fortified castor oil that has been sufficiently heated to obtain the hot mixture of the tin-based solder alloy melt and the fortified castor oil, characterized by a ratio of 0.2 kg to 5 kg of tin-based solder alloy melt per liter of fortified castor oil. The hot mixture of the tin-based solder alloy melt and the fortified castor oil is converted into a dispersion of molten spherical tin-based solder alloy particles in fortified castor oil by passing the hot mixture through a rotor-stator until a particle size of molten spherical tin-based solder alloy particles in the range of 2 µm to 38 µm, preferably 2 µm to 25 µm, is obtained.

[0028] In step (2) of the method of the present invention, the dispersed molten spherical tin-based solder alloy particles (i.e., liquid spherical tin-based solder alloy droplets) formed in step (1) are solidified in vegetable oil. For this purpose, the dispersion is cooled to a temperature below the liquidus temperature of the tin-based solder alloy. Active cooling is possible, but not necessary; that is, simply waiting for the temperature to drop below the liquidus temperature is sufficient. Advantageously, the dispersion is kept in motion, for example, by stirring, until the temperature drops below the liquidus temperature.

[0029] The solidified spherical tin-based solder alloy particles settle under the influence of gravity, and the impurities are largely retained in the liquid vegetable oil phase.

[0030] In step (3) of the method of the present invention, the solidified spherical tin-based solder alloy particles are separated from the vegetable oil to obtain solid spherical tin-based solder alloy particles covered with residual vegetable oil. Examples of separation techniques include centrifugation, filtration, and preferably sieving.

[0031] The residual vegetable oil covering the solid spherical tin-based solder alloy particles is excessive, and its appropriate proportion is rinsed off in subsequent step (4). In step (4) of the method of the present invention, the solid spherical tin-based solder alloy particles are repeatedly rinsed and finally dried until they have a thin outer surface layer of vegetable oil to exhibit an O / C weight ratio in the range of 1.0 to 2.2, preferably 1.0 to 2.0, wherein a liquid composition consisting of at least one halogenated hydrocarbon solvent of 50% to 100% by weight and at least one organic solvent different from the halogenated hydrocarbon solvent of 0% to 50% by weight is used as the rinsing fluid, wherein the total weight percentage is 100% by weight.

[0032] Importantly, a liquid composition consisting of 50% to 100% by weight of at least one halogenated hydrocarbon solvent and 0% to 50% by weight of at least one organic solvent other than the halogenated hydrocarbon solvent is used as the rinsing fluid, wherein the total weight percentage is 100% by weight. The liquid composition is homogeneous, i.e., if it contains two or more solvents, all solvents are completely miscible with each other without any miscibility gaps. All solvents are volatile organic solvents with boiling points below (especially well below) the liquidus temperature but generally above 40°C. Preferably, the liquid composition consists of at least one halogenated hydrocarbon solvent, particularly only one halogenated hydrocarbon solvent. Chlorinated hydrocarbon solvents are preferred.

[0033] Examples of available chlorinated hydrocarbon solvents include carbon tetrachloride (CCl4), chloroform (CHCl3), 1,1,1-trichloroethane (CCl3CH3), trichloroethylene (C2HCl3), and tetrachloroethylene (C2Cl4).

[0034] Examples of organic solvents that are different from halogenated hydrocarbon solvents include acetone, toluene, benzene, dimethyl sulfoxide, petroleum ether, and ethanol.

[0035] The repeated rinsing comprises multiple (e.g., 5 to 20) consecutive rinsing steps. Those skilled in the art will determine the number of consecutive rinsing steps; such a determination depends at least on the type of vegetable oil involved; in the case of castor oil, the number of consecutive rinsing steps can range, for example, from 8 to 20 consecutive rinsing steps, and in the case of fortified castor oil, the number of consecutive rinsing steps can range, for example, from 8 to 15, or preferably from 10 to 15. Typically, after such a number of consecutive rinsing steps, a thin outer surface layer of vegetable oil is left on the surface of the spherical tin-based solder alloy particles to exhibit the stated O / C weight ratio in the range of 1.0 to 2.2, preferably 1.0 to 2.0. Each rinsing step in the consecutive rinsing steps can be performed, for example, at a ratio of 2 to 5 liters of rinsing fluid per kilogram of spherical tin-based solder alloy particles. Each rinsing step in the consecutive rinsing steps can be performed by stirring the mixture of rinsing fluid and spherical tin-based solder alloy particles for a period of time, for example, in the range of 20 to 60 minutes, and then allowing the mixture to stand to allow the spherical tin-based solder alloy particles to sink into the rinsing fluid. Afterward, the flushing fluid can be drained. Each flushing step in the continuous flushing process can be carried out under ambient conditions, such as at a temperature in the range of 20°C to 40°C; it is not necessary to raise the temperature or apply heat.

