JP2003282766A - Container for housing electronic component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a container for housing an electronic component that can secure airtightness in an electronic device by preventing a sealant from melting even when a heat history of 230-240°C is applied at the time of mounting the electronic device. <P>SOLUTION: This container is composed of an insulating substrate 2 having a mounting section A on which an electronic component 5 is mounted, and a frame-like metallized metal layer 2a surrounding the mounting section A and a metal cap 3 which is bonded to the metal layer 2a through the sealant 4 and airtightly houses the component 5 in a space formed between the cap 3 and substrate 2. The sealant 4 has a first area 4a disposed on the mounting section A side, and a second area 4b disposed on the outside of the area 4a. The material forming the first area 4a is prepared by adding a filler F containing silver and having a higher melting point than tin has to solder S composed mainly of tin in an amount of 25-50 mass%. The material forming the second area 4b is prepared by adding the filler F to the solder S in an amount of ≤5 mass% (including 0 mass%). <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic component storage container for hermetically sealing and storing electronic components such as semiconductor elements and piezoelectric vibrators, and in particular, a low melting point alloy is used as a sealing material. The present invention relates to a container for storing electronic components for sealing.

[0002]

2. Description of the Related Art Conventionally, an electronic component housing container for housing electronic components such as semiconductor elements is made of an electrically insulating material such as an aluminum oxide sintered body, and the electronic components are provided in the substantially central portion of its upper surface. An insulating substrate having a concave portion for accommodating and a plurality of metallized wiring layers made of a high melting point metal such as tungsten and molybdenum led out from the periphery to the lower surface, and an encapsulating material adhered to the outer peripheral portion of the upper surface. , A lid made of a metallic material and having a sealing material applied to the outer peripheral portion of the lower surface. Then, an electronic component such as a semiconductor element is attached to the bottom surface of the concave portion of the insulating substrate via an adhesive, and each electrode of the electronic component is connected to the metallized wiring layer via a bonding wire. The respective sealing materials adhered to the main surfaces facing each other are fused and integrated, and the container including the insulating base and the lid is sealed to form an electronic device as a final product. In such a conventional electronic device, solder containing lead as a main component has been used as a sealing material for joining the insulating base and the lid.

However, when lead contained in the encapsulant is designated as an environmental pollutant and, for example, an electronic device using a solder containing lead is discarded or left outdoors and exposed to the wind and rain, it is exposed to the environment. There is a risk that lead will melt out and pollute the environment, and in recent years, a lead-free encapsulant has been required due to the increasing global environmental protection movement.

Therefore, various encapsulating materials which do not use lead, which is harmful to the human body, have been developed and proposed. As such a sealing material, for example, various solders containing tin, bismuth-silver, zinc-aluminum or the like as a main component are adopted.

[0005]

[Problems to be Solved by the Invention] However, tin and silver
Solder made of an alloy containing zinc has a temperature of -55 to 125
Although they show excellent reliability in heat fatigue resistance under temperature cycle conditions of ℃, the solders containing them all have melting points of 225 ℃ or less, and the solder is hermetically sealed between the insulating base and the lid. When used as an encapsulant for encapsulation, the encapsulant should be 230 to 240 ° C when the electronic device is mounted on an external electrical circuit, etc.
Has a problem that the sealing material itself is melted due to the heat history of 1. and the airtight sealing inside the electronic device is broken.

Further, the solder containing bismuth-silver as a main component is inferior in heat fatigue resistance under a temperature cycle condition of -55 to 125 ° C. and lacks airtight reliability, so that it is used for hermetically sealing an electronic device. Had the problem of being inappropriate.

Further, the solder containing zinc-aluminum as a main component easily corrodes when left in the air, and lacks long-term airtight reliability, so that it is necessary to use it for airtight sealing of electronic devices. Was also inappropriate.

The present invention has been completed in view of the above conventional problems, and an object thereof is to mount an electronic device on an external electric circuit or the like even if a thermal history of 230 to 240 ° C. is applied. Another object of the present invention is to provide a container for accommodating electronic components, which can secure the airtightness of the material, has excellent corrosion resistance of the sealing material itself, and is environmentally friendly and does not contain lead.

