JP6428409B2 - Solder powder and solder paste using this powder - Google Patents

Description

  The present invention relates to a solder powder used for mounting electronic components and the like, and a solder paste using the powder. More particularly, the present invention relates to a solder powder suitable for mounting an electronic component or the like exposed to a high temperature atmosphere, and a solder paste using this powder, which hardly remelts and lowers bonding strength after reflow.

  Solder used for joining electronic parts and the like has been made lead-free from the viewpoint of the environment, and at present, solder powder mainly composed of tin is used. As a method for obtaining a fine metal powder such as a solder powder, in addition to an atomizing method such as a gas atomizing method or a rotating disk method, a melt spinning method, a rotating electrode method, a mechanical process, a chemical process, or the like is known. . As the gas atomizing method, a method is known in which after melting a metal in an induction furnace or a gas furnace, the molten metal is caused to flow down from a nozzle at the bottom of a tundish, and high pressure gas is sprayed from the surroundings to pulverize. The rotating disk method is also called a centrifugal atomizing method, and is a method in which a molten metal is dropped onto a rotating disk at high speed, and a shearing force is applied in a tangential direction to break and make a fine powder.

  In addition to the above-mentioned environmental characteristics, the solder is required to have various characteristics depending on the use of the electronic component to be mounted. For example, information electronic devices such as mobile phones and personal computers are required to be thinner and lighter with an emphasis on portability, and electronic components used in these manufactures are becoming smaller and finer pitches for joining components. Therefore, a solder powder having a finer particle size is required.

  On the other hand, in electronic parts used at high temperatures for in-vehicle applications, etc., it is necessary to prevent the solder after mounting from remelting by being exposed to a high-temperature atmosphere and reducing the bonding strength. High heat resistance is required. In the case of the most common Sn—Pb type eutectic solder (composition ratio Sn: Pb = 63: 37% by mass), the melting point is about 187 ° C., and in the case of a general Sn—Ag—Cu type solder, It is about 217 ° C. In contrast, Au—Sn high temperature solder (composition ratio Sn: Au = 20: 80 mass%) known as heat resistant high temperature solder has a melting point of about 280 ° C., and Sn—Pb high temperature solder. (Composition ratio Sn: Pb = 5: 95% by mass) shows a high melting point of about 310 to 315 ° C.

  However, since the Au—Sn solder uses very expensive Au, there is a problem that the manufacturing cost increases. In addition, although Sn—Pb-based high-temperature solder exhibits very high heat resistance, it uses lead, and thus the above-mentioned environmental problems remain. In order to solve such problems, a technology is known that improves heat resistance, bonding strength, etc. by mixing low-melting and low-cost material powders that do not contain lead or gold with other material powders with high melting points. (For example, refer to Patent Documents 1 to 6.)

  Patent Document 1 discloses a joining method using a metal material in which a metal Cu powder, a metal Sn powder, and an Ag—Cu—Ti alloy powder are mixed for contact between a ceramic member and a metal member. Further, Patent Document 2 does not contain lead, and Sn—Bi or the like, which has a melting point lower than that of a conventional tin-lead eutectic alloy, Sn—Ag or the like whose melting point is higher than that of a conventional tin-lead eutectic alloy. A solder paste containing a mixed powder obtained by mixing these alloy powders is disclosed. Patent Document 3 discloses cream solder in which metal particles such as Ag, Sn, and Cu having a higher melting point than eutectic solder are mixed in addition to eutectic solder. Patent Document 4 discloses a soldering composition in which a second metal component such as Ag or Cu having a melting point of 400 ° C. or higher is mixed with a first metal component such as Sn that does not contain lead. ing. Patent Document 5 includes a first metal component and a second metal component in powder form, and the first metal component is either a Sn—Cu based alloy or a Sn—Cu—Sb based alloy, or of these. One or more of Ag, In, Bi, Zn, or Ni is added to any of the above, and the second metal component is one or more metals of Cu, Sn, Sb, Ag, Zn, Ni, or these A composition for high-temperature cream soldering, which is an alloy of two or more of these metals, is disclosed. Patent Document 6 includes a first metal powder that does not contain lead and contains tin as a main component, and a solder that contains a second metal powder that has a higher melting point than the first metal powder and mainly contains copper. A composition is disclosed.

