A palladium-plated bonded copper wire and its preparation process

Abstract

The present invention belongs to the technical field of bonded copper wires, and particularly relates to a palladium-plated bonded copper wire and its preparation process. Copper, platinum, cobalt, zirconium, magnesium, strontium, rare earth metal elements and silicon are mixed to form a copper-based bonded wire material. The functions of the metals platinum, cobalt and zirconium and the combined silicon help to improve the internal grain distribution structure of the copper wire, improve the mechanical properties such as the tensile strength of the copper wire, the balling degree and the oxidation resistance; the magnesium, strontium and rare earth metal elements used help to improve the bonding performance of the bonded copper wire. The primary and secondary palladium plating processes are adopted to make the bonded copper wire have excellent tensile properties, bonding properties and oxidation resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of bonding copper wires, and particularly relates to a palladium-plated bonding copper wire and a preparation process thereof. Background Art

[0002] In recent years, with the rapid development of the integrated circuit packaging industry, the performance requirements for bonding wire materials have become increasingly high. All along, bonding gold wires have the advantages of high electrical conductivity, corrosion resistance, good toughness, etc., and are widely used in integrated circuit packaging. However, due to the lack of resources, under the dual pressures of rising material prices and falling prices of electronic products, it has invisibly promoted professional institutions and units in the industry to continuously seek new low-cost alternative materials, which has become the law of market development. Bonding copper wires have obvious advantages compared with gold wires. Using bonding copper wires can not only greatly reduce the manufacturing cost of devices and improve the competitive advantage, but also the excellent material properties of bonding copper wires have accelerated their application in the electronic packaging industry. The main advantages are low material cost, excellent mechanical properties, electrothermal properties, and bonding properties. However, bonding copper wires also have some defects, such as high hardness, which is likely to cause problems such as substrate fracture and false soldering, and easy oxidation, which affects the quality of products. Summary of the Invention

[0003] In view of the above problems, the purpose of the present invention is to provide a palladium-plated bonding copper wire and a preparation process thereof.

[0004] The technical content of the present invention is as follows:

[0005] The present invention provides a preparation process of a palladium-plated bonding copper wire, including the following steps:

[0006] 1) Pretreatment of raw materials: The used raw materials are ultrasonically cleaned with absolute ethanol and dried.

[0007] 2) Synthesis of bonding copper wire: Metal copper, platinum, cobalt, zirconium, magnesium, strontium, rare earth metal elements and silicon are mixed, and vacuum melting and continuous casting are carried out to obtain bonding copper wire.

[0008] By mass fraction, the metal copper ≥ 99.215%, platinum accounts for 0.04 - 0.06%, cobalt accounts for 0.05 - 0.08%, zirconium accounts for 0.03 - 0.05%, magnesium accounts for 0.05 - 0.07%, strontium accounts for 0.03 - 0.05%, rare earth metal elements account for 0.005 - 0.006% and silicon accounts for 0.3 - 0.5%;

[0009] The rare earth elements include cerium, gadolinium and yttrium;

[0010] When carrying out the vacuum melting, the vacuum degree is 10 -3 ~10 -2MPa, the melting temperature is 1780 - 1810 °C, and the melting time is 60 - 80 minutes;

[0011] The diameter of the obtained bonded copper wire is 0.08 - 0.1 mm;

[0012] 3) Surface activation treatment of the bonded copper wire: After the bonded copper wire is cooled to 100 - 120 °C, it is evenly sprayed with a surface active solution by aerosol, left standing, and then dried;

[0013] The components of the surface active solution include a mixture of sulfuric acid, potassium dihydrogen phosphate, and ethylmalonic acid, and the mixing volume ratio is (2 - 3):(3 - 5):(1 - 2);

[0014] The concentration of the sulfuric acid is 5 - 10 mol / L;

[0015] The concentration of the potassium dihydrogen phosphate is 30 - 40 mol / L;

[0016] The concentration of the ethylmalonic acid is 20 - 30 mol / L;

[0017] 4) Primary palladium plating treatment: Electroplate a palladium layer on the surface of the bonded copper wire that has undergone surface activation treatment, using an alkaline palladium electroplating solution;

[0018] The concentration of palladium in the alkaline palladium electroplating solution is 30 - 40 g / L, and the thickness of the palladium layer is 0.001 - 0.003 mm;

