JP2006140039A - Lead wire and solar cell using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lead wire which can be jointed properly, regardless of the thickness of a silicon crystal wafer, and to provide a solar cell that uses it. <P>SOLUTION: In the lead wire 10, at least a part of the periphery of a conductor 11 is covered with a solder film 15. The solder film 15 is constituted of a solder alloy, where Young's modulus is 40 GPa or smaller at normal temperature, the tensile strength is 40 MPa or smaller, and 0.2% proof stress is 30 MPa or smaller. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a lead wire, and more particularly to a power transmission lead wire connected to a silicon crystal wafer of a solar cell.

  A semiconductor chip in which a silicon crystal is grown on a substrate is used for a solar cell. A connecting lead wire is joined by soldering or the like to a predetermined region (contact region) of the silicon crystal wafer obtained by cutting and dividing the semiconductor chip, and electric power is transmitted (output) through the connecting lead wire.

  Usually, a solder plating film for connection to a silicon crystal wafer is formed on the surface of a conductor as a connecting lead wire. For example, there is a connecting lead wire using a flat copper rectangular conductor such as tough pitch copper or oxygen-free copper as a conductor, and Sn—Pb eutectic solder as a constituent material of a solder plating film (see, for example, Patent Document 1).

  In recent years, switching to a solder containing no Pb (Pb-free solder) as a constituent material of a solder plating film has been studied in consideration of the environment (for example, see Patent Document 2).

  Of the members constituting the solar cell, the silicon crystal wafer accounts for the majority of the material cost. For this reason, in order to reduce the manufacturing cost, the thinning of the silicon crystal wafer has been studied. However, if the silicon crystal wafer is thinned, the silicon crystal wafer may be deformed (warped, etc.) or damaged due to the heating process when soldering the connecting lead wires and the temperature change when using the solar cell. A malfunction occurs. These cause a decrease in productivity of the solar cell module, and also cause a decrease in power generation efficiency when the solar cell module is used for a long period of time, resulting in a decrease in reliability.

  Therefore, in order to deal with these problems, a connection lead wire having a low thermal expansion and a solder plating film having a low melting temperature have been adopted. As an example of a lead wire having a small thermal expansion, there is one using a strip (copper-invar (registered trademark) -copper) clad with a copper material and an Invar (registered trademark) material having a small thermal expansion as a lead frame ( For example, see Patent Document 3).

Japanese Patent Laid-Open No. 11-21660 JP 2002-263880 A JP 2002-164560 A

  However, if the silicon crystal wafer is further thinned in the future, even if a lead wire conductor having a low thermal expansion is used, the lead is deformed into a silicon crystal wafer as long as the thermal expansion coefficient of the silicon crystal wafer and the lead wire is not the same ( There is a risk that a problem such as warping or damage may occur again.

  An object of the present invention created in view of the above circumstances is to provide a lead wire that can be satisfactorily bonded regardless of the thickness of the silicon crystal wafer, and a solar cell using the lead wire.

  In order to achieve the above object, the lead wire according to the present invention is a lead wire in which at least a part of the outer periphery of the conductor is covered with a solder film. The solder film has a Young's modulus at room temperature of 40 GPa or less and a tensile strength of 40 MPa. The following is composed of a solder alloy having a 0.2% proof stress of 30 MPa or less.

Moreover, the lead wire according to the present invention is a lead wire in which at least a part of the outer periphery of the conductor is covered with a solder film.
Sn-0.10 ～ 0.80mass% Cu alloy,
Sn-0.10 ~ 0.80mass% Cu-0.1 ~ 0.55mass% Ag alloy,
Or Sn-0.10 ~ 0.80mass% Cu-0.1 ~ 0.35mass% Sb alloy,
It is composed of

  Here, the conductor is preferably composed of annealed copper or a clad material of a copper material and an Fe—Ni alloy material. The conductor is preferably a flat wire.

  On the other hand, a solar cell according to the present invention is obtained by joining the above-described lead wire and a silicon crystal wafer via a solder alloy constituting the solder film.

  According to the present invention, the excellent effect that the lead wire and the silicon crystal wafer can be satisfactorily soldered regardless of the thickness of the silicon crystal wafer is exhibited.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, a preferred embodiment of the invention will be described with reference to the accompanying drawings.

  FIG. 1 shows a cross-sectional view of a lead wire according to a preferred embodiment of the present invention.

