WO2017104562A1

WO2017104562A1 - Solder powder, method for manufacturing same, and method for preparing solder paste using this powder

Info

Publication number: WO2017104562A1
Application number: PCT/JP2016/086694
Authority: WO
Inventors: 隆二植杉
Original assignee: 三菱マテリアル株式会社
Priority date: 2015-12-15
Filing date: 2016-12-09
Publication date: 2017-06-22
Also published as: TW201739545A; TWI668065B; KR102189367B1; CN108602121A; JP2017110251A; CN108602121B; KR20180093979A; JP6587099B2

Abstract

In the present invention, a nickel metal salt is added to and mixed with a dispersion of copper powder, and the metal salt is dissolved to obtain a first solution in which the copper powder is dispersed. After adjusting the pH of the solution, a first reducing agent is added and mixed so as to reduce the nickel ions, and a dispersion is obtained in which the precipitated nickel coats the copper powder and is dispersed. This solution is subjected to solid-liquid separation and the solids are dried to obtain a metal powder in which copper nuclei are coated with a diffusion preventing layer made of nickel. A tin metal salt is added to and mixed with a dispersion liquid of the metal powder, and the metal salt is dissolved to obtain a second solution in which the metal powder is dispersed. After adjusting the pH of the solution, a second reducing agent is added and mixed so as to reduce the tin ions, and a dispersion is obtained in which the precipitated tin coats the metal powder and is dispersed. This solution is subjected to solid-liquid separation and the solids are dried to obtain a solder powder in which the metal powder is coated with a tin layer.

Description

Solder powder, method for producing the same, and method for preparing solder paste using the powder

The present invention relates to a solder powder used for mounting electronic components and the like, the core of which is made of copper, and a coating layer containing a tin layer, a manufacturing method thereof, and a method of preparing a solder paste using this powder. Note that this international application claims priority based on Japanese Patent Application No. 244270 (Japanese Patent Application No. 2015-244270) filed on December 15, 2015. Incorporated into this international application.

Conventionally, a solder powder having an average particle diameter of 5 μm or less, in which the central core is made of copper and the coating layer is made of tin, has been disclosed (for example, see Patent Document 1). This solder powder is excellent in printability because it is lead-free and fine in terms of environment. Further, by using copper as the metal element constituting the central core, not only the coating layer but also the central core melts during reflow to form a Cu—Sn alloy. Strength is improved.

However, the solder powder in which the central core described in Patent Document 1 is made of copper and the coating layer is made of tin, when the solder powder is manufactured and stored for a long period of time, the diffusion coefficient of copper to tin becomes smaller than that of tin to copper. Since the diffusion coefficient is larger, the central core copper diffuses into the tin of the coating layer, and an intermetallic compound layer having a high melting point such as Cu ₃ Sn or Cu ₆ Sn ₅ is formed between the central core and the coating layer, Alternatively, all the copper in the central core may diffuse into the tin of the coating layer, and the entire coating layer may become an intermetallic compound of copper and tin.

In order to solve this problem, a diffusion layer made of Ni is provided between the Cu ball and the solder layer, including a core layer made of Cu balls and a solder layer mainly composed of tin covering the core layer. A Cu core ball in which a prevention layer is formed is disclosed (for example, see Patent Document 2). This diffusion prevention layer prevents Cu constituting the Cu ball from diffusing into the solder layer. In Patent Document 2, as a method of forming a solder layer on a Cu ball, a known electrolytic plating method such as barrel plating, a pump connected to a plating tank generates a high-speed turbulent flow in the plating solution, A method of forming a plating film on a Cu ball by turbulent plating solution, a plating plate is vibrated at a predetermined frequency by providing a vibration plate in a plating tank, and the plating solution is stirred at high speed turbulence. A method of forming a plating film is shown, and it is disclosed that a solder layer is formed after coating a Cu ball with Ni plating.

Patent Document 2 describes that the solder layer contains Sn of 40% or more and Ge of 20 ppm or more and 220 ppm or less, and after coating a Cu ball having a diameter of 100 μm with a Ni plating with a film thickness (one side) of 2 μm. A Sn-Ag-Cu-Ge solder plating film having a film thickness (one side) of 18 μm is formed, and a Cu core ball having a diameter of about 140 μm is shown.

JP 2008-138266 A (Claim 1, paragraph [0005], paragraph [0014]) Japanese Patent No. 5562560 (Claim 1, paragraph [0033], paragraph [0035], paragraph [0067], paragraph [0068])

However, as described in Patent Document 2, in the method of covering Cu balls with a diffusion prevention layer composed of Ni by Ni plating such as barrel plating, it is easy to form agglomerated balls due to adhesion between the balls. In addition, there is a problem that the film thickness of the plating film tends to vary. Further, the diffusion prevention layer made of Ni of the Cu core ball shown in Patent Document 2 has a thickness of 0.02 when the radius of the Cu ball (copper core) is 1, so that the Cu to tin tin There is a problem that it is difficult to prevent diffusion.