  On the other hand, the present applicant, in a solder powder composed of a central core and a coating layer covering the central core, having an average particle size of 5 μm or less, a metal element core in which the central core is made of nickel, and an outer periphery of the metal element core Patent application was made for a solder powder having a two-layer structure having an intermetallic compound layer of the metal element and tin and the coating layer being made of tin (see Patent Document 7). The content ratio of nickel in the solder powder is 0.1 to 1.0% by mass with respect to 100% by mass of the total amount of the solder powder. By limiting the nickel content to the above range, the composition is prevented from deviating from the eutectic point to lower the melting point of the solder powder, and the increase in the electrical resistance of the solder alloy in the formed solder bump is suppressed. Strength is improved. Also, this solder powder has better melt diffusibility during reflow, easier composition control during solder bump formation, better wettability, and better average solder powder than the conventional solder powder whose central core is composed only of metal elements. Since it is a fine powder having a particle size of 5 μm or less, it has an effect that it can be printed with a fine pitch pattern when a solder paste made from this powder is printed on a substrate or the like.

JP-A-5-24943 (Claim 2, paragraph [0015]) JP-A-11-186712 (claim 2, paragraph [0018], paragraph [0023]) JP 2000-176678 A (Claims 1 and 3, paragraph [0010]) JP 2002-254195 A (Claims 1 to 5, paragraphs [0011] to [0013]) JP 2003-154485 A (Claim 1, paragraph [0009]) Japanese Patent No. 3782743 (Claims 1-4, paragraph [0005]) JP 2012-157870 A (Claim 1, Claim 2, Paragraph [0022], Paragraph [0014])

  In the above-mentioned conventional Patent Documents 1 to 6, all use a solder powder obtained by mixing a low melting point metal powder not containing lead or gold and a high melting point metal powder, etc. As described above, in the solder powder obtained by mixing two or more powders having different melting points, compositions, etc., nonuniformity is likely to occur in the mixing of the powders. When non-uniformity occurs, partial melting unevenness and compositional deviation occur during reflow, which causes a problem that sufficient strength cannot be obtained at the joint portion.

  Further, in the invention of Patent Document 7, when the content ratio of nickel is 0.1 to 1.0% by mass, when the solder after mounting is exposed to a high temperature atmosphere, the solder after mounting remelts, or Since a liquid phase is generated in a part of the solder, the bonding strength with the substrate or the like may be lowered, and further improvement is necessary.

  An object of the present invention is to provide a solder powder suitable for mounting an electronic component or the like exposed to a high-temperature atmosphere, and a solder paste using this powder, which hardly remelts and lowers bonding strength after reflow. .

  As a result of intensive studies by the present inventors, even when the content ratio of the upper limit value of nickel exceeds 1.0% by mass, at least 35% by mass of tin is included and reflow treatment is performed. The solder powder can be sufficiently melted and bonded to a substrate or the like, and even after being exposed to a high temperature atmosphere, the solder after mounting does not remelt or a liquid phase does not occur in a part of the solder. Thus, the present inventors have found that no joint failure occurs.

  As shown in FIG. 1, the first aspect of the present invention is composed of a central core 11 and a coating layer 12 covering the central core 11, and the central core 11 is composed of nickel and an intermetallic compound of nickel and tin. In the solder powder 10 in which the coating layer 12 is made of tin, the average particle size of the solder powder 10 is 1 to 30 μm, and the nickel content exceeds 3.0% by mass with respect to 100% by mass of the total amount of the solder powder 10. It is 65 mass% or less, It is characterized by the above-mentioned.

A second aspect of the present invention is the invention based on the first aspect, wherein the intermetallic compound of nickel and tin is further selected from the group consisting of Ni ₃ Sn, Ni ₃ Sn ₂ , Ni ₃ Sn ₄ and NiSn _3. It is one or two or more selected compounds.

