JP2002120086A - Lead-free solder and its manufacturing method - Google Patents

Abstract

(57) [Problem] To provide a lead-free solder having a eutectic temperature of about 183 ° C., excellent in wettability and chemical stability, and capable of obtaining high bonding strength. SOLUTION: The lead-free solder according to the present invention is Sn-Zn.
Sn, Ag, and C are formed on the surface of the solder particles 10 made of the eutectic alloy.
u, Bi, Ni, a Sn—Bi alloy, a Sn—Cu alloy or a Sn—Ag alloy to form a coating layer 20, which prevents a Zn reaction contained in the solder particles 10. At the same time, the wettability when the solder particles 10 are melted is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solder used in a process of mounting various electronic components on a circuit board, and more particularly to a lead-free solder containing no lead (Pb) toxic to the human body. .

[0002]

2. Description of the Related Art In various industrial fields, Pb is widely used as a material for parts and the like. However, since Pb is toxic to the human body, it has been studied to regulate its use.
For example, in the electronics industry, soldering is used to mount various electronic components on a circuit board,
Since a lot of Pb is contained in the solder, when such an electronic device is discarded or left outdoors, the solder used for soldering the electronic components reacts with the acid rain and reacts with the Pb.
The ions are eluted, and the eluted Pb ions penetrate into the ground together with the rainwater to contaminate the groundwater, and Pb may be ingested by the human body from the groundwater. Therefore, the development of lead-free solder containing no Pb has been promoted.

By the way, the solder used in the mounting process of electronic parts is chemically stable and does not deteriorate,
Appropriate soldering temperature, good wettability with the joint surface when the solder is melted, and sufficient soldering strength are required. For example, Sn-37% Pb solder is widely used.

[0004] Since the Sn-37% Pb solder is a eutectic alloy whose composition does not change with a phase change between a solid phase and a liquid phase, a so-called lift-off in which a void is formed at the joint surface between the substrate and the solder. No phenomena occur, which provides sufficient bonding strength. The eutectic temperature of Sn-37% Pb solder is 1
Since the temperature is 83 ° C., which is lower than that of other eutectic alloys, damage to electronic components due to heat during soldering can be prevented. If the eutectic temperature is higher than 183 ° C., the soldering temperature is also high, and accordingly, a change in the heat-resistant specification of the electronic component and a modification of the soldering apparatus are required. Conversely, when the eutectic temperature is lower than 183 ° C., the heat resistance of the solder joint decreases. Furthermore, since the Sn-37% Pb solder has excellent wettability with the mounting substrate, it comes into contact with the substrate surface over a large area, and exhibits sufficient bonding strength. Furthermore, Sn and Pb have excellent chemical stability and are not easily affected by acids or alkalis, so they do not deteriorate in the storage state before soldering, and have high reliability in the joint after soldering. can get.

[0005] Therefore, even in the case of lead-free solder, Sn-3
Properties equivalent to 7% Pb solder, ie eutectic temperature is 183 ° C
In the vicinity, it is necessary to be excellent in wettability and chemical stability, and to have high bonding strength.

Conventionally, lead-free eutectic alloys include Sn-0.7% Cu, Sn-3.5% Ag, and Sn-9.
% Zn and Sn-57% Bi are known. or,
Lead-free solders obtained by adding various elements as third components to these eutectic alloys are also being studied (see Nikkei Electronics, no. 775, 2000.7.31, pages 49 to 56). For example,
The Sn-Ag-Bi alloy solder focuses on the effect of lowering the melting temperature of the Bi element and the effect of improving the wettability,
Bi is added to a Sn-Ag eutectic alloy as a third component (Electronic Packaging Technology, vol. 14, No.
6, 1998.6, pages 53-56).