  As shown in FIG. 1, the lead wire 10 according to the present embodiment has an entire outer periphery of the rectangular conductor 11 having a Young's modulus at room temperature of 40 GPa or less, a tensile strength of 40 MPa or less, and a 0.2% proof stress of 30 MPa or less. It is coated with a solder film 15 made of a solder alloy.

Specifically, the solder film 15 is
Sn-0.10 ～ 0.80mass% Cu alloy,
Sn-0.10 ~ 0.80mass% Cu-0.1 ~ 0.55mass% Ag alloy,
Or Sn-0.10 ~ 0.80mass% Cu-0.1 ~ 0.35mass% Sb alloy,
Consists of.

  Here, the Cu content of the Sn-Cu alloy, Sn-Cu-Ag alloy, or Sn-Cu-Sb alloy constituting the solder film 15 is set to 0.10 to 0.80 mass% or less because the Cu content is 0.10 mass. If it is less than%, whisker is likely to be generated from the surface of the solder film 15, so that the whisker property is increased and the reliability is lowered. Moreover, it is because the liquidus temperature of a solder alloy will raise large when Cu content exceeds 0.80 mass%.

  In addition, the Ag content of the Sn—Cu—Ag alloy (or Sn—Cu—Sb alloy) constituting the solder film 15 is set to 0.10 to 0.55 (or Sb content is 0.10 to 0.35) mass% or less. (Or Sb) If the content is less than 0.10 mass%, the effect of addition cannot be obtained. In addition, if the Ag content exceeds 0.55 (or Sb content is 0.35) mass%, at least one of Young's modulus, tensile strength, or 0.2% proof stress exceeds the above-mentioned specified value.

  The film thickness of the solder film 15 is 1/4 to 1/20, preferably 1/5 to 1/10, particularly preferably 1/6 to 1/8 of the thickness of the flat conductor 11.

  The flat conductor 11 is made of a flat wire made of annealed copper. The constituent material of the flat wire is not limited to soft copper, and has, for example, high conductivity and low thermal expansion (or the same thermal expansion as that of a silicon crystal wafer), low resistance (or Low strength).

  Moreover, as the flat conductor 11, in addition to a general flat wire, a wire obtained by flattening a wire (round wire) or a wire obtained by flattening a stranded wire obtained by twisting a plurality of wires is used. You can also. In this case, examples of the strand include a Cu strand, an Al strand, an Ag strand, an Au strand, or an Fe—Ni alloy strand. The Cu strand, the Al strand, the Ag strand, and the Au strand referred to here are strands made of Cu or Cu alloy, Al or Al alloy, Ag or Ag alloy, Au or Au alloy. As a constituent material of the Fe—Ni alloy strand, Invar (registered trademark) is preferable. The stranded wire is composed of a Cu strand, an Al strand, an Ag strand, an Au strand, an Fe—Ni alloy strand, or a combination thereof.

  A solder film 15 is formed by performing solder plating on the entire outer periphery of the flat conductor 11, and the lead wire 10 according to the present embodiment shown in FIG. 1 is obtained. At this time, the method for forming the solder film 15 is not particularly limited, and all conventional plating methods for conductors can be applied.

  The lead wire 10 according to the present embodiment manufactured in this way is applied to a predetermined contact region (for example, an Ag plating region) on the cell surface and a predetermined contact region of a frame member (for example, a lead frame) on the silicon crystal wafer. A solar cell (solar cell module (not shown)) can be obtained by connecting via a solder alloy constituting the solder film 15. A solder film 15 for bonding may also be provided in predetermined contact areas of the cell surface and the lead frame.

  In the present embodiment, the case where the flat conductor 11 is made of a flat wire made of soft copper alone has been described. However, the present invention is not particularly limited thereto. For example, as shown in FIG. 2, a rectangular conductor 21 made of a rectangular wire made of a clad material of copper materials 23a and 23b and an Fe—Ni alloy material 22 may be used. The layer structure of the clad material is not limited to three layers, and may be two layers or four or more layers.

  In the present embodiment, the case where the entire outer periphery of the rectangular conductor 11 is covered with the solder film 15 has been described. However, the present invention is not particularly limited thereto. For example, only a part of the outer periphery of the flat conductor 11 (for example, the upper surface and / or the lower surface of the flat conductor 11) may be covered with the solder film 15.

