Solder and electronic components

Abstract

To provide a solder with a reduced linear expansion coefficient without a significant change to the composition of the alloy.SOLUTION: A solder includes a Sn alloy phase and particles. The particles have a higher Young's modulus than that of the Sn alloy phase. The particles have a lower linear expansion coefficient than that of the Sn alloy phase.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to solder and electronic components.

Background Art

[0002] As a method of mounting an electronic component such as a multilayer ceramic capacitor on a substrate using solder, there are a method of surface mounting on the substrate by a reflow solder process, and a method of mounting on the substrate by joining a metal terminal for mounting to the terminal electrode of the multilayer ceramic capacitor with solder as shown in Patent Document 1. Solder is used in any method. In particular, the latter method is advantageous in that it is easy to suppress the vibration of the multilayer ceramic capacitor from being transmitted to the substrate and generating sound from the substrate.

[0003] Generally, the linear expansion coefficient of solder is higher than that of each material to be joined. This is because the melting point of solder is lower than that of each material, and generally, the lower the melting point, the greater the tendency of the linear expansion coefficient.

[0004] When two members with different linear expansion coefficients are joined by solder, the thermal stress due to thermal shock is particularly relaxed when the linear expansion coefficient of the solder is between the linear expansion coefficients of the two members. Generally, since the linear expansion coefficient of solder is large, a method of reducing the linear expansion coefficient of solder is required so that the linear expansion coefficient of the solder is between the linear expansion coefficients of the two members.

[0005] However, when the composition of the alloy constituting the solder is changed to reduce the linear expansion coefficient, the characteristics of the solder itself may change significantly, such as an increase in the melting point of the solder.

[0006] Therefore, it is required to reduce the linear expansion coefficient without significantly changing the composition of the alloy.

Prior Art Documents

Patent Documents

[0007] [Patent Document 1] Japanese Patent Publication No. 2000-182888 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] The present invention aims to provide a solder with a reduced coefficient of thermal expansion without significantly altering the alloy composition. [Means for solving the problem]

[0009] To achieve the above objective, the solder according to the present invention is Containing a Sn alloy phase and particles, The Young's modulus of the aforementioned particles is higher than that of the Sn alloy phase. The coefficient of linear expansion of the aforementioned particles is smaller than the coefficient of linear expansion of the aforementioned Sn alloy phase.

[0010] The aforementioned particles may contain one or more elements selected from C, N, and O.

[0011] The aforementioned particles may contain C.

[0012] The aforementioned particles may contain SiC as the main component.

[0013] The content ratio of the aforementioned particles may be 15% by volume or more and 40% by volume or less.

[0014] The coefficient of linear expansion of the aforementioned particles may be 10 ppm / °C or less.

[0015] The Young's modulus of the aforementioned particles may be 60 GPa or higher.

[0016] The aforementioned solder does not need to substantially contain Pb.

[0017] The electronic component according to the present invention has metal terminals joined by the solder described above. [Modes for carrying out the invention]

[0018] Hereinafter, embodiments of the present invention will be described.

[0019] The solder according to this embodiment contains a Sn alloy phase and particles. The Young's modulus of the particles is higher than that of the Sn alloy phase, and the linear expansion coefficient of the particles is smaller than that of the Sn alloy phase.

[0020] The solder according to this embodiment has a lower linear expansion coefficient compared to a solder composed only of a Sn alloy phase and containing no particles. As a result, without significantly changing the composition of the Sn alloy phase, the linear expansion coefficient of the solder can be made closer to the linear expansion coefficient of the member joined by the solder. In particular, the linear expansion coefficient of the solder can be made between the linear expansion coefficient of a generally used metal terminal and the linear expansion coefficient of an electronic component.

[0021] When the Young's modulus of the particles is less than or equal to that of the Sn alloy phase, even if the linear expansion coefficient of the particles is smaller than that of the Sn alloy phase, the linear expansion coefficient of the solder containing the Sn alloy phase and the particles is difficult to change. This is because when the solder is used, the particles with a small Young's modulus are deformed due to the deformation of the solder.

