Photovoltaic solder and preparation method thereof, main grid-free photovoltaic welding strip and preparation method thereof

Abstract

The present invention belongs to the field of photovoltaic new energy technology, and specifically relates to a photovoltaic solder and a preparation method thereof, a main-grid-free photovoltaic welding strip and a preparation method thereof. The photovoltaic solder includes at least tin, lead, and bismuth, wherein the content of the bismuth does not exceed 25% by mass fraction, the main-grid-free photovoltaic welding strip includes photovoltaic solder and a conductive substrate, and the preparation method of the photovoltaic solder includes: step a, weighing; step b, preparing a first molten liquid; step c, preparing photovoltaic solder; the preparation method of the main-grid-free photovoltaic welding strip includes the following steps: step S1, preparing solder; step S2, preparing a conductive substrate; step S3, pretreating the conductive substrate; step S4, coating solder. The embodiment of the present invention provides two solder ratios, both of which strictly control the bismuth content to not exceed 25%, and can obtain solders in the solid-liquid phase temperature zone required by different 0BB main-grid-free welding technologies, which can meet the requirements of different welding equipment and welding processes.

Description

Technical Field

[0001] The present invention belongs to the technical field of photovoltaic new energy, and in particular relates to a photovoltaic solder and a preparation method thereof, a main grid-free photovoltaic welding strip and a preparation method thereof. Background Art

[0002] In order to further reduce the cost of photovoltaic power generation, improve battery power, design more efficient current transmission paths, and reduce the material costs of each link of photovoltaic modules, 0BB busbar-free technology has become the most core technical development direction. The characteristic of this technology is to replace the main grid with a thin grid and weld it with a welding ribbon, thereby greatly reducing the consumption of silver paste and saving costs. In order to ensure the safe current carrying capacity, the number of welding ribbons is increased and the diameter of the welding ribbon is reduced. Therefore, the wire diameter of the busbar-free welding ribbon is basically below 0.2mm.

[0003] Since 0BB technology can be used not only for HJT cells, but also for TOPCON cells and BC cells, the different types of cells determine the diversity of 0BB solder strips. There are also many technical solutions for 0BB battery welding technology on the market, such as welding + dispensing, coating + lamination, dispensing + lamination, etc. Different technical routes have different process parameters and different requirements for solder strips. Since 0BB technology reduces the consumption of silver paste and the thickness of the encapsulation film, in the absence of a main grid, it is very easy to have problems such as broken grid and over-welding when welding the solder strip to the fine grid, so the welding temperature needs to be lowered and low-temperature solder needs to be used.

[0004] In the prior art, solders containing metallic indium or solders with low melting points are often prepared based on Sn42Bi58 alloys. However, the high price of indium leads to rising costs. Low-temperature solders based on Sn42Bi58 alloys can achieve low-temperature welding. However, in practice, it is found that high-bismuth solders have low welding strength, coarse bismuth particles exist in IMC (intermetallic compounds), solder joints are brittle, and the metal diffusion layer is shallow. In addition, the solid-liquid phase of Sn42Bi58-based solders is around 138°C. When hot spots occur, the local heating temperature of the photovoltaic panel can reach above 140°C. At this time, the solder of some solder strips will melt, and then the solder joints will become loose. At the same time, tin-bismuth solders have poor oxidation resistance, and yellowing will occur during aging tests, making it difficult to pass component reliability tests. Therefore, high-bismuth low-temperature solders are not very suitable for the welding of photovoltaic cells. Summary of the invention

[0005] In order to solve at least one technical problem in the prior art, a photovoltaic solder and a preparation method thereof, a main grid-free photovoltaic solder strip and a preparation method thereof are provided, which are low in cost and applicable to 0BB main grid-free technology. To achieve the above technical objectives, the technical solution adopted in the embodiment of the present invention is: In a first aspect, an embodiment of the present invention provides a photovoltaic solder comprising at least tin, lead, and bismuth, wherein the content of the bismuth does not exceed 25% by mass.

[0006] Furthermore, the following components are included by mass fraction: 39% to 41% tin, 39% to 41% lead, 19% to 21% bismuth, and 0 to 0.5% silver, and the solid-liquid phase of the photovoltaic solder is 132 to 154° C. The photovoltaic solder is suitable for the connection process of dispensing + welding, and the welding temperature is 175 to 185° C.