  A third aspect of the present invention is a solder paste obtained by mixing the solder powder of the first or second aspect and a solder flux into a paste.

  A fourth aspect of the present invention is a method for mounting an electronic component using the solder paste according to the third aspect.

  The solder powder according to the first aspect of the present invention is composed of a central core and a coating layer covering the central core, the central core is made of nickel and an intermetallic compound of nickel and tin, and the coating layer is made of tin. The average particle size of the solder powder is 1 to 30 μm, and the nickel content is more than 3.0% by mass and 65% by mass or less with respect to 100% by mass of the total amount of the solder powder. Thus, in the solder powder of the present invention, the powder surface is structured from tin having a low melting point, so that even when the content ratio of tin is reduced compared to the content ratio of the solder powder described in Patent Document 7, On the other hand, after reflow, the content ratio of nickel is increased compared to the content ratio of 0.1 to 1.0% by mass of the solder powder described in Patent Document 7, so Due to the existing intermetallic compound, an intermetallic compound having a high melting point is formed at a higher ratio than the solder powder described in Patent Document 7.

In the solder powder according to the second aspect of the present invention, the intermetallic compound of nickel and tin is one or more selected from the group consisting of Ni ₃ Sn, Ni ₃ Sn ₂ , Ni ₃ Sn ₄ and NiSn _3. It is a compound of this. For example, the melting point of Ni ₃ Sn is about 1000 ° C., the melting point of Ni ₃ Sn ₂ is 1200 ° C., and the melting point of Ni ₃ Sn ₄ is 784.5 ° C. Thus, when the solidification start temperature rises to about 600 to 1200 ° C., remelting hardly occurs. For this reason, the solder powder of the present invention can be suitably used as a high-temperature solder used for mounting electronic parts and the like exposed to a high-temperature atmosphere. In addition, since nickel and tin are contained in one metal particle constituting the powder, it is possible to prevent a decrease in bonding strength due to melting unevenness or composition shift during reflow.

  The solder paste according to the third aspect of the present invention is obtained using the solder powder of the present invention. Therefore, this solder paste is fast melting at reflow and excellent in meltability, but after reflowing, unlike solder powder described in Patent Document 7, it forms a bonding layer made of an intermetallic compound and nickel and melts solder. Since the powder forms an intermetallic compound having a higher melting point than the solder powder described in Patent Document 7 and the heat resistance is increased, remelting due to heat hardly occurs. For this reason, the solder paste of the present invention can be suitably used for mounting electronic components exposed to a high temperature atmosphere.

  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. It can be mounted with high heat resistance after mounting.

It is the figure which represented typically an example of the section structure of solder powder solder powder of an embodiment of the present invention.

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

[Solder powder]
As shown in FIG. 1, the solder powder of the present invention comprises a central core 11 and a coating layer 12 covering the central core 11, and the central core 11 is made of nickel and an intermetallic compound of nickel and tin. 12 is a solder powder made of tin. As described above, the solder powder of the present invention has a structure in which a central core made of nickel and an intermetallic compound of nickel and tin is covered with a coating layer made of tin having a low melting point, and thus melts during reflow. Excellent in properties. In addition, the conventional solder powder is not a powder in which two or more kinds of metal powders having different melting points or the like are mixed, but nickel and tin are contained in one metal particle constituting the powder. Melting unevenness and composition deviation hardly occur, and high bonding strength can be obtained. Furthermore, since a part of the central core has already formed an intermetallic compound of nickel and tin before reflowing, for example, compared to a powder having a structure in which the central core made of nickel is covered with tin, melting at the time of reflowing Good diffusibility, easy composition control during solder bump formation, and excellent wettability.

And the solder powder 10 of this invention is 1-30 micrometers in average particle diameter. The reason why the average particle size of the solder powder is limited to 1 to 30 μm is that when it exceeds 30 μm, the bump coplanarity is lowered when the bump is formed, and the uneven coating is applied when the pattern surface is coated with solder. This is because a problem arises that the entire pattern surface cannot be uniformly coated. When the thickness is less than 1 μm, the specific surface area increases, and the meltability of the solder decreases due to the influence of the surface oxide layer of the powder. The average particle size of the solder powder is preferably in the range of 2 to 18 μm. In the present specification, the average particle size of the powder was measured with a particle size distribution measuring device (Horiba Seisakusho, laser diffraction / scattering particle size distribution measuring device LA-950) using a laser diffraction scattering method. Volume cumulative median diameter (Median diameter, D ₅₀ ).