  Furthermore, in the present embodiment, the case where the solder film 15 is directly formed on the entire outer periphery of the flat conductor 11 has been described, but the present invention is not particularly limited thereto. For example, a pure Sn plating film is formed on the entire periphery of the flat conductor 11 to form a pure Sn plating film, and then a heat treatment such as energization heating is performed to diffuse Cu and Sn to each other, thereby forming the solder film 15. It may be. At this time, the applied voltage and / or energization time during energization heating is controlled so that the Cu content in the solder film 15 is 0.10 to 0.80 mass%.

  Next, the operation of the lead wire 10 according to the present embodiment will be described.

  When soldering the lead wire 10 and the silicon crystal wafer, the flat conductor 11 of the lead wire 10 is thermally expanded by heat application. For example, in the case of a Cu conductor, the coefficient of thermal expansion is about 0.3%. That is, the lead wire 10 is soldered to the silicon crystal wafer in a state where the flat conductor 11 is thermally expanded. Thereafter, the solder is cooled and solidified by stopping the application of heat, but the rectangular conductor 11 is thermally contracted during this cooling. Along with this, residual stress is generated inside the rectangular conductor 11 after heat application. This residual stress is a cause of warping of the silicon crystal wafer, and the magnitude of the residual stress determines the warp amount of the silicon crystal wafer.

  Here, in the lead wire 10 according to the present embodiment, the solder alloy constituting the solder film 15 has a Young's modulus at room temperature, a tensile strength, and a 0.2% proof stress, such as a general Pb-free solder alloy. The chemical composition is adjusted to be lower than that of the Sn-0.5mass% Cu-3mass% Ag alloy. Specifically, the Young's modulus at room temperature of the solder alloy constituting the solder film 15 is 40 GPa or less, the tensile strength is 40 MPa or less, and the 0.2% proof stress is 30 MPa or less. As the Young's modulus is smaller, elastic deformation is possible with a smaller external force, and as the tensile strength (or 0.2% yield strength) is smaller, plastic deformation is possible with a smaller external force. From this, the solder alloy constituting the solder film 15 can be elastically deformed and plastically deformed with a small external force.

  Therefore, when the lead wire 10 and the silicon crystal wafer according to the present embodiment are solder-bonded, the solder alloy constituting the solder film 15 is deformed following the thermal expansion and contraction of the flat conductor 11 to form the flat conductor 11. Absorbs and relieves the generated residual stress. That is, the load applied to the silicon crystal wafer during solder bonding is remarkably reduced regardless of the thickness of the silicon crystal wafer. As a result, there is almost no risk that the silicon crystal wafer will be deformed (warped or the like) or damaged. Therefore, even if there is a difference in thermal expansion coefficient between the lead wire 10 and the silicon crystal wafer, the lead wire 10 and the silicon crystal wafer can be soldered satisfactorily.

  Moreover, the solder alloy which comprises the solder film 15 of the lead wire 10 which concerns on this Embodiment is Ag compared with the conventional general Pb free solder (for example, Sn-0.50mass% Cu-3.0mass% Ag). Low content. Therefore, the lead wire 10 according to the present embodiment is lower in manufacturing cost than a conventional lead wire using a general Pb-free solder.

  By using the lead wire 10 according to the present embodiment as a lead wire for connecting a solar cell, almost no warpage occurs in the silicon crystal wafer after soldering, so that the productivity of the solar cell module can be improved. .

  The lead wire 10 according to the present embodiment can be used as a lead wire of a solar cell module, or can be used as a lead wire of a component in which warpage after solder joining becomes a problem.

  As described above, the present invention is not limited to the above-described embodiment, and it goes without saying that various other things are assumed.

  Next, although this invention is demonstrated based on an Example, this invention is not limited to this Example.

Example 1
Solder plating is applied to the entire periphery of a flat conductor made of annealed copper, containing 0.30 mass% Cu, the balance being composed of an alloy of Sn and unavoidable impurities, forming a solder film with a thickness of 30 μm, thickness 0.2 mm A lead wire having a width of 2 mm was produced. The solder alloy constituting this solder film had a Young's modulus at room temperature of 36 GPa, a tensile strength of 30 MPa, and a 0.2% proof stress of 19 MPa.

(Example 2)
A lead wire was produced in the same manner as in Example 1 except that the solder film contained 0.75 mass% Cu and the balance was composed of an alloy of Sn and inevitable impurities. The solder alloy constituting this solder film had a Young's modulus at room temperature of 35 GPa, a tensile strength of 32 MPa, and a 0.2% proof stress of 22 MPa.