The first object of the present invention is to provide a solder powder in which the central core copper does not diffuse into the tin layer tin during powder storage, the powder does not adhere to each other, and the diffusion prevention layer made of nickel has a small variation in thickness. It is in providing the soldering paste using the manufacturing method and this powder. In addition, the second object of the present invention is a poor bonding caused by the fact that the solder does not melt sufficiently even if the solder powder stored for a long period of time is reflowed at a temperature at which the solder powder with a short storage period is melted. It is an object of the present invention to provide a solder powder that does not cause soldering, a method for producing the same, and a method for preparing a solder paste using the powder. Furthermore, the third object of the present invention is to use solder powder suitable for mounting electronic parts and the like that are not easily remelted and reduced in bonding strength after reflow and particularly exposed to a high-temperature atmosphere, a manufacturing method thereof, and this powder. It is to provide a method for preparing a solder paste.

As shown in FIG. 1, the first aspect of the present invention is a step S1 for preparing a first dispersion in which copper powder is dispersed, and a nickel metal salt is added to and mixed with the first dispersion of copper powder. Step S2 for preparing the first solution in which the metal salt of the copper is dissolved and the copper powder is dispersed, Step S3 for adjusting the pH of the first solution, and adding and mixing the first reducing agent to the pH-adjusted first solution Step S4 for preparing a second dispersion in which nickel ions are reduced and the deposited nickel is coated with copper powder and dispersed, and the second dispersion is solid-liquid separated, and the solid content separated by solid-liquid separation is obtained. Step S5 for producing a metal powder in which a copper core is coated with a diffusion prevention layer made of nickel by drying, Step S6 for preparing a third dispersion in which this metal powder is dispersed, and a metal salt of tin as a metal powder In the third dispersion, the metal salt of tin is dissolved and the metal powder is dissolved. Step S7 for preparing the second solution to be dispersed, Step S8 for adjusting the pH of the second solution, and adding and mixing the second reducing agent to the pH-adjusted second solution, thereby reducing tin ions. Step S9 of preparing a fourth dispersion in which the deposited tin is coated with and dispersed by the metal powder, and solid-liquid separation of the fourth dispersion, and drying the solid content separated by solid-liquid separation, the metal powder becomes a tin layer. And a step S10 of producing a solder powder coated with a solder powder.

The second aspect of the present invention is a solder powder 10 in which a metal powder 13 formed by coating a copper core 11 with a diffusion preventing layer 12 made of nickel is coated with a tin layer 14 as shown in FIG. The characteristic configuration is that the average particle size of the solder powder is 1 μm or more and 30 μm or less, and the copper content is 2% by mass or more and 70% by mass or less with respect to 100% by mass of the total amount of the solder powder, The thickness of the diffusion preventing layer made of nickel is in a ratio of 0.04 to 0.51 when the radius of the copper nucleus is 1.

A third aspect of the present invention is a method for preparing a solder paste by mixing the solder powder manufactured by the method of the first aspect or the solder powder of the second aspect and a solder flux into a paste. is there.

A fourth aspect of the present invention is a method of mounting an electronic component using a solder paste manufactured by the method of the third aspect.

The method for producing a solder powder according to the first aspect of the present invention comprises forming a diffusion preventing layer made of nickel on the surface of a copper nucleus by reducing nickel ions in the first solution in which the copper powder is dispersed, Further, a tin layer is formed on the surface of a metal powder formed by coating a copper core with a diffusion preventing layer made of nickel, thereby producing a solder powder. As a result, unlike the method such as the barrel plating method described in Patent Document 2, the layer structure of the solder powder has little variation among individual powders, the powders do not adhere to each other, and the diffusion prevention layer is made of nickel. Variation in the thickness of the film is small.

As shown in FIG. 2, the solder powder 10 according to the second aspect of the present invention is made of nickel, in which a copper powder 11 is coated with a metal layer 13 formed by coating a diffusion prevention layer 12 made of nickel with a tin layer 14. The thickness of the diffusion preventing layer 12 is a ratio of 0.04 to 0.51 when the radius of the copper nucleus is 1. That is, in the solder powder 10 of the present invention, since the diffusion prevention layer 12 made of nickel having a predetermined thickness is interposed between the copper core 11 and the tin layer 14, the conventional copper core and the tin layer covering the copper core Without drastically changing the solder characteristics of the solder powder made of the above, it is possible to prevent the copper in the central core from diffusing into the tin in the tin layer, and to prevent the tin in the tin layer from diffusing into the copper in the central core. As a result, even if the solder powder stored for a long time is reflowed at a temperature at which the solder powder having a short storage period or before storage is melted, it does not cause poor bonding due to the fact that the solder is not sufficiently melted. There is an effect. Further, after _{_{_{_{reflow, Cu 3 Sn, Cu 6 Sn}}}} 5, Ni 3 Sn, Ni 3 Sn 2, Ni 3 Sn 4, NiSn 3, (Ni, Cu) 3 Sn 4, (Ni, Cu) 6 Sn 5 or the like Since a bonding layer made of an intermetallic compound and copper having a high melting point is formed, remelting and a decrease in bonding strength are unlikely to occur after reflow, and it is particularly suitable for electronic parts exposed to a high temperature atmosphere.

The solder paste prepared by the method of the third aspect of the present invention is obtained using the solder powder of the present invention. Therefore, this solder paste is rapidly melted during reflow and has excellent meltability.

In the method of mounting the electronic component according to the fourth aspect of the present invention, the solder paste of the present invention is used. Therefore, during reflow, the melting speed of the solder paste and the excellent meltability make it easy and highly accurate. Can be implemented. In the joined body on which this electronic component is mounted, not only the coating layer but also the central core melts during reflow to form a Cu—Sn alloy or Sn—Ni—Cu alloy, so that the formed Cu—Sn alloy or Sn—Ni is formed. -Due to the Cu alloy, even if it is exposed to a high temperature atmosphere after soldering, remelting and reduction in bonding strength are unlikely to occur.