  Moreover, the solder powder 10 of this invention is more than 3.0 mass% and 65 mass% or less of nickel content rate with respect to 100 mass% of powder whole quantity. 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 eutectic composition is required. The proportion is about 0.7% by mass and relatively small. On the other hand, in the solder powder of the present invention, a Sn—Ni alloy having a high solidification start temperature of about 600 to 1200 ° C. is formed after reflowing by including a proportion of nickel in the above-mentioned range which is larger than that in Patent Document 7. To do. Even if the content ratio of nickel is small, after reflowing, a Sn—Ni alloy having a higher solidification start temperature than tin is formed. However, if more nickel is contained, the solidification start temperature is further increased. This is because the ratio of the intermetallic compound 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 parts and the like exposed to a high-temperature atmosphere. When the nickel content is 3.0% by mass or less, the solidification start temperature becomes low, so that sufficient heat resistance cannot be obtained in the solder bump formed after reflow, and remelting occurs when used in a high temperature atmosphere. Can not be used as high temperature solder. On the other hand, if it exceeds 65% by mass, the solidification start temperature becomes too high and the solder is not sufficiently melted, resulting in a problem that bonding failure occurs. Among these, it is preferable that the content rate of nickel which occupies for 100 mass% of the whole amount of powder shall be 5-65 mass%.

  Further, the content ratio of tin in the solder powder is 35% by mass or more and less than 97% by mass, preferably 35 to 95% by mass with respect to the remainder other than nickel in the powder, that is, 100% by mass of the total amount of the solder powder. is there. This is because if the tin content is less than 35% by mass, the low melting point required for the solder powder is not exhibited during reflow. On the other hand, when the content is 97% by mass or more, the nickel content decreases as a result, and the heat resistance of the solder bump formed after reflowing is lowered. 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.

As an intermetallic compound of nickel and tin constituting a part of the central core, one or more compounds selected from the group consisting of Ni ₃ Sn, Ni ₃ Sn ₂ , Ni ₃ Sn ₄ and NiSn ₃ In practice, however, Ni ₃ Sn and Ni ₃ Sn ₂ mainly form the central core depending on the composition ratio of nickel and tin.

[Method for producing solder powder]
Subsequently, a method for producing the solder powder of the present invention will be described. First, nickel powder is dispersed in a solvent to prepare a first dispersion. A dispersant may be mixed in the solvent. Next, a metal salt of tin is added to and mixed with the first dispersion of nickel powder to prepare a mixed solution in which the metal salt of tin is dissolved and the nickel powder is dispersed. Next, the pH of the mixed solution is adjusted to 0.1 to 2.0. After adding a reducing agent to the pH adjusted liquid mixture and mixing, tin ions are reduced, and after preparing a second liquid dispersion in which the deposited tin disperses the metal powder covering the nickel powder, (2) The dispersion is subjected to solid-liquid separation, and the solid content thus separated is dried to obtain a solder powder. The ratio of the nickel powder and the compound containing tin in the mixed liquid is adjusted so that the nickel content is in the range of more than 3% by mass and less than 65% by mass after the solder powder is manufactured.

  The nickel powder preferably has an average particle diameter of 1 to 21 μm. If the lower limit is less than 1 μm, the average particle size of the solder powder produced by the method of the present invention does not reach the minimum value of 1 μm. When the average particle size of the solder powder is less than the minimum value of 1 μm, the specific surface area of the solder powder increases, and the meltability of the solder is lowered due to the influence of the surface oxide layer of the powder. Moreover, if it is less than 1 micrometer of the said lower limit, it will become difficult to exceed content of 3.0 mass% of nickel which comprises solder powder. If the upper limit of 21 μm is exceeded, the average particle size of the solder powder tends to exceed the maximum value of 30 μm. When the average particle size of the solder powder exceeds the maximum value of 30 μm, the bump coplanarity is lowered when the bump is formed by the solder paste using the solder powder, and the uneven coating is applied when the pattern surface is coated with the solder. As a result, the entire surface of the pattern cannot be uniformly coated. The average particle diameter of the nickel powder is more preferably 1.2 to 16.0 μm. This nickel 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.