(Example 3)
A lead wire was produced in the same manner as in Example 1 except that the solder film contained 0.70 mass% Cu, 0.2 mass% Sb, and the balance was composed of an alloy of Sn and inevitable impurities. The solder alloy constituting this solder film had a Young's modulus at room temperature of 32 GPa, a tensile strength of 30 MPa, and a 0.2% proof stress of 21 MPa.

Example 4
A lead wire was produced in the same manner as in Example 1 except that the solder film contained 0.70 mass% Cu, 0.3 mass% Sb, and the balance was composed of an alloy of Sn and inevitable impurities. The solder alloy constituting this solder film had a Young's modulus at room temperature of 32 GPa, a tensile strength of 31 MPa, and a 0.2% proof stress of 22 MPa.

(Example 5)
A lead wire was prepared in the same manner as in Example 1 except that the solder film contained 0.70 mass% Cu and 0.3 mass% Ag, and the balance was composed of an alloy of Sn and inevitable impurities. The solder alloy constituting this solder film had a Young's modulus at room temperature of 38 GPa, a tensile strength of 37 MPa, and a 0.2% proof stress of 25 MPa.

(Example 6)
A lead wire was produced in the same manner as in Example 1 except that the solder film contained 0.70 mass% Cu, 0.5 mass% Ag, and the balance was composed of an alloy of Sn and inevitable impurities. The solder alloy constituting this solder film had a Young's modulus at room temperature of 39 GPa, a tensile strength of 39 MPa, and a 0.2% proof stress of 29 MPa.

(Example 7)
A lead wire was produced in the same manner as in Example 1 except that the solder film contained 0.75 mass% Cu, 0.5 mass% Ag, and the balance was composed of an alloy of Sn and inevitable impurities. The solder alloy constituting this solder film had a Young's modulus at room temperature of 40 GPa, a tensile strength of 40 MPa, and a 0.2% proof stress of 30 MPa.

(Comparative Example 1)
A lead wire was produced in the same manner as in Example 1 except that the composition of the solder film was Sn-0.70Cu-0.7Ag (unit: mass%). The solder alloy constituting this solder film had a Young's modulus at room temperature of 41 GPa, a tensile strength of 41 MPa, and a 0.2% proof stress of 33 MPa.

(Comparative Example 2)
A lead wire was produced in the same manner as in Example 1 except that the composition of the solder film was Sn-0.50Cu-1.0Ag (unit: mass%). The solder alloy constituting this solder film had a Young's modulus at room temperature of 37 GPa, a tensile strength of 46 MPa, and a 0.2% proof stress of 28 MPa.

(Comparative Example 3)
A lead wire was produced in the same manner as in Example 1 except that the composition of the solder film was Sn-0.50Cu-2.0Ag (unit: mass%). The solder alloy constituting this solder film had a Young's modulus at room temperature of 42 GPa, a tensile strength of 46 MPa, and a 0.2% proof stress of 35 MPa.

(Conventional example 1)
A lead wire was produced in the same manner as in Example 1 except that the composition of the solder film was Sn-0.50Cu-3.0Ag (unit: mass%). The solder alloy constituting this solder film had a Young's modulus at room temperature of 42 GPa, a tensile strength of 53 MPa, and a 0.2% proof stress of 40 MPa.

(Conventional example 2)
A lead wire was produced in the same manner as in Example 1 except that the composition of the solder film was Sn-0.75Cu-3.5Ag (unit: mass%). The solder alloy constituting this solder film had a Young's modulus at room temperature of 45 GPa, a tensile strength of 53 MPa, and a 0.2% proof stress of 40 MPa.

(Conventional example 3)
A lead wire was produced in the same manner as in Example 1 except that the composition of the solder film was Sn-3.5Ag (unit: mass%). The solder alloy constituting this solder film had a Young's modulus at room temperature of 44 GPa, a tensile strength of 41 MPa, and a 0.2% proof stress of 37 MPa.

  The lead wires of Examples 1 to 7, Comparative Examples 1 to 3, and Conventional Examples 1 to 3 were each solder-connected to the center of a silicon crystal wafer having a side of 100 mm and a thickness of 2 mm. The warp of the silicon crystal wafer after connection was evaluated. Table 1 shows specifications of the lead wires (composition and mechanical characteristics of the solder film) and evaluation results of Examples 1 to 7, Comparative Examples 1 to 3, and Conventional Examples 1 to 3. Here, the warp of the silicon crystal wafer was a relative evaluation based on the warp of Conventional Example 1.

