JPH1192727A - Conductive adhesive - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a cured adhesive which has high dispersibility of conductive particles, creates large volume shrinkage in the course of curing, causes relatively high frequency of contact among nickel particles, and has lowered volume resistivity by incorporating nickel particles and particles having a lipophilic surface into a resin binder. SOLUTION: The particles having a lipophilic surface contain particles of a metal which is base relative to silver (e.g. Mn, Cr, or Fe). The surface of the nickel particles have protrusions which increase the frequency of contact among the particles and enhance the conductivity. E.g. the surface of a four-legged zinc oxide whisker is coated with nickel. A resin having high adhesive strength and high heat resistance, e.g. epoxy or acrylic resin, is selected as the resin binder from thermosetting resins. The particle diameter of the nickel particles is 3 to 20 μm, and the particle diameter of the base metal particles is 3 to 15 μm. The mixing ratio of the particles having a lipophilic surface to the nickel particles is 0.05 to 0.15. The total volume of both the particles is 0.25 to 0.40 based on the total volume of the adhesive.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a conductive adhesive used for bonding electronic components to a printed circuit board instead of solder in the field of electronic component mounting.

[0002]

2. Description of the Related Art Recently, awareness of environmental issues has increased,
In the field of electronics mounting, regulations on lead in solder alloys are about to be enforced, and there is an urgent need to establish a joining technology that does not use lead for mounting electronic components. Lead-free mounting techniques mainly include lead-free solder and conductive adhesives. Attention has been paid to conductive adhesives, which are expected to have advantages such as flexibility of bonding portions and lower mounting temperatures.

A conventional conductive adhesive is generally one in which conductive particles are dispersed in a resin-based adhesive component. After connecting electrodes with the adhesive, the resin is cured, and the particles are brought into contact with each other. , To ensure conduction of the connecting portion. Therefore, since the joint is bonded with a resin, it has a merit that the joint can be flexibly responded to deformation due to heat or external force, and a crack is less likely to occur in the joint as compared with a solder in which the conductive part is an alloy. Therefore, it is expected as a substitute material for solder.

[0004]

The filler metals generally used as the conductive particles of the conductive adhesive include silver and nickel. For example,
As disclosed in Japanese Patent No. 53234, a large number of conductive adhesives using silver have been reported. However, silver is liable to cause migration, so that it is difficult to put it into practical use as a solder substitute material. was there.

On the other hand, as disclosed in JP-A-3-40899, a conductive adhesive using nickel does not cause migration, and is therefore most expected as a solder substitute material. However, the conductive adhesive using nickel has an extremely high volume resistance value as compared with the adhesive using silver, and an improvement for lowering the volume resistance value has been desired. Therefore, an object of the present invention is to provide a nickel-based conductive adhesive having a lower volume resistivity.

[0006]

In order to solve the above-mentioned problems, the conductive adhesive of the present invention is characterized in that the resin binder contains lipophilic particles together with nickel particles.

In the present invention, the particles to be added together with the nickel particles are made of a substance having a lipophilic surface because the resin of the binder is an oleaginous substance. The reason for this is that, by adding particles having a surface lipophilic property, the particles are uniformly dispersed in the binder. By kneading the nickel particles into the resin binder together with the surface lipophilic particles, the dispersibility of the conductive particles in the resin binder becomes higher than when only the nickel particles are used as the conductive particles, and the adhesive accordingly. Since the volume shrinkage during the curing process increases, the frequency of contact between nickel particles becomes relatively high, and after the adhesive is cured, the volume resistance value of the adhesive can be reduced as compared with the case of only nickel particles. is there.

In the present invention, first, the surface lipophilic particles include solid resin particles. The resin particles have high wettability with the resin binder and enhance the dispersibility of the conductive particles in the resin, thereby increasing the dispersibility of the nickel particles.

In the present invention, second, the particles whose surface is lipophilic include silver-based base metal particles. Here, the term “base metal” refers to a metal that is electrochemically base on the basis of silver, contains silver, and has a greater ionization tendency than silver, but includes alkali metals, alkaline earth metals, and Ni. Not specified. Such a base metal disperses the base metal particles in the resin binder by utilizing the fact that a natural oxide film is easily formed on the surface and the oxygen content of the oxide film of the base metal increases the wettability to the resin binder. And the dispersibility of nickel is also improved accordingly.

Third, the conductive adhesive of the present invention is characterized in that the nickel particles have projections on the surface. In the nickel particles having projections on the surface, a large number of nickel particles first come into contact with the respective projections, thereby increasing the frequency of contact between the particles and increasing the conductivity. Further, the surface-projecting nickel particles have an effect of increasing the frequency of conduction with the base metal particles by coexistence with the surface oxidizable base metal particles.