  Examples of the solvent of the first dispersion liquid in which the nickel powder is dispersed include water, alcohol, ether, ketone, and ester. The prepared mixed solution is adjusted to a pH of 0.1 to 2.0 in consideration of re-dissolution of the generated solder powder. Note that a complexing agent may be added to the mixed solution to complex the tin element. By adding the complexing agent, tin 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. Examples of the dispersant include cellulose-based, vinyl-based, and polyhydric alcohols. In addition, gelatin, casein, and the like can be used. The dispersant may be added after adding the complexing agent.

  Examples of the tin compound used for preparing the mixed solution include tin metal salts such as tin (II) chloride, tin (II) sulfate, tin (II) acetate, and tin (II) oxalate. Among these, it is particularly preferable to use tin (II) chloride as a hydrochloride as the compound containing tin.

  Next, when adding a reducing agent to the pH-adjusted mixed solution, it is preferable to prepare an aqueous solution in which the reducing agent is dissolved in advance and adjust the pH of the aqueous solution to the same level as the mixed solution. Examples of the reducing agent include 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, and the like.

  Next, a reducing agent aqueous solution is added to and mixed with the above mixed solution, whereby tin ions in the mixed solution are reduced, and a second dispersion in which the metal powder covering the nickel powder with the deposited tin is dispersed is obtained. . By this reduction reaction, metal powder composed of a central core made of nickel and a coating layer made of tin covering the central core is formed. As a method of mixing the mixed solution and the reducing agent aqueous solution, the reducing agent aqueous solution is dropped into the mixed solution in the container at a predetermined addition rate and stirred with a stirrer or a reaction tube having a predetermined diameter. Examples include a method of pouring both solutions into a reaction tube at a predetermined flow rate and mixing them.

  Next, the second dispersion liquid in which the metal powder is dispersed is subjected to solid-liquid separation by decantation or the like, and the recovered solid content is water or a hydrochloric acid aqueous solution, a nitric acid aqueous solution, a sulfuric acid aqueous solution whose pH is adjusted to 0.5 to 2, or Wash with methanol, ethanol, acetone, etc. After washing, the solid content is recovered by solid-liquid separation again. The solder powder of the present invention can be obtained by repeating the steps from washing to solid-liquid separation, preferably 2 to 5 times, and then vacuum-drying the collected solid content.

  Further, the heat treatment described below may be performed. A high boiling point solvent having a boiling point of 100 ° C. or higher is added to and dispersed in the collected solid content before vacuum drying, and heated at a predetermined temperature in an inert gas atmosphere. By applying this heat treatment, the central core made of nickel of the metal powder formed by the above reduction reaction reacts with a part of the coating layer made of tin covering the central core, and compared with the case where the heat treatment is not performed. Thus, a central core having a layer made of a thicker nickel-tin intermetallic compound is formed.

  Examples of the high boiling point solvent used include ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, and castor oil.

  The heat treatment is preferably performed at a temperature of 100 to 130 ° C. for 20 minutes to 1 hour in an inert gas atmosphere such as nitrogen or argon. When the treatment temperature or holding time is less than the lower limit value, an intermetallic compound may not be formed in the central core. On the other hand, when the treatment temperature exceeds the upper limit value, tin in the coating layer is oxidized, resulting in a problem that the meltability is lowered. The effect does not change even if the holding time is set to the upper limit value or more. Among these, it is particularly preferable to carry out at a temperature of 115 to 125 ° C. for 30 to 40 minutes. The reason why the reaction is performed in an inert gas atmosphere is to prevent oxidation of tin.

