JPWO2020004512A1 - Solder paste - Google Patents

Abstract

The present invention relates to a solder paste containing solder particles and flux, wherein the solder particles have a flat portion on a part of the surface. By using solder particles having a flat surface on a part of the surface, a solder paste having excellent wettability and spreading property can be obtained.

Description

The present invention relates to a solder paste.

Conventionally, the use of solder particles has been studied as conductive particles to be blended in anisotropic conductive materials such as anisotropic conductive films and anisotropic conductive pastes. For example, Patent Document 1 describes a conductive paste containing a thermosetting component and a plurality of solder particles that have been subjected to a specific surface treatment.

Japanese Unexamined Patent Publication No. 2016-76494

An object of the present invention is to provide a novel solder paste.

One aspect of the present invention relates to a solder paste containing solder particles and flux, wherein the solder particles have a flat portion on a part of the surface.

In one embodiment, the average particle size of the solder particles is 1 to 30 μm, and C.I. V. The value may be 20% or less.

In one aspect, the ratio (A / B) of the diameter A of the flat surface portion to the diameter B of the solder particles may satisfy the following formula.
0.01 <A / B <1.0

In one aspect, when a quadrangle circumscribing the projected image of the solder particles is created by two pairs of parallel lines, and the distances between the opposing sides are X and Y (where Y <X), X and Y are The following formula may be satisfied.
0.8 <Y / X <1.0

In one aspect, the solder particles may comprise at least one selected from the group consisting of tin, tin alloys, indium and indium alloys.

In one embodiment, the solder particles are In-Sn alloy, In-Sn-Ag alloy, In-Bi alloy, Sn-Au alloy, Sn-Bi alloy, Sn-Bi-Ag alloy, Sn-Ag-Cu alloy and Sn-. It may contain at least one tin alloy selected from the group consisting of Cu alloys.

According to the present invention, a novel solder paste is provided. By using solder particles having a flat surface on a part of the surface, a solder paste having excellent wettability and spreading property can be obtained.

FIG. 1 is a diagram schematically showing an example of solder particles. FIG. 2 is a diagram showing distances X and Y (provided that Y ≦ X) between opposite sides when a quadrangle circumscribing a projected image of solder particles is created by two pairs of parallel lines. FIG. 3A is a plan view schematically showing an example of a substrate, and FIG. 3B is a cross-sectional view taken along the line IIIb-IIIb shown in FIG. 3A. 4 (a) to 4 (d) are cross-sectional views schematically showing an example of the cross-sectional shape of the concave portion of the substrate. FIG. 5 is a cross-sectional view schematically showing a state in which solder fine particles are contained in the recesses of the substrate. FIG. 6 is a cross-sectional view schematically showing a state in which solder particles are formed in the recesses of the substrate. FIG. 7 is an SEM image showing an example of solder particles. FIG. 8 is a diagram showing the results of evaluation of wettability.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments. Unless otherwise specified, the materials exemplified below may be used alone or in combination of two or more. The content of each component in the composition means the total amount of the plurality of substances present in the composition when a plurality of substances corresponding to each component are present in the composition, unless otherwise specified. The numerical range indicated by using "~" indicates a range including the numerical values before and after "~" as the minimum value and the maximum value, respectively. In the numerical range described stepwise in the present specification, the upper limit value or the lower limit value of the numerical range of one step may be replaced with the upper limit value or the lower limit value of the numerical range of another step. In the numerical range described in the present specification, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples.

<Solder paste>
The solder paste according to this embodiment contains solder particles and a flux. The paste may further contain a thermosetting component and other components.

(Solder particles)
The solder particles have a flat surface on a part of the surface. Details of the solder particles and the method for producing the same will be described later.

The content of the solder particles may be 1% by mass or more, 10% by mass or more, 20% by mass or more, or 30% by mass or more based on the total mass of the solder paste. .. Further, the content may be 99% by mass or less, 90% by mass or less, or 85% by mass or less based on the total mass of the solder paste. When the content of the solder particles is within the above range, it becomes easier to arrange the solder particles on the electrodes, and it becomes easier to arrange a large amount of the solder particles between the electrodes, which tends to improve the conduction reliability. ..

(flux)
Examples of the flux include compounds generally used for solder bonding and the like, for example, a mixture of zinc chloride, zinc chloride and an inorganic halide, a mixture of zinc chloride and an inorganic acid, a molten salt, phosphoric acid and phosphoric acid. Derivatives of, organic halides, hydrazines, organic acids, pine fats and the like. One of these fluxes may be used alone, or two or more of these fluxes may be used in combination.

Examples of the molten salt include ammonium chloride and the like. Examples of the organic acid include lactic acid, citric acid, stearic acid, glutamic acid, glutaric acid and the like. Examples of pine fat include activated pine fat and non-activated pine fat. More specifically, examples of pine fat include rosins containing abietic acid as a main component. The flux is preferably rosins, more preferably abietic acid. The use of these fluxes further increases the reliability of conduction between the electrodes.

The melting point of the flux may be 50 ° C. or higher, 70 ° C. or higher, or 80 ° C. or higher. Still, the melting point may be 200 ° C. or lower, 160 ° C. or lower, 150 ° C. or lower, or 140 ° C. or lower. When the melting point of the flux is within the above range, the flux effect is likely to be exhibited, and the solder particles tend to be efficiently arranged on the electrode. For example, fluxes having a melting point within the above range include succinic acid (melting point 186 ° C.), glutaric acid (melting point 96 ° C.), adipic acid (melting point 152 ° C.), pimelic acid (melting point 104 ° C.), and suberic acid (melting point 142 ° C.). Dicarboxylic acids such as ° C., benzoic acid (melting point 122 ° C.), pimelic acid (melting point 130 ° C.), and the like can be mentioned.

Further, as the flux, an organic acid having a hydroxyl group and a carboxyl group can be used. Aliphatic dihydroxycarboxylic acid is preferable from the viewpoint of efficiently removing the surface oxide film of the solder particles and giving good bonding reliability. For example, tartaric acid, 2,2-bis (hydroxymethyl) propionic acid, 2,2-bis (hydroxymethyl) butanoic acid, 2,2-bis (hydroxymethyl) pentanoic acid and the like can be mentioned.

The flux is not only contained in the solder paste, but may be attached to the surface of the solder particles.