[0009]

A container for storing electronic parts according to the present invention comprises a mounting part on which an electronic part is mounted, an insulating base having a frame-shaped metallized metal layer surrounding the mounting part, and a frame-shaped container. A container for electronic parts, which is joined to a metallized metal layer via a sealing material and comprises a metal lid for hermetically housing electronic parts in a space between the insulating base body and the sealing material And a second region disposed outside the first region. The first region contains a solder containing tin as a main component and a filler containing silver and having a melting point higher than that of the solder. -50% by mass, and second
The region is characterized by comprising solder containing tin as a main component and containing 5% by mass or less (including 0% by mass) of a filler containing silver and having a melting point higher than that of the solder.

In the container for storing electronic parts of the present invention, in the above constitution, the filler has silver particles having an average particle size of 5 to 40 μm, or the surface of copper particles is coated with silver and the average particle size is 5 to 5. It is characterized by being metal particles of 40 μm.

According to the container for storing electronic parts of the present invention, the solder containing tin as a main component in the first region arranged on the mounting portion side of the sealing material is filled with a filler containing silver and having a higher melting point than this solder.
Since the content of the filler is 25 to 50% by mass, the silver contained in the filler easily reacts with tin, which is the main component of the solder, and the melting point of the first region of the sealing material is The melting point is higher than the melting point, and the first region of the sealing material does not melt at a temperature lower than 250 ° C. As a result, the sealing material melts even if a thermal history of 230 to 240 ° C. is applied to the electronic device that houses the electronic parts in the electronic part housing container when it is mounted on an external electric circuit or the like. The airtight seal inside the electronic device is not broken, and it is possible to obtain an electronic component storage container with high airtightness and reliability.

The second region of the encapsulant is composed of a solder containing tin as a main component and a filler containing silver and having a higher melting point than that of silver.
Since the content is less than or equal to wt% (including 0 wt%), the melting point of the second region can be made low, and the sealing material in the second region can have good fluidity in the sealing step. In addition, it shows good wettability with the metallized metal layer of the insulating substrate and the metal lid, and as a result, the container can be hermetically sealed in a good manner, and it is airtight and reliable even in thermal fatigue resistance such as temperature cycling. The container for storing electronic components having high property can be provided.

Further, according to the container for storing electronic parts of the present invention, in the above constitution, the filler has an average particle size of 5 to 40.
Since the metal particles having an average particle size of 5 to 40 μm are formed by coating silver on the surface of μm silver particles or copper particles, the silver easily reacts with tin, which is the main component of the solder, to form a solid layer wire. Temperature is 25
By forming a eutectic alloy layer having a temperature of 0 ° C. or higher, it is possible to obtain a container for storing electronic components with higher airtightness and reliability.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Next, a container for storing electronic parts according to the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a cross-sectional view showing an example of an embodiment of a container for storing electronic components of the present invention, and FIG. 2 is an enlarged cross-sectional view of the main part thereof. In these figures, 1 is a container for storing electronic components, 2 is an insulating substrate, 3 is a metal lid, 4 is a sealing material, 5 is an electronic component, and the container 1 for storing electronic components is mainly an insulating substrate 2, It is composed of a metal lid 3 and a sealing material 4.

The insulating base 2 is provided with a mounting portion A for mounting the electronic component 5 on the center of the upper surface thereof. The electronic component 5 is attached to the mounting portion A by an adhesive such as glass, resin or brazing material. It is adhesively fixed through.

The insulating base 2 is formed with a plurality of metallized wiring layers 6 deposited from the vicinity of the mounting portion A to the lower surface thereof. The respective electrodes of the component 5 are electrically connected via the bonding wires 7, and the portion leading to the lower surface is connected to an external electric circuit (not shown) via an electric connecting member such as solder. Further, a frame-shaped metallized metal layer 2a for joining with a metal lid 3 described later is attached to the outer peripheral portion of the upper surface of the insulating substrate 2.

