JP4656100B2 - Solder-plated wire for solar cell and manufacturing method thereof - Google Patents

Description

  The present invention relates to a solar cell solder plated wire for connecting Si cells of a solar cell and a method for manufacturing the same.

  For solar cells, polycrystalline or single crystal Si crystal wafers are used.

  As shown in FIG. 2, the solar cell joins the connection lead wire 2 to a predetermined region on the back surface of the Si crystal wafer (Si cell) 1 with solder, and transmits the generated power through the connection lead wire 2. It has a configuration.

  As shown in FIG. 3, the connecting lead wire 2 is a solder plating wire comprising a conductor 3 and a solder plating 4 covering the conductor 3, and the conductor 3 is made of pure Cu such as tough pitch Cu or oxygen-free Cu. Sn-Pb eutectic solder is used as the solder plating 4 (see, for example, Patent Document 1).

  In addition, switching from solder plated wires to those using solder that does not contain Pb has been studied in consideration of the environment (see, for example, Patent Document 2).

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

  By the way, Si cells occupy most of the material cost among the members constituting the solar cell. Therefore, thinning of the Si cell has been studied.

  However, the thinned Si cell may suffer from problems such as warpage of the Si crystal wafer and damage due to the warpage due to a heating process at the time of joining the solder plating wires and a temperature change at the time of use. For example, as shown in FIG. 4A, by soldering the Si cell 1 and the solder plated wire 2 which were flat before solder connection, as shown in FIG. Alternatively, stress is generated in the Si cell 1 due to heat shrinkage after bonding, and warpage occurs.

  In order to prevent the warpage of the cell due to such heat shrinkage, there is an increasing need for a material having a low thermal expansion coefficient as a material used for a solder plating wire. For example, as an example of a solder plating wire having a small thermal expansion coefficient, as shown in FIG. 5, solder plating in which a rectangular conductor 7 clad with Cu 5 -invar 6 -Cu 5 (hereinafter, Invar: registered trademark) is coated with a solder film 8 is coated. There is a line 9.

  However, since the invar 6 has a low thermal expansion, the solder plating wire 9 in FIG. 4 can be matched with the Si cell in thermal expansion, but the electrical conductivity is lowered and the power generation efficiency as a solar cell is reduced. End up.

  Accordingly, an object of the present invention is to solve the above-described problems and provide a solar cell solder-plated wire having a low conductivity and high conductivity even in thermal contraction after the Si cell and solder connection, and a method for manufacturing the same. There is.

  In order to achieve the above object, a first aspect of the present invention provides a solder plating wire for a solar battery in which the surface of a conductor whose cross-sectional shape is processed into a flat rectangular shape is coated with solder plating so as to be joined to a solar battery cell. The conductor has a two-layer structure having a volume resistivity of 30 μΩ · mm or less and a surface layer and an inner layer, and the crystal grain size of the surface layer is larger than the crystal grain size of the inner layer. It is.

The invention according to claim 2 is a solder plating wire for solar battery in which the surface of a conductor whose cross-sectional shape is processed into a flat rectangular shape is coated with solder plating so as to be bonded to a solar battery cell.
The conductor has a two-layer structure having a volume resistivity of 30 μΩ · mm or less and a surface layer and an inner layer, the crystal grain size of the surface layer being larger than the crystal grain size of the inner layer, and the thickness of the inner layer. This is a solder-plated wire for solar cells in which the ratio r2 / (r1 + r2) of the thickness of the surface layer to the sum of the half thickness r1 and the thickness r2 of the surface layer is 0.1 to 0.3.

  The invention according to claim 3 is the solder plated wire for solar cells according to claim 1 or 2, wherein the 0.2% proof stress of the conductor in a tensile test is 16 to 70 MPa.

  Invention of Claim 4 is a solder plating wire for solar cells in any one of Claims 1-3 whose 0.2% yield strength in a tension test is 37-100 Mpa.