The flux content may be 0.5% by mass or more based on the total mass of the solder paste. Further, the content may be 30% by mass or less, or 25% by mass or less, based on the total mass of the solder paste. When the flux content is within the above range, it becomes difficult for an oxide film to be formed on the surfaces of the solder and the electrode, and it is easy to effectively remove the oxide film formed on the surface of the solder and the electrode.

(Thermosetting component)
The thermosetting component can include a thermosetting compound and a thermosetting agent.
Examples of the thermosetting compound include oxetane compounds, epoxy compounds, episulfide compounds, (meth) acrylic compounds, phenol compounds, amino compounds, unsaturated polyester compounds, polyurethane compounds, silicone compounds, polyimide compounds and the like.

The content of the thermosetting compound may be 10% by mass or more, 40% by mass or more, or 50% by mass or more based on the total mass of the solder paste. Further, the content may be 99% by mass or less, 98% by mass or less, 90% by mass or less, or 80% by mass or less based on the total mass of the solder paste.

Examples of the heat curing agent include an imidazole curing agent, an amine curing agent, a phenol curing agent, a polythiol curing agent, an acid anhydride, a thermal cation initiator, a thermal radical generator and the like. One of these thermosetting agents may be used alone, or two or more thereof may be used in combination.

From the viewpoint that the solder paste can be easily cured at a low temperature, an imidazole curing agent, a polythiol curing agent, an amine curing agent and the like can be used as the thermosetting agent. From the viewpoint of storage stability, a latent thermosetting agent can be used. As the latent curing agent, a latent imidazole curing agent, a latent polythiol curing agent, a latent amine curing agent and the like can be used.

The reaction start temperature of the thermosetting agent may be 50 ° C. or higher, 70 ° C. or higher, or 80 ° C. or higher. Further, the same temperature may be 250 ° C. or lower, 200 ° C. or lower, 150 ° C. or lower, or 140 ° C. or lower. When the reaction start temperature of the thermosetting agent is within the above range, the solder particles are likely to be efficiently arranged on the electrodes. In the present specification, the reaction start temperature of the thermosetting agent refers to the temperature at which the exothermic peak starts to rise in DSC.

The content of the thermosetting compound may be 0.01 part by mass or more or 1 part by mass or more with respect to 100 parts by mass of the thermosetting compound. Further, the content may be 200 parts by mass or less, 100 parts by mass or less, or 75 parts by mass or less. When the content of the thermosetting agent is within the above range, the solder paste is easily cured, and it is difficult for excess thermosetting agent that was not involved in curing to remain after curing, and the heat resistance of the cured product is high. Tends to be.

(Other ingredients)
The solder paste may further contain an organic solvent, a thixotropy control agent, a coupling agent, a reactive diluent and the like, if necessary.

<Solder particles>
The solder particles according to this embodiment have a flat surface portion on a part of the surface. At this time, it is preferable that the surface other than the flat surface portion has a spherical crown shape. That is, the solder particles may have a flat surface portion and a spherical crown-shaped curved surface portion. When an anisotropic conductive material is obtained by using such solder particles, it is possible to realize excellent conduction reliability and insulation reliability. The reason for this will be described below.

First, when the flat surface portion of the solder particles comes into contact with the electrode, a wide contact area can be secured between the flat surface portion and the electrode. For example, when connecting an electrode formed of a material in which solder easily wets and spreads and an electrode formed of a material in which solder does not easily wet and spread, adjustment is made so that a flat portion of solder particles is arranged on the latter electrode side. Therefore, the connection between the two electrodes can be preferably performed.

Further, the large contact area between the solder particles and the electrodes makes it easy for the solder to get wet and spread. For example, as a method for wetting and spreading the solder particles arranged on the electrode (board) on the electrode, there is a method in which flux is applied to the solder particles themselves or the electrodes in advance and the solder particles are melted by reflow (heating). At this time, when the oxide film of the solder particles is thick, the flux is weak, or the like, if the contact area between the solder particles and the electrode is narrow, it is difficult for the solder to get wet and spread. On the other hand, if a flat portion is provided on a part of the surface of the solder particles, the contact area between the electrode and the solder particles becomes wide, and the solder particles tend to get wet and spread easily. This is because when the electrode and the surface of the solder particles come into contact with each other while the removal of the oxide film is in progress, the thinned oxide film is cracked and the melted solder or flux flows to remove the oxide film. It is presumed that this is because it becomes easier to proceed. As described above, when the contact area between the solder particles and the electrode is large, the contact points between the electrodes and the surface of the solder particles increase, so that the timing of wet spread becomes earlier and the wet spread becomes easier. Since the solder particles are easily wet and spread, the amount of flux can be reduced, and the occurrence of ion migration due to the residue of the flux can be suppressed.

Further, since the solder particles having a flat surface portion are comfortable to sit on, when the solder particles are arranged on the electrodes so that the flat surface portion and the electrode are in contact with each other, the stable shape of the solder particles makes it difficult for the solder particles to be displaced. That is, it is easy to prevent the solder particles from rolling out of the electrodes before reflow, causing a decrease in conduction reliability between the opposing electrodes and a decrease in insulation reliability between adjacent electrodes.

FIG. 1 is a diagram schematically showing an example of solder particles 1 according to the present embodiment. As shown in FIG. 1, the solder particles 1 have a shape in which a flat surface portion 11 having a diameter A is formed on a part of the surface of a sphere having a diameter B. From the viewpoint of achieving excellent conduction reliability and insulation reliability, the ratio (A / B) of the diameter A of the flat surface portion to the diameter B of the solder particles 1 is, for example, more than 0.01 and less than 1.0 (0.01 < A / B <1.0), and may be 0.1 to 0.9, 0.1 to 0.5, 0.1 to 0.3 or 0.1 to 0.2. The diameter B of the solder particles and the diameter A of the flat surface portion can be observed by, for example, a scanning electron microscope or the like.
Specifically, arbitrary particles are observed with a scanning electron microscope and an image is taken. From the obtained image, the diameter B of the solder particles and the diameter A of the flat surface portion are measured, and the A / B of the particles is obtained. This operation is performed on 300 solder particles to calculate an average value, which is used as the A / B of the solder particles.

In the solder particles 1, the average particle size is 1 to 30 μm, and C.I. V. The value may be 20% or less. Such solder particles have both a small average particle size and a narrow particle size distribution, and can be suitably used as conductive particles applied to an anisotropic conductive material having high conductivity reliability and insulation reliability.