  As shown in Table 1, each lead wire of Examples 1 to 7 has a solder alloy constituting the solder film having a Young's modulus of 32 to 40 GPa, a tensile strength of 30 to 40 MPa, a 0.2% proof stress of 19 to 19 at room temperature. The warpage of the silicon crystal wafer was remarkably small because it was 30 MPa and each satisfied the specified value. Therefore, the solder alloy constituting the solder film has a Young's modulus at room temperature of 40 GPa or less, a tensile strength of 40 MPa or less, and a 0.2% proof stress of 30 MPa or less. It was confirmed that the warpage of the crystal wafer can be remarkably reduced.

  On the other hand, in the lead wire of Conventional Example 2, the Ag content of the solder alloy constituting the solder film was 3.5 mass%, significantly exceeding the specified range (0.1 to 0.55 mass%). For this reason, the solder film in the lead wire of Conventional Example 2 has a Young's modulus, tensile strength, and 0.2% proof stress at room temperature that are all greater than the specified values, and the warp of the silicon crystal wafer is as large as that of Conventional Example 1. .

  Moreover, although the solder film in the lead wire of Conventional Example 3 is the same as the solder film in the lead wire of Conventional Example 2 in terms of Ag content, the Cu content was extremely small. As a result, the tensile strength at room temperature of the solder film in the lead wire of Conventional Example 3 was significantly lower than that of the lead wire in Conventional Example 2. For this reason, the warp of the silicon crystal wafer due to the lead wire of Conventional Example 3 is smaller than that in the case of using the lead wires of Conventional Examples 1 and 2. However, the warpage was not small enough.

  On the other hand, the solder film in each lead wire of Comparative Examples 1 to 3 has smaller Young's modulus, tensile strength, and 0.2% proof stress at room temperature than the solder film in each lead wire of Conventional Examples 1 and 2. Therefore, the warp of the silicon crystal wafer due to the lead wires of Comparative Examples 1 to 3 was smaller than that in the case of using the lead wires of Conventional Examples 1 and 2. However, each lead wire of Comparative Examples 1 to 3 has an Ag content of 0.7 to 2.0 mass% although the Cu content of the solder alloy constituting the solder film satisfies the specified range (0.1 to 0.80 mass%). It was over the specified range (0.1-0.55 mass%). For this reason, it was not possible to satisfy the specified values for the Young's modulus, the tensile strength, and the 0.2% proof stress at room temperature. Therefore, the warp of the silicon crystal wafer due to the lead wires of Comparative Examples 1 to 3 was not yet sufficiently small.

It is a cross-sectional view of a lead wire according to a preferred embodiment of the present invention. It is a modification of FIG.

Explanation of symbols

10 Lead wire 11 Conductor 15 Solder film

Claims (8)

  In a lead wire in which at least a part of the outer periphery of the conductor is covered with a solder film, the solder film is composed of a solder alloy having a Young's modulus at room temperature of 40 GPa or less, a tensile strength of 40 MPa or less, and a 0.2% proof stress of 30 MPa or less. Lead wire characterized by that.   A lead wire in which at least a part of the outer periphery of a conductor is covered with a solder film, wherein the solder film is composed of a Sn-0.10 to 0.80 mass% Cu alloy.   A lead wire in which at least a part of the outer periphery of a conductor is covered with a solder film, wherein the solder film is composed of an Sn-0.10 to 0.80 mass% Cu-0.1 to 0.55 mass% Ag alloy.   A lead wire in which at least a part of the outer periphery of a conductor is covered with a solder film, wherein the solder film is composed of a Sn-0.10 to 0.80 mass% Cu-0.1 to 0.35 mass% Sb alloy.   The lead wire according to any one of claims 1 to 4, wherein the conductor is made of soft copper.   The lead wire according to any one of claims 1 to 4, wherein the conductor is made of a clad material made of a copper material and an Fe-Ni alloy material.   The lead wire according to any one of claims 1 to 6, wherein the conductor is a rectangular wire. 8. A solar cell, wherein the lead wire according to claim 1 and a silicon crystal wafer are joined via a solder alloy constituting the solder film.

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