[0007] Furthermore, the following components are included by mass fraction: 36% to 38% tin, 36% to 38% lead, 23% to 25% bismuth, and 0 to 1% silver. The solid-liquid phase of the photovoltaic solder is 125 to 145°C. The photovoltaic solder is suitable for the dispensing / film coating + lamination welding process route, which specifically includes the following steps: (1) The soldering tape is attached to the battery cell by adhesive or film fixing. Then, during the lamination packaging process, the solder on the surface of the soldering tape is melted and the soldering tape and the gate line are welded under vacuum pressure to achieve electrical connection; The lamination packaging process is divided into two stages of heating, the first temperature zone is 100-120°C, and the second temperature zone is 135-145°C.

[0008] In a second aspect, an embodiment of the present invention provides a method for preparing the photovoltaic solder according to the first aspect, comprising the following steps: Step a, weighing: weigh tin, lead, bismuth and silver respectively according to the proportions and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250-270° C. to melt the raw materials, stirring for 25-35 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing a second molten liquid: adding metallic bismuth into the first molten liquid, and covering the surface of the molten metal with mica sheets and / or quartz sand, stirring for 40 to 60 minutes until all the metal is melted to obtain a second molten liquid, and cooling the tin furnace to 220 to 230° C.; Step d, preparing photovoltaic solder: adding weighed silver particles into the second molten liquid, and continuing to fully stir for 30 to 40 minutes to obtain a finished photovoltaic solder.

[0009] In a third aspect, an embodiment of the present invention provides a method for preparing the photovoltaic solder according to the first aspect, comprising the following steps: Step a, weighing: weigh tin, lead and bismuth respectively according to the ratio and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250-270° C. to melt the raw materials, stirring for 25-35 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing photovoltaic solder: adding metallic bismuth into the first molten liquid, and covering the metal liquid surface with mica sheets and / or quartz sand, stirring for 40 to 60 minutes until all the metal is melted, to obtain a finished photovoltaic solder.

[0010] In a fourth aspect, an embodiment of the present invention further provides a busbar-free photovoltaic welding ribbon, comprising a conductive substrate and a photovoltaic solder coated on a surface of the conductive substrate; The conductive substrate includes a wire or strip made of copper, aluminum, or a copper-aluminum composite material; The photovoltaic solder adopts the photovoltaic solder in the second aspect or the third aspect.

[0011] In a fifth aspect, an embodiment of the present invention further provides a method for preparing a busbar-free photovoltaic welding ribbon, which is obtained by coating the photovoltaic solder in the second aspect or the third aspect on the surface of a conductive substrate, comprising the following steps: Step S1, preparing solder: putting photovoltaic solder into the tin furnace of the tin plating production line, and setting the temperature of the tin furnace to 190-220° C.; Step S2, preparing a conductive substrate: processing the conductive substrate by a wire drawing process and / or a calendering process; Step S3, pre-treating the conductive substrate: annealing and softening the conductive substrate prepared in step S2, and then surface treating the conductive substrate with a low-temperature soldering flux; Step S4, coating solder: passing the pretreated conductive substrate in step S3 through the tin furnace in step S1, setting a pressure air knife at the tin outlet to control the thickness of the photovoltaic solder layer on the surface of the conductive substrate, and obtaining a finished busbar-free photovoltaic solder strip after cooling.

[0012] Further, in step S2, the diameter of the conductive substrate is 0.16 to 0.2 mm; And / or, in step S3, the current of the annealing treatment is 9 to 14A; And / or, in step S3, the running speed of the conductive substrate during annealing is 250-350 m / min; and / or, in step S3, the yield strength of the conductive substrate after annealing is 55 to 75 MPa; and / or, in step S3, the elongation of the conductive substrate after annealing is 20-30%; And / or, in step S3, the activity range of the low temperature soldering flux is 150 to 240°C; And / or, in step S4, the running speed of the conductive substrate when coating the photovoltaic solder layer is 250-350 m / min; And / or, in step S4, the thickness of the photovoltaic solder layer is 5 to 15 μm; And / or, in step S4, the diameter of the busbar-free photovoltaic welding ribbon is 0.18-0.22 mm.

[0013] The technical solution provided by the embodiment of the present invention has the following beneficial effects: The embodiment of the present invention provides two solder formulas, both of which control the bismuth content to no more than 25%, and can obtain solders in the solid-liquid phase temperature zone required by different 0BB busbar-free technologies. This content ratio reduces the proportion of bismuth grains and bismuth-based solid solution phases in the solder as much as possible, and also reduces the generation of coarse particles of tin-lead-bismuth ternary eutectic to the greatest extent, while also ensuring the proportion of tin in the solder, providing a strong guarantee for the formation of the IMC alloy layer. The embodiment of the present invention provides targeted ratio classification according to different 0BB welding technology routes, which can meet the requirements of different welding equipment and welding processes, and obtain welding quality with good mechanical properties, firm solder joints, and high reliability at a lower welding temperature. DETAILED DESCRIPTION

[0014] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0015] In addition, it should be understood that after reading the contents disclosed in the present invention, those skilled in the art may make various changes or modifications to the present invention, and these equivalent forms also fall within the protection scope defined by the present invention.