The average particle size of the solder particles is not particularly limited as long as it is in the above range, but is preferably 30 μm or less, more preferably 25 μm or less, and further preferably 20 μm or less. The average particle size of the solder particles is preferably 1 μm or more, more preferably 2 μm or more, and further preferably 4 μm or more.

The average particle size of the solder particles can be measured by using various methods according to the size. For example, a dynamic light scattering method, a laser diffraction method, a centrifugal sedimentation method, an electrical detection band method, a resonance type mass measurement method, or the like can be used. Further, a method of measuring the particle size from an image obtained by an optical microscope, an electron microscope, or the like can be used. Specific devices include a flow-type particle image analyzer, a microtrack, a Coulter counter, and the like.

C.I. of solder particles. V. The value is preferably 20% or less, more preferably 10% or less, still more preferably 7% or less, and most preferably 5% or less from the viewpoint of achieving more excellent conductivity reliability and insulation reliability. In addition, C.I. V. The lower limit of the value is not particularly limited. For example, C.I. V. The value may be 1% or more, and may be 2% or more.

C.I. of solder particles. V. The value is calculated by dividing the standard deviation of the particle size measured by the method described above by the average particle size and multiplying by 100.

When a quadrangle circumscribing the projected image of solder particles is created by two pairs of parallel lines, and the distance between the opposing sides is X and Y (where Y <X), the ratio of Y to X (Y / X) may be more than 0.8 and less than 1.0 (0.8 <Y / X <1.0), and may be 0.81 to 0.99 or 0.81 to 0.95. .. Such solder particles can be said to be particles closer to a true sphere. When the solder particles are re-accommodated in the recess 62 of the substrate 60, if the solder particles are close to spherical, they tend to be easily accommodated. Further, since the solder particles are close to a true sphere, when a plurality of opposing electrodes are electrically connected via the solder particles, the contact between the solder particles and the electrodes is less likely to be uneven, and a stable connection can be obtained. Tend. Further, when a conductive film or paste in which solder particles are dispersed in a resin material is produced, high dispersibility is obtained, and dispersion stability during production tends to be obtained. Furthermore, when a film or paste in which solder particles are dispersed in a resin material is used for connection between electrodes, even if the solder particles rotate in the resin, if the solder particles have a spherical shape, when viewed in a projected image, The projected areas of the solder particles are close to each other. Therefore, when connecting the electrodes, it tends to be easy to obtain a stable electrical connection with little variation. From the above viewpoint, the roundness of the solder particles (the ratio r / R of the radii of the two concentric circles of the solder particles (the radius r of the minimum circumscribed circle and the radius R of the maximum inscribed circle)) is 0.85 or more. It may be 0.90 or more.

FIG. 2 is a diagram showing distances X and Y (provided that Y <X) between opposite sides when a quadrangle circumscribing a projected image of solder particles is created by two pairs of parallel lines. For example, an arbitrary particle is observed with a scanning electron microscope to obtain a projected image. Two pairs of parallel lines are drawn with respect to the obtained projected image, and the pair of parallel lines are arranged at the position where the distance between the parallel lines is the minimum, and the other pair of parallel lines are arranged at the position where the distance between the parallel lines is the maximum. , Find the Y / X of the particle. This operation is performed on 300 solder particles, the average value is calculated, and the Y / X of the solder particles is obtained.

The solder particles may contain tin or a tin alloy. As the tin alloy, for example, In-Sn alloy, In-Sn-Ag alloy, Sn-Au alloy, Sn-Bi alloy, Sn-Bi-Ag alloy, Sn-Ag-Cu alloy, Sn-Cu alloy and the like are used. be able to. Specific examples of these tin alloys include the following examples.
-In-Sn (In 52% by mass, Bi48% by mass, melting point 118 ° C)
-In-Sn-Ag (In 20% by mass, Sn77.2% by mass, Ag 2.8% by mass, melting point 175 ° C.)
-Sn-Bi (Sn43% by mass, Bi57% by mass, melting point 138 ° C.)
-Sn-Bi-Ag (Sn42% by mass, Bi57% by mass, Ag1% by mass, melting point 139 ° C.)-Sn-Ag-Cu (Sn96.5% by mass, Ag3% by mass, Cu0.5% by mass, melting point 217 ° C.)
-Sn-Cu (Sn99.3% by mass, Cu0.7% by mass, melting point 227 ° C)
-Sn-Au (Sn21.0% by mass, Au79.0% by mass, melting point 278 ° C)
The solder particles may contain indium or an indium alloy. As the indium alloy, for example, an In-Bi alloy, an In-Ag alloy, or the like can be used. Specific examples of these indium alloys include the following examples.
-In-Bi (In66.3% by mass, Bi33.7% by mass, melting point 72 ° C.)
-In-Bi (In33.0% by mass, Bi67.0% by mass, melting point 109 ° C)
-In-Ag (In97.0% by mass, Ag3.0% by mass, melting point 145 ° C)

The tin alloy or indium alloy can be selected according to the use of the solder particles (temperature at the time of use) and the like. For example, when solder particles are used for fusion at a low temperature, an In—Sn alloy or a Sn—Bi alloy may be adopted, and in this case, the solder particles can be fused at 150 ° C. or lower. When a material having a high melting point such as Sn-Ag-Cu alloy or Sn-Cu alloy is used, high reliability can be maintained even after being left at a high temperature.

The solder particles may contain one or more selected from Ag, Cu, Ni, Bi, Zn, Pd, Pb, Au, P and B. Among these elements, Ag or Cu may be contained from the following viewpoints. That is, when the solder particles contain Ag or Cu, the melting point of the solder particles can be lowered to about 220 ° C., and the bonding strength with the electrode is further improved, so that better conduction reliability can be obtained. It will be easier.

The Cu content of the solder particles is, for example, 0.05 to 10% by mass, and may be 0.1 to 5% by mass or 0.2 to 3% by mass. When the Cu content is 0.05% by mass or more, it becomes easy to achieve better solder connection reliability. Further, when the Cu content is 10% by mass or less, the melting point is low and the solder particles tend to have excellent wettability, and as a result, the connection reliability of the joint portion by the solder particles tends to be good.

The Ag content of the solder particles is, for example, 0.05 to 10% by mass, and may be 0.1 to 5% by mass or 0.2 to 3% by mass. When the Ag content is 0.05% by mass or more, it becomes easy to achieve better solder connection reliability. Further, when the Ag oil content is 10% by mass or less, the melting point is low and the solder particles tend to have excellent wettability, and as a result, the connection reliability of the joint portion by the solder particles tends to be good.