In a solder cream used in a reflow method or a flow method in a mounting process of an electronic component, a thixotropic agent, a rosin, and an activator (Cl, Br, etc.) are added to granular Sn-Pb solder. Kneading, thixotropicity is imparted by a thixotropic agent, wettability is improved by rosin, and an oxide film at a solder joint is removed by an activator and wettability is improved. Therefore, it has been studied to provide a lead-free solder cream that can exhibit required characteristics by using a granular lead-free solder as a main material and adding various additives thereto.

[0008]

However, Sn-0.7% C, which is known as a eutectic alloy lead-free solder, has been proposed.
u, Sn-3.5% Ag and Sn-57% Bi have eutectic temperatures of 227 ° C., 221 ° C. and 138, respectively.
183 ° C, which is the eutectic temperature of the Sn-Pb solder
Therefore, there is a problem in practical use. Also, Sn
The -9% Zn solder has a eutectic temperature of 198 ° C and Sn-P
Although the eutectic temperature of solder b is close to 183 ° C, it is considered promising, but the wettability is inferior to that of Sn-Pb solder, and the Zn contained in Sn-9% Zn solder is chemically imperfect. Due to the drawbacks of being stable and having high reactivity, it has not been put to practical use.

Also, Sn-0.7% Cu, Sn-3.5% A
g, Sn-9% Zn and Sn-57% Bi lead-free solder in which the third component is added deviates from the eutectic composition due to the addition of the third component. As a result, the problem of lift-off occurs.

In a solder cream made of lead-free solder, a component of the lead-free solder (eg, Zn) and a component of the additive (eg, Cl or Br as an activator) react with each other to change the alloy composition. Accordingly, there is a problem that the preservability and wettability of the solder cream are reduced.

An object of the present invention is to provide a lead-free solder which has a eutectic temperature of about 183 ° C., is excellent in wettability and chemical stability, and can obtain a high bonding strength, and a method for producing the same. That is.

[0012]

Means for Solving the Problems The present inventors have intensively studied to achieve the above object, and have developed a lead-free solder capable of obtaining necessary characteristics without changing the eutectic state of the solder material. The structure was revealed. That is, in the lead-free solder according to the present invention, to prevent the reaction of Zn contained in the solder grains on the surface of the solder grains made of the Sn-Zn eutectic alloy and to improve the wettability when the solder grains are melted. Was formed.

Since the lead-free solder of the present invention uses a Sn-Zn eutectic alloy as a material for the solder particles, there is no fear that a lift-off phenomenon occurs, and a sufficient bonding strength can be obtained. The eutectic temperature of the Sn—Zn eutectic alloy is 198 ° C., and the eutectic temperature of the Sn—Pb solder is 18 ° C.
Since there is no great difference from 3 ° C., the specifications and mounting equipment of the conventional electronic components can be used. Furthermore, since the surface of the solder particles is covered with the coating layer, the chemically unstable Zn component contained in the solder particles does not come into direct contact with the atmosphere in an acid or alkali atmosphere. ,
Therefore, there is no possibility that the Zn component reacts with an acid or an alkali.
Furthermore, the coating layer contains a component having an effect of improving the wettability when the solder particles are melted, and at the time of soldering, the component is mixed into the Sn-Zn eutectic alloy to wet the lead-free solder. Improve sex.

In a specific configuration, the coating layer is made of S
n, Ag, Cu, Bi, Ni, Sn-Bi alloy, Sn-
It is formed from one or a plurality of materials selected from Cu alloy and Sn-Ag alloy. In this specific configuration, since the composition forming the coating layer is a chemically stable metal, it does not react even when it comes into contact with an acid or alkali in the atmosphere. Further, the coating layer is made of Bi or Sn-Bi.
When formed from an alloy, Bi in the coating mixes with the Sn-Zn eutectic alloy at the time of soldering to improve the wettability of the solder.

[0015] In another specific configuration, the coating layer is formed by plating. According to this specific configuration, since a dense coating layer is formed to cover the entire surface of the solder particle, there is no possibility that a defect such as a pinhole will occur in the coating layer.