The base metal particles, which act as an insulating material due to the oxide film on spherical nickel particles, also act as a conductive material on the protruding nickel particles. For this reason, the volume resistance of the adhesive can be further reduced due to the coexistence of the protruding nickel particles and the surface oxidizable metal particles. In the present invention, as the nickel particles having projections on the particle surface, those obtained by coating the surface of a quadruped zinc oxide whisker with nickel can be used. As shown schematically in FIG. 1, such quadruped nickel particles have the protrusions 11 of the nickel particles 1 in the resin binder 3 in point contact with the oxide film 21 of the base metal particles 2. It penetrates mechanically and comes into contact with the internal metal 20 of the nickel particles 2, that is, metal nickel, and conducts, and the base metal particles also contribute to conductivity.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The conductive adhesive of the present invention is described below.
This will be described with reference to the drawings. In the present invention, the first
Is to knead resin particles as nickel particles and surface lipophilic particles in a resin binder to obtain an adhesive paste. The resin particles are not particularly limited, but among them, those having high wettability with a resin binder are preferable, and from this point, the same type of resin as the resin binder is preferably used for the resin particles. As the resin binder used in the present invention, a resin having a high adhesive strength and particularly a high heat resistance is selected from thermosetting resins. In particular, a resin such as an epoxy resin or an acrylic resin is used. As the resin particles, corresponding to the resin binder, solid particles of the same type of resin, for example, the above-mentioned epoxy-based or acrylic-based resin can be used.

As base metal particles having a lipophilic surface, metal particles having a greater ionization tendency than silver are widely available. Examples of such metals include Mn, Cr,
Examples include, but are not limited to, Fe, Co, Zn, Cd, Sn, Pb, Cu, and Ag. However, alkali metals, alkaline earth metals, and nickel are excluded. In particular, copper, zinc, tin, and iron are preferably used as the base metal.

These base metals are apt to form an oxide film on the surface in the air at room temperature, and a natural oxide film formed by oxidation of the surface of the metal particles in the air is used as the oxide film. Is done. In the present invention, the thickness of the oxide film may be a thickness enough for the oxide film to increase the wettability to the resin binder. If such an oxide film is secured, as will be described later, when nickel particles having protrusions on the surface are applied, the wettability to the resin binder is ensured, and at the same time, the protrusions of the nickel particles are formed at the tips of the protrusions. This facilitates contact and destruction of the coating, which can ensure conduction between the nickel particles and the inside of the base metal.

The oxide film of the base metal particles may be further thickened. For this purpose, the base metal particles may be formed by subjecting the surface-cleaned metal particles to a heat treatment at room temperature or higher in advance to oxidize the surface. In the heat treatment, an oxide film is formed and grown by heating at an appropriate temperature of room temperature or higher, for example, at a temperature of about 50 to 200 ° C. in air or an appropriate oxidizing atmosphere. By the oxide film formed and grown in this manner, the dispersibility of the conductive particles in the resin is further improved, and the degree of reduction in the volume resistance value can be increased.

In the conductive adhesive of the present invention, the shape of the nickel particles is not particularly limited, but usually, granular or spherical particles can be used. As the nickel particles, those having projections outward on the surface are used.

As the nickel particles having such projections, those obtained by coating the surface of a quadruped zinc oxide whisker with a nickel layer can be used. The quadruped zinc oxide has needles in which zinc oxide crystals radially extend from a center position in four directions, and the intervals between the needles are equiangularly oriented.

A nickel film is formed on the surface of such zinc oxide crystal particles by a vapor deposition method or other methods to obtain nickel particles. Although the Ni film on the zinc oxide provides conductivity, the ridges of the zinc oxide whiskers are sharp and can more effectively penetrate the thin oxide film of the base metal particles,
The volume resistivity can be further reduced as compared with the case of a conductive adhesive using granular or surface-protruding nickel particles.

On the other hand, the shape of the base metal particles is granular,
A spherical shape is better. Spherical particles have the smallest specific surface area among many shapes and have excellent wettability with resin,
By kneading the nickel particles of the conductive particles together with the base metal particles of the spherical particles into the resin binder, the dispersibility of the conductive particles in the resin is most excellent. Is the largest.

The conductive adhesive of the present invention preferably has a particle size of 3 to 20 μm as nickel particles and a particle size of 3 to 15 μm as base metal particles. In the case where the particle diameter has the above condition, the dispersibility of the conductive particles in the resin is most improved, and the volume resistance value is lowest.

