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Tungsten wire diamond line's permeability improved, achieving cost-effective efficiency.

published: 2024-03-22 15:48

Diamond wire is an important consumable in the photovoltaic silicon wafer manufacturing process.

Electroplated diamond wire is currently used for cutting hard and brittle materials such as crystalline silicon, sapphire, and precision ceramics. The production process of the silicon wafer segment includes the steps of crystalline silicon drawing/casting → block cutting, sectioning → surface grinding and edge chamfering → rod sticking → wafer slicing → adhesive removal from the silicon wafer → silicon wafer cleaning → inspection sorting → finished product packaging. As an important cutting consumable in the production of silicon wafers, diamond wire is mainly used in the photovoltaic field for the block cutting, sectioning, and slicing of crystalline silicon. In the block cutting and sectioning stages, to ensure cutting speed and efficiency, a thicker diamond wire with a diameter of 250μm or more is used; in the slicing stage, due to the higher requirements of manufacturers for raw material utilization rate and unit cutting cost, a thinner diamond wire is used. Diamond wire cutting significantly reduces costs and improves efficiency, making it the mainstream route for silicon wafer cutting at present. Looking at the development history of crystalline silicon cutting technology, the methods of silicon wafer cutting have undergone a technological upgrade from inner circle saw cutting, free abrasive slurry cutting to diamond wire cutting. The inner circle saw blade process has problems of large kerf and high silicon material loss, and also imposes restrictions on the size of silicon, resulting in high production costs of silicon wafers. Free abrasive slurry cutting has reduced costs to a certain extent, but due to the still low efficiency of slurry cutting, large cutting loss, and high environmental treatment costs, there is little room for further significant reduction in slicing costs. Compared to the above two cutting methods, diamond wire cutting technology has significant advantages: 1) greatly reducing wire consumption costs; 2) improving material utilization rate, significantly reducing cutting wear, and increasing the yield rate (an additional 15%-20% of silicon wafers); 3) increasing cutting speed (4-5 times the cutting speed), greatly improving slicing efficiency; 4) causing less environmental pollution.

Based on the type of bus wire used, photovoltaic diamond wires are primarily made from high-carbon steel and tungsten wire.

The main costs in producing diamond wire include raw materials, direct labor, energy and power, and manufacturing expenses. The primary raw materials are diamond micro powder, bus wire, and nickel, with direct materials accounting for the largest proportion of costs. For carbon steel wire diamond lines: taking the cost data from Ju Cheng Technology in 2022 as an example, the cost of raw materials and direct labor accounted for 50.69% and 9.55% of the total diamond wire cost, respectively. For tungsten wire diamond lines: the cost of direct materials accounts for about 79%, of which the bus wire cost accounts for 70%. According to Ju Cheng Technology's 2022 data, the cost of raw materials and direct labor accounted for 78.68% and 4.04% of the total diamond wire cost, respectively.

The trend towards larger sizes and thinner slices for silicon wafers is intensifying. With the acceleration of larger sizes and thinner slices, reducing the thickness of silicon wafers can save on material usage without changing the area, thereby reducing the cost of silicon wafers. In 2021, as the price of silicon materials rose, the process of thinning silicon wafers accelerated. According to the China Photovoltaic Industry Association (CPIA), the mainstream P-type monocrystalline silicon wafer thickness decreased from 170μm in 2020 to 155μm in 2023, the thickness of silicon wafer cells used for heterojunction decreased from 150μm in 2021 to 120μm in 2023, and the thickness of silicon wafer cells used for TOPCon decreased from 165μm in 2021 to 125μm in 2023. The fine wire trend is a rigid demand for the development of thinner silicon wafers. According to the formula, the number of slices produced = the length of each kilogram of square rod / pitch (silicon slice thickness + diamond wire diameter + grit size). The finer the diamond wire diameter, the smaller the cutting slot, resulting in less loss of silicon material during the slicing process and more slices from the same volume of silicon ingot. At the same time, a finer wire diameter means lower breakage force and higher resistance, requiring higher operating speeds and compatibility for equipment, accurate control of coating thickness, and matching with the corresponding diamond models. The mainstream conventional product for diamond wire currently has a high-carbon steel wire as its base. By 2023, the mainstream specifications for carbon steel wire diamond lines have evolved to 36μm.

The performance indicators of the diamond wire directly affect the quality and cost of the slices. During the cutting process of silicon wafers, the diamond wire must withstand high-frequency reciprocating motion and significant tension. The distribution density of diamond particles on the wire, their bonding strength, cutting ability of the diamonds, and the fatigue resistance of the wire all directly impact the performance of the diamond wire, which in turn determines the quality and cost of the slices. In practical production applications, the performance indicators of diamond wire are mainly reflected in cutting capacity, cutting quality, and breakage rate. The current main trends in diamond wire cutting are: fine wire, fast cutting, wire conservation, low TTV (Total Thickness Variation), and high stability in cutting.