[0022] There is no particular limitation on the Young's modulus of the particles. For example, it may be 60 GPa or more. It is preferably 100 GPa or more, and more preferably 300 GPa or more. There is no particular limitation on the linear expansion coefficient of the particles. For example, it may be 10.0 ppm / °C or less. It is preferably 7.0 ppm / °C or less.

[0023] When containing a plurality of types of particles with different Young's moduli, a value obtained by weighted-averaging the Young's moduli of the respective particles according to the volume ratio of each particle may be used as the Young's modulus of the above particles.

[0024] When containing a plurality of types of particles with different linear expansion coefficients, a value obtained by weighted-averaging the linear expansion coefficients of the respective particles according to the volume ratio of each particle may be used as the linear expansion coefficient of the above particles.

[0025] There is no particular limitation on the method for measuring the Young's modulus, Poisson's ratio, and coefficient of linear expansion of the Sn alloy phase contained in the solder. For example, the Young's modulus, Poisson's ratio, and coefficient of linear expansion of an alloy having the same composition as the Sn alloy phase contained in the solder may be measured by a well-known method.

[0026] There is no particular limitation on the method for measuring the Young's modulus, Poisson's ratio, and coefficient of linear expansion of the particles contained in the solder. Hereinafter, an example of the method for measuring the Young's modulus, Poisson's ratio, and coefficient of linear expansion of the particles will be described.

[0027] Prepare a cylinder having the same composition as the particles contained in the solder and having two opposing surfaces that are substantially parallel. Measure the Young's modulus, Poisson's ratio, and coefficient of linear expansion of the cylinder by a well-known method. The Young's modulus, Poisson's ratio, and coefficient of linear expansion of the cylinder obtained by the measurement can be regarded as the Young's modulus, Poisson's ratio, and coefficient of linear expansion of the particles contained in the solder.

[0028] There is no particular limitation on the composition of the Sn alloy phase, and generally, any composition of the Sn alloy used as solder may be used. For example, JIS Z 3282:2017 describes the compositions of Sn-Pb-based, Sn-Pb-Bi-based, and Sn-Pb-Ag-based as the compositions of lead-containing solders. Further, as the compositions of lead-free solders, Sn-Sb-based, Sn-Cu-based, Sn-Cu-Ni-based, Sn-Ag-Cu-In-based, Sn-Ag-based, Sn-Cu-Ag-P-Ga-based, Sn-Ag-Cu-based, Sn-Ag-Cu-Ni-Ge-based, Sn-Bi-Cu-In-based, Sn-Ag-Cu-based, Sn-Cu-Ni-P-Ga-based, Sn-Ag-Bi-Cu-based, Sn-Bi-Ag-Cu-In-based, Sn-Bi-Ag-Cu-based, Sn-In-Ag-Bi-based, Sn-Zn-based, Sn-Zn-Bi-based, Bi-Sn-based, and Sn-In-based compositions are described. For example, the composition may be Sn-Sb-based or Sn-Ag-Cu-based.

[0029] In consideration of the environmental aspect, it is preferable that the solder does not substantially contain Pb. This means that the Pb content is 0.10 mass% or less.

[0030] There are no particular restrictions on the type of particles. For example, the particles may contain one or more selected from C, N, and O, or they may contain C. In other words, the particles may contain one or more compounds selected from carbides, nitrides, and oxides, or they may contain carbides. The strength tends to be higher compared to when the particles are metal particles. This is because when the particles are metal particles, the metal particles may react with the Sn alloy to form intermetallic compounds, in which case the strength of the solder tends to decrease. Furthermore, it is particularly preferable when the particles contain C, i.e., carbides, because the Young's modulus of carbides is larger than that of nitrides and oxides, and the coefficient of linear expansion of carbides is smaller than that of nitrides and oxides.

[0031] The particles may contain SiC, AlN, Al2O3, and / or BaTiO3 as their main components, or they may contain SiC. For example, containing SiC as the main component means that the SiC content in the total particles is 50% by mass or more.