It is a figure which shows the manufacturing process of the solder powder of this embodiment. 1 is a cross-sectional structure diagram of a solder powder in which a metal powder obtained by coating a copper core of this embodiment with a diffusion prevention layer made of nickel is coated with a tin layer.

Next, modes for carrying out the present invention will be described with reference to the drawings.

[Method for producing solder powder]
Copper powder is used for the formation of the central core. First, as shown in steps S1 and S2 of FIG. 1, a first dispersion in which copper powder is dispersed is prepared by adding and mixing copper powder and a dispersant to a solvent, and a compound containing nickel is added and mixed therein. A first solution in which the copper powder is dispersed and the nickel-containing compound is dissolved is prepared. The copper powder preferably has an average particle size of 0.1 μm or more and 27 μm or less. Below this lower limit, the average particle size of the solder powder tends to be less than 1 μm, the specific surface area becomes high, and the meltability of the solder is lowered due to the influence of the surface oxide layer of the powder. If the upper limit is exceeded, the average particle size of the solder powder tends to exceed 30 μm. When the thickness exceeds 30 μm, there is a problem that the coplanarity of the bump is lowered when the bump is formed, and there is a problem that coating unevenness occurs when the pattern surface is coated with solder, and the entire pattern cannot be coated uniformly. The average particle size of the copper powder is more preferably 2 to 20 μm. This copper powder can be obtained not only by a chemical method using a reduction reaction but also by a physical method such as an atomizing method. In the present specification, the average particle diameter of the powder was measured by a particle size distribution measuring apparatus using a laser diffraction / scattering method (manufactured by Horiba, Ltd., laser diffraction / scattering particle diameter distribution measuring apparatus LA-950). Volume cumulative median diameter (Median diameter, D ₅₀ ).

The ratio of the copper powder and the nickel compound in the first solution is adjusted so that the content ratio of each metal element is within the range described later after the solder powder is manufactured. Examples of the nickel compound include nickel chloride (II), nickel sulfate (II), nickel nitrate (II) and the like. Examples of the solvent include water, alcohol, ether, ketone, ester and the like. Examples of the dispersant include cellulose-based, vinyl-based, and polyhydric alcohols. In addition, gelatin, casein, and the like can be used.

ＰＨ As shown in step S3 of FIG. 1, pH adjustment of the prepared first solution is performed. The pH is preferably adjusted to a range of 0.1 to 2.0 in consideration of redissolution of the generated solder powder. In addition, after adding the said nickel compound to a solvent and making it melt | dissolve, after adding a complexing agent and complexing nickel, you may add a dispersing agent. By adding a complexing agent, nickel ions do not precipitate even when the pH is alkaline, and synthesis in a wide range is possible. Examples of the complexing agent include succinic acid, tartaric acid, glycolic acid, lactic acid, phthalic acid, malic acid, citric acid, oxalic acid, ethylenediaminetetraacetic acid, iminodiacetic acid, nitrilotriacetic acid, and salts thereof.

Next, an aqueous solution in which the reducing agent is dissolved is prepared, and the pH of the aqueous solution is adjusted to the same level as the first dissolved solution. Reducing agents include phosphoric acid compounds such as sodium phosphinate, boron hydrides such as sodium tetrahydroborate and dimethylamine borane, nitrogen compounds such as hydrazine, metal ions such as trivalent titanium ions and divalent chromium ions. Etc.

Next, as shown in step S4 of FIG. 1, by adding and mixing the reducing agent aqueous solution to the first solution containing nickel ions, the nickel ions in the first solution are reduced and precipitated nickel. Prepare a second dispersion in which the copper powder is coated and dispersed. As a method of mixing the first solution and the reducing agent aqueous solution, a method in which the reducing agent aqueous solution is dropped into the solution in the container at a predetermined addition rate and stirred with a stirrer or a reaction tube having a predetermined diameter is used. A method of pouring both solutions into the reaction tube at a predetermined flow rate and mixing them may be mentioned.

Next, as shown in step S5 of FIG. 1, the second dispersion is subjected to solid-liquid separation by decantation or the like, and the recovered solid content is water or a hydrochloric acid aqueous solution adjusted to pH 0.5 to 2, nitric acid. Wash with aqueous solution, sulfuric acid aqueous solution, methanol, ethanol, acetone or the like. After washing, the solid content is recovered by solid-liquid separation again. The steps from washing to solid-liquid separation are preferably repeated 2 to 5 times. The collected solid content is vacuum-dried to form a metal powder with a Cu core Ni layer composed of a central core made of copper (copper core) and a nickel layer (a diffusion prevention layer made of nickel) covering the central core. Is made.

Next, as shown in steps S6 and S7 of FIG. 1, the above-mentioned metal powder and dispersant are added to a solvent and mixed to prepare a third dispersion in which the metal powder with the Cu core Ni layer is dispersed. A second solution is prepared in which a metal powder with a Cu core Ni layer is dispersed and a compound containing tin is dissolved. Examples of tin compounds include tin (II) chloride, tin (II) sulfate, tin (II) acetate, and tin (II) oxalate. The addition ratio of the compound containing tin is adjusted so that the content ratio of each metal element is in the range described later after the solder powder is manufactured. As the dispersion medium and the solvent, the same dispersion medium and solvent as described above are used.