The use of the solder particles is not particularly limited, and for example, they can be suitably used as conductive particles for an anisotropic conductive material. In addition, applications such as ball grid array connection method (BGA connection), which is widely used for mounting semiconductor integrated circuits, are used to electrically connect electrodes, and parts such as MEMS are sealed, sealed, brazed, and high. It can also be suitably used for applications such as a spacer for controlling a sheath gap. That is, the solder particles can be used for general applications in which conventional solder is used.

<Manufacturing method of solder particles>
The method for producing the solder particles according to the present embodiment is not particularly limited, but an example of the production method will be described below. For example, the solder particles according to the present embodiment are housed in a preparatory step of preparing a substrate having a plurality of recesses and solder fine particles, a storage step of storing at least a part of the solder fine particles in the recesses of the substrate, and a recess. It can be manufactured by a method for manufacturing solder particles, which comprises a fusion step of fusing the solder fine particles to form the solder particles inside the recess. According to this manufacturing method, solder particles having a flat surface portion on a part of the surface are manufactured.

Hereinafter, a method for producing solder particles will be described with reference to FIGS. 3 to 6.

First, the solder fine particles and the substrate 60 for accommodating the solder fine particles are prepared. FIG. 3A is a plan view schematically showing an example of the substrate 60, and FIG. 3B is a cross-sectional view taken along the line IIIb-IIIb shown in FIG. 3A. The substrate 60 shown in FIG. 3A has a plurality of recesses 62. The plurality of recesses 62 may be regularly arranged in a predetermined pattern.

The recess 62 of the base 60 is preferably formed in a tapered shape in which the opening area expands from the bottom 62a side of the recess 62 toward the surface 60a side of the base 60. That is, as shown in FIG. 3, the width of the bottom portion 62a of the recess 62 (width a in FIG. 3) is preferably narrower than the width of the opening on the surface 60a of the recess 62 (width b in FIG. 3). The size of the recess 62 (width a, width b, volume, taper angle, depth, etc.) may be set according to the size of the target solder particles.

The shape of the recess 62 may be a shape other than the shape shown in FIG. For example, the shape of the opening on the surface 60a of the recess 62 may be an ellipse, a triangle, a quadrangle, a polygon, or the like, in addition to the circle as shown in FIG.

Further, the shape of the recess 62 in the cross section perpendicular to the surface 60a may be, for example, the shape shown in FIG. 4 (a) to 4 (d) are cross-sectional views schematically showing an example of the cross-sectional shape of the concave portion of the substrate. The bottom surface of each of the cross-sectional shapes shown in FIGS. 4 (a) to 4 (d) is flat. As a result, a flat surface portion is formed on a part of the surface of the solder particles. Further, in any of the cross-sectional shapes shown in FIGS. 4A to 4D, the width (width b) of the opening on the surface 60a of the recess 62 is the maximum width in the cross-sectional shape. As a result, the solder particles formed in the recess 62 can be easily taken out, and workability is improved.

As the material constituting the substrate 60, for example, an inorganic material such as silicon, various ceramics, glass, a metal such as stainless steel, and an organic material such as various resins can be used. Of these, the substrate 60 is preferably made of a heat-resistant material that does not deteriorate at the melting temperature of the solder fine particles. Further, the recess 62 of the substrate 60 can be formed by a known method such as a photolithography method, an imprint method, a machining method, an electron beam processing method, or a radiation processing method.

The solder fine particles prepared in the preparatory step may contain fine particles having a particle size smaller than the width (width b) of the opening on the surface 60a of the recess 62, and may contain more fine particles having a particle size smaller than the width b. Is preferable. For example, in the solder fine particles, the D10 particle size of the particle size distribution is preferably smaller than the width b, the D30 particle size of the particle size distribution is more preferably smaller than the width b, and the D50 particle size of the particle size distribution is smaller than the width b. More preferred.

The particle size distribution of the solder fine particles can be measured by using various methods according to the size. For example, a dynamic light scattering method, a laser diffraction method, a centrifugal sedimentation method, an electrical detection band method, a resonance type mass measurement method, or the like can be used. Further, a method of measuring the particle size from an image obtained by an optical microscope, an electron microscope, or the like can be used. Specific devices include a flow-type particle image analyzer, a microtrack, a Coulter counter, and the like.

C.I. of solder fine particles prepared in the preparation process. V. The value is not particularly limited, but from the viewpoint of improving the filling property into the recess 62 by the combination of large and small fine particles, C.I. V. The value is preferably high. For example, C.I. V. The value may exceed 20%, preferably 25% or more, more preferably 30% or more.

C.I. of solder fine particles. V. The value is calculated by dividing the standard deviation of the particle size measured by the above method by the average particle size (D50 particle size) and multiplying by 100.

The solder fine particles may contain tin or a tin alloy. As the tin alloy, for example, In-Sn alloy, In-Sn-Ag alloy, Sn-Au alloy, Sn-Bi alloy, Sn-Bi-Ag alloy, Sn-Ag-Cu alloy, Sn-Cu alloy and the like are used. be able to. Specific examples of these tin alloys include the following examples.
-In-Sn (In 52% by mass, Bi48% by mass, melting point 118 ° C)
-In-Sn-Ag (In 20% by mass, Sn77.2% by mass, Ag 2.8% by mass, melting point 175 ° C.)
-Sn-Bi (Sn43% by mass, Bi57% by mass, melting point 138 ° C.)
-Sn-Bi-Ag (Sn42% by mass, Bi57% by mass, Ag1% by mass, melting point 139 ° C.)-Sn-Ag-Cu (Sn96.5% by mass, Ag3% by mass, Cu0.5% by mass, melting point 217 ° C.)
-Sn-Cu (Sn99.3% by mass, Cu0.7% by mass, melting point 227 ° C)
-Sn-Au (Sn21.0% by mass, Au79.0% by mass, melting point 278 ° C)
The solder particles may contain indium or an indium alloy. As the indium alloy, for example, an In-Bi alloy, an In-Ag alloy, or the like can be used. Specific examples of these indium alloys include the following examples.
-In-Bi (In66.3% by mass, Bi33.7% by mass, melting point 72 ° C.)
-In-Bi (In33.0% by mass, Bi67.0% by mass, melting point 109 ° C)
-In-Ag (In97.0% by mass, Ag3.0% by mass, melting point 145 ° C)

The tin alloy or indium alloy can be selected according to the use of the solder particles (temperature at the time of use) and the like. For example, when it is desired to obtain solder particles used for fusion at a low temperature, an In—Sn alloy or a Sn—Bi alloy may be adopted. In this case, solder particles that can be fused at 150 ° C. or lower can be obtained. When a material having a high melting point such as Sn-Ag-Cu alloy or Sn-Cu alloy is used, solder particles capable of maintaining high reliability even after being left at a high temperature can be obtained.