[0019] 5) Rough wire drawing and medium wire drawing: Heat the palladium-plated bonded copper wire to 500 - 600 °C for rough wire drawing, and cool it to 400 - 500 °C for medium wire drawing;

[0020] The diameter of the bonded copper wire after rough wire drawing is 0.06 - 0.08 mm;

[0021] The diameter of the bonded copper wire after medium wire drawing is 0.04 - 0.06 mm;

[0022] 6) Secondary palladium plating treatment: Electroplate a secondary palladium layer on the surface of the palladium-plated bonded copper wire that has undergone rough wire drawing and medium wire drawing;

[0023] The electroplating is carried out using an alkaline palladium electroplating solution containing an antioxidant;

[0024] The concentration of palladium in the alkaline palladium electroplating solution is 30 - 40 g / L, and the thickness of the palladium layer is 0.001 - 0.003 mm;

[0025] The antioxidant includes pyrrole-2-sulfonic acid or pyrrole-3-sulfonic acid, and the concentration is 150 - 200 g / L;

[0026] 7) Fine wire drawing;

[0027] The diameter of the bonded copper wire after fine drawing is 0.01 - 0.03 mm;

[0028] 8) Annealing: Nitrogen is used as the annealing atmosphere, with a flow rate of 5 - 8 L / min, an annealing temperature of 400 - 600 °C, and an annealing rate of 1 - 2 m / s;

[0029] For the electroplating conditions described in step 4) and step 6), the pH of the electroplating solution is 7 - 8, the current density of electroplating is 2 - 5 A / dm 2 , the electroplating temperature is 60 - 70 °C, and the electroplating speed is 30 - 40 m / s.

[0030] The present invention also provides a palladium-plated bonded copper wire obtained by the above preparation process.

[0031] The beneficial effects of the present invention are as follows:

[0032] In the preparation process of the palladium-plated bonded copper wire of the present invention, copper, platinum, cobalt, zirconium, magnesium, strontium, rare earth metal elements and silicon are mixed to form a copper-based bonding wire material. The roles of the metal platinum, cobalt, zirconium and combined silicon used help to improve the internal grain distribution structure of the copper wire, improve the mechanical properties such as the tensile strength of the copper wire, the ball forming property and the oxidation resistance; the magnesium, strontium and rare earth metal elements used help to improve the bonding property of the bonded copper wire. By using the primary and secondary palladium plating processes, the bonded copper wire has excellent tensile properties, bonding properties and oxidation resistance. The palladium-plated bonded copper wire prepared by the present invention solves the defects such as strong hardness, poor bonding ability and poor oxidation resistance of the copper wire compared with the traditional copper-based bonding wire, comprehensively improves the use performance of the bonded copper wire, meets high standards and is suitable for actual production applications. Brief Description of the Drawings

[0033] Figure 1 It is a cross-sectional electron microscope image of the bonded copper wire prepared by the present invention. Detailed Description of the Embodiments

[0034] The present invention will be further described in detail below through specific embodiments and the description of the drawings. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the protection scope of the present invention. After reading the present invention, various equivalent modifications made by those skilled in the art to the present invention all fall within the scope defined by the appended claims of this application.

[0035] Unless otherwise specified, all raw materials and reagents of the present invention are raw materials and reagents in the conventional market.

[0036] Example 1

[0037] A preparation process of a palladium-plated bonded copper wire

[0038] 1) Pretreatment of raw materials: The used raw materials are ultrasonically cleaned with absolute ethanol and dried;

[0039] 2) Synthesis of bonded copper wire: 99.404% metallic copper, 0.06% platinum, 0.08% cobalt, 0.05% zirconium, 0.07% magnesium, 0.03% strontium, 0.006% rare earth metal elements and 0.3% silicon are mixed and subjected to vacuum melting. The vacuum degree is 10 -2 MPa, the melting temperature is 1800 °C, the melting time is 70 minutes, and then continuous casting is carried out to obtain bonded copper wire with a diameter of 0.08 mm;

[0040] The rare earth elements include cerium, gadolinium and yttrium metals, and the mass ratio is 1:1:1;

[0041] 3) Surface activation treatment of bonded copper wire: After the bonded copper wire is cooled to 110 °C, it is subjected to aerosol uniform spraying treatment with a surface active solution, allowed to stand, and dried;