  According to a fifth aspect of the present invention, there is provided a method for producing a solder-plated wire for a solar battery in which the surface of a conductor whose cross-sectional shape is processed into a flat rectangular shape to be bonded to a solar battery cell is coated with a solder plating wire for a solar battery. After casting a metal material of mm or less, a conductor is formed by rolling or slitting, heat-treating the conductor, skin-passing only the vicinity of the surface of the conductor, and heat-treating again to thereby form the conductor. Is formed into a two-layer structure in which the crystal grain size of the surface layer is larger than the crystal grain size of the inner layer.

  The invention according to claim 6 is the method for producing a solder plated wire for a solar cell according to claim 5, wherein a processing rate of the conductor by the skin pass processing is 0.5 to 5%.

  Invention of Claim 7 performs the said heat processing for 40 to 60 minutes at 650-750 degreeC, and performs the reheat processing for 40 to 60 minutes at 850-900 degreeC, The manufacturing method of the solder plating wire for solar cells of Claim 5 or 6 It is.

  According to the present invention, there is an excellent effect that there is little warping of the cell and high conductivity even in thermal contraction after the solder connection with the Si cell.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a cross-sectional view showing a preferred embodiment of a solar cell solder plating wire according to the present invention.

  As shown in FIG. 1, the solder plating wire for solar battery cells (hereinafter referred to as solder plating wire) of this embodiment is a solder plating wire that joins a solar battery cell (Si cell) produced from a Si wafer, and is a conductor. 10 and solder plating (not shown) covering the conductor 10.

  The conductor 10 is formed of a conductor material having a rectangular cross section so that solder connection to the Si cell is easy, and a volume resistivity of 30 μΩ · mm or less. Cu is used as a conductor material having a volume resistivity of 30 μΩ · mm or less.

  Further, the conductor 10 is formed in a two-layer structure including a surface layer 11 and an inner layer (inner layer portion) 12 having a crystal grain size larger than that of the surface layer (outer layer portion) 11. The two-layer structure of the conductor 10 is formed by subjecting the conductor to heat treatment and skin pass processing, which will be described later. 12 In the present embodiment, there are three layers of surface layer-inner layer-surface layer in which the surface layer 11 is formed on both surfaces of the flat conductor 10.

  In the solder plated wire of the present embodiment, the ratio of the thickness r2 of the inner layer 12 to the total of the thickness r2 of the surface layer 11 and the thickness r2 of the inner layer 12 (referred to as the thickness ratio) r2 / (r1 + r2) is 0.1. It was set to -0.3.

  The thickness ratio is determined by observing the cross section of the conductor 10 to determine a region 13 having a small crystal grain size and a region having a large crystal grain size to determine the boundary 13 between the two layers. And the thickness r2 of the region (surface layer 11) having a large particle diameter are respectively measured and obtained from the measured r1 and r2.

  Moreover, the 0.2% yield strength in the tensile test of the conductor 10 is 16-70 Mpa in the solder plating wire of this embodiment. Moreover, the 0.2% yield strength in the tensile test of a solder plating wire is 37-100 Mpa. Here, when the 0.2% yield strength by the tensile test is obtained, the 0.2% yield strength of the conductor is obtained by using the cross-sectional area of the conductor 10, and the 0.2% yield strength of the solder plated wire is The cross-sectional area of only the conductor 10 is used to divide the load in the tensile test.

  A method for manufacturing the solder plated wire of this embodiment will be described.

  First, a conductor material (Cu in this embodiment) having a volume resistivity of 30 μΩ · mm or less constituting the conductor 10 is cast into a long length, and then the cast conductor material is subjected to rolling or slitting to obtain a flat conductor 10. Form.

  Next, the conductor 10 is heat-treated. This heat treatment is performed in order to recover (correct) strain and grow the crystal grain size of the rolled or slit conductor. The heat treatment method may be an electric heating method in which current is directly applied to the conductor 10 to heat it, or a batch heating method in which the conductor 10 is heated in a high temperature furnace. The heat treatment conditions are preferably 650 to 750 ° C. and 40 to 60 minutes.

  Next, the heat-treated conductor 10 is subjected to skin pass processing. Skin pass processing here is a process of rolling only the surface layer 11 of the conductor 10 lightly. The skin pass processing method for the conductor 10 may be either rolling or pressing. The processing rate of skin pass processing is preferably 0.5 to 5%.