The solder fine particles may contain one or more selected from Ag, Cu, Ni, Bi, Zn, Pd, Pb, Au, P and B. Among these elements, Ag or Cu may be contained from the following viewpoints. That is, when the solder fine particles contain Ag or Cu, the melting point of the obtained solder particles can be lowered to about 220 ° C., and the solder particles having excellent bonding strength with the electrode can be obtained, so that good conduction reliability can be obtained. The effect of being able to obtain is achieved.

The Cu content of the solder fine particles is, for example, 0.05 to 10% by mass, and may be 0.1 to 5% by mass or 0.2 to 3% by mass. When the Cu content is 0.05% by mass or more, it becomes easy to obtain solder particles capable of achieving good solder connection reliability. Further, when the Cu content is 10% by mass or less, solder particles having a low melting point and excellent wettability can be easily obtained, and as a result, the connection reliability of the joint portion by the solder particles tends to be good.

The Ag content of the solder fine particles is, for example, 0.05 to 10% by mass, and may be 0.1 to 5% by mass or 0.2 to 3% by mass. When the Ag content is 0.05% by mass or more, it becomes easy to obtain solder particles capable of achieving good solder connection reliability. Further, when the Ag oil content is 10% by mass or less, solder particles having a low melting point and excellent wettability can be easily obtained, and as a result, the connection reliability of the joint portion by the solder particles tends to be good.

In the accommodating step, the solder fine particles prepared in the preparatory step are accommodating in each of the recesses 62 of the substrate 60. The accommodating step may be a step of accommodating all the solder fine particles prepared in the preparatory step into the recess 62, and a part of the solder fine particles prepared in the preparatory step (for example, the width b of the opening of the recess 62 among the solder fine particles). It may be a step of accommodating a smaller particle) in the recess 62.

FIG. 5 is a cross-sectional view schematically showing a state in which the solder fine particles 111 are housed in the recess 62 of the substrate 60. As shown in FIG. 5, a plurality of solder fine particles 111 are housed in each of the plurality of recesses 62.

The amount of the solder fine particles 111 contained in the recess 62 is, for example, preferably 20% or more, more preferably 30% or more, still more preferably 50% or more, based on the volume of the recess 62. , 60% or more is most preferable. As a result, the variation in the accommodating amount is suppressed, and it becomes easy to obtain solder particles having a smaller particle size distribution.

The method of accommodating the solder fine particles in the recess 62 is not particularly limited. The accommodating method may be either dry or wet. For example, the solder fine particles prepared in the preparation step are placed on the substrate 60, and the surface 60a of the substrate 60 is rubbed with a squeegee to remove the excess solder fine particles while accommodating sufficient solder fine particles in the recess 62. can do. When the width b of the opening of the recess 62 is larger than the depth of the recess 62, solder fine particles may pop out from the opening of the recess 62. When a squeegee is used, the solder fine particles protruding from the opening of the recess 62 are removed. Examples of the method of removing the excess solder fine particles include a method of blowing compressed air, a method of rubbing the surface 60a of the substrate 60 with a non-woven fabric or a bundle of fibers, and the like. Since these methods have a weaker physical force than the squeegee, they are preferable for handling easily deformable solder fine particles. Further, in these methods, the solder fine particles protruding from the opening of the recess 62 can be left in the recess.

The fusion step is a step of fusing the solder particles 111 housed in the recess 62 to form the solder particles 1 inside the recess 62. FIG. 6 is a cross-sectional view schematically showing a state in which the solder particles 1 are formed in the recess 62 of the substrate 60. The solder fine particles 111 housed in the recess 62 are united by melting and spheroidized by surface tension. At this time, at the contact portion of the recess 62 with the bottom portion 62a, the molten solder follows the bottom portion 62a to form the flat surface portion 11. As a result, the formed solder particles 1 have a shape having a flat surface portion 11 on a part of the surface.

FIG. 1 is a view of the solder particles 1 viewed from the side opposite to the opening of the recess 62 in FIG.

Examples of the method of melting the solder fine particles 111 contained in the recess 62 include a method of heating the solder fine particles 111 to a temperature equal to or higher than the melting point of the solder. Due to the influence of the oxide film, the solder fine particles 111 may not melt even when heated at a temperature equal to or higher than the melting point, may not spread when wet, or may not coalesce. Therefore, the solder fine particles 111 are exposed to a reducing atmosphere to remove the surface oxide film of the solder fine particles 111, and then heated to a temperature equal to or higher than the melting point of the solder fine particles 111 to melt the solder fine particles 111 and spread them wet. It can be unified. Further, it is preferable that the solder fine particles 111 are melted in a reducing atmosphere. By heating the solder fine particles 111 to a temperature equal to or higher than the melting point of the solder fine particles 111 and creating a reducing atmosphere, the oxide film on the surface of the solder fine particles 111 is reduced, and the solder fine particles 111 are efficiently melted, wetted and spread, and unified. It becomes easier to proceed.

The method for creating a reducing atmosphere is not particularly limited as long as the above-mentioned effect can be obtained, and for example, there is a method using hydrogen gas, hydrogen radical, formic acid gas and the like. For example, by using a hydrogen reduction furnace, a hydrogen radical reduction furnace, a formic acid reduction furnace, or a conveyor furnace or a continuous furnace thereof, the solder fine particles 111 can be melted in a reducing atmosphere. These devices may be equipped with a heating device, a chamber filled with an inert gas (nitrogen, argon, etc.), a mechanism for evacuating the inside of the chamber, etc., which makes it easier to control the reducing gas. Become. Further, if the inside of the chamber can be evacuated, the voids can be removed by reducing the pressure after the solder fine particles 111 are melted and united, and the solder particles 1 having further excellent connection stability can be obtained.