In the present invention, the mixing ratio of the surface lipophilic particles to the nickel particles is preferably a ratio of the surface lipophilic particle volume to the nickel particle volume,
It is adopted in the range of 0.05 to 0.15. If the volume ratio is less than 0.05, improvement in dispersibility of the conductive particles in the resin cannot be expected, and the degree of decrease in the volume resistance value is unfavorably small.

If the volume ratio exceeds 0.15, the amount of the base metal particles is too large, the probability of contact between the base metal particles increases, and the frequency of contact between the nickel particles decreases.
It is not preferable because the degree of decrease in the volume resistance value becomes small. In particular, when a base metal such as copper or silver is used as the surface lipophilic particles, the weight ratio of the base metal particles to the nickel particles is determined in accordance with the range of the volume ratio,
It is preferred to be in the range of 0.05 to 0.15.

As for the content of the surface lipophilic particles and the nickel particles in the adhesive, the ratio of the total of the volume of the surface lipophilic particles and the volume of the nickel particles to the total volume of the adhesive is 0. .25 to 0.40. If this volume ratio is less than 0.25, the frequency of mutual contact of nickel particles in the adhesive decreases, and if this volume ratio exceeds 0.40, the adhesive paste has too high a viscosity,
It becomes difficult to handle. In particular, copper,
When a base metal such as silver is used, the above volume ratio of 0.1 is used.
It is possible to adjust the composition so that the total weight of the nickel particles and the base metal particles is 0.75 to 0.85 in terms of the weight ratio to the total weight of the adhesive corresponding to 25 to 0.40. preferable.

As described above, the conductive adhesive of the present invention is prepared by kneading nickel particles and surface lipophilic particles in a resin binder, preparing a paste, and forming an electrode on a substrate and an electrode of an electronic component. Used for adhesive fixation. This conductive adhesive has a remarkably low volume resistance value as compared with a conventional conductive adhesive using only nickel as conductive particles, and can be used as a highly practical conductive adhesive.

[0025]

Next, the conductive particles of the present invention will be described with reference to examples. First, the constituent materials of the adhesive used in the following examples are shown below.

A resin binder; epoxy / acrylic resin (Nippon Loctite Co., Ltd.)
Manufacturing, product name "Loctite 3016")

B Nickel particles; ・ Spherical particles; three types having average particle diameters of 1 μm, 5 μm, and 30 μm (manufactured by Inco SP Co.) ・ Protruding particles; average particle diameter of 5 μm (New Metals End Chemicals Corporation) ・ Legged particles; zinc oxide whiskers with an average length of 5 μm (manufactured by Matsushita Amtech Co., Ltd.) Whisker particles whose surface is electrolessly plated with nickel

C Particles of lipophilic substance on the surface; Resin particles; Spherical epoxy particles (average particle size: 5 μm) Base metal particles; Copper particles; Spherical particles; Two types having average particle sizes of 1 μm and 5 μm (manufactured by Kyoto Elex) 20 μm diameter (New Metals En Chemicals Corporation) Scaly particles; average particle size 5 μm ・ Silver particles: spherical particles with average particle size 5 μm (manufactured by Fukuda Metal Foil & Powder Co., Ltd.)

As an example, as described below, a paste-like conductive adhesive was prepared by kneading nickel particles and surface lipophilic particles in a resin binder from the above-mentioned materials in the composition shown in Table 1.

In the comparative example, only a spherical nickel particle having a particle diameter of 5 μm was kneaded with an epoxy / acrylic resin as a resin binder to prepare a paste-like conductive adhesive.

The conductive adhesives of Examples and Comparative Examples were measured for volume resistance and cure shrinkage by the methods described below.

(Method of Measuring Volume Resistance) The method of measuring volume resistance is schematically shown in FIG. 2, and each of the preparations is performed so as to cross the silver-palladium electrodes 5 and 6 provided on the ceramic substrate 4. The obtained conductive adhesive 7 was printed using a metal plate. This is heat-treated at 150 ° C. for 30 minutes to completely cure the adhesive, and then the resistance between the electrodes 5 and 6 is measured by a four-terminal method, the thickness of the adhesive is measured, and the volume resistance is measured. Was determined by calculation. Table 1 shows the measurement results of the volume resistance value.

[0033]

[Table 1]

(Measurement Method of Curing Shrinkage Ratio) In the above-described volume resistance measuring method, the size of the cross-sectional area of the adhesive before and after curing is measured by using a non-contact type surface roughness meter (manufactured by Rodenstock) and cured. The volume shrinkage was calculated. Table 2 shows the measurement results of the cure shrinkage of the adhesive for some examples.