  After the skin pass processing, the conductor 10 is heat treated again (reheat treatment). This reheating treatment is performed in order to generate recrystallization in the conductor 10 after the skin pass processing. As the re-heat treatment method, any one of an electric heating method and a batch heating method may be used in the same manner as the heat treatment performed before the skin pass processing. The reheating condition is preferably 850 to 900 ° C. and 40 to 60 minutes.

  By performing heat treatment, skin pass processing, and reheat treatment on the rolled or slit conductor 10, a conductor having a two-layer structure in which the crystal grain size of the surface layer 11 is larger than the crystal grain size of the inner layer 12 can be formed.

  After the reheating treatment, the conductor is coated with a solder plating to obtain a solder plated wire.

  Next, the operation of the present embodiment will be described.

  The solder plated wire is soldered (joined) to the Si cell at a high temperature. In general, when dissimilar metals having different thermal expansion coefficients, such as Si cells and solder plated wires, are connected at a high temperature, a value obtained by integrating the temperature change, the thermal expansion coefficient, and the Young's modulus is shown in FIG. It becomes the force that generates the warp as shown in. The members (Si cell and solder plating wire) connected to each other have significantly different rigidity, and when the solder connection temperature is as high as 200 ° C. or higher, the conductor having a smaller cross-sectional area than the Si cell yields.

  When the yield stress is small, the conductor is plastically deformed with a small force (the force that generates the above-described warpage), and even if a force beyond that is applied, the increase in deformation resistance is small. The index indicating the stress that causes the plastic deformation is 0.2% proof stress, and the smaller the 0.2% proof stress of the conductor, the lower the warpage amount after solder connection to the Si cell.

  The 0.2% proof stress is known to have a large correlation with the crystal grain size of the conductor, and the smaller the grain boundary, that is, the larger the crystal grain size, the smaller the deformation resistance of the conductor. That is, when the crystal grain size of the conductor is increased, the yield strength of the conductor is reduced, and the thermal stress (force to warp the cell) generated in the Si cell after connecting the solder plating wire to the Si cell can be reduced.

  On the other hand, if the crystal grain size of the conductor becomes too large, the elongation of the conductor decreases, cracks are likely to occur, and the conductor becomes brittle.

  Therefore, the crystal grain size needs to be within a certain range of conditions. Factors that cause the crystal grain size to fall within the above-mentioned range of conditions include conductor rolling or slit processing conditions, heat treatment conditions (temperature, time, etc.), skin pass processing conditions (processing rate), and reheat treatment conditions (temperature, time). Etc.), and the crystal grain size is determined by a combination thereof.

  The solder plated wire of the present embodiment achieves high conductivity by using a conductor material of 30 μΩ · mm or less such as copper, and the conductor 10 has a crystal grain size smaller than that of the surface layer 11 having a large crystal grain size and the surface layer 11. By forming the inner layer 12 in a two-layer structure, in the surface layer 11 having a large crystal grain size, the yield strength of the conductor is reduced to prevent cell warpage, and the inner layer 12 has a crystal grain size comparable to that of the prior art. By doing so, embrittlement of the conductor 10 can be prevented.

  Moreover, since the conductor 10 having the selected crystal grain size (within the above-described certain condition range) can obtain a sufficient elongation, the conductor 10 has excellent molding processability and long-term reliability.

  In the solder plated wire of the present embodiment, the ratio (thickness ratio) r2 / (r1 + r2) of the thickness of the surface layer 11 to the sum of the half r1 of the thickness of the inner layer 12 and the thickness r2 of the surface layer 11 is 0.1 to 0.3.

  If the thickness ratio is less than 0.1, there are too few regions where the crystal grain size of the surface layer 11 is large, and the effect of reducing cell warpage cannot be obtained. On the other hand, if the thickness ratio is greater than 0.3, the region where the crystal grain size of the surface layer 11 is large is too wide, and cracks are generated. Therefore, the thickness ratio is preferably 0.1 to 0.3.