  After heating, the steps from washing to solid-liquid separation are preferably repeated 2 to 5 times, and then the collected solid content is vacuum dried to obtain the solder powder of the present invention.

[Solder paste and its preparation method]
The solder powder of the present invention obtained by the above steps 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 preparing 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.

  Further, as the thixotropic agent, 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- Examples include D-sorbitol and its derivatives.

  The activator is preferably a hydrohalic acid amine salt, specifically, triethanolamine, diphenylguanidine, ethanolamine, butylamine, aminopropanol, polyoxyethyleneoleylamine, polyoxyethylenelaurylamine, 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, isopropylamine , Dimethyl hexyl amines, hydrochloric acid salt or hydrobromide of an amine such as cyclohexylamine. In addition, organic acids, amines and salts thereof can be used.

  The soldering flux can be obtained by mixing the above components at a predetermined ratio. It is preferable that the ratio of the solvent in the total amount of flux of 100% by mass is 30 to 60% by mass, the ratio of the thixotropic agent is 1 to 10% by mass, and the ratio of the activator is 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, ascorbic acid derivatives, and the like. 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 components exposed to a high temperature atmosphere.

[Electronic component mounting method and joint using solder paste]
To mount electronic components on silicon chips, LED chips, etc. on various heat dissipation boards, FR4 (Flame Retardant Type 4) boards, Kovar boards, etc. using the solder paste prepared by the above method, pin transfer The solder paste is transferred to a predetermined position of the substrate by the method. Next, a chip element which is an electronic component is mounted on the transferred 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, a first dispersion was prepared by dispersing 0.10 g of nickel powder having an average particle size of 1.1 μm in 50 mL of water. To this dispersion, 2.56 × 10 ⁻² mol of tin (II) chloride 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 hydrochloric 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 / sec. Reduced. A second dispersion was obtained in which the metal powder covering the nickel powder was dispersed by tin deposited by this reduction. The dispersion was allowed to stand for 60 minutes to settle the metal powder, and then the supernatant was discarded, 100 mL of water was added thereto, and the mixture was stirred for 10 minutes at a rotational speed of 300 rpm, and washed four times. It was. Thereafter, 100 mL of ethylene glycol was added and dispersed, and heating was performed in a nitrogen atmosphere at 120 ° C. for 30 minutes while stirring at a rotation speed of 300 rpm. Finally, this is dried by a vacuum dryer, whereby the Ni nucleus and the central nucleus having an intermetallic compound (Ni ₃ Sn, Ni ₃ Sn ₂ ) on the outer periphery of the Ni element nucleus and Sn are coated with the coating layer. A metal powder was obtained.

<Examples 2-35 and Comparative Examples 1-18>
As shown in Tables 1 to 3 below, nickel powders having different average particle diameters were used, or the amount of nickel contained in 100% by mass of solder powder was changed by adjusting the amount of nickel powder added. In addition, a solder powder was obtained in the same manner as in Example 1 except that the average particle size of the solder powder was controlled to a predetermined particle size.

<Comparison test and evaluation>
With respect to the solder powders obtained in Examples 1 to 35 and Comparative Examples 1 to 18, the structure of the metal particles constituting the powder, the average particle diameter of the powder, and the composition were analyzed or measured by the method described below. These results are shown in Tables 1 to 3 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 4 to 11 below.

  (1) Structural analysis of solder powder: Structural analysis was performed with a powder X-ray diffractometer (manufactured by Rigaku Corporation: RINT Ultimate + / PC).

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

  (3) Composition of solder powder: The metal element content was measured by inductively coupled plasma emission spectroscopic analysis (ICP emission analyzer: ICPS-7510 manufactured by Shimadzu Corporation).

  (4) Bonding 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 powder obtained in Examples 1 to 35 and Comparative Examples 1 to 18 are mixed in a proportion of 88% by mass of flux and 12% by mass of solder powder, respectively. Prepared.