[0032] There are no particular restrictions on the particle content in solder. It may be 6% by volume or more, 10% by volume or more, or 15% by volume or more. It may also be 40% by volume or less, or 35% by volume or less. There are no particular restrictions on the method for measuring the particle content in solder. For example, first, the mass ratio of the constituent elements of the solder is measured. The mass ratio of the constituent elements of the solder can be measured by using an electron probe microanalyzer (EPMA) or the like on a cross-section of the solder containing the Sn alloy phase and particles. Then, the particle content in the solder can be calculated as a volume ratio from the solder density and particle density. Note that changing the particle content in solder does not significantly change the melting point of the solder.

[0033] There are no particular restrictions on the average particle size of solder. For example, it may be between 0.3 μm and 20 μm. There are no particular restrictions on the method for measuring the average particle size of solder. For example, first, a cross-section of the solder is photographed using a scanning electron microscope or the like. At this time, the cross-section of the solder is photographed at a magnification that allows for the measurement of the circular area equivalent diameter of each particle in the solder. Then, the circular area equivalent diameter of each particle in the solder is analyzed using image analysis software or the like on the image obtained from the photograph. By calculating the circular area equivalent diameter for 50 or more particles and averaging them, the average particle size of the particles in the solder can be measured.

[0034] There are no particular restrictions on the type and shape of the electronic components according to this embodiment. Examples include multilayer ceramic capacitors.

[0035] A multilayer ceramic capacitor generally consists of a main element and a pair of external electrodes. The main element has a structure in which dielectric layers and internal electrode layers are alternately stacked along the stacking direction.

[0036] Generally, the dielectric material accounts for the largest volume ratio in a multilayer ceramic capacitor. Therefore, the coefficient of thermal expansion of a multilayer ceramic capacitor tends to be close to that of the dielectric material. The main component of the dielectric material most commonly used in multilayer ceramic capacitors is BaTiO3. The coefficient of thermal expansion of BaTiO3 is 9.4 ppm / °C. Therefore, the coefficient of thermal expansion of a multilayer ceramic capacitor tends to be close to 9.4 ppm / °C.

[0037] One method for mounting multilayer ceramic capacitors onto a substrate is to directly surface mount the multilayer ceramic capacitor onto the substrate. Another method involves attaching mounting terminal electrodes to the external electrodes of the multilayer ceramic capacitor, and then mounting the multilayer ceramic capacitor with the terminal electrodes onto the substrate.

[0038] When directly surface-mounting multilayer ceramic capacitors onto a substrate, one method is to use the reflow soldering process. Alternatively, when joining terminal electrodes for mounting to the external electrodes of a multilayer ceramic capacitor, solder can be used to connect the external electrodes and the terminal electrodes. In either case, the aforementioned solder can be used.

[0039] When using the above-mentioned solder to join the external electrode and the terminal electrode, there are no particular restrictions on the material of the external electrode. For example, resin electrodes can be used. There are no particular restrictions on the material of the metal terminal. For example, metals mainly composed of Fe (e.g., stainless steel) or metals mainly composed of Cu (e.g., phosphor bronze) can be used. There are also no particular restrictions on the thickness of the solder. For example, it may be 1 μm or more and 20 μm or less.

[0040] There are no particular limitations on the method for producing the solder according to this embodiment. For example, it can be produced by mixing solder balls and particles of a tin alloy.

[0041] First, prepare Sn alloy solder balls, flux, and particles, and weigh them to achieve the desired composition. Next, mix the Sn alloy solder balls, flux, and particles to obtain solder paste. Then, melt the solder paste to produce the solder according to this embodiment. There are no particular restrictions on the type of flux; well-known fluxes can be used. [Examples]

[0042] The present invention will be specifically described below based on examples.

[0043] (Physical properties of Sn alloys) Solder balls of Sn alloys A and B, as shown in Table 1, were prepared.

[0044] The Young's modulus, Poisson's ratio, coefficient of thermal expansion, and melting point of Sn alloys A and B were measured. Cylindrical samples for measuring Young's modulus and Poisson's ratio, and cylindrical samples for measuring the coefficient of thermal expansion were prepared by melting solder balls of Sn alloys A and B and then casting them. Young's modulus and Poisson's ratio were measured using the ultrasonic method. The coefficient of thermal expansion was measured using a thermomechanical analyzer (TMA). The results are shown in Table 1. In addition, the melting points of Sn alloys A and B were measured using a thermogravimetric differential thermal analyzer (TG-DTA) after preparing solder balls of Sn alloys A and B. The melting point of Sn alloy A was 245°C, and the melting point of Sn alloy B was 217°C.