ＰＨ As shown in step S8 of FIG. 1, pH adjustment of the prepared second solution is performed. The pH is preferably adjusted to a range of 0.1 to 2.0 in consideration of redissolution of the generated solder powder. In addition, after adding the said tin compound to a solvent and making it melt | dissolve, after adding a complexing agent and complexing tin, you may add a dispersing agent. By adding a complexing agent, tin ions do not precipitate even when the pH is alkaline, and synthesis in a wide range is possible. As the complexing agent, the same complexing agent as described above is used.

Next, as shown in step S9 of FIG. 1, a reducing agent aqueous solution in which the same reducing agent as the reducing agent described above is dissolved in the second dissolving liquid containing the tin ions is added and mixed by the same method as described above. Then, a fourth dispersion is prepared in which tin ions in the second solution are reduced, and the precipitated tin coats and disperses the metal powder with the Cu core Ni layer.

Finally, as shown in step S10 of FIG. 1, this fourth dispersion is washed by the same method as described above, and solid-liquid separated to recover the solid content. The steps from washing to solid-liquid separation are preferably repeated 2 to 5 times. The collected solid content is vacuum dried to produce a solder powder in which a metal powder with a Cu core Ni layer is coated with a tin layer.

[Solder powder]
As shown in FIG. 2, the solder powder 10 manufactured by the above method is coated with a tin layer 14 with a metal powder 13 in which a copper core 11 as a central core is covered with a diffusion prevention layer 12 made of nickel. As described above, the solder powder has a structure in which the central core made of copper is coated with the coating layer of the tin layer having a low melting point, and therefore, the solder powder has excellent meltability at the time of reflow. In addition, since copper and tin are contained in one metal particle constituting the powder, melting unevenness and composition deviation during reflow hardly occur, and high bonding strength can be obtained. Furthermore, since the solder powder has a diffusion preventing layer made of nickel between the central core and the coating layer, diffusion of copper into tin and diffusion of tin into copper can be prevented. Furthermore, after reflow, Cu ₃ Sn, Cu ₆ Sn ₅ , Ni ₃ Sn, Ni ₃ Sn ₂ , Ni ₃ Sn ₄ , NiSn ₃ , (Ni, Cu) ₃ Sn ₄ , (Ni, Cu) ₆ Sn _5, etc. Since a bonding layer made of an intermetallic compound and copper having a high melting point is formed, remelting and a decrease in bonding strength are unlikely to occur after reflow, and it is particularly suitable for electronic parts exposed to a high temperature atmosphere.

The diffusion preventing layer 12 made of nickel has a thickness of 0.04 or more and 0.51 or less when the radius of the copper nucleus is 1. The ratio is preferably 0.05 or more and 0.20 or less. If it is less than 0.04, it becomes impossible to prevent the diffusion of copper or tin, and if it exceeds 0.51, the meltability of the solder powder decreases.
from here

The solder powder 10 produced by the above method has an average particle size of 1 μm or more and 30 μm or less. The reason why the average particle size of the solder powder is limited to 1 μm or more and 30 μm or less is as described above.

The solder powder 10 produced by the above method has a copper content of 2% by mass or more and 70% by mass or less with respect to 100% by mass of the total amount of the powder. Since conventional solder powder is used as an alternative to Sn—Pb eutectic solder (composition ratio Sn: Pb = 63: 37% by mass), the melting point is close and the eutectic composition is required. The proportion is contained in a proportion of 0.5 to 1.5% by mass. On the other hand, in the solder powder produced by the above method, by adding more than 2% by mass, a Sn—Cu alloy having a high solidification start temperature of about 880 to 600 ° C. or about 800 to 400 ° C. is obtained after reflow. An Sn—Ni—Cu alloy having a high solidification start temperature is formed. Even if the content of copper is small, after reflowing, Sn—Cu alloy or Sn—Ni—Cu alloy having a higher solidification start temperature than tin is formed, but solidification starts by containing more copper. The temperature is further increased because the ratio of intermetallic compounds having a high melting point in the alloy is further increased. Thereby, in the solder bump formed by reflow of the solder paste containing this solder powder, the heat resistance is greatly improved, and remelting and reduction in bonding strength can be prevented. For this reason, it can be suitably used as a high-temperature solder used for mounting electronic components exposed to a high-temperature atmosphere. If the copper content is less than 2% by mass, the solidification start temperature will be low, so that sufficient heat resistance will not be obtained in the solder bumps formed after reflow, and remelting will occur when used in a high temperature atmosphere. It cannot be used as solder. On the other hand, if it exceeds 70% by mass, the solidification start temperature becomes too high, and the solder does not melt sufficiently, resulting in a problem that bonding failure occurs. Of these, the copper content in the total amount of powder of 100% by mass is preferably 10 to 60% by mass.

Further, the content ratio of nickel in the solder powder is 1% by mass or more and less than 15% by mass, preferably 2 to 10% by mass with respect to 100% by mass of the total amount of the solder powder. According to this content ratio, the thickness of the diffusion preventing layer made of nickel is determined. If the nickel content is less than 1% by mass, it is difficult to prevent the diffusion of copper or tin. If the nickel content exceeds 15% by mass, the meltability of the solder powder decreases.