The insulating base 2 has a length and width of 2 to 20 m.
A rectangular parallelepiped with a height of 0.5 to 2.0 mm and aluminum oxide
Um, mullite, aluminum nitride, silicon carbide, glass
Electrically insulating materials such as sintered bodies whose main component is ceramics
, For example, an aluminum oxide sintered body
In the case of alumina (Al₂O₃) And silica (SiO
₂), Calcia (CaO), magnesia (MgO), etc.
Appropriate organic solvent / solvent is added to the raw material powder and mixed to form a slurry
This is done by the well-known doctor blade method and curren
Ceramics formed into a sheet shape by using the Darroll method
After obtaining the green sheet, after that, the ceramic green sheet
Punched into a predetermined shape and laminated multiple sheets
It is manufactured by firing at a temperature of about 1600 ° C.

The metallized wiring layer 6 and the frame-shaped metallized metal layer 2a are made of a refractory metal such as tungsten, molybdenum and manganese, and a metal paste prepared by adding and mixing an organic solvent and a solvent to these powders is used as ceramic green. It is formed by depositing or filling a predetermined pattern on a predetermined position of the sheet by a conventionally known screen printing method, and is fired at the same time as the ceramic green sheet.

The metallized wiring layer 6 and the frame-shaped metallized metal layer 2a are coated with a metal such as nickel or gold, which has excellent corrosion resistance and has good wettability with a brazing material such as solder, by a plating method or the like. , 1.0 to 20 μm in thickness can effectively prevent oxidative corrosion of the metallized wiring layer 6 and the frame-shaped metallized metal layer 2a.

On the other hand, the metal lid 3 is made of metal such as 42 alloy and has a length and width of 2 to 20 mm and a thickness of 0.1.
Approximately 0.5 mm, the metal lid 3 is used as a plate material that becomes the metal lid 3.
It is manufactured by punching with a punching die having a shape corresponding to. A metal layer may be deposited on the surface by electrolytic nickel plating or the like. In addition, in the present embodiment, the metal lid 3 has a concave shape so as to accommodate the electronic component 5 mounted on the insulating base 2.

The metal lid 3 is used to bond and fix the electronic component 5 to the mounting portion A of the insulating substrate 2 via an adhesive such as glass, resin or brazing material, and each electrode of the electronic component 5 and a metallized wiring layer. 6 are electrically connected to each other via a bonding wire 7, and then bonded to a frame-shaped metallized metal layer 2a formed on the insulating substrate 2 via a sealing material 4.

In the container 1 for storing electronic components of the present invention, the sealing material 4 has the first region 4a arranged on the mounting portion A side of the electronic component 5 and the first region 4a arranged outside the first region 4a. The second region 4b has a second region 4b, the first region 4a is composed of a solder S containing tin as a main component and having a melting point higher than that of the solder S and containing 25 to 50 mass% of a filler F containing silver. The second region 4b has a melting point higher than that of the solder S containing tin as a main component, and the filler F containing silver is 5% by mass or less (0% by mass).
Including)). This is also important in the present invention.

According to the electronic component storage container 1 of the present invention,
The first region 4a arranged on the mounting portion A side of the encapsulant 4 contains solder S containing tin as a main component and 25 to 50 mass% of a filler F containing silver and having a higher melting point than the solder S. Therefore, the silver contained in the filler F easily reacts with tin, which is the main component of the solder S, and the melting point of the first region 4a of the sealing material is higher than the melting point of the solder S whose main component is tin. Therefore, the first region 4a of the sealing material does not melt at a temperature lower than 250 ° C. As a result, the electronic component 5 is placed in the electronic component storage container 1.
Even if a thermal history of 230 to 240 ° C. is applied to an electronic device that accommodates the above when mounting it on an external electric circuit or the like, the sealing material 4 melts and the hermetic seal inside the electronic device is broken. The electronic component storage container 1 can be made airtight and highly reliable without being damaged.

Further, the second region 4b of the sealing material is made of the solder S containing tin as a main component or the solder S containing 5% by mass or less of a filler F having a higher melting point and containing silver. Therefore, the melting point of the second region 4b can be made low, the sealing material 4b in the second region exhibits good fluidity in the sealing step, and the metallized metal layer 2a and the metal lid of the insulating base 2 can be formed. It shows good wettability with the body 3, and as a result, it is possible to make the container hermetically sealed,
The container 1 for housing electronic components can be highly airtight and reliable in terms of thermal fatigue resistance such as temperature cycle.