[0042] The components of the surface active solution include a mixture of sulfuric acid, potassium dihydrogen phosphate and ethylmalonic acid, and the mixing volume ratio is 3:4:2;

[0043] The concentration of the sulfuric acid is 8 mol / L;

[0044] The concentration of the potassium dihydrogen phosphate is 35 mol / L;

[0045] The concentration of the ethylmalonic acid is 25 mol / L;

[0046] 4) Primary palladium plating treatment: A 0.002 mm palladium layer is electroplated on the surface of the bonded copper wire after surface activation treatment. An alkaline palladium electroplating solution is used, the concentration of palladium is 35 g / L, the pH is 8, and the current density of electroplating is 3 A / dm 2 , the electroplating temperature is 65 °C, and the electroplating speed is 35 m / s;

[0047] 5) Rough drawing and medium drawing: The palladium-plated bonded copper wire is heated to 550 °C and subjected to rough drawing until the copper wire diameter is 0.07 mm, and then cooled to 450 °C and subjected to medium drawing until the copper wire diameter is 0.05 mm;

[0048] 6) Secondary palladium plating treatment: After the palladium-plated bonded copper wire after rough drawing and medium drawing is cooled, a 0.002 mm secondary palladium layer is electroplated on the surface. An alkaline palladium electroplating solution containing 18 g / L pyrrole-2-sulfonic acid is used, the pH is 8, the concentration of palladium is 35 g / L, and the current density of electroplating is 3 A / dm 2 , the electroplating temperature is 65 °C, and the electroplating speed is 35 m / s;

[0049] 7) The diameter of the bonded copper wire is 0.02 mm after fine drawing;

[0050] 8) Annealing: Nitrogen is used as the annealing atmosphere with a flow rate of 6 L / min, an annealing temperature of 500 °C, and an annealing rate of 1.5 m / s.

[0051] Example 2

[0052] A preparation process of palladium-plated bonded copper wire

[0053] 1) Pretreatment of raw materials: The used raw materials are ultrasonically cleaned with absolute ethanol and then dried.

[0054] 2) Synthesis of bonded copper wire: 99.335% metallic copper, 0.05% platinum, 0.06% cobalt, 0.04% zirconium, 0.06% magnesium, 0.05% strontium, 0.005% rare earth metal elements, and 0.4% silicon are mixed and subjected to vacuum melting. The vacuum degree is 10 -3 MPa, the melting temperature is 1780 °C, and the melting time is 80 minutes. Then, continuous casting is carried out to obtain the bonded copper wire with a diameter of 0.08 mm.

[0055] The rare earth elements include cerium, gadolinium, and yttrium metals with a mass ratio of 1:1:1.

[0056] 3) Surface activation treatment of bonded copper wire: After the bonded copper wire is cooled to 100 °C, it is evenly sprayed with a surface active solution by aerosol, allowed to stand, and then dried.

[0057] The components of the surface active solution include a mixture of sulfuric acid, potassium dihydrogen phosphate, and ethylmalonic acid with a mixing volume ratio of 2:3:1.

[0058] The concentration of the sulfuric acid is 5 mol / L.

[0059] The concentration of the potassium dihydrogen phosphate is 30 mol / L.

[0060] The concentration of the ethylmalonic acid is 30 mol / L.

[0061] 4) Primary palladium plating treatment: A 0.001-mm-thick palladium layer is electroplated on the surface of the bonded copper wire after surface activation treatment. An alkaline palladium electroplating solution is used with a palladium concentration of 30 g / L, a pH of 7, an electroplating current density of 2 A / dm 2 , an electroplating temperature of 70 °C, and an electroplating speed of 30 m / s.

[0062] 5) Rough drawing and medium drawing: The palladium-plated bonded copper wire is heated to 500 °C and subjected to rough drawing until the copper wire diameter is 0.06 mm, and then cooled to 400 °C and subjected to medium drawing until the copper wire diameter is 0.04 mm.