[0035]

[Table 2]

The contents of the embodiment will be described in detail below with reference to Tables 1 and 2. (Example 1) As conductive particles, spherical nickel particles having an average particle size of 5 µm and spherical epoxy particles having an average particle size of 5 µm were prepared at a weight ratio of 7: 1.
A conductive adhesive was prepared by kneading an acrylic resin such that the weight of the conductive particles was 80% of the total weight of the adhesive. A lower volume resistance was obtained as compared with the comparative example.

Example 2 In place of the copper particles in place of the epoxy particles in Example 1, only conventional spherical nickel particles having a particle size of 5 μm were used as conductive particles, and the other points were the same as in Example 1. To prepare a conductive adhesive. As compared with the comparative example, a lower volume resistance was obtained.

Example 3 A conductive adhesive was prepared in the same manner as in Example 2 except that the copper particles in Example 2 were heat-treated at 100 ° C. for 3 hours in advance. Compared with the comparative example and the example 1, a lower volume resistance was obtained.

(Example 4) In place of the spherical nickel particles in Example 2, spherical nickel particles having projections having an average particle diameter of 5 µm were used as conductive particles, and the other points were the same as in Example 2. Thus, a conductive adhesive was prepared. With this adhesive, a lower volume resistance was obtained as compared with Comparative Example and Example 2.

(Example 5) In place of the spherical nickel particles in Example 2, tetrapod-shaped zinc oxide particles having an average particle diameter of 5 µm were subjected to electroless plating of nickel. In the same manner as in Example 2, a conductive adhesive was prepared. This adhesive provided the lowest volume resistivity in the series of examples.

Example 6 A conductive adhesive was prepared in the same manner as in Example 2 except that the shape of the copper particles in Example 2 was changed to a scaly shape. This adhesive had a lower volume resistance than the conventional comparative example using only spherical nickel particles having a particle size of 5 μm as conductive particles.
The volume resistance was higher than those of Examples 1 and 2.

Example 7 A conductive adhesive was used in the same manner as in Example 2 except that the average particle diameter of nickel particles in Example 2 was changed to 5 μm and the average particle diameter was changed to 1 μm. Prepared. In this adhesive, since the particle size of the nickel particles was smaller than 3 μm, a lower volume resistance was obtained than the conventional comparative example using only spherical nickel particles having a particle size of 5 μm as the conductive particles. It was higher than 1.

Example 8 Except that the average particle size of nickel particles in Example 2 was changed from 5 μm to 25 μm,
A conductive adhesive was prepared in the same manner as in Example 2. This adhesive obtained a lower volume resistance than the conventional comparative example using only spherical nickel particles having a particle size of 5 μm as conductive particles, but the particle size of the nickel particles was larger than 20 μm. Volume resistance was higher than 2.

(Example 9) In this example, the average particle diameter of copper particles was changed to 1 μm instead of the average particle diameter of copper particles of 5 μm in Example 2, except that the average particle diameter of copper particles was changed to 5 μm. A conductive adhesive was prepared in the same manner as in Example 2.
This adhesive had a lower volume resistance than the conventional comparative example using only spherical nickel particles having a particle size of 5 μm as conductive particles, but the particle size of the base metal particles was smaller than 3 μm. Volume resistance is higher than 2

Example 10 A conductive adhesive was prepared in the same manner as in Example 2 except that the average particle diameter of the copper particles in Example 2 was changed to 25 μm instead of 5 μm.
This adhesive had a lower volume resistance than the conventional case where only spherical nickel particles having a particle size of 5 μm were used as the conductive particles (Comparative Example), but the particle size of the base metal particles was 15 μm.
Therefore, the volume resistance was higher than that of Example 2.

Example 11 The composition of Example 2 was changed so that the weight ratio of nickel particles to spherical copper particles having an average particle diameter of 5 μm was 8: 1, and the conductive adhesive was used. Although it was prepared, a lower volume resistance was obtained than the comparative example,
Since the ratio of the weight of the base metal particles to the weight of the nickel particles was smaller than 0.05, the ratio was higher than that of Example 2.

Example 12 A conductive adhesive was prepared by changing the composition of Example 2 so that the weight ratio of nickel particles to spherical copper particles having an average particle size of 5 μm was 4: 1. Although the volume resistance was lower than that of the comparative example, the ratio of the weight of the base metal particles to the weight of the nickel particles was larger than 0.15, and was higher than that of the second embodiment.