  The solder plated wire of the present embodiment has a 0.2% yield strength of the conductor 10 of 16 to 70 MPa, or a 0.2% yield strength of the solder plated wire of 37 to 100 MPa. Both warpage of the Si cell and generation of cracks can be reliably suppressed.

  The solder covering the conductor 10 has been required to have a composition capable of low-temperature connection in consideration of thermal expansion matching with the Si cell. By using the two-layer structure conductor 10 shown in FIG. A Sn-Ag-Cu composition having a high connection temperature can also be selected.

  Further, in the method of manufacturing a solder plated wire according to this embodiment, when the conductor 10 is heat-treated and then subjected to a skin pass process in which only the surface layer 11 is lightly rolled, the inner layer 12 is not hardened and only the surface layer is hardened. In general, when the cured conductor material is heat-treated, crystals grow and the crystal grain size increases. However, when the conductor 10 having only the surface layer 11 cured is reheated as in the manufacturing method of the present embodiment, only the surface layer 11 is treated. Crystal growth is promoted, and the crystal grain size of the surface layer 11 can be made larger than before the skin pass processing. On the other hand, since the inner layer 12 is not hardened by the skin pass process, the crystal growth by the reheat treatment is reduced. Therefore, the average 0.2% proof stress of the conductor 10 can be reduced as compared with that before the skin pass processing, and the crystal grains in the inner layer 12 are not grown extremely, so that the generation of cracks is suppressed. Can do.

  That is, according to the method of manufacturing a solder plated wire of the present embodiment, it is possible to prevent the warpage of the cell even in the thermal contraction after the solder connection with the Si cell, and to prevent the conductor from becoming brittle due to the occurrence of a crack. And a solder plated wire having high conductivity can be obtained.

  The skin pass processing degree of the conductor 10 is suitably 0.5 to 5%, more preferably 3 to 5%. When the degree of work is less than 0.5%, the effect of reducing the warpage of the Si cell cannot be obtained. By setting the workability within the range of 0.5 to 5%, it is possible to prevent cracks in the conductor while significantly reducing the proof stress than before, and to prevent deterioration in reliability over a long period of time.

  Moreover, regarding the heat treatment conditions, in the manufacturing method of this embodiment, after rolling or slitting, heat treatment is performed at 650 to 750 ° C. for 40 to 60 minutes, then skin pass processing is performed, and after skin pass processing, 850 to 900 ° C. is performed for 40 to 60 minutes. Re-heat treatment. The reason why the appropriate range of heat treatment conditions after rolling or slitting is set to 650 ° C. to 750 ° C. is that when the heat treatment temperature is lower than 650 ° C., the crystal grains in the center (inner layer 12) become as small as 200 μm or less, and 750 ° C. This is because cracks are likely to occur when the value exceeds. The reason why the appropriate range of the heat treatment temperature after the skin pass is 850 to 900 ° C. is that if the temperature is lower than 850 ° C., the crystal grains of the surface layer 11 do not increase, and if it exceeds 900 ° C., cracks are likely to occur.

  As described above, the embodiment of the present invention is not limited to the above-described embodiment, and various other embodiments are assumed.

  Although Cu was used as a conductor material having a volume resistivity of 30 μΩ · mm or less, Au, Ag, Al, or an alloy containing these elements can also be used.

  In particular, the conductor material having the lowest volume resistivity is Ag, which can maximize the power generation efficiency of the solar cell. Moreover, when priority is given to cost reduction, it is good to use Cu as a conductor material. In order to reduce the weight, it is desirable to use Al as the conductor material.

  As the kind of Cu, any of tough pitch Cu, oxygen-free Cu, phosphorus deoxidized Cu, and Cu (purity 99.9999% or more) may be used. In order to minimize the 0.2% proof stress, Cu having a high purity is preferably used, and Cu having a purity of 99.9999% or more is selected. On the other hand, when impurities are added, the 0.2% proof stress increases, but to reduce costs, tough pitch Cu or phosphorus deoxidized Cu is selected.