In still another specific configuration, a diffusion layer in which atoms contained in the solder particles and atoms contained in the coating layer are mixed is formed at the interface between the solder particles and the coating layer. In this specific configuration, the diffusion layer has a large adhesive strength of the coating layer to the solder particles, and there is no possibility that the coating layer is peeled off.

The solder cream according to the present invention is a solder cream containing a lead-free solder as a main component, and the lead-free solder is provided on the surface of a solder particle made of a Sn--Zn eutectic alloy with Zn contained in the solder particle. A coating layer is formed to prevent the reaction and improve the wettability when the solder particles are melted.

The coating layer is made of Sn, Ag, Cu, B
i, Ni, Sn—Bi alloy, Sn—Cu alloy, and Sn
It is formed in a single layer or a plurality of layers by, for example, plating using one or a plurality of materials selected from an Ag alloy.

The solder cream of the present invention is produced by using the lead-free solder of the present invention as a main material and adding an additive to solder particles made of the lead-free solder, and thus is obtained by the lead-free solder of the present invention. At the same time as good characteristics are obtained, the Zn component contained in the solder particles reacts with Cl or Br added as an activator to the solder cream because the solder particles are covered by the coating layer. Therefore, the preservability and wettability of the solder cream can be improved.

In a specific configuration, a diffusion layer in which atoms contained in the solder particles and atoms contained in the coating layer are mixed is formed at the interface between the solder particles and the coating layer. In this specific configuration, the diffusion layer has a high adhesion strength of the coating layer to the surface of the solder particle, and there is no possibility that the coating layer is peeled off in a process of kneading an additive into the solder particle to produce a solder cream.

The method for producing a lead-free solder according to the present invention comprises:
A solder grain preparation step of preparing solder grains made of an n-Zn eutectic alloy, and a step of preventing the reaction of Zn contained in the solder grains on the surface of the solder grains produced by the step and melting the solder grains. The method includes a plating step of forming a coating layer by plating a material capable of improving wettability, and a heating step of heating the solder particles on which the coating layer is formed.

In the method for producing a lead-free solder according to the present invention, the entire area of the surface of the solder grain is covered with a coating layer having a dense and uniform thickness by the plating step. Therefore, there is no possibility that defects such as pinholes occur in the coating layer. In the heating step, the atoms contained in the solder particles and the atoms contained in the coating layer are thermally diffused, and a diffusion layer in which the two atoms are mixed is formed at the interface between the solder particles and the coating layer. The diffusion layer,
Since the adhesion strength of the coating layer to the solder particles is large, there is no possibility that the coating layer will peel off.

[0023]

The lead-free solder according to the present invention has a eutectic temperature of about 183 ° C., is excellent in wettability and chemical stability, and can obtain a high bonding strength. Further, according to the method for producing a lead-free solder according to the present invention, the lead-free solder according to the present invention can be easily produced by adding a simple process.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below with reference to the drawings. As shown in FIG.
The lead-free solder (1) according to the present invention covers the entire surface of the solder particle (10) having a particle diameter of 20 to 40 μm made of a Sn—Zn eutectic alloy, and has a thickness of about 2 μm made of Bi or Sn—Bi alloy.
The coating layer prevents the reaction of Zn contained in the solder particles (10) and improves the wettability when the solder particles (10) are melted. Have been.

The solder cream used in the step of mounting the electronic component on the circuit board is prepared by adding a thixotropic agent, a rosin as an additive, and Cl or Br as an activator to the powder of the lead-free solder (1) of the present invention. It is manufactured by kneading.