[0045] (Particle properties) Particles A through D, as shown in Table 1, were prepared. The average particle size of particles A through D was 3 μm for all of them.

[0046] The Young's modulus, Poisson's ratio, and coefficient of thermal expansion of particles A to D were measured. First, cylinders (hereinafter sometimes simply referred to as cylindrical samples) with the same composition as each particle and two opposing faces that are approximately parallel were prepared.

[0047] Specifically, for each composition, cylindrical samples were prepared for measuring Young's modulus and Poisson's ratio, and for measuring the coefficient of thermal expansion. The Young's modulus, Poisson's ratio, and coefficient of thermal expansion were then measured using the same method as for Sn alloys A and B. The results are shown in Table 1.

[0048] (Physical properties of metal terminals) Metal terminals A and B, as shown in Table 1, were prepared. The Young's modulus, Poisson's ratio, and coefficient of thermal expansion of metal terminals A and B were measured. First, cylindrical samples made of the same material as metal terminals A and B were prepared for measuring Young's modulus and Poisson's ratio, and a cylindrical sample was prepared for measuring the coefficient of thermal expansion. Then, the Young's modulus, Poisson's ratio, and coefficient of thermal expansion were measured in the same manner as for Sn alloy A and B. The results are shown in Table 1. The material of metal terminal A is phosphor bronze (C5212), and the material of metal terminal B is stainless steel (SUS304).

[0049] (Physical properties of glass composite substrates) CEM-3 was prepared as the glass composite substrate. The coefficient of linear expansion of the glass composite substrate was then measured using a thermomechanical analyzer (TMA). In Experimental Example 2, described later, the direction of measurement of the coefficient of linear expansion was made the same as the longitudinal direction of the multilayer ceramic capacitor. The results are shown in Table 1.

[0050] The Young's modulus of the glass composite substrate was measured using a tensile test. The Poisson's ratio of the glass composite substrate was calculated from the values ​​obtained by performing a tensile test with an orthogonal strain gauge attached to the center of the substrate. The results are shown in Table 1.

[0051] Furthermore, in the composition of the Sn alloy and the metal terminals, the percentage of elements for which no numerical value is listed represents the actual remainder.

[0052] [Table 1]

[0053] (Experimental Example 1) Solder samples were prepared for each sample by adding the particles shown in Table 2 to the Sn alloys shown in Table 2. The specific procedure is described below.

[0054] Particles A to D with the compositions shown in Table 1 were prepared.

[0055] The preparation of Sn alloys A and B and the addition of particles A to D were carried out simultaneously. Specifically, solder balls of Sn alloys A and B with the compositions shown in Table 1 were prepared. Then, solder balls of Sn alloys A and B, flux, and particles A to D were added and mixed in appropriate amounts to prepare solder paste. The amount of particles A to D added was set so that the particle content in the solder obtained when the solder paste was melted was the value shown in Table 2. The type of flux used was rosin. The amount of flux added was set so that the flux content in the solder obtained when the solder paste was melted was 5% by mass. In sample number 10, the volume ratio of particles A, B, and C was set to 1:0.5:0.5.

[0056] The linear expansion coefficient of each obtained solder sample was measured using a thermomechanical analyzer (TMA) in the same manner as the linear expansion coefficients of Sn alloys A and B. The results are shown in Table 2. A linear expansion coefficient of 20.0 ppm / °C or less was considered good, and 18.2 ppm / °C or less was considered even better.

[0057] The melting points of the solder obtained for each sample were measured using a thermogravimetric differential thermal analyzer (TG-DTA) in the same manner as the melting points of Sn alloys A and B. It was confirmed that the melting points of all samples remained unchanged from those of the Sn alloys in Table 1 without the addition of particles.

[0058] Next, thermal shock tests and tensile strength tests were conducted.