Further, the content ratio of tin in the solder powder is 29% by mass or more and less than 97% by mass, preferably 40 to 90% by mass with respect to the balance other than the copper and nickel in the powder, that is, 100% by mass of the total amount of the solder powder. %. This is because when the tin content is less than 29% by mass, the low melting point required for the solder powder is not exhibited during reflow. Moreover, if it is 97 mass% or more, the content rate of copper will decrease as a result and the heat resistance of the solder bump formed after reflow will fall. That is, when the solder after mounting is exposed to a high temperature atmosphere, the solder after mounting may be remelted or a liquid phase may be generated in a part of the solder, which may reduce the bonding strength with the substrate or the like.

[Solder paste and its preparation method]
The solder powder produced by the above method is suitably used as a material for a solder paste obtained by mixing with a solder flux to form a paste. The solder paste is prepared by mixing solder powder and solder flux at a predetermined ratio to form a paste. The solder flux used for the preparation of the solder paste is not particularly limited, but a flux prepared by mixing components such as a solvent, rosin, thixotropic agent and activator can be used.

Solvents suitable for the preparation of the solder flux include diethylene glycol monohexyl ether, diethylene glycol monobutyl ether, diethylene glycol monobutyl ether acetate, tetraethylene glycol, 2-ethyl-1,3-hexanediol, α-terpineol and the like having a boiling point of 180. An organic solvent having a temperature of not lower than ° C. can be mentioned. Examples of the rosin include gum rosin, hydrogenated rosin, polymerized rosin, and ester rosin.

Examples of thixotropic agents include hardened castor oil, fatty acid amide, natural fats and oils, synthetic fats and oils, N, N′-ethylenebis-12-hydroxystearylamide, 12-hydroxystearic acid, 1,2,3,4-dibenzylidene- And D-sorbitol and its derivatives.

The activator is preferably a hydrohalic acid amine salt, specifically, triethanolamine, diphenylguanidine, ethanolamine, butylamine, aminopropanol, polyoxyethylene oleylamine, polyoxyethylene laurelamine, polyoxyethylene. Stearylamine, diethylamine, triethylamine, methoxypropylamine, dimethylaminopropylamine, dibutylaminopropylamine, ethylhexylamine, ethoxypropylamine, ethylhexyloxypropylamine, bispropylamine, isopropylamine, diisopropylamine, piperidine, 2,6-dimethyl Piperidine, aniline, methylamine, ethylamine, butylamine, 3-amino-1-propene, isopropyl alcohol Emissions, dimethyl hexyl amines, hydrochloric acid salt or hydrobromide of an amine such as cyclohexylamine.

The solder flux can be obtained by mixing the above components at a predetermined ratio. The proportion of the solvent in the total amount of flux of 100% by mass is preferably 30 to 60% by mass, the proportion of the thixotropic agent is 1 to 10% by mass, and the proportion of the activator is preferably 0.1 to 10% by mass. If the solvent ratio is less than the lower limit, the viscosity of the flux will be too high, so the viscosity of the solder paste using this will also increase accordingly, resulting in poor printability, such as poor solder filling and uneven coating. May cause malfunctions. On the other hand, when the upper limit is exceeded, the viscosity of the flux becomes too low, and the viscosity of the solder paste using the flux also decreases accordingly, which may cause a problem that the solder powder and the flux component settle and separate. In addition, when the ratio of the thixotropic agent is less than the lower limit, the viscosity of the solder paste becomes too low, which may cause a problem that the solder powder and the flux component are settled and separated. On the other hand, when the upper limit is exceeded, the viscosity of the solder paste becomes too high, which may cause problems such as poor solderability and poor printability such as coating unevenness. In addition, if the ratio of the activator is less than the lower limit value, the solder powder may not be melted and a sufficient bonding strength may not be obtained. On the other hand, if the upper limit value is exceeded, the activator may become solder powder during storage. This may cause a problem that the storage stability of the solder paste is lowered. In addition, a viscosity stabilizer may be added to the solder flux. Examples of the viscosity stabilizer include polyphenols that can be dissolved in a solvent, phosphoric acid compounds, sulfur compounds, tocophenols, tocophenol derivatives, ascorbic acid, and ascorbic acid derivatives. If there are too many viscosity stabilizers, problems such as a decrease in the meltability of the solder powder may occur.

The amount of solder flux mixed when preparing the solder paste is preferably such that the proportion of the flux in 100% by mass of the prepared paste is 5 to 30% by mass. If it is less than the lower limit, it becomes difficult to form a paste due to insufficient flux, while if it exceeds the upper limit, the content of the flux in the paste is too high and the content of metal is reduced, and solder bumps of the desired size when melting the solder This is because it becomes difficult to obtain.

Since this solder paste is made of the above-described solder powder of the present invention, melting at the time of reflow is fast and excellent in meltability, and after reflow, the melting solder powder forms an intermetallic compound having a high melting point, Since heat resistance increases, remelting due to heat hardly occurs. For this reason, the solder paste of the present invention can be suitably used for mounting electronic parts and the like exposed to a high temperature atmosphere.

[Electronic component mounting method and joint using solder paste]
To mount electronic parts such as silicon chips and LED chips on various heat dissipation boards, FR4 (Flame Retardant Type 4) boards, and Kovar boards using the solder paste prepared by the above method, use the pin transfer method. The solder paste is transferred to a predetermined position of the substrate, or the solder paste is printed at a predetermined position by a printing method. Next, a chip element which is an electronic component is mounted on the transferred or printed paste. In this state, the solder powder is reflowed by holding in a reflow furnace in a nitrogen atmosphere at a temperature of 250 to 400 ° C. for 5 to 120 minutes. In some cases, the chip and the substrate may be joined while being pressed. Thus, the chip element and the substrate are bonded to obtain a bonded body, and the electronic component is mounted on the substrate.