In the lead-free solder (1) of the present invention,
Since the Sn-Zn eutectic alloy is used as the material of the solder particles (10), there is no possibility that a lift-off phenomenon occurs, and thereby sufficient bonding strength can be obtained. Also, Sn-Zn
Since the eutectic temperature of the eutectic alloy is 198 ° C., which is not much different from the eutectic temperature of 183 ° C. of the Sn—Pb solder, the specifications and mounting equipment of the conventional electronic components can be used. Also, since the entire surface of the solder particle (10) is covered by the coating layer (20), the chemically unstable Z
There is no possibility that the n component reacts with an acid or an alkali in a storage state before soldering, and excellent storage stability is obtained.

Further, Bi contained in the coating layer (20) is
Since it has the effect of improving the wettability of the molten solder, the solder particles (10) and the coating layer (20) are melted during soldering, so that Bi contained in the coating layer (20) becomes Sn-
It is mixed with Zn eutectic alloy to improve the wettability of lead-free solder. Furthermore, in the solder cream of the present invention containing the lead-free solder (1) as a main component, since the solder particles (10) are covered by the coating layer (20), Zn contained in the solder particles (10) is not included.
Does not react with Cl or Br added as an activator, thereby improving the preservability and wettability of the solder cream.

FIG. 3 shows another embodiment of the lead-free solder according to the present invention. In the lead-free solder (11),
An inner layer (4) of Ni or Sn having a thickness of about 2 μm is formed to cover the entire surface of the solder particle (10), and Bi or Sn-Bi is further covered to cover the entire surface of the inner layer (4). An outer layer (40) made of an alloy and having a thickness of about 2 μm is formed to form a coating layer having a two-layer structure. In the above lead-free solder (11), the inner layer (4) made of Ni or Sn has a solder particle (1).
0) and the outer layer (40) has high adhesion strength by plating, so that the outer layer (40) is directly on the surface of the solder particles (10).
The outer layer (40) is less likely to be peeled off as compared with the case of plating.

Next, the lead-free solder (1) of the present invention shown in FIG.
A method for producing a solder cream mainly composed of a solder cream will be described. As shown in FIG. 2A, first, solder particles (10) made of Sn-9% Zn and having a particle size of 20 to 40 μm are prepared by a known method. Subsequently, as shown in FIG.
Electroplating is performed on the entire surface of (0) using a Sn—Bi alloy as a plating material to form a coating layer (20) having a thickness of about 2 μm. Then, heat treatment is performed on the solder particles (10) on which the coating layer (20) is formed. As a result, the solder particles (10) and the coating layer (2
At the interface of (0), Sn and Zn contained in the solder particles (10)
And Sn and Bi contained in the coating layer (20) are thermally diffused in both directions, so that the solder particles (10) and the coating layer (20) as shown in FIG.
In between, a diffusion layer (30) in which these atoms are mixed is formed.

Further, the lead-free solder obtained through the above steps
A thixotropic agent, a rosin, and an activator are added as additives to the powder of (1) according to a well-known method, and these are uniformly kneaded. Thereby, a solder cream (not shown) according to the present invention is obtained.

In the method for producing a lead-free solder according to the present invention, the entire surface of the solder particles (10) is covered with the covering layer (10) having a dense and uniform thickness by the plating step.
There is no possibility that a defect such as a pinhole occurs in 0). Further, by the heating step, the diffusion layer (3) is formed between the solder particles (10) and the coating layer (20).
0) is formed, so that the coating layer (20) for the solder particles (10) is formed.
Has high adhesion strength. Therefore, after that, lead-free solder (1)
There is no possibility that the coating layer (20) will be peeled off in the step of producing a solder cream by kneading the additive into the powder. In addition, when the temperature of the solder particles (10) rises in the plating step, a diffusion layer (30) may be formed between the solder particles (10) and the coating layer (20). A heating step after the step can be omitted.

The solder cream is, for example, shown in FIGS.
It is used in the mounting process of the electronic component by the reflow method shown in (d). Specifically, after the solder cream (51) is screen-printed on the surface of the substrate (50) shown in FIG. 4A as shown in FIG. 4B, the solder cream (51) is printed as shown in FIG. The electronic component (52) is mounted on the top by a chip mounter. afterwards,
As shown in FIG. 3D, by heating in a reflow furnace, the solder cream (51) is melted, and the electronic component (5) is melted.
2) is bonded onto the substrate (50).