When the content of the filler F in the first region 4a is less than 25% by mass, the melting point of the reaction layer of silver contained in the filler F and tin which is the main component of the solder S becomes low. The heat resistance of the first region 4a tends to deteriorate, and if it exceeds 50% by weight, the bubbles in the first region 4a increase and the reliability of hermetic sealing tends to decrease. Therefore, the content of the filler F in the first region 4a is preferably 25 to 50% by mass. When the content of the filler F in the second region 4b exceeds 5% by mass, the wettability with the metallized metal layer 2a and the metal lid 3 decreases, and the thermal fatigue resistance such as temperature cycle tends to deteriorate. . Therefore, the content of the filler F in the second region is preferably 5% by mass or less.

Examples of the solder S containing tin as a main component include a eutectic composed of 96.5% by mass of tin and 3.5% by mass of silver, or 95.75% by mass of tin, 3.5% by mass of silver and 0.75% by mass of silver. Copper, 92% by weight tin, 8% by weight zinc, 99.3% by weight tin
A eutectic composed of 0.7% by mass of copper, or a lead-free solder having a composition close to that of a eutectic is used.

From the viewpoint of thermal protection of the electronic component 5 and residual stress, such a solder S is preferably one that can be melted at a temperature as low as possible, for example, 96.5 mass% tin and 3.5 mass% silver. The eutectic made of consists of a melting temperature of about 221 ° C, contains silver with excellent thermal conductivity, forms an alloy layer with excellent heat resistance, and is resistant to thermal history during mounting. Also, since it effectively releases heat, it is preferably used.

As the filler F, various fillers having a higher melting point than the solder S containing tin as a main component and containing a silver component can be applied. Examples of such a filler F include silver metal particles or silver-containing alloy particles. In particular, silver particles and silver-coated copper metal particles have good wettability with solder containing tin as a main component and have little influence on the fluidity of the solder. It is optimal because it forms an alloy with a high solidus temperature and has excellent thermal conductivity.

The shape of the filler F is basically preferably spherical, but it is not necessarily required to be a true sphere, and the shape of the filler F in the present invention is not limited to elliptic sphere and cylinder.
Various irregular shapes may be used as long as they can function as particles.

Further, those having an average particle size of about 5 to 40 μm are preferable from the viewpoint of the fluidity of the sealing material 4 at the time of hermetic sealing, and in particular, tin in the sealing material 4 from the surface thereof is preferable. Easily reacts with the solder whose main component is to form a eutectic alloy with tin, and these eutectic alloys bond to each other to form an alloy layer with excellent heat resistance, which improves heat resistance and heat resistance. It is more preferable in terms of excellent conductivity.

When the average particle size is less than 5 μm, the filler F is melted into the solder S in the heat step of sealing, and the mixed region of the above-mentioned alloy layer having excellent heat resistance and the filler F is insufficient and the heat resistance is high. Tends to deteriorate, and if it exceeds 40 μm, the first
Bubbles in the region 4a increase and the reliability of hermetic sealing tends to decrease. Therefore, the average particle size of the filler F is 5 to 40 μm.
It is preferably about m.

Also, the same effect can be obtained by using, as the filler F, inorganic material particles to which a metal film such as silver or a silver alloy is deposited. In particular, from the viewpoint of thermal conductivity, as the inorganic material particles, aluminum nitride having a thermal conductivity of 58.6 W / mK and silicon carbide having a thermal conductivity of 268 W / mK are suitable. Further, from the viewpoint of wettability and fluidity of the first region 4, the average particle size of the inorganic material particles coated with the metal film is not problematic as long as it is the same particle size as the metal material.
Therefore, the content of the inorganic material may be equivalent to that of the metal material, while having the same effect as that of the metal material.

In particular, when silver particles or metal particles obtained by coating copper particles with silver are used as the filler F, the particle size is such that good flowability of the sealing material 4 at the time of hermetic sealing and It easily reacts with the solder S containing tin as a main component in the encapsulant 4 to form a eutectic alloy with tin, and these eutectic alloys are bonded to each other to form a strong alloy layer having excellent heat resistance. From the point that it is easy to do,
It is preferably 5 to 40 μm.