Next, examples of the present invention will be described in detail together with comparative examples.

<Example 1>
First, 4.35 × 10 ⁻³ mol of nickel (II) sulfate, 9.66 × 10 ⁻⁴ mol of sodium phosphinate and 3.29 × 10 ⁻⁴ mol of sodium citrate are added to 50 mL of water, and a stirrer is added. The mixture was stirred at a rotational speed of 300 rpm for 5 minutes to prepare a solution. After adjusting the pH of this solution to 5.0 with sulfuric acid, 0.2 g of polyvinyl alcohol 500 (polyvinyl alcohol having an average molecular weight of 500) was added as a dispersant, and the mixture was further stirred at a rotation speed of 300 rpm for 10 minutes. Next, 0.2 g of polyvinyl alcohol 500 (polyvinyl alcohol having an average molecular weight of 500) as a dispersant is dissolved in 50 mL of water in this solution, and 3.40 g of copper powder having an average particle size of 0.18 μm is dispersed. The dispersion was added and stirred at a rotational speed of 500 rpm for 10 minutes to obtain a dispersion in which nickel-coated copper powder having nickel deposited on the surface of the copper powder was dispersed. The dispersion was allowed to stand for 60 minutes, and the produced powder was allowed to settle. Then, the supernatant was discarded, and 100 mL of water was added thereto, followed by stirring for 10 minutes at a rotational speed of 300 rpm, and washing was performed four times. . Finally, this was dried with a vacuum dryer to obtain a powder having copper as the central core and nickel as the first coating layer (diffusion prevention layer).

Subsequently, 0.37 g of the above powder was dispersed in 50 mL of water to prepare a dispersion. To this dispersion, 2.56 × 10 ⁻² mol of tin (II) sulfate was added and stirred for 5 minutes at a rotational speed of 300 rpm using a stirrer to prepare a mixed solution. After adjusting the pH of this mixed solution to 0.5 with sulfuric acid, 0.5 g of polyvinyl alcohol 500 (polyvinyl alcohol having an average molecular weight of 500) was added as a dispersant, and the mixture was further stirred at a rotation speed of 300 rpm for 10 minutes. Next, 50 mL of a 1.58 mol / L divalent chromium ion aqueous solution whose pH was adjusted to 0.5 was added to this mixed solution at an addition rate of 50 mL / min, and stirred at a rotational speed of 500 rpm for 10 minutes to remove tin ions. By carrying out reduction, a dispersion liquid in which the nickel-coated copper powder having tin as the outermost layer in which tin was deposited on the surface of the nickel-coated copper powder was obtained. The dispersion was allowed to stand for 60 minutes, and the produced powder was allowed to settle. Then, the supernatant was discarded, and 100 mL of water was added thereto, followed by stirring for 10 minutes at a rotational speed of 300 rpm, and washing was performed four times. . Finally, this is dried in a vacuum dryer, the average particle size is 1.1 μm, copper is the central core, nickel is the first coating layer (diffusion prevention layer), and tin is the second coating layer. Solder powder formed on each (outermost layer) was obtained.

<Examples 2 to 28, Comparative Examples 1 to 55>
Also in Examples 2 to 28 and Comparative Examples 1 to 55, the particle diameter of the copper powder to be used, the addition amount of the copper powder, the addition amounts of nickel (II) sulfate and tin (II) sulfate, and the ratio of other components are adjusted. Thus, a solder powder was obtained in the same manner as in Example 1 except that the predetermined copper central core radius, the thickness of the nickel diffusion preventing layer and the tin outermost layer, and the solder powder having a predetermined particle diameter were controlled. It was.

<Comparative Example 56>
First, 4.35 × 10 ⁻³ mol of nickel (II) sulfate, 9.66 × 10 ⁻⁴ mol of sodium phosphinate and 3.29 × 10 ⁻⁴ mol of sodium citrate are added to 50 mL of water, and a stirrer is added. The mixture was stirred at a rotational speed of 300 rpm for 5 minutes to prepare a solution. After adjusting the pH of this solution to 5.0 with sulfuric acid, 0.2 g of polyvinyl alcohol 500 (polyvinyl alcohol having an average molecular weight of 500) was added as a dispersant, and the mixture was further stirred at a rotation speed of 300 rpm for 10 minutes. Next, 0.2 g of polyvinyl alcohol 500 (polyvinyl alcohol having an average molecular weight of 500) as a dispersant is dissolved in 50 mL of water in this solution, and 3.40 g of copper powder having an average particle size of 0.18 μm is dispersed. The dispersion was added and stirred at a rotational speed of 500 rpm for 10 minutes to obtain a dispersion in which nickel-coated copper powder having nickel deposited on the surface of the copper powder was dispersed. The dispersion was allowed to stand for 60 minutes, and the produced powder was allowed to settle. Then, the supernatant was discarded, and 100 mL of water was added thereto, followed by stirring for 10 minutes at a rotational speed of 300 rpm, and washing was performed four times. . Finally, this was dried with a vacuum dryer to obtain a powder having copper as the central core and nickel as the first coating layer (diffusion prevention layer).