[0059] First, a multilayer ceramic capacitor with dimensions of 5.7 mm × 5.0 mm × 2.0 mm and a dielectric material whose main component is BaTiO3 was prepared. Next, two metal terminals of the types shown in Table 2 were fixed to the resin electrodes, which are the external electrodes of the multilayer ceramic capacitor, using the solder of each sample shown in Table 2.

[0060] A multilayer ceramic capacitor with two metal terminals fixed to it was placed on a ceramic plate and subjected to a thermal shock test. The thermal shock test conditions were a temperature range of -55°C to 125°C and a cycle count of 200.

[0061] Tensile strength tests were performed on multilayer ceramic capacitors after thermal shock testing. The tensile strength tests were conducted using a universal material testing machine. Specifically, a jig was attached to each of the two metal terminals fixed to the multilayer ceramic capacitor after thermal shock testing. The strength at which the jig broke when pulled was measured. The results are shown in Table 2. A tensile strength of 29.0 N or higher was considered good, 30.0 N or higher was considered even better, and 32.0 N or higher was considered particularly good.

[0062] [Table 2]

[0063] Table 2 shows that when using the solders from each example in which the coefficient of linear expansion was reduced by adding specific particles to the Sn alloy, the tensile strength after the thermal shock test improved. Samples 4 to 12, in which the coefficient of linear expansion of the solder was lower than that of the metal terminals, showed particularly improved tensile strength after the thermal shock test. This is thought to be because the coefficient of linear expansion of the solders from samples 4 to 12 fell between the coefficient of linear expansion of the dielectric used in the multilayer ceramic capacitor and the coefficient of linear expansion of the metal terminals, thus easing the stress generated between the multilayer ceramic capacitor and the metal terminals during thermal shock.

[0064] (Experimental Example 2) Solder samples for each of the materials shown in Table 3 were prepared using the same method as in Experimental Example 1.

[0065] Next, thermal shock tests and adhesion strength tests were conducted.

[0066] First, we prepared multilayer ceramic capacitors with dimensions of 3.2 mm × 1.6 mm × 1.6 mm and a dielectric material whose main component is BaTiO3, as well as substrates of the types shown in Table 3. Then, using the solder of each sample shown in Table 3, we fixed and mounted the resin electrodes, which are the external electrodes of the multilayer ceramic capacitors, onto the substrates using a reflow soldering process. We ensured that the direction of measurement for the linear expansion coefficient of the substrate was the same as the longitudinal direction of the multilayer ceramic capacitor.

[0067] Next, a thermal shock test was performed on the multilayer ceramic capacitors that were fixed and mounted on a circuit board. The thermal shock test conditions were a temperature range of -55°C to 125°C and a cycle count of 200.

[0068] A bonding strength test was performed on multilayer ceramic capacitors after thermal shock testing. The bonding strength test was conducted using a universal material testing machine. Specifically, the strength at which a jig was applied to the side of a multilayer ceramic capacitor mounted on a substrate and pressed was measured. The results are shown in Table 3. A bonding strength of 110 N or higher was considered good.

[0069] [Table 3]

[0070] Table 3 shows that when solder with a reduced coefficient of thermal expansion due to the addition of specific particles to the Sn alloy was used, the bonding strength after thermal shock testing improved. This is thought to be because the coefficient of thermal expansion of the solder in each example fell between the coefficient of thermal expansion of the dielectric used in the multilayer ceramic capacitor and the coefficient of thermal expansion of the substrate, thus easing the stress generated between the multilayer ceramic capacitor and the substrate during thermal shock.

Claims

1. Containing Sn alloy phase and particles, The Young's modulus of the aforementioned particles is higher than that of the Sn alloy phase. The linear expansion coefficient of the aforementioned particles is smaller than that of the linear expansion coefficient of the Sn alloy phase. The aforementioned particles contain SiC as the main component, The content ratio of the aforementioned particles is 15% by volume or more and 40% by volume or less. The linear expansion coefficient of the aforementioned particles is 10.0 ppm / °C or less. The solder having a Young's modulus of 60 GPa or more, Solder having a coefficient of thermal expansion of 14.1 ppm / °C or higher and 19.9 ppm / °C or lower.

2. The solder according to claim 1, which is substantially free of Pb.

3. An electronic component having metal terminals joined by solder, as described in claim 1 or 2.

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