Further, it is necessary that the amount of the solder S is such that it can spread between the fillers F and fill the gaps. Therefore, the thickness of the sealing material 4 is 50 μm to 150 μm.
It is preferably about m. The thickness of the sealing material 4 is 50 μm
If it is less than 100 μm, bubbles in the first region 4a increase, and the reliability of hermetic sealing tends to decrease.
The region 4b tends to flow out to the outer periphery of the insulating base 2 and cause an electrical problem. Therefore, the thickness of the solder S is 50 to
It is preferably about 150 μm.

The sealing width of the first region 4a and the second region 4b of the sealing material is preferably 100 μm or more. If the sealing width of the first region 4a and the second region 4b of the sealing material is less than 100 μm, the reliability of the hermetic sealing will be improved due to the influence of bubbles contained in the first region 4a and the second region 4b of the sealing material. Tends to decline. Therefore, the first region 4 of the sealing material
The sealing width of a and the second region 4b is preferably 100 μm or more.

Further, in order to improve the wettability between the solder S and the filler F and facilitate the uniform formation of an alloy layer on the surface of the filler F, various flux components represented by pine resin or the like are used as the sealing material 4. It is also effective to add such additives as in the range that does not lower the solidus temperature of the alloy layer and to heat the sealing material 4 in an inert gas atmosphere or a reducing gas atmosphere.

To join the insulating base 2 and the metal lid 3, first, the first region 4a of the frame-shaped sealing material is applied to the joining portion of the metal lid 3 with the insulating base 2 by the screen printing method. Then, the second region 4b of the frame-shaped sealing material is applied to the joint portion of the metal lid 3 with the insulating substrate 2 by using the screen printing method. After that, the metal lid 3 is placed on the insulating base 2 so that the sealing material 4 is sandwiched between the frame-shaped metallized metal layer 2a of the insulating base 2 and the metal lid 3, and then the metal lid 3 is placed. Approximately 300 to 3 while pressing 3 on the insulating base 2 side with a constant pressure
By heating the sealing material 4 by heating to a temperature of 50 ° C., an alloy with tin is formed on the surface of the filler F in the first region 4a of the sealing material to improve heat resistance, and The second region 4b is a frame-shaped metallized metal layer 2a of the insulating substrate 2.
The metal lid 3 is airtightly bonded to the upper surface of the insulating substrate 2 with good wettability.

Thus, according to the electronic component storage container 1 of the present invention, the electronic component 5 such as a semiconductor element is adhered and fixed to the mounting portion A of the insulating substrate 2 through an adhesive such as glass, resin or brazing material. Each electrode of the electronic component 5 is connected to the metallized wiring layer 6 via a connecting agent 7 such as a bonding wire, and then the metal lid 3 is sealed on the upper surface of the insulating substrate 1 so as to cover the mounting portion A. And the insulating base 2
The electronic component 5 is hermetically sealed in the container 1 composed of the metal lid 3 and the metal lid 3 to form an electronic device as a final product.

The present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention. In the above embodiments, a semiconductor element is used as an electronic component. Although the electronic device housing this is described in detail above, it is also applicable to an electronic device housing electronic components such as a piezoelectric vibrator and a surface acoustic wave element.

[0040]

EXAMPLE The following experiment was conducted to confirm the effect. An experimental example in which the content of the filler F in the first region 4a and the second region 4b of the sealing material 4 and the range of the average particle diameter of the filler F are determined will be shown. Further, the evaluation of the electronic component storage container of the present invention was conducted by a helium gas leak test of the sealed container after 300 temperature cycles. As the evaluation container, the longitudinal dimension of the insulating substrate is 12.0 mm,
A container having a lateral dimension of 8.0 mm, a height of 1.0 mm, and a width of a joint surface with the lid body of 0.7 mm was used.

(Experiment 1) The filler F content of the second region 4b of the encapsulating material 4 and the average particle diameter of the filler F are kept constant, and the filler F content of the second region 4a of the encapsulating material 4 is 20 to 20%.
The airtightness after the temperature cycle was confirmed by changing between 55% by mass. The results are shown in Table 1.

[0042]

[Table 1]

From the experimental results, it was found that the content of the filler F in the second region 4a of the sealing material 4 was in the range of 25 to 50% by mass, and good airtightness reliability was exhibited.