Tungsten wire has a higher tensile strength, allowing for a thinner wire diameter under the same breaking force, which means it has greater potential for fine wire development.

Tungsten, as a non-renewable and scarce resource, possesses physical properties such as high density, high melting point, high wear resistance, high electrical conductivity, and high hardness. Tungsten-based diamond wires are more heat-resistant and have a stronger tensile strength, enabling a thinner wire diameter under the same breaking force. (1) The breaking tensile force of tungsten wire is 1.2 to 1.3 times that of carbon steel of the same specifications, its high torsion value is more than 10 times that of the same specifications, and the Young's modulus of alloyed tungsten wire is 1.7 times that of steel wire, with an elongation rate of 60% compared to carbon steel; (2) Tungsten has a body-centered cubic lattice structure, and after doping modification, the alloyed tungsten wire belongs to a microcrystalline structure with a grain size of about 100nm, making its tissue uniform without inclusions, and its purity can reach up to 99.95%, which is conducive to the drawing of ultra-fine tungsten wires; (3) The resistivity of alloyed tungsten wire is 5.4x10^-6Ω/cm, only 55.7% of that of carbon steel, which allows it to carry twice the current, and the nickel plating is more uniform and dense; (4) High corrosion resistance: it does not corrode in sulfuric acid or hydrochloric acid, effectively avoiding defects and breakages in the mother wire due to acid corrosion during the production process. For the slicing end, the advantages of tungsten wire diamond cutting include: (1) Improvement of A-grade rate: under the same wire diameter, tungsten wire has the advantages of high strength and low breakage rate, thereby improving the A-grade rate during the slicing process. Especially against the backdrop of rapidly increasing penetration rates of N-type silicon wafers, further catalyzing the demand for tungsten wire diamond lines. (2) Increase in the number of slices: the finer the diamond wire diameter, the smaller the cutting slot, resulting in less loss of silicon material in the slicing process and a higher number of slices from the same volume of silicon ingot; (3) Cost savings on silicon materials: it can be used to cut thinner silicon wafers, producing more silicon wafers and saving on silicon material costs; (4) Reduction in equipment depreciation and wear: slicing equipment lasts longer; (5) Reduction in auxiliary material costs: the amount of diamond micropowder used is reduced, and the demand for cooling liquid is reduced by 30%-50%.

The preparation of tungsten wire mother lines is much more difficult than that of carbon steel wires, with key indicators lying in the success rate of wire drawing. (1) Longer manufacturing process: the manufacturing process of carbon steel mother lines is relatively simple, mainly involving purchasing carbon raw materials followed by wire drawing through dies, with the whole process being relatively short. However, for tungsten wire production, it involves a series of complex steps such as purchasing tungsten powder, then smelting and subsequent forming processes (reduction → doping → reduction → isostatic pressing → sintering → vertical melting → rotary forging → annealing → wire drawing → electrolysis, etc.), with dozens of procedures. Maintaining consistency during the single wire drawing process is challenging. (2) Hot drawing process: unlike carbon steel wire, tungsten has a higher melting point and is brittle at low temperatures, so tungsten wire mother lines can only be forged at high temperatures. The difficulty of drawing at high temperatures far exceeds that at low temperatures (impurity control, production environment requirements, etc.), hence the manufacturing barriers for tungsten wire mother lines are much higher than those for carbon steel mother lines. (3) Workmanship skills: higher technical level requirements. According to industry chain research, the training period for production line workers usually takes about three months. (4) Equipment: higher requirements for core equipment like dies.

The thinning of wires is driving the accelerated expansion of industry demand, and the penetration rate of tungsten wire is expected to increase rapidly.

With the gradual prominence of photovoltaic economics, the global photovoltaic installation market continues to thrive. We expect that by 2024-2026, the global photovoltaic new installations will reach 530, 630, and 730GW, respectively. The usage of silicon wafers is calculated based on a ratio of 1:1.3 for capacity matching plus loss. The consumption of diamond wire will increase with the reduction of wire diameter and the thinning of silicon wafers. Combining the industry's tungsten wire production capacity planning and downstream application scenarios, assuming the penetration rates of tungsten wire in 2024-2026 are 30%/50%/60%, the estimated demand for tungsten wire will be 12,273/25,044/35,868 million km, with a 3-year CAGR of 93% from 2023 to 2026. The corresponding market sizes for 2024-2026 are approximately 43/83/108 billion yuan; the total industry demand for diamond wire will be 40,910/50,088/59,780 million km, with market sizes of 106/135/155 billion yuan, respectively.