  The conductor material is stored in a molten metal storage means (for example, a molten copper storage means) of a continuous casting and rolling apparatus, and from a Ti, Zr, V, Ta, Fe, Ca, Mg or Ni into a molten copper having an oxygen content of 5 ppm or more. Contains at least one or two selected metals (sulfur affinity metal), adjusts the ratio of the metal contained in the molten copper to 0.0007 to 0.05% by weight, and uses the molten copper A roughened material may be manufactured, and the roughened material may be subjected to a cold wire drawing process with a surface reduction rate of 30% or more and a heat treatment.

  This sulfur-affinity metal (for example, Ti) reacts with S dissolved in a copper melt having an oxygen content of 5 ppm or more, so that S precipitates as a sulfide (for example, TiS), and the solid solution amount of S Decrease. Therefore, even if tough pitch copper (TPC) having a relatively high oxygen content is used as the conductor material, S can be precipitated from the copper matrix, and high purity copper (for example, OFC) can be used in terms of softening characteristics. This is because the characteristics almost equivalent to those of (1) can be obtained. According to this, even if it is not expensive OFC, 0.2% yield strength can be further reduced using inexpensive TPC.

  As a conductor material, about 200 ppm of P may be added to tough pitch copper, and a copper alloy may be used so that the P content finally becomes 100 ppm or less. In this case, the added P is combined with O, and impurities are reduced. Therefore, a material with low yield strength can be obtained inexpensively without using an expensive OFC.

  The conductor 10 may be either a method of rolling a round wire obtained by casting to make it flat or a method of slitting. The rolling method can produce a long and uniform product. The slit method has the merit that it can cope with materials of various widths.

  As a heat treatment method for reducing the 0.2% proof stress, either an electric heating method or a batch heating method can be applied. When processing continuously over a long length, current heating is suitable. When stable heat treatment is required, a batch-type heating method is desirable. In addition, a furnace in a hydrogen reducing atmosphere may be used from the viewpoint of preventing oxidation.

  In the heat treatment and reheat treatment, the crystal grain size can be controlled by a combination of the heating temperature and the heating time. The heat treatment conditions and reheat treatment conditions are the same for conductors using Au, Ag, and Al other than Cu.

  The solder used for covering the conductor 10 is Sn-based solder (first component) or at least one element selected from Pb, In, Bi, Sb, Ag, Zn, Ni, and Cu as the second component. Although it is Sn type alloy solder containing 1 wt% or more, you may use what contains 1000 ppm or less of trace elements as a 3rd component further.

  Further, the wiring conductor for connecting solar cells and the solder plating wire according to the present invention may include a deformed portion which is easily deformed at a plurality of locations where the cell is joined. Etching, pressing, bending as a processing method. Any one or a plurality of moldings may be used in combination.

Next, embodiments of the present invention will be described based on examples.
(Example)
In Example 1-14, a Cu material having a width of 2.0 mm and a thickness of 0.16 mm was first rolled into a rectangular wire shape, and the strain was recovered and the crystal grain size was grown by heat treatment (heat treatment step). Next, only the surface layer of the conductor is lightly rolled (skin pass processing step), followed by heat treatment to generate recrystallization (reheat treatment step), and finally the periphery is Sn-3% Ag-0. What was coated with 5% Cu-based lead-free solder was produced. However, the heat treatment conditions, skin pass conditions, and reheat treatment conditions of the examples and comparative examples were different from each other.
(Comparative example)
The comparative example was manufactured in the same manner as the example except that skin pass processing and re-heat treatment were not performed.

  The results of Example 1-14 and Comparative Example 1 are shown in Tables 1 and 2. In the table, 10 crystal grain sizes are selected from those having large crystal grains in the cross section of the solder plating wire, and 10 crystal grain sizes are averaged.

  As shown in Table 1 and Table 2, when samples were prepared under various heat treatment conditions, skin pass conditions, and reheat treatment conditions, there were conditions that could prevent cracking of the conductor while reducing 0.2% proof stress. It could be confirmed. In Example 1-14, the conductor is formed in a two-layer structure of a surface layer (outer layer portion) having a crystal grain size of 450 μm and an inner layer (inner layer portion) having a crystal grain size of 250 μm, thereby maintaining high conductivity. It has been confirmed that the warpage of the cell can be reduced as compared with the conventional solder-plated wire, and that the conductor used for the solder-plated wire for connecting solar cells can be made highly reliable.