The lead-free solder of the present invention includes Sn-Zn
The eutectic alloy contains traces (for example, about 0.1% by weight) of lead or other metal elements as inevitable impurities.
A substantially lead-free solder is also included, and in this case, the same effect can be obtained.

The configuration of each part of the present invention is not limited to the above embodiment, and various modifications can be made within the technical scope described in the claims. For example, the material of the coating layer (20) is not limited to the above-described Bi and Sn-Bi alloy, but may be Sn, Ag, C
u, Ni, Sn-Cu alloy, or Sn-Ag alloy can also be adopted. Further, the plating method for forming the coating layer (20) is not limited to the electrolytic plating method, and an electroless plating method can also be used.
It is also possible to adopt a thin film forming method such as a vacuum evaporation method, an ion plating method, or a sputtering method.

[Brief description of the drawings]

FIG. 1 is an enlarged sectional view of a lead-free solder according to the present invention.

FIG. 2 is a process chart showing a method for producing a lead-free solder according to the present invention.

FIG. 3 is an enlarged sectional view of another lead-free solder according to the present invention.

FIG. 4 is a process chart of a reflow method using a solder cream.

[Explanation of symbols]

 (1) Lead-free solder (10) Solder grains (11) Lead-free solder (20) Coating layer (30) Diffusion layer (4) Inner layer (40) Outer layer (50) Board (51) Solder cream (52) Electronic components

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) // B22F 1/02 B22F 1/02 A

Claims (12)

[Claims] A coating layer is formed on a surface of a solder particle made of an Sn—Zn-based alloy to prevent a reaction of Zn contained in the solder particle and to improve wettability when the solder particle is melted. Lead-free solder. 2. A coating layer for preventing the reaction of Zn contained in the solder particles and improving the wettability when the solder particles are melted is formed on the surface of the solder particles made of the Sn-Zn eutectic alloy. Have lead-free solder. 3. The lead-free solder according to claim 2, wherein the coating layer has a two-layer structure including an inner layer and an outer layer. 4. The coating layer is made of Sn, Ag, Cu, Bi, N
The lead-free solder according to claim 2 or 3, wherein the lead-free solder is made of one or more materials selected from i, Sn-Bi alloy, Sn-Cu alloy, and Sn-Ag alloy. 5. The lead-free solder according to claim 2, wherein the coating layer is formed by plating. 6. A diffusion layer in which atoms contained in the solder particles and atoms contained in the coating layer are formed at the interface between the solder particles and the coating layer. Lead-free solder as described. 7. A solder cream containing a lead-free solder as a main component, wherein the lead-free solder prevents the reaction of Zn contained in the solder particles on the surface of the solder particles made of a Sn—Zn eutectic alloy, and A solder cream having a coating layer for improving the wettability when the particles are melted. 8. The coating layer is made of Sn, Ag, Cu, Bi, N
8. The solder cream according to claim 7, wherein the solder cream is made of one or more materials selected from i, Sn-Bi alloy, Sn-Cu alloy and Sn-Ag alloy. 9. The solder cream according to claim 7, wherein the coating layer is formed by plating. 10. A diffusion layer in which atoms contained in the solder particles and atoms contained in the coating layer are formed at the interface between the solder particles and the coating layer. The described solder cream. 11. A solder particle producing step for producing a solder particle made of a Sn—Zn eutectic alloy, and a step of preventing a reaction of Zn contained in the solder particle on a surface of the solder particle produced by the step, And a plating step of forming a coating layer by plating a material capable of improving wettability when the solder is melted. 12. The method for producing a lead-free solder according to claim 11, further comprising a heating step of heating the solder particles having the coating layer formed by the plating step.