[0036] After the final rinsing step, the rinsed spherical tin-based solder alloy particles are dried. To do this, they can be transferred to a drying oven and kept there at a temperature range of, for example, 40°C to 80°C. This oven drying can, for example, last for 8 to 10 hours. Oven drying can be supported by applying reduced pressure or a vacuum.

[0037] If necessary, any of steps (1) to (4) may be performed in the absence of air or oxygen, i.e., by means of working in an inert atmosphere. However, it is preferred that all steps (1) to (4) be performed in the presence of air, i.e., air or oxygen can come into contact with all materials used in the method of the present invention.

[0038] After step (4) is completed, the small spherical tin-based solder alloy particles of 2µm to 38µm, preferably 2µm to 25µm, thus obtained can be used directly to prepare solder paste, or they can be stored first in an airtight package having a thin outer surface layer of vegetable oil (e.g., castor oil, especially fortified castor oil) and exhibiting an O / C weight ratio in the range of 1.0 to 2.2, preferably 1.0 to 2.0.

[0039] As already noted, the tin-based solder alloy powder or the spherical tin-based solder alloy particles of the present invention can be used to prepare solder paste. Within this scope, the invention also relates to solder paste comprising the spherical tin-based solder alloy particles of the present invention. Such solder paste may comprise, for example, 82% to 92% by weight of the spherical tin-based solder alloy particles of the present invention and 8% to 18% by weight of flux, or be composed thereof. Such solder paste can be produced by mixing the components of a flux and adding the tin-based solder alloy powder in the form of the spherical tin-based solder alloy particles of the present invention. The tin-based solder alloy powder is preferably added in multiple parts to the mixture of already provided flux components while stirring (typically without heating). The flux is not particularly limited in its composition, and therefore conventional fluxes known to those skilled in the art can be used. Typically, the flux may comprise one or more base resins (e.g., rosin, acrylic resins), activators (e.g., hydrogen halides of amines, organic carboxylic acids), thixotropic agents (e.g., hydrogenated castor oil, beeswax, carnauba wax), and (in many cases) organic solvents.

[0040] The tin-based solder alloy powder of the present invention, in the form of spherical tin-based solder alloy particles, is characterized by considerable storage stability, which can be determined as described above. This storage stability is demonstrated in the excellent results of the aforementioned solderability and wetting tests. Example

[0041] Fortified castor oil is prepared from fresh castor oil:

[0042] Fresh castor oil with an acid value of 1.8 mg KOH / g was poured into a container and heated to 240°C. The mixture was then subjected to a rotor-stator process for 60 minutes until the acid value of the castor oil increased to 3.0 mg KOH / g.

[0043] Preparation of spherical tin-based solder alloy powder:

[0044] 2 kg of SnAg3Cu0.5 alloy was added to 1 L of fresh castor oil in container #1, and the mixture was heated to 240°C. The molten solder was transferred to container #2, which was filled with 5 L of fortified castor oil with an acid value of 3.0 mg KOH / g. The hot mixture was then passed through a rotor-stator device for 15 minutes, resulting in molten SnAg3Cu0.5 alloy particles with a particle size between 2 µm and 11 µm. The hot dispersion was then cooled to below 217°C, and the solidified spherical SnAg3Cu0.5 alloy particles thus formed settled under gravity. The solidified spherical SnAg3Cu0.5 alloy particles were separated from the liquid fortified castor oil by sieving.

[0045] Example 1 (according to the present invention): 8 L of chloroform was added to the separated SnAg3Cu0.5 alloy particles, and the mixture was stirred for 40 minutes. Afterward, the mixture was allowed to stand for 10 hours, and then the chloroform was drained. The rinsing cycle was repeated 10 times with fresh chloroform each time. After draining the chloroform in the final rinsing cycle (the 11th rinsing cycle), the SnAg3Cu0.5 alloy particles were dried in a drying oven at 60°C under vacuum for 10 hours. Solder paste was then prepared using the SnAg3Cu0.5 alloy particles as described below.

[0046] Example 2 (according to the present invention): It operates as in Example 1; however, 6L of chloroform is used instead of 8L of chloroform, and a total of 15 flushing cycles are performed.