  In particular, in Examples 2-4, 7, 8, 10, 11, 13, and 14, the thickness ratio between the inner layer portion and the outer layer portion is set within a range of 0.1 to 0.3, thereby preventing cracks. It was particularly reliable in both cell warpage.

On the other hand, as shown in Table 1, in the comparative example in which the conductor is not formed in the two-layer structure, a crack is generated.
(Conventional example)
In the conventional example, a conductor is formed by rolling and molding a Cu material having a width of 2.0 mm and a thickness of 0.16 mm into a rectangular wire, and the lead-free solder of Sn-3% Ag-0.5% Cu is formed around the conductor. The various 0.2% proof stress shown in Table 3 was produced. The warpage of each solder plated wire produced by solder connection to a Si cell having a length of 150 mm × width of 150 mm and a thickness of 200 μm was examined.

  As shown in Table 3, the warpage is reduced as the 0.2% proof stress is reduced, and the warpage of the solder plating coated Cu rectangular wire of 70 MPa can be reduced to about 1/2 of that of 160 MPa.

It is a cross-sectional view which shows the conductor of the solder plating wire for solar cells of suitable one Embodiment which concerns on this invention. It is a perspective view which shows the joining state of Si cell and a solder plating wire. It is a cross-sectional view which shows the conventional solder plating wire. 4A is a cross-sectional view showing a state before joining the Si cell and the solder plating wire, and FIG. 4B is a cross-sectional view showing a state where warpage occurs after joining the Si cell and the solder plating wire. FIG. It is a cross-sectional view showing a solder plating wire having a Cu-Invar-Cu structure.

Explanation of symbols

10 conductor 11 surface layer 12 inner layer

Claims (7)

In the solder plating wire for solar battery in which the surface of the conductor whose cross-sectional shape is processed into a flat rectangular shape to coat the solar battery cell is coated with solder plating,
The conductor has a volume resistivity of 30 μΩ · mm or less and a two-layer structure including a surface layer and an inner layer, and the crystal grain size of the surface layer is larger than the crystal grain size of the inner layer. Solder plating wire for solar cells. In the solder plating wire for solar battery in which the surface of the conductor whose cross-sectional shape is processed into a flat rectangular shape to coat the solar battery cell is coated with solder plating,
The conductor has a two-layer structure having a volume resistivity of 30 μΩ · mm or less and a surface layer and an inner layer, the crystal grain size of the surface layer being larger than the crystal grain size of the inner layer, and the thickness of the inner layer. A solder plated wire for a solar cell, wherein a ratio r2 / (r1 + r2) of the thickness of the surface layer to the sum of the half of the thickness r1 and the thickness r2 of the surface layer is 0.1 to 0.3.   The solder plated wire for a solar cell according to claim 1 or 2, wherein the 0.2% proof stress of the conductor in a tensile test is 16 to 70 MPa.   The 0.2% yield strength in a tensile test is 37-100 Mpa, The solder plating wire for solar cells in any one of Claims 1-3. In the method for producing a solder-plated wire for a solar battery in which the surface of a conductor whose cross-sectional shape is processed into a flat rectangular shape is coated with solder plating in order to join the solar battery cell,
After casting a metal material having a volume resistivity of 30 μΩ · mm or less, a conductor is formed by rolling or slitting, heat-treating the conductor, skin-passing only near the surface of the conductor, and heat-treating again. A method for producing a solder plated wire for a solar cell, wherein the conductor is formed into a two-layer structure in which the crystal grain size of the surface layer is larger than the crystal grain size of the inner layer.   The manufacturing method of the solder plating wire for solar cells of Claim 5 which makes the processing rate of the conductor by the said skin pass process 0.5 to 5%.   The manufacturing method of the solder plating wire for solar cells of Claim 5 or 6 which performs the said heat processing for 40 to 60 minutes at 650-750 degreeC, and performs the said reheat processing for 40 to 60 minutes at 850-900 degreeC.

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