Profiles such as reduction, melting conditions, temperature, and atmosphere adjustment in the furnace of the solder fine particles 111 may be appropriately set in consideration of the melting point, particle size, recess size, material of the substrate 60, and the like of the solder fine particles 111. For example, the substrate 60 in which the solder fine particles 111 are filled in the recesses is inserted into the furnace, vacuumed, and then the reducing gas is introduced to fill the inside of the furnace with the reducing gas, and the surface oxide film of the solder fine particles 111 is formed. After removing, the reducing gas is removed by vacuuming, and then the gas is heated to a temperature equal to or higher than the melting point of the solder fine particles 111 to dissolve and coalesce the solder fine particles to form the solder particles in the recess 62. After filling with nitrogen gas, the temperature inside the furnace is returned to room temperature to obtain solder particles 1. Further, for example, the substrate 60 in which the solder fine particles 111 are filled in the recesses is inserted into the furnace, and after vacuuming, the reducing gas is introduced to fill the inside of the furnace with the reducing gas, and the in-core heater is used. The solder fine particles 111 are heated to remove the surface oxide film of the solder fine particles 111, then the reducing gas is removed by vacuuming, and then the solder fine particles 111 are heated to a temperature equal to or higher than the melting point of the solder fine particles 111 to dissolve and coalesce the solder fine particles. After forming the solder particles in the recess 62, the temperature inside the furnace is returned to room temperature after filling with nitrogen gas to obtain the solder particles 1. By heating the solder fine particles in a reducing atmosphere, there is an advantage that the reducing power is increased and the surface oxide film of the solder fine particles can be easily removed.

Further, for example, the substrate 60 in which the solder fine particles 111 are filled in the recesses is inserted into the furnace, and after vacuuming, the reducing gas is introduced to fill the inside of the furnace with the reducing gas, and the in-core heater is used. The substrate 60 is heated to a temperature equal to or higher than the melting point of the solder fine particles 111 to remove the surface oxide film of the solder fine particles 111 by reduction, and at the same time, the solder fine particles are melted and united to form solder particles in the recess 62 and evacuated. After removing the reducing gas and further reducing the voids in the solder particles, the temperature in the furnace is returned to room temperature after filling with nitrogen gas to obtain the solder particles 1. In this case, since it is sufficient to adjust the temperature rise and fall in the furnace once, there is an advantage that the processing can be performed in a short time.

After forming the solder particles in the recess 62, the inside of the furnace is once again made into a reducing atmosphere to remove the surface oxide film that could not be completely removed. Residues such as a part of the oxide film remaining on the solder can be reduced.

When an atmospheric pressure conveyor furnace is used, the substrate 60 in which the solder fine particles 111 are filled in the recesses is placed on the conveyor and passed through a plurality of zones in succession to obtain the solder particles 1. For example, the substrate 60 in which the solder fine particles 111 are filled in the recesses is placed on a conveyor set at a constant speed and passed through a zone filled with an inert gas such as nitrogen or argon at a temperature lower than the melting point of the solder fine particles 111. Subsequently, the surface oxide film of the solder fine particles 111 is removed by passing through a zone in which a reducing gas such as formic acid gas having a temperature lower than the melting point of the solder fine particles 111 exists, and then nitrogen having a temperature equal to or higher than the melting point of the solder fine particles 111 or the like. Solder particles 1 are obtained by passing through a zone filled with an inert gas such as argon to melt and coalesce the solder fine particles 111, and then passing through a cooling zone filled with an inert gas such as nitrogen or argon to obtain solder particles 1. be able to. For example, the substrate 60 in which the solder fine particles 111 are filled in the recesses is placed on a conveyor set at a constant speed and passed through a zone filled with an inert gas such as nitrogen or argon having a temperature equal to or higher than the melting point of the solder fine particles 111. Subsequently, the surface oxide film of the solder fine particles 111 is removed, melted and coalesced by passing through a zone in which a reducing gas such as an inert gas having a temperature equal to or higher than the melting point of the solder fine particles 111 exists, and then nitrogen, argon or the like, etc. The solder particles 1 can be obtained by passing through a cooling zone filled with the inert gas. Since the conveyor furnace can be processed at atmospheric pressure, a film-like material can be continuously processed by roll-to-roll. For example, a continuous roll product of the substrate 60 in which the solder fine particles 111 are filled in the recesses is produced, a roll unwinder is installed on the inlet side of the conveyor furnace, and a roll winder is installed on the outlet side of the conveyor furnace to maintain a constant speed. By transporting the base 60 and passing through each zone in the conveyor furnace, the solder fine particles 111 filled in the recesses can be fused.

The formed solder particles 1 are taken out from the recess 62 and collected, and are used for preparing a solder paste. FIG. 7 is an SEM image showing an example of the solder particles thus obtained. The solder particles 1 may be transported and stored in a state of being housed in the recess 62 of the substrate 60.

According to the manufacturing method of the present embodiment, solder particles having a uniform size can be formed regardless of the material and shape of the solder fine particles. For example, indium-based solder can be precipitated by plating, but it is difficult to precipitate it in the form of particles, and it is soft and difficult to handle. However, in the production method of the present embodiment, indium-based solder particles having a uniform particle size can be easily produced by using the indium-based solder fine particles as a raw material. Further, since the formed solder particles 1 can be handled in a state of being housed in the recess 62 of the substrate 60, the solder particles can be transported and stored without being deformed. Further, since the formed solder particles 1 are simply housed in the recesses 62 of the substrate 60, they can be easily taken out, and the solder particles can be collected, surface-treated, etc. without being deformed.

Further, the solder fine particles 111 may have a large variation in particle size distribution or a distorted shape, and can be used as a raw material for the manufacturing method of the present embodiment as long as they can be accommodated in the recess 62.

Further, in the manufacturing method of the present embodiment, the shape of the recess 62 of the substrate 60 can be freely designed by a lithography method, an imprint method, an electron beam processing method, a radiation processing method, a machining method, or the like. Since the size of the solder particles 1 depends on the amount of the solder fine particles 111 accommodated in the recesses 62, the size of the solder particles 1 can be freely designed by designing the recesses 62 in the manufacturing method of the present embodiment.