Example 13 The composition of Example 2 was changed so that the ratio of the total weight of nickel particles and copper particles to the total weight of the adhesive was 0.60, and the conductive adhesive was changed. Although it was prepared, the volume resistance was lower than that of the comparative example, but the ratio of the total weight of the nickel particles and the base metal particles to the weight of the adhesive was smaller than 0.75, so that the volume resistance was higher than that of Example 2. became.

(Example 14) A conductive adhesive was prepared by mixing the composition of Example 2 so that the ratio of the total weight of nickel particles and copper particles to the total weight of the adhesive was 0.90. Since the ratio of the total weight of nickel particles and base metal particles to the weight of the adhesive exceeded 0.85, a lower volume resistance was obtained than in the comparative example, but the volume resistance was higher than in Example 2.

From the above examples, among the conductive adhesives of the present invention, Example 5 in which a combination of nickel particles and copper particles each having a quadruple-shaped projection has the lowest volume resistance has the lowest connection reliability. Because it is excellent, it is most preferable as the conductive adhesive of the present invention.

Further, as is clear from Table 2, the curing shrinkage ratio and the volume resistance value have a good correlation, and the conductive adhesive of the present invention has a higher curing shrinkage than the conventional conductive adhesive. High in nature. Therefore, it can be seen that the conductive adhesive of the present invention was able to reduce the resistance by improving the wettability of the conventional conductive adhesive and increasing the curing shrinkage.

In this embodiment, an epoxy / acrylic resin is used as a synthetic resin component of the resin binder. However, the resin is not limited to this as long as it is a resin having excellent insulating properties and sufficient adhesive strength. Although epoxy particles were used as the resin particles, any resin may be used, and the present invention is not limited to this.

[0053]

The conductive adhesive of the present invention contains nickel particles as conductive particles and particles whose surface is a lipophilic substance in the resin binder, so that the conductive particles are dispersed in the resin binder. The adhesive property is increased, the volume shrinkage during the curing process is increased, the frequency of contact between the nickel particles is relatively increased, and a cured adhesive having a reduced volume resistance value can be obtained.

Further, according to the present invention, since the particles whose surface is a lipophilic substance are solid resin particles or base metal particles such as copper, the conductive adhesive having a effectively reduced volume resistance value by a simple blending. Can be prepared.

In the present invention, in particular, a conductive adhesive having a low volume resistance can be obtained by using nickel-coated quadruped zinc oxide whiskers as nickel particles.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating the interrelationship between nickel particles having tetrapod projections and copper particles having an oxide film in the conductive adhesive of the present invention.

FIG. 2 is a schematic view showing a volume resistance measuring method used in an example of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Nickel particle 2 Copper particle 3 Projection 4 Ceramic base material 5 Silver-palladium electrode 6 Silver-palladium electrode 7 Conductive adhesive 8 Resin binder

Claims (12)

[Claims] 1. A conductive adhesive comprising nickel particles and surface lipophilic particles in a resin binder. 2. The conductive adhesive according to claim 1, wherein the surface lipophilic particles are resin particles. 3. The conductive adhesive according to claim 1, wherein the surface lipophilic particles are particles of a base metal (but not containing nickel) based on silver. 4. The conductive adhesive according to claim 3, wherein said base metal particles are particles of one or more metals among silver, copper, zinc, tin and iron. 5. The conductive adhesive according to claim 3, wherein said base metal particles are copper particles. 6. The conductive adhesive according to claim 3, wherein a natural oxide film is formed on the surface of the base metal particles. 7. The conductive adhesive according to claim 3, wherein the base metal particles are previously heat-treated at a temperature higher than room temperature to form a surface oxide film. 8. The conductive adhesive according to claim 1, wherein the nickel particles have projections on the surface. 9. The conductive adhesive according to claim 8, wherein the nickel particles are formed by coating a surface of a quadruped zinc oxide whisker with nickel. 10. The conductive adhesive according to claim 3, wherein the shape of the metal particles having at least the surface oxide film is spherical. 11. The method according to claim 3, wherein the nickel particles have a particle size of 3 to 20 μm, and the metal particles having an oxide film formed on at least the surface thereof have a particle size of 3 to 15 μm. 3. The conductive adhesive according to 1.). 12. The ratio of the weight of the base metal particles to the weight of the nickel particles is 0.05 to 0.15, and the ratio of the total weight of the nickel particles and the base metal particles to the weight of the adhesive is 0. The conductive adhesive according to any one of claims 3 to 10, wherein the conductive adhesive has a ratio of 0.75 to 0.85.