[0063] 6) Secondary palladium plating treatment: After cooling the palladium-plated bonded copper wire that has undergone rough wire drawing and medium wire drawing, a secondary palladium layer of 0.001 mm is electroplated on the surface. An alkaline palladium electroplating solution containing 150 g / L of pyrrole-3-sulfonic acid is used, with a pH of 7, a palladium concentration of 30 g / L, and an electroplating current density of 2 A / dm 2 , the electroplating temperature is 70 °C, and the electroplating speed is 30 m / s;

[0064] 7) After the bonded copper wire is finely drawn, its diameter is 0.01 - 0.03 mm;

[0065] 8) Annealing: Nitrogen is used as the annealing atmosphere, with a flow rate of 5 L / min, an annealing temperature of 400 °C, and an annealing rate of 1 m / s.

[0066] Example 3

[0067] A preparation process of palladium-plated bonded copper wire

[0068] 1) Pretreatment of raw materials: The used raw materials are ultrasonically cleaned with anhydrous ethanol and then dried;

[0069] 2) Synthesis of bonded copper wire: 99.215% metallic copper, 0.06% platinum, 0.07% cobalt, 0.04% zirconium, 0.07% magnesium, 0.04% strontium, 0.005% rare earth metal elements, and 0.5% silicon are mixed and subjected to vacuum melting. The vacuum degree is 10 -2 MPa, the melting temperature is 1780 °C, the melting time is 60 minutes, and then continuous casting is carried out to obtain a bonded copper wire with a diameter of 0.1 mm;

[0070] The rare earth elements include cerium, gadolinium, and yttrium, with a mass ratio of 1:1:1;

[0071] 3) Surface activation treatment of bonded copper wire: After the bonded copper wire is cooled to 120 °C, it is subjected to aerosol uniform spraying treatment with a surface activation solution, allowed to stand, and then dried;

[0072] The components of the surface activation solution include a mixture of sulfuric acid, potassium dihydrogen phosphate, and ethylmalonic acid, with a mixing volume ratio of 3:5:2;

[0073] The concentration of the sulfuric acid is 10 mol / L;

[0074] The concentration of the potassium dihydrogen phosphate is 40 mol / L;

[0075] The concentration of the ethylmalonic acid is 20 mol / L;

[0076] 4) First palladium plating treatment: Electroplate a 0.003 mm thick palladium layer on the surface of the surface-activated bonding copper wire. Use an alkaline palladium electroplating solution with a palladium concentration of 40 g / L, a pH of 8, and an electroplating current density of 5 A / dm 2 , the electroplating temperature is 60 °C, and the electroplating speed is 40 m / s;

[0077] 5) Rough wire drawing and medium wire drawing: Heat the palladium-plated bonding copper wire to 600 °C, perform rough wire drawing until the wire diameter is 0.08 mm, cool down to 500 °C, and perform medium wire drawing until the wire diameter is 0.06 mm;

[0078] 6) Second palladium plating treatment: After cooling the palladium-plated bonding copper wire that has undergone rough wire drawing and medium wire drawing, electroplate a second 0.003 mm thick palladium layer on its surface. Use an alkaline palladium electroplating solution containing 200 g / L pyrrole-2-sulfonic acid, with a pH of 8, a palladium concentration of 40 g / L, and an electroplating current density of 5 A / dm 2 , the electroplating temperature is 60 °C, and the electroplating speed is 40 m / s;

[0079] 7) The diameter of the bonding copper wire after fine wire drawing is 0.03 mm;

[0080] 8) Annealing: Use nitrogen as the annealing atmosphere with a flow rate of 8 L / min, an annealing temperature of 600 °C, and an annealing rate of 2 m / s.

[0081] Example 4

[0082] A preparation process of palladium-plated bonding copper wire

[0083] 1) Pretreatment of raw materials: Ultrasonically clean and dry the used raw materials with absolute ethanol;

[0084] 2) Synthesis of bonding copper wire: Mix 99.374% metallic copper, 0.04% platinum, 0.05% cobalt, 0.03% zirconium, 0.05% magnesium, 0.05% strontium, 0.006% rare earth metal elements, and 0.4% silicon, and perform vacuum melting. The vacuum degree is 10 -3 MPa, the melting temperature is 1800 °C, the melting time is 73 minutes, and then continuous casting is carried out to obtain bonding copper wire with a diameter of 0.08 mm;

[0085] The rare earth elements include cerium, gadolinium, and yttrium, and the mass ratio is 1:1:1;

[0086] 3) Surface activation treatment of bonding copper wire: After the bonding copper wire is cooled to 105 °C, perform aerosol uniform spraying treatment with a surface activation solution, let it stand, and then dry it;