[0047] Example 3 (according to the present invention): It operates as in Example 1; however, thereafter, the SnAg3Cu0.5 alloy particles are aged by storing them at 55°C in the presence of air with 50% relative humidity for 16 days (equivalent to storing them at room temperature of 25°C in the presence of air with 50% relative humidity for 3 months).

[0048] Example 4 (according to the present invention): It operates as in Example 1; however, thereafter, the SnAg3Cu0.5 alloy particles are aged by storing them at 55°C in the presence of air with 50% relative humidity for 31 days (equivalent to storing them at room temperature of 25°C in the presence of air with 50% relative humidity for 6 months).

[0049] Comparative Example 5:It operates as in Example 1; however, thereafter, the SnAg3Cu0.5 alloy particles are aged by storing them at 55°C in the presence of air with 50% relative humidity for 63 days (equivalent to storing them at room temperature of 25°C in the presence of air with 50% relative humidity for 12 months).

[0050] Comparative Example 6: It operates as in Example 1; however, 2L of chloroform is used instead of 8L of chloroform, and only one flushing cycle is performed.

[0051] Comparative Example 7: It operates as in Example 1; however, 2L of chloroform is used instead of 8L of chloroform, and two flushing cycles are performed.

[0052] Comparative Example 8: It works as in Example 1; however, castor oil with an acid value of 4.5 mg KOH / g is used.

[0053] Comparative Example 9: It operates as in Example 1; however, a total of 30 flushing cycles are performed.

[0054] The oxygen and carbon content of the spherical tin-based solder alloy powders of Examples 1 to 9 were measured as described above. Based on the analytical results, the O / C ratio was calculated.

[0055] Preparation of solder paste:

[0056] The flux is formed by melting 45 pbw (parts by weight) of hydrogenated rosin (hydrogenated rosin resin) with an acid value of 240 mg KOH / g at 170°C, and then adding 28 pbw of ethylene glycol dimethyl ether, 12 pbw of polyamide thickener, 5 pbw of succinic acid, 8 pbw of surfactant and 2 pbw of antioxidant at 140°C.

[0057] The nine solder pastes were prepared by mixing 15 pbw of flux thus prepared with 85 pbw of SnAg3Cu0.5 alloy solder particles from Examples 1 to 9.

[0058] Solderability test (solder ball test):

[0059] Nine different solder pastes were each printed onto a 5cm x 5cm ceramic substrate. The printed solder paste was then soldered by heating it in a reflow oven at 250°C in a nitrogen atmosphere. After the solder paste melted, the ceramic substrate was removed and cooled.

[0060] After the ceramic sheet cooled, the solderability of each solder paste was evaluated. To do this, the ceramic sheet was visually inspected under an optical microscope for any solder beads that formed around the solder paste.

[0061] Wetting test:

[0062] Nine different solder pastes were each printed onto a 2.5cm x 2.5cm copper sheet. The printed solder paste was then soldered by heating it in a reflow oven at 250°C in a nitrogen atmosphere. After the solder paste melted, the copper sheet was removed and cooled.

[0063] After the copper sheets cooled, the wetting properties of each solder paste were evaluated. To do this, the copper sheets were visually inspected under an optical microscope for any dehumidification from the initially printed area.

[0064] The oxygen content, carbon content, and O / C weight ratio of the spherical tin-based solder alloy powders of Examples 1 to 9, as well as the solderability and wetting properties of the nine solder pastes prepared therefrom, are listed in the table below:

[0065]

[0066] Definition of rating:

[0067] Solderability properties:

[0068] "++++": 0 to <2 solder balls observed

[0069] "++": 2 to 4 solder balls

[0070] "+": Cluster of 5 to 8 solder balls

[0071] "--": >A cluster of 8 to 20 solder balls

[0072] "---": >A cluster of 20 solder balls, or remain unsoldered.

[0073] Wetting properties:

[0074] "++++": No dehumidification observed

[0075] "+++": >0% to <5% dehumidification from the original printing area

[0076] "+": 5% to 10% dehumidification from the original printing area

[0077] "--": >10% to 40% dehumidification from the original printing area

[0078] "---": >40% dehumidification from the original printing area

Claims

1. A tin-based solder alloy powder, said tin-based solder alloy powder being in the form of small spherical tin-based solder alloy particles of 2µm to 38µm, said small spherical tin-based solder alloy particles exhibiting an O / C weight ratio in the range of 1.0 to 2.2, wherein said spherical tin-based solder alloy particles have a thin vegetable oil outer surface layer.