Next, the following experiment was conducted on the content of the filler F in the second region 4b of the sealing material 4. (Experiment 2) The content of the filler F in the second region 4a of the encapsulating material 4 and the average particle diameter of the filler F were made constant, and the encapsulating material 4
The filler F content of the second region 4b was changed between 0 and 7% by mass, and the airtightness after the temperature cycle was confirmed. The results are shown in Table 2.

[0045]

[Table 2]

From the experimental results, it was found that the content of the filler F in the second region 4b of the sealing material 4 was in the range of 0 to 5% by mass, and the airtightness reliability was good.

(Experiment 3) Furthermore, the second region 4 of the sealing material 4
Filler F content of a and the second region 4b of the sealing material 4
The content of the filler F was maintained constant, the average particle size of the filler F was varied between 3 and 45 μm, and the airtightness after the temperature cycle was confirmed. The results are shown in Table 3.

[0048]

[Table 3]

From the experimental results, it was found that the average particle size of the filler F is in the range of 5 to 40 μm and shows good reliability of airtightness.

[0050]

According to the electronic component storage container of the present invention,
The first region arranged on the mounting portion side of the encapsulant contains 25 to 50% by mass of a filler containing tin as a main component and a filler containing silver and having a melting point higher than that of the solder. The silver contained easily reacts with tin, which is the main component of the solder, and the melting point of the first region of the encapsulant becomes higher than the melting point of the solder containing tin as the main component. It does not melt below 250 ° C. As a result, the sealing material melts even if a thermal history of 230 to 240 ° C. is applied to the electronic device that houses the electronic parts in the electronic part housing container when it is mounted on an external electric circuit or the like. The airtight seal inside the electronic device is not broken, and it is possible to obtain an electronic component storage container with high airtightness and reliability.

In addition, the second region of the encapsulant is mixed with a solder containing tin as a main component and a filler containing silver and having a melting point higher than that of silver.
Since the content is less than or equal to wt% (including 0% by mass), it is possible to lower the melting point of the second region, and the sealing material in the second region has good fluidity in the sealing step. In addition, it shows good wettability with the metallized metal layer of the insulating substrate and the metal lid, and as a result, the container can be hermetically sealed in a good manner, and it is airtight and reliable even in thermal fatigue resistance such as temperature cycling. The container for storing electronic components having high property can be provided.

Further, according to the container for storing electronic parts of the present invention, in the above structure, the filler has an average particle size of 5 to 40.
Eutectic with a solid layer temperature of 250 ° C or higher due to the fact that silver easily reacts with tin, which is the main component of solder, because silver particles with a diameter of μm or copper particles are coated with silver. By forming an alloy layer, it is possible to obtain a container for storing electronic components with higher airtightness and reliability.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of an embodiment of a container for storing electronic components of the present invention.

FIG. 2 is an enlarged sectional view of an essential part of a container for storing electronic components of the present invention.

[Explanation of symbols]

1 --- Container for storing electronic components 2 ... Insulating substrate 2a --- Frame-shaped metallized metal layer A ・ ・ ・ ・ Mounting part 3 ... Metal lid 4 ... Encapsulating material 4a ... First area 4b ... ・ Second area F ... Filler S ... Solder 5 ... Electronic parts 6 ... Metallized wiring layer 7 ... Bonding wire

Claims (2)

[Claims] 1. An insulating substrate having a mounting portion on which an electronic component is mounted and a frame-shaped metallized metal layer surrounding the mounting portion, and an insulating substrate bonded to the frame-shaped metallized metal layer via a sealing material. A container for storing an electronic component, comprising a metal lid that hermetically accommodates an electronic component in a space between the insulating base, and the sealing material is disposed on the mounting portion side.
And a second region arranged outside the first region, wherein the first region contains 25 to 50% by mass of a filler containing silver as a main component and containing silver and having a melting point higher than that of the solder.
And the second region is formed by adding, to the solder containing tin as a main component, 5% by mass or less (including 0% by mass) of a filler containing silver and having a melting point higher than that of the solder. A container for storing electronic components characterized by: 2. The filler has an average particle size of 5 to 40 μm.
2. The container for storing electronic parts according to claim 1, which is a metal particle having an average particle diameter of 5 to 40 μm, which is formed by coating the surface of silver particles of m or copper particles with silver.