Although the preferred embodiment of the present invention has been described above, the present invention is not limited to the above embodiment.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

<Manufacturing of solder particles>
[Example 1]
(Step a1: Classification of solder fine particles)
100 g of Sn-Bi solder fine particles (manufactured by 5N Plus, melting point 139 ° C., Type 8) were immersed in distilled water, ultrasonically dispersed, and then allowed to stand to recover the solder fine particles floating in the supernatant. This operation was repeated to recover 10 g of solder fine particles. The average particle size of the obtained solder fine particles was 1.0 μm, and C.I. V. The value was 42%.
(Step b1: Arrangement on the substrate)
A substrate having a plurality of recesses having an opening diameter of 1.2 μmφ, a bottom diameter of 1.0 μmφ, and a depth of 1.0 μm (the bottom diameter of 1.0 μmφ is located at the center of the opening diameter of 1.2 μmφ when the opening is viewed from the top surface). (Polyimide film, thickness 100 μm) was prepared. The plurality of recesses were regularly arranged at intervals of 1.0 μm. The solder fine particles (average particle diameter 1.0 μm, CV value 42%) obtained in step a were placed in the recesses of the substrate. By rubbing the surface side of the substrate on which the recesses were formed with a fine adhesive roller, excess solder fine particles were removed, and a substrate in which the solder fine particles were arranged only in the recesses was obtained.
(Step c1: Formation of solder particles)
The substrate in which the solder fine particles were arranged in the recesses in step b1 was placed in a hydrogen reduction furnace, evacuated, and then hydrogen gas was introduced into the furnace to fill the inside of the furnace with hydrogen. Then, after keeping the inside of the furnace at 280 ° C. for 20 minutes, the solder particles were formed by drawing a vacuum again, introducing nitrogen to return to atmospheric pressure, and then lowering the temperature inside the furnace to room temperature.

By tapping the substrate that had undergone step c1 from the back side of the recess, solder particles were recovered from the recess. The obtained solder particles were observed by the following method. Place the obtained solder particles on a conductive tape fixed to the surface of the scanning electron microscope (SEM) observation pedestal, and tap the SEM observation pedestal on a stainless steel plate with a thickness of 5 mm to spread the solder particles on the conductive tape. Spread evenly. Then, compressed nitrogen gas was sprayed on the surface of the conductive tape to fix the solder particles on the conductive tape in a single layer. The observation results are shown in FIG. FIG. 7 is an SEM image of the solder particles obtained in Example 1. As shown in the figure, the obtained solder particles had a shape in which a flat portion was formed on a part of the surface of the sphere. The solder particles obtained in the other examples also had the same shape.

<Evaluation of solder particles>
The conductive tape was attached so as to face the concave portion of the substrate that had undergone step c1 and then peeled off to obtain an observation sample in which the flat portions of the solder particles were arranged in parallel with the conductive tape surface. After Pt sputtering was applied to the surface of the observation sample on which the solder particles were arranged, SEM observation was performed. Observing 300 solder particles, the average diameter B (average particle diameter) of the solder particles, the average diameter A of the flat surface portion A, C.I. V. Values, roundness, A / B and Y / X were calculated. The results are shown in Table 1.
Roundness: The ratio r / R of the radii of two concentric circles (radius r of the minimum circumscribed circle, radius R of the maximum inscribed circle) of the solder particles.
A / B: The ratio of the diameter A of the flat surface to the diameter B of the solder particles.
Y / X: When a quadrangle circumscribing the projected image of the solder particles is created by two pairs of parallel lines, and the distance between the opposing sides is X and Y (where Y <X), Y with respect to X ratio.

[Examples 2 to 12]
Solder particles were produced in the same manner as in Example 1 except that the recess size was changed as shown in Table 1, and the solder particles were evaluated.

<Making solder paste>
[Production Example 1]
YL980 (trade name of bisphenol A type epoxy resin manufactured by Mitsubishi Chemical Co., Ltd.) 16.0 parts by mass, 2P4MHZ-PW (trade name of imidazole compound manufactured by Shikoku Kasei Co., Ltd.) 0.8 parts by mass, and flux activator The mixture was mixed with 3.2 parts by mass of adipic acid, and the mixture was passed through three rolls three times to prepare an adhesive component.

Next, 80 parts by mass of Sn-Bi solder particles having a flat surface formed on a part of the surface of the sphere produced in Example 1 was added to 20 parts by mass of the adhesive component, and the obtained mixture was planetary. The solder paste was obtained by stirring and kneading with a mixer and performing a defoaming treatment at 500 Pa or less for 10 minutes.

[Production Examples 2 to 12]
A solder paste was produced by the same method as in Production Example 1 except that Sn-Bi solder particles having a flat surface formed on a part of the surface of the sphere obtained in Examples 2 to 12 were used.

[Production Example 13]
YL980 (trade name of bisphenol A type epoxy resin manufactured by Mitsubishi Chemical Co., Ltd.) 16.7 parts by mass, 2P4MHZ-PW (trade name of imidazole compound manufactured by Shikoku Kasei Co., Ltd.) 0.8 parts by mass, and flux activator The mixture was mixed with 2.5 parts by mass of adipic acid as a compound, and the mixture was passed through three rolls three times to prepare an adhesive component.

Next, 80 parts by mass of Sn-Bi solder particles having a flat surface formed on a part of the surface of the sphere, which was produced in Example 5, was added to 20 parts by mass of the adhesive component, and the obtained mixture was used as a planet. The solder paste was obtained by stirring and kneading with a Lee mixer and performing a defoaming treatment at 500 Pa or less for 10 minutes.

[Comparative Production Example 1]
Except for the use of SnBi solder particles (“ST-3” manufactured by Mitsui Metal Co., Ltd.) that do not have such a flat surface instead of the Sn-Bi solder particles having a flat surface formed on a part of the surface of the sphere. Made a solder paste by the same method as in Production Example 1. The average particle size of the SnBi solder particles was 4 μm, and C.I. V. The value was 31%.

[Comparative Production Example 2]
Except for the use of SnBi solder particles (“Type-4” manufactured by Mitsui Metal Co., Ltd.) that do not have such a flat surface instead of the Sn-Bi solder particles having a flat surface formed on a part of the surface of the sphere. Made a solder paste by the same method as in Production Example 1. The average particle size of the SnBi solder particles was 26 μm, and C.I. V. The value was 25%.

[Comparative Production Example 3]
YL980 (trade name of bisphenol A type epoxy resin manufactured by Mitsubishi Chemical Co., Ltd.) 15.4 parts by mass, 2P4MHZ-PW (trade name of imidazole compound manufactured by Shikoku Kasei Co., Ltd.) 0.8 parts by mass, and flux activator Adipic acid (3.9 parts by mass) was mixed, and the mixture was passed through 3 rolls 3 times to prepare an adhesive component.