[0087] The components of the surface active solution include a mixture of sulfuric acid, potassium dihydrogen phosphate, and ethylmalonic acid, with a mixing volume ratio of 2:4:1.5;

[0088] The concentration of the sulfuric acid is 6 mol / L;

[0089] The concentration of the potassium dihydrogen phosphate is 36 mol / L;

[0090] The concentration of the ethylmalonic acid is 28 mol / L;

[0091] 4) Primary palladium plating treatment: Electroplate a 0.002 mm palladium layer on the surface of the surface-activated bonding copper wire. Use an alkaline palladium electroplating solution with a palladium concentration of 35 g / L, a pH of 7, and an electroplating current density of 3 A / dm 2 , an electroplating temperature of 65°C, and an electroplating speed of 35 m / s;

[0092] 5) Rough wire drawing and medium wire drawing: Heat the palladium-plated bonding copper wire to 500°C, perform rough wire drawing until the wire diameter reaches 0.07 mm, cool it to 400°C, and perform medium wire drawing until the wire diameter reaches 0.05 mm;

[0093] 6) Secondary palladium plating treatment: After cooling the palladium-plated bonding copper wire that has undergone rough wire drawing and medium wire drawing, electroplate a secondary 0.002 mm palladium layer on its surface. Use an alkaline palladium electroplating solution containing 170 g / L pyrrole-3-sulfonic acid, with a pH of 7 and a palladium concentration of 35 g / L, and an electroplating current density of 3 A / dm 2 , an electroplating temperature of 65°C, and an electroplating speed of 35 m / s;

[0094] 7) The diameter of the bonding copper wire after fine wire drawing is 0.015 mm;

[0095] 8) Annealing: Use nitrogen as the annealing atmosphere with a flow rate of 6 L / min, an annealing temperature of 450°C, and an annealing rate of 2 m / s.

[0096] Perform the following performance tests on the palladium-plated bonding copper wires prepared in Examples 1 to 4, and select specifications of φ0.01 mm, φ0.02 mm, and φ0.03 mm.

[0097] 1. Mechanical property test

[0098] Table 1 Mechanical properties of palladium-plated bonding copper wires

[0099]

[0100] Among them, the standard for the tensile force is > 4 gf, the standard for the elongation rate is 10 - 14%, and the standard for the bonding thrust is ≥ 25 CN.

[0101] As can be seen from Table 1, the palladium-plated bonded copper wire prepared by the present invention exhibits excellent performance in terms of tensile strength, elongation rate, and bonding thrust, and has excellent mechanical properties. Combining Figure 1 As can be seen, it is the SEM scan image of the cross-section of the bonded copper wire. It can be seen that the internal particle structure is evenly distributed, closely arranged, and the stable structure lays a good foundation for excellent mechanical properties.

[0102] 2. Ball forming degree and anti-oxidation and anti-sulfuration tests

[0103] Table 2 Ball forming degree and anti-oxidation and anti-sulfuration properties of palladium-plated bonded copper wire

[0104]

[0105] The ball forming degree is calculated by the ball forming rate of the burned ball;

[0106] The anti-oxidation test is carried out by the oxidation weight gain experiment. 20 g of bonded copper wire is placed in an oven at 40 °C for 7 days, taken out and weighed, and the difference before and after is compared;

[0107] The anti-sulfuration test is to place the lamp bead in a sealed container with a certain sulfur concentration inside, and under the environment of 85 °C, after sulfuration for 2 hours, the light decay is tested;

[0108] As can be seen from Table 2, the palladium-plated bonded copper wire prepared by the present invention has excellent ball forming degree, anti-oxidation performance, and anti-sulfuration performance, and the wire quality is excellent, meeting the application requirements of the packaging industry.

[0109] 3. High and low temperature cycle test

[0110] The TQFP packaging form is adopted for packaging, and the high and low temperature cycle test is carried out.

[0111] Table 3 High and low temperature cycle performance of palladium-plated bonded copper wire

[0112]

[0113] As can be seen from Table 3, the palladium-plated bonded copper wire prepared by the present invention can all undergo more than 400 cycles exceeding the standard, and has excellent service performance.