2. The tin-based solder alloy powder according to claim 1, wherein the O / C weight ratio is in the range of 1.0 to 2.

0.

3. The tin-based solder alloy powder according to claim 1 or 2, wherein the tin-based solder alloy is a solder alloy containing at least 42% by weight of tin or a tin-rich solder alloy with a tin content of 90% to 99.5% by weight.

4. The tin-based solder alloy powder according to any one of the preceding claims, wherein the vegetable oil is castor oil.

5. The tin-based solder alloy powder according to claim 4, wherein the castor oil is selected from the group consisting of fresh castor oil with an acid value in the range of 0.14 mg KOH / g to 1.97 mg KOH / g and reinforced castor oil with an acid value in the range of 3 mg KOH / g to 4 mg KOH / g.

6. The tin-based solder alloy powder according to any one of the preceding claims, wherein the thin outer surface layer has a thickness in the range of 0.5 nm to 5.2 nm.

7. A method for manufacturing tin-based solder alloy powder, said tin-based solder alloy powder being in the form of small spherical tin-based solder alloy particles of 2µm to 38µm, said small spherical tin-based solder alloy particles exhibiting an O / C weight ratio in the range of 1.0 to 2.2, said spherical tin-based solder alloy particles having a thin vegetable oil outer surface layer, said method comprising the following sequential steps: (1) The hot mixture of tin-based solder alloy melt and vegetable oil is converted into a dispersion of molten spherical tin-based solder alloy particles in the vegetable oil by subjecting the hot mixture to a rotor-stator process until a particle size in the range of 2µm to 38µm is obtained, wherein the ratio of the tin-based solder alloy melt to the vegetable oil is in the range of 0.2 kg to 5 kg of the tin-based solder alloy melt per liter of the vegetable oil. (2) The dispersed molten spherical tin-based solder alloy particles formed in step (1) are solidified in the vegetable oil. (3) Separate the cured spherical tin-based solder alloy particles from the vegetable oil to obtain solid spherical tin-based solder alloy particles covered with residual vegetable oil, and (4) Repeat rinsing and finally drying the solid spherical tin-based solder alloy particles until they have a thin outer surface layer of vegetable oil to exhibit an O / C weight ratio in the range of 1.0 to 2.

2. The flushing fluid is a liquid composition consisting of 50% to 100% by weight of at least one halogenated hydrocarbon solvent and 0% to 50% by weight of at least one organic solvent other than the halogenated hydrocarbon solvent, wherein the total weight percentage is 100%.

8. The method of claim 7, wherein the O / C weight ratio is in the range of 1.0 to 2.

0.

9. The method according to claim 7 or 8, wherein the tin-based solder alloy is a solder alloy containing at least 42% by weight of tin or a tin-rich solder alloy with a tin content of 90% to 99.5% by weight.

10. The method according to any one of claims 7 to 9, wherein the vegetable oil is castor oil.

11. The method according to claim 10, wherein the castor oil is selected from the group consisting of fresh castor oil with an acid value in the range of 0.14 mg KOH / g to 1.97 mg KOH / g and fortified castor oil with an acid value in the range of 3 mg KOH / g to 4 mg KOH / g.

12. The method according to any one of claims 7 to 11, wherein the thin outer surface layer has a thickness in the range of 0.5 nm to 5.2 nm.

13. The method according to any one of claims 7 to 12, wherein the liquid composition comprises at least one halocarbon solvent.

14. The method according to any one of claims 7 to 9, 12 or 13, wherein the repeated rinsing comprises 5 to 20 consecutive rinsing steps. Or, according to the method of claim 10, wherein the repeated rinsing comprises 8 to 20 consecutive rinsing steps. Or the method according to claim 11, wherein the castor oil is fortified castor oil, and wherein the repeated rinsing comprises 8 to 15 consecutive rinsing steps.

15. The method according to any one of claims 7 to 14, wherein each rinsing step in the continuous rinsing step is performed by: stirring the mixture of the rinsing fluid and the spherical tin-based solder alloy particles for a period of time ranging from 20 to 60 minutes, and then allowing the mixture to stand so that the spherical tin-based solder alloy particles sink into the rinsing fluid.

16. A solder paste comprising tin-based solder alloy powder according to any one of claims 1 to 6 or tin-based solder alloy powder manufactured by any one of claims 7 to 15.

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