Subsequently, tin plating was performed on the powder having the above copper as the central core and nickel as the diffusion preventing layer, using a mini barrel apparatus manufactured by Yamamoto Gold Tester Co., Ltd. As a plating solution composition used, a solution in which 5.0 g of powder was dispersed in 150 mL of a plating solution of sodium stannate 100 g / L, sodium hydroxide 10 g / L and sodium acetate 15 g / L was used. Further, the plating conditions were tin for the anode, bath temperature of 50 ° C., barrel rotation speed of 19 rpm, and voltage of 0.8V. The tin plating film thickness is adjusted by the treatment time. In this comparative example, the nickel-coated copper powder with tin as the outermost layer on which the tin is deposited on the nickel-coated copper powder surface is dispersed by performing the treatment for 0.3 hour. A dispersion was obtained. The dispersion was allowed to stand for 60 minutes, and the produced powder was allowed to settle. Then, the supernatant was discarded, and 100 mL of water was added thereto, followed by stirring for 10 minutes at a rotational speed of 300 rpm, and washing was performed four times. . Finally, this is dried in a vacuum dryer, the average particle size is 3.3 μm, copper is the central core, nickel is the first coating layer (diffusion prevention layer), and tin is the second coating layer. Solder powder formed on each (outermost layer) was obtained.

<Comparative Examples 57 to 65>
Also in Comparative Examples 57 to 65, by adjusting the particle size of the copper powder to be used and the addition amount of the copper powder, the addition amount of nickel (II) sulfate and tin (II) sulfate, the ratio of other components and the treatment conditions, A solder powder was obtained in the same manner as in Comparative Example 56, except that the predetermined radius of the copper core, the thickness of the nickel diffusion preventing layer and the outermost tin layer, and the solder powder having a predetermined particle diameter were controlled.

<Comparison test and evaluation>
For the solder powders obtained in Examples 1 to 28 and Comparative Examples 1 to 65, the copper content of the solder powder [% by mass], the average particle size [μm], The average radius [μm], the average thickness [μm] of the diffusion preventing layer made of nickel, and the average thickness [μm] of the coating layer made of tin were measured. These results are shown in Tables 1 to 5 below. In addition, solder pastes were prepared using these solder powders, and the bonding strength when the maximum holding temperature during reflow was changed was evaluated. These results are shown in Tables 6 to 10 below. The sum of the average radius of the central core made of copper, the average thickness of the diffusion preventing layer made of nickel, and the average thickness of the coating layer made of tin was taken as the average radius of the solder powder.

(1) Analysis of copper content in solder powder: The copper content of solder powder was analyzed by inductively coupled plasma emission spectroscopic analysis (ICP emission analysis device: ICPS-7510 manufactured by Shimadzu Corporation).

(2) Average particle size of solder powder: The particle size distribution was measured with a particle size distribution measuring device using a laser diffraction scattering method (Horiba, Ltd., laser diffraction / scattering type particle size distribution measuring device LA-950). The volume cumulative median diameter (Median diameter, D ₅₀ ) was defined as the average particle diameter of the solder powder.

(3) Measurement of the radius of the central core made of copper, the thickness of the diffusion prevention layer made of nickel, and the thickness of the coating layer made of tin: the solder powder was embedded in a thermosetting epoxy resin, and the cross section of the solder powder was dry-polished Then, using an electron microscope (Scanning Electron Microscope, SEM), about 30 solder particles, the radius of the central core made of copper, the thickness of the diffusion prevention layer made of nickel, and the thickness of the coating layer made of tin The thickness was measured and the average value of each was obtained. Further, the ratio of the average thickness value (the thickness of the diffusion prevention layer / the radius of the central core) is calculated from the thickness of the diffusion prevention layer made of nickel obtained from the above measurement and the average value of the radius of the central core made of copper. did.

(4) Aggregation of solder powder: Obtained by measuring the particle size distribution with a particle size distribution measuring device (Horiba, Ltd., laser diffraction / scattering particle size distribution measuring device LA-950) using a laser diffraction scattering method. In the obtained particle size distribution profile, in addition to the distribution peak of the desired particle size, the case where there are two or more particle size profiles having a distribution peak on the larger particle size side is regarded as “aggregation”, as such A case where no peak was observed was judged by a method of “no aggregation”.

(5) Cohesive strength: 50% by mass of diethylene glycol monohexyl ether as a solvent, 46% by mass of polymerized rosin (softening point 95 ° C.) as rosin, and 1.0% by mass of cyclohexylamine hydrobromide as an activator A flux was prepared by mixing 3.0% by mass of hardened castor oil as a thixotropic agent. Next, this flux and the solder powders obtained in Examples 1 to 28 and Comparative Examples 1 to 65 were mixed at a ratio of 88 mass% flux and 12 mass% solder powder to obtain solder pastes, respectively. Prepared.

The paste prepared above was transferred to a predetermined position on a 0.5 mm thick Kovar (Fe—Ni—Co alloy) substrate using a pin having a diameter of 100 μm by a pin transfer method. The Kovar substrate was Ni-plated, and further Au flash-plated. Subsequently, a 0.9 mm □ LED chip was mounted on the transferred paste. Further, using a pressurizing jig, the LED chip and the substrate are pressed at a pressure of 1.0 MPa, and reflowed at a predetermined maximum holding temperature in a nitrogen atmosphere in an infrared heating furnace for 0.17 hours, and the LED A bonded sample was obtained by bonding the chip and the Kovar substrate. In addition, the maximum holding temperature at the time of the reflow was set to different temperatures of 250 ° C., 300 ° C., and 350 ° C., and three bonded samples were obtained for each of the examples or comparative examples.