Next, 80 parts by mass of SnBi solder particles (“ST-3” manufactured by Mitsui Mining & Smelting Co., Ltd.) having no flat surface portion were added to 20 parts by mass of the adhesive component, and the obtained mixture was mixed with a planetary mixer. The solder paste was obtained by stirring and kneading with and defoaming at 500 Pa or less for 10 minutes.

[Comparative Production Example 4]
YL980 (trade name of bisphenol A type epoxy resin manufactured by Mitsubishi Chemical Co., Ltd.) 16.7 parts by mass, 2P4MHZ-PW (trade name of imidazole compound manufactured by Shikoku Kasei Co., Ltd.) 0.8 parts by mass, and flux activator The mixture was mixed with 2.5 parts by mass of adipic acid as a compound, and the mixture was passed through three rolls three times to prepare an adhesive component.

Next, 80 parts by mass of SnBi solder particles (“ST-3” manufactured by Mitsui Mining & Smelting Co., Ltd.) having no flat surface portion were added to 20 parts by mass of the adhesive component, and the obtained mixture was mixed with a planetary mixer. The solder paste was obtained by stirring and kneading with and defoaming at 500 Pa or less for 10 minutes.

The characteristics of the solder pastes of the above-mentioned production examples and comparative production examples were measured by the following methods. The results are summarized in Table 2.

(1) Evaluation of Adhesive Strength 0.5 mg of solder paste was applied on a silver-plated copper plate (10 mm x 15 mm, thickness: 0.20 mm), and a rectangular flat plate-shaped tin-plated copper chip (2 mm x 2 mm, After laminating (thickness 0.25 mm), a test piece was obtained by applying a heat history at 150 ° C. for 10 minutes using a hot plate. For this test piece, the shear strength at 25 ° C. was measured using a bond tester (2400 manufactured by DAGE) under the conditions of a shear rate of 500 μm / sec and a clearance of 100 μm.

(2) Temperature cycle test A rectangular flat plate-shaped thin FR4 having two adjacent copper foil lands (0.2 mm × 0.4 mm) and having a distance between the copper foil lands of 100 μm of 100 mm × 50 mm × 0.5 mm. The board was prepared. Next, the solder paste was printed on the copper foil land using a metal mask (thickness 100 μm, opening size 0.2 mm × 0.3 mm), and a small chip resistor (0.2 mm × 0.4 mm) with an electrode-to-electrode distance of 100 μm was printed. Was installed. The same thermal history as in (1) above was added to this component mounting substrate to obtain a test substrate for TCT resistance evaluation. After confirming the initial resistance of this test substrate using a simple tester, a thermal shock tester (1 cycle: hold at -55 ° C for 30 minutes, heat up to 125 ° C in 5 minutes, hold at 125 ° C for 30 minutes, -55 The temperature was lowered to ℃ in 5 minutes), and the connection resistance was measured. The evaluation of TCT resistance was based on the number of cycles showing a resistance change rate within ± 10% of the initial resistance.

(3) Insulation resistance test After measuring the insulation resistance value of the component mounting substrate manufactured by the same method as in (2) above, a migration test (temperature 60 ° C., humidity 90%, 50V application) was carried out. Evaluation of migration resistance, the leakage occurs between the electrodes, the insulation resistance value was used as an index of the test time showing the following 10 ⁹ Omega.

In Production Examples 1 to 13, good adhesive strength was shown, and good results were shown in both the temperature cycle test and the insulation resistance test.

In Comparative Preparation Examples 1 and 2, it was confirmed that the adhesive strength was remarkably lowered and the temperature cycle resistance was lowered.

In Comparative Production Example 3, although the connectivity was improved due to the increase in the flux component, the decrease in the insulating property due to the migration was confirmed.

In Comparative Fabrication Example 4, although the insulation reliability was improved by reducing the flux component, it was confirmed that the connection strength was lowered due to the lowering of the solder wettability and the temperature cycle resistance was lowered.

Comparison between Fabrication Example 5 and Comparison Fabrication Example 1 shows a difference in connection strength and connection reliability, which are affected by the wettability and spreadability of the solder particles, even though solder particles having an average particle diameter of 4 μm are used. Wet spreadability of Production Example 5 and Comparative Production Example 1 was evaluated.

A predetermined amount of solder paste is printed on a rolled copper foil (10 mm × 15 mm, thickness: 0.04 mm) using a printing metal mask (opening: 5 mm × 5 mm, thickness: 0.1 mm), and a hot plate is used. Then, a heat history of 150 ° C. for 3 minutes was added to evaluate the wet spreadability. The wett spreadability evaluation result is shown in FIG.

FIG. 8A is a diagram showing the wettability and spreading property of Production Example 5. It can be confirmed that the copper foil was quickly wetted and spread while maintaining the square shape after printing.

FIG. 8B is a diagram showing the wettability and spreading property of Comparative Production Example 1. It can be confirmed that the wet spreadability to the copper foil is poor and the wet spread spreads sparsely.

1 ... Solder particles, 11 ... Flat surface, 111 ... Solder fine particles, 60 ... Base, 60a ... Surface, 62 ... Recesses, 62a ... Bottom.

Claims (6)

Contains solder particles and flux,
A solder paste in which the solder particles have a flat portion on a part of the surface. The average particle size of the solder particles is 1 to 30 μm, and C.I. V. The solder paste according to claim 1, wherein the value is 20% or less. The solder paste according to claim 1 or 2, wherein the ratio (A / B) of the diameter A of the flat surface portion to the diameter B of the solder particles satisfies the following formula.
0.01 <A / B <1.0 When a quadrangle circumscribing the projected image of the solder particles is created by two pairs of parallel lines, and the distance between the opposing sides is X and Y (where Y <X), X and Y have the following equations. The solder paste according to any one of claims 1 to 3.
0.8 <Y / X <1.0 The solder paste according to any one of claims 1 to 4, wherein the solder particles contain at least one selected from the group consisting of tin, tin alloys, indium and indium alloys. The solder particles are In-Sn alloy, In-Sn-Ag alloy, In-Bi alloy, Sn-Au alloy, Sn-Bi alloy, Sn-Bi-Ag alloy, Sn-Ag-Cu alloy and Sn-Cu alloy. The solder paste according to claim 5, which comprises at least one tin alloy selected from the group consisting of.

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