  The prepared paste 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. Furthermore, while pressing the LED chip and the substrate at a pressure of 1.0 MPa using a pressurizing jig, a predetermined maximum holding temperature for 0.5 hours in a nitrogen atmosphere in a reflow furnace (SIKAMA Falcon 8500) The sample was obtained by reflowing and bonding the LED 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 the LED chip, in accordance with “Lead-free solder test method described in JIS Z 3198-7—Part 7:“ Measurement method of shear strength of solder joint in chip parts ” The joint shear strength after holding at room temperature and 300 ° C. for 0 hour and 500 hours was measured, and the relative shear strength after holding at 300 ° C. for 0 hour and 500 hours when the shear strength at room temperature was taken as 100 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 4 to 11, Examples 1-35 and Comparative Examples 1-18 were compared, and the following was found.

(1) Nickel content ratio In Comparative Example 1 and Comparative Example 3 where the nickel content ratio is 3.0 mass%, the solder remelts because the solidification start temperature is low, and after holding at 300 ° C for 500 hours Since the relative shear strength of the sheet became low as 55 to 68, all the joint strength evaluation judgments after holding for 500 hours were “impossible”. On the other hand, in Comparative Example 2 and Comparative Example 4 in which the content ratio of nickel exceeds 65% by mass, since the solidification start temperature is too high, the solder powder does not melt at the time of reflow, and sufficient bonding cannot be obtained. The joint shear strength could not be measured, and all of the joint strength evaluation judgments after holding for 500 hours were “impossible”. On the other hand, in Examples 1 to 35 in which the nickel content ratio exceeds 3.0 mass% and is 65 mass% or less, the relative shear strength is improved, and the evaluation for the bonding strength in a high-temperature atmosphere is 0 hour and 500 All the determinations after holding time were “OK”, “Good” or “Excellent”, and good results were obtained. In particular, in Examples 1, 8, 15, 22, and 29 where the nickel content was 3.2% by mass, all the judgments after holding for 0 hour and 500 hours in the evaluation of the bonding strength in the high temperature atmosphere were “possible”. In contrast, Examples 2-7, Examples 9-14, Examples 16-21, Examples 23-28, and Example 30 in which the nickel content is in the range of 5.0 to 65.0% by mass. In 35, the above determinations were all “good”, “good”, or “excellent”, and good results were obtained.

(2) Average particle size of solder powder In Comparative Examples 5 to 11 in which the average particle size of the solder powder is 0.6 to 0.7 μm, the specific surface area of the solder powder is increased, and the ratio of the oxide film on the surface to the powder is Since the solder was not melted due to this, the initial (0 hour, 300 ° C.) bonding was poor and the bonding strength could not be determined. Further, in Comparative Examples 12 to 18 in which the average particle size of the solder powder was 31.9 to 40.0 μm, the relative shear strength after holding at 300 ° C. for 500 hours was as low as 52 to 65, so that the holding time was 500 hours. All the subsequent evaluations of the bonding strength were “impossible”. On the other hand, in Examples 1 to 35 in which the average particle size of the solder powder is 1 to 30 μm, the relative shear strength is improved, and the evaluation after holding for 0 hours and 500 hours for the evaluation of the bonding strength in a high temperature atmosphere is performed. All were “good”, “good” or “excellent” and good results were obtained.

  The present invention can be suitably used for mounting electronic components that are exposed to a high-temperature atmosphere.

10 Solder powder 11 Central core 12 Coating layer

Claims (4)

In a solder powder comprising a central core and a coating layer covering the central core, wherein the central core is composed of nickel and an intermetallic compound of nickel and tin, and the coating layer is composed of tin,
The average particle diameter of the solder powder is 1 to 30 μm,
Solder powder characterized in that the nickel content is more than 3.0% by mass and 65% by mass or less with respect to 100% by mass of the total amount of the solder powder. The solder powder according to claim 1, wherein the intermetallic compound of nickel and tin is one or more compounds selected from the group consisting of Ni ₃ Sn, Ni ₃ Sn ₂ , Ni ₃ Sn ₄ and NiSn _3. .   A solder paste obtained by mixing the solder powder according to claim 1 and a solder flux into a paste. A method for mounting an electronic component using the solder paste according to claim 3 .