Regarding the bonding strength between the above-mentioned Kovar substrate and LED chip, in accordance with “Lead-free solder test method described in JIS Z 3198-7—Part 7:“ Measurement method of solder joint shear strength in chip components ” , Measured the joint shear strength under the conditions after storage at room temperature and 300 ° C for 0 day and 30 days, respectively, and relative share after storage at 300 ° C for 0 day and 30 days when the shear strength at room temperature is 100 The strength was determined. In the table, “excellent” indicates a case where the relative share strength is 90 or more, “good” indicates a case where it is less than 90 to 80 or more, and “good” indicates less than 80 to 70 or more. “Not possible” indicates a case of less than 70.

From Tables 6 to 10, the following was found when Examples 1 to 28 and Comparative Examples 1 to 65 were compared.

比較 In the comparative example in which the average particle size of the solder powder was 0.5 μm, the particle size was too small, so the solder powder did not melt before storing the solder powder due to the effect of the oxide film on the powder surface. When the thickness of the diffusion prevention layer made of nickel is 0.03 when the radius of the copper nucleus is 1, the diffusion prevention layer is too thin to prevent the diffusion of copper or tin. In some cases, the bonding strength becomes impossible. Moreover, in the comparative example with a copper content of about 75% by mass, since the copper content is too high, the solidification start temperature becomes too high, and the solder may not melt during reflow. Moreover, in the comparative example in which the copper content is about 1.5% by mass, the copper content is too low, the solidification start temperature becomes low, and sufficient heat resistance cannot be obtained, so that the bonding strength becomes impossible. was there.

In the comparative example in which the average particle size of the solder powder is about 40 μm, the particle size is too large, and after the reflow, a bonding layer having large voids (voids) is obtained and a dense bonding layer cannot be obtained. There was a case. Further, in the comparative example in which the ratio of the thickness of the diffusion prevention layer to the radius of the central core is smaller than 0.04, the diffusion prevention effect is small, so that copper diffuses into the coating layer made of tin, so that the meltability of the solder powder is increased. In some cases, it declined and became impossible. On the contrary, in the comparative example in which the ratio of the thickness of the diffusion prevention layer and the radius of the central core exceeds 0.51, the nickel ratio becomes too high and the meltability of the solder powder is lowered, so that the bonding strength may be impossible. there were. In the comparative example in which the coating layer made of tin was formed by barrel plating, a large amount of strongly agglomerated powder was generated. Therefore, the average particle size of the solder powder obtained using the laser diffraction scattering method was measured by SEM observation. While the value greatly different from the average particle size of the solder powder obtained was obtained, a good printed film could not be obtained, and the bonding strength could not be measured in any sample.

On the other hand, the average particle size of the solder powder is in the range of 1 to 30 μm, and the copper content is in the range of 2 to 70% by mass with respect to 100% by mass of the total amount of the solder powder. In Examples 1-28 in which the thickness of the diffusion-preventing layer made of nickel is in the range of 0.04 to 0.51 when the radius of the copper nucleus is 1, the aggregated powder is almost free and good At the same time, the joint strength was good, good or excellent at all reflow temperatures of 250 ° C., 300 ° C., and 350 ° C. before and after storing the solder powder for 30 days.

The present invention can be suitably used for producing solder powder that can be stored for a long period of time. Moreover, it can utilize suitably for mounting of electronic components, especially mounting of electronic components exposed to a high temperature atmosphere.

Claims

Preparing a first dispersion of copper powder;
Adding a nickel metal salt to the first dispersion of copper powder to prepare a first solution in which the nickel metal salt is dissolved and the copper powder is dispersed;
Adjusting the pH of the first solution;
A step of preparing a second dispersion in which nickel ions are reduced by adding and mixing the first reducing agent to the pH-adjusted first solution, and the deposited nickel covers and disperses the copper powder;
Solid-liquid separation of the second dispersion, drying the solid-solid separated solids to produce a metal powder having a copper core coated with a diffusion preventing layer made of nickel;
Preparing a third dispersion of the metal powder;
Adding and mixing a tin metal salt to the third dispersion of the metal powder to prepare a second solution in which the metal salt of the tin is dissolved and the metal powder is dispersed;
Adjusting the pH of the second solution;
A step of preparing a fourth dispersion in which tin ions are reduced by adding and mixing a second reducing agent to the pH-adjusted second solution, and the deposited tin covers and disperses the metal powder;
Solid-liquid separation of the fourth dispersion, and drying the solid-liquid separated solid to produce a solder powder coated with the tin powder with the metal powder.
In the solder powder in which the metal powder formed by coating the copper core with the diffusion preventing layer made of nickel is coated with the tin layer,
The average particle size of the solder powder is 1 μm or more and 30 μm or less,
The total content of the solder powder is 100% by mass, and the copper content is 2% by mass or more and 70% by mass or less.
Solder powder characterized in that the thickness of the diffusion preventing layer made of nickel is a ratio of 0.04 or more and 0.51 or less when the radius of the copper nucleus is 1.
A method for producing a solder paste by mixing the solder powder produced by the method of claim 1 or the solder powder of claim 2 and a solder flux into a paste.
A method for mounting an electronic component using the solder paste manufactured by the method according to claim 3.