Claims

1. A preparation process of palladium-plated bonded copper wire, characterized in that, It includes the following steps: 1) Pretreatment of raw materials; 2) Synthesis of bonding copper wire: Mix metallic copper, platinum, cobalt, zirconium, magnesium, strontium, rare earth metal elements and silicon, and conduct vacuum melting and continuous casting to obtain bonding copper wire; By mass fraction, metallic copper ≥ 99.215%, platinum accounts for 0.04 - 0.06%, cobalt accounts for 0.05 - 0.08%, zirconium accounts for 0.03 - 0.05%, magnesium accounts for 0.05 - 0.07%, strontium accounts for 0.03 - 0.05%, rare earth metal elements account for 0.005 - 0.006% and silicon accounts for 0.3 - 0.5%; 3) Surface activation treatment of bonding copper wire: After the bonding copper wire is cooled to 100 - 120 °C, conduct aerosol uniform spraying treatment with a surface activation solution, let it stand, and then dry it; 4) Primary palladium plating treatment: Electroplate a palladium layer on the surface of the bonding copper wire that has undergone surface activation treatment, using an alkaline palladium electroplating solution; 5) Rough drawing and medium drawing: Heat the palladium-plated bonding copper wire to 500 - 600 °C for rough drawing, and cool it to 400 - 500 °C for medium drawing; 6) Secondary palladium plating treatment: Electroplate a secondary palladium layer on the surface of the palladium-plated bonding copper wire that has undergone rough drawing and medium drawing; The electroplating uses an alkaline palladium electroplating solution containing an antioxidant; The antioxidant includes pyrrole-2-sulfonic acid or pyrrole-3-sulfonic acid; 7) Fine drawing; 8) Annealing.

2. The preparation process of the palladium-plated bonded copper wire according to claim 1, characterized in that, During the vacuum melting in step 2), the vacuum degree is 10 -3 ~10 -2 MPa, the melting temperature is 1780 - 1810 °C, and the melting time is 60 - 80 minutes. The diameter of the obtained bonding copper wire is 0.08 - 0.1 mm.

3. The preparation process of the palladium-plated bonded copper wire according to claim 1, wherein The rare earth elements in step 2) include cerium, gadolinium and yttrium.

4. The preparation process of the palladium-plated bonded copper wire according to claim 1, characterized in that, The components of the surface activation solution in step 3) include a mixture of sulfuric acid, potassium dihydrogen phosphate and ethylmalonic acid, and the mixing volume ratio is (2 - 3):(3 - 5):(1 - 2); The concentration of the sulfuric acid is 5 - 10 mol / L; The concentration of the potassium dihydrogen phosphate is 30 - 40 mol / L; The concentration of the ethylmalonic acid is 20 - 30 mol / L.

5. The preparation process of the palladium-plated bonded copper wire according to claim 1, wherein Under the electroplating conditions described in step 4) and step 6), the pH of the electroplating solution is 7 - 8, the current density of electroplating is 2 - 5 A / dm 2 , the electroplating temperature is 60 - 70 °C, and the electroplating speed is 30 - 40 m / s.

6. The preparation process of the palladium-plated bonded copper wire according to claim 1, characterized in that, The diameter of the bonding copper wire after rough drawing in step 4) is 0.06 - 0.08 mm; The diameter of the bonding copper wire after medium drawing is 0.04 - 0.06 mm.

7. The preparation process of the palladium-plated bonded copper wire according to claim 1, characterized in that, The diameter of the bonding copper wire after fine drawing in step 6) is 0.01 - 0.03 mm.

8. The preparation process of the palladium-plated bonded copper wire according to claim 1, wherein In step 7), the annealing uses nitrogen as the annealing atmosphere, the flow rate is 5 - 8 L / min, the annealing temperature is 400 - 600 °C, and the annealing rate is 1 - 2 m / s.

9. A palladium-plated bonded copper wire obtained by the preparation process according to any one of claims 1 to 8, characterized in that, In the bonding copper wire, by mass fraction, metallic copper ≥ 99.2%, platinum accounts for 0.04 - 0.06%, cobalt accounts for 0.05 - 0.08%, zirconium accounts for 0.03 - 0.05%, magnesium accounts for 0.05 - 0.07%, strontium accounts for 0.03 - 0.05%, rare earth metal elements account for 0.005 - 0.006% and silicon accounts for 0.3 - 0.5%.

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