Traditional tungsten product manufacturers and diamond wire companies are both laying out tungsten wire mother line capacities, with Xiamen Tungsten taking the lead in mass supply. Currently, Xiamen Tungsten, Zhongtung High & New Technology, Xianglu Tungsten, and other traditional tungsten product manufacturers, as well as Meichang Shares, Daile New Materials, and other diamond wire companies, have all laid out tungsten wire mother line capacities. However, due to the significantly higher technical difficulty of tungsten wire mother lines compared to carbon steel wires, Xiamen Tungsten is currently the first to achieve large-scale supply, and the tungsten wire mother line is in a state of supply not meeting demand. According to disclosures from Xiamen Tungsten, the company has an ample backlog of orders for fine tungsten wires used in photovoltaics. The company has already built new production lines with annual outputs of 8.8 billion meters and 20 billion meters of fine tungsten wires, and a new 60 billion meters photovoltaic fine tungsten wire production line has been put into operation. The company is also planning a new construction project for a 100 billion meters photovoltaic fine tungsten wire production line. From January to September 2023, the initial phase of tungsten wire penetration has seen a higher premium, which is expected to increase the profits of the tungsten wire industry.

Tungsten ore resources are strategically scarce assets. Since 2002, China has implemented a total control on the mining of tungsten ore. Tungsten is a non-renewable and irreplaceable strategic metal, known as the "industrial teeth," with small supply elasticity. The amount of tungsten resources globally is limited, but China's reserves rank first in the world, accounting for 52%. In September 2023, the Ministry of Natural Resources of China issued the 2023 annual tungsten ore mining total control indicator, with the national tungsten concentrate (65% tungsten trioxide content) mining total indicator set at 111,000 tons. Against the backdrop of a small increase in the mining total control indicator, it is difficult for China's tungsten concentrate output to have significant growth, showing a fluctuating downward trend. The price of tungsten has risen rapidly. The price of tungsten concentrate has risen from 115,900 yuan/ton at the beginning of 2023 to 126,000 yuan/ton in mid-March 2024.

The domestic diamond wire industry has high concentration, presenting a "one super, many strong" pattern. Currently, major domestic diamond wire industry manufacturers include Meichang Shares, Gaoce Shares, Daile New Materials, Sanchao New Materials, Jucheng Technology, etc., among which Meichang Shares leads far ahead in diamond wire shipments. In 2022, the diamond wire sales volumes of Meichang Shares, Gaoce Shares, Jucheng Technology, Daile New Materials, and Sanchao New Materials were 9615.60, 2539.73, 2141.14, 1310.19, and 640.22 million kilometers, respectively. The overall industry basically presents a "one super, many strong" pattern. The gross profit margin of the diamond wire industry is relatively high, and leading enterprises mainly reduce costs through self-supply of raw materials. Diamond wire is an important consumable in the silicon wafer preparation process, accounting for 6.25% of the silicon wafer production cost according to Solarzoom data, equivalent to 1.34 cents/W. In 2022, the average gross profit margin of the diamond wire industry was 38.7% (sample data includes Meichang Shares, Daile New Materials, Sanchao New Materials, Gaoce Shares, Jucheng Technology, five companies), while the gross profit margin of Meichang Shares' diamond wire products was 55.8%, 17.1 percentage points higher than the industry average, mainly due to significantly lower unit production costs compared to other enterprises in the industry. On one hand, Meichang Shares extends its business upstream, reducing raw material costs such as yellow wire and mother lines through increased self-supply rates; on the other hand, it adopts a fifteen-wire-per-machine production process to improve production efficiency.

Domestic diamond wire enterprises are actively introducing and validating, and the industrial application demand for tungsten wire diamond wire is strong. From 2021 to the present, Jucheng Technology, Daile New Materials, Sanchao New Materials, Yuan Shi New Materials, and others have gradually carried out the research and development and production of tungsten wire lines. With the increasing acceptance and penetration rate of tungsten wire diamond lines, diamond wire enterprises that can obtain or self-produce tungsten wire mother lines in bulk are expected to gain a first-mover advantage. Silicon wafer enterprises are gradually using tungsten wire diamond lines for slicing. According to disclosures from Jucheng Shares, the main customers of the company's diamond wire products show a general trend towards finer wire diameter specifications. The main customers of the company's different specifications of tungsten wire lines include major silicon wafer enterprises in the market such as TCL Zhonghuan, Jingyunton, Xie Xin, Gaojing Solar Energy, Jingying Optoelectronics, etc. In 2022, the company's tungsten wire line customers numbered 28, with 11 customers achieving large-scale sales of more than 100,000 kilometers.

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