[0016] In a first aspect, an embodiment of the present invention provides a photovoltaic solder, comprising at least tin (Sn), lead (Pb), and bismuth (Bi), wherein the content of the bismuth does not exceed 25% by mass.

[0017] The present application can be applied to the 0BB process technology. By preparing photovoltaic solder with tin-lead-bismuth ternary alloy as the main component, low-temperature solder can be generated and the preparation cost is low. Since the liquidus temperature of the tin-lead-bismuth ternary eutectic point is only 96°C, and the phase contains coarse bismuth grains and has poor mechanical properties, it is necessary to avoid the generation of eutectic components. The present application designs the composition ratio within the tin-based solid solution interval with tin as the solvent to reduce the appearance of lead-bismuth solid solution phase that is not beneficial to welding. When the bismuth content in the solder exceeds 20%, bismuth will appear in a single crystal state. When the bismuth content in the solder exceeds 25%, there will be obvious coarse bismuth grains, and the bismuth grains are easy to gather on the welding joint surface, which has a negative impact on welding. Therefore, the present application strictly limits the bismuth content in the solder to less than 25%, which not only ensures the proportion of tin in the solder to ensure the formation of the IMC alloy layer, but also reduces the proportion of bismuth grains and bismuth-based solid solution phase in the solder to reduce the generation of coarse particles of tin-lead-bismuth ternary eutectic.

[0018] Silver may also be added to the solder configuration of the present application to refine the bismuth grains and hinder the diffusion of bismuth into the IMC layer.

[0019] Further, the photovoltaic solder comprises the following components in parts by mass: Tin 39% to 41%, lead 39% to 41%, bismuth 19% to 21%, silver 0 to 0.5%, the solid-liquid phase line of the photovoltaic solder is 132 to 154°C. The applicable welding temperature range is 175 to 185°C, which is suitable for the process technology of dispensing + welding, and is further suitable for cells that can withstand a welding temperature higher than 170°C. The solder under this composition ratio ensures that sufficient tin participates in the IMC reaction, so a solder joint with good bonding strength can be obtained.

[0020] Furthermore, the photovoltaic solder includes the following components by mass fraction: 36% to 38% tin, 36% to 38% lead, 23% to 25% bismuth, and 0 to 1% silver. The solid-liquid phase line of the photovoltaic solder is 125 to 145°C.

[0021] It should be noted that the principle of the glue dispensing + lamination welding process technology or the film coating + lamination welding process technology is to first attach the solder strip to the battery cell by adhesive or film fixing, and then in the lamination packaging process, use the lamination temperature (135-145°C) to melt the solder on the surface of the solder strip, and achieve the welding of the solder strip and the grid line under vacuum pressure to achieve electrical connection. Usually the lamination welding process is divided into two-stage heating, the first temperature zone is 100-120°C, and the second temperature zone is 135-145°C. This process needs to ensure that the solder does not melt in the first temperature zone and is completely melted or partially liquefied in the second temperature zone. In the first temperature zone, the solder does not melt, which can prevent the solder from flowing over a large area due to the softening and spreading of the encapsulation film, thereby causing undesirable phenomena such as tin accumulation and tin beads; in the second temperature zone, the flow of the film basically stops, and the solder will not flow over a long distance, but will only spread slightly along the grid line direction. At this time, the solder is completely or mostly liquefied, and can form a good electrical connection with the battery cell. Therefore, the solid-liquid phase line of the solder under this component ratio is 125-145°C, which is very consistent with the temperature requirements of the lamination welding process, and a good fusion layer can be obtained. It is suitable for dispensing + lamination welding process technology or film coating + lamination welding process technology, and is suitable for batteries that cannot withstand welding temperatures of 170°C and above. This welding process can avoid over-welding, broken grid, hidden cracks in the battery cell, etc., and has high reliability. At the same time, when conducting hot spot tests, compared with tin-bismuth solders, higher liquidus temperatures have stronger anti-hot spot capabilities, and the relatively low bismuth content and the presence of lead elements can also greatly improve the solder's antioxidant ability and better ensure the reliability of the components.

[0022] It should be noted here that even solder with a solid-liquid phase line of 132-154°C may partially melt during the lamination process. However, since the lamination temperature is still quite far from the liquid phase temperature, the overall surface of the solder is still mainly solid at this time, and secondary melting and desoldering will not occur. Therefore, it is basically possible to avoid an increase in the rate of false solder joints during lamination.

[0023] The above two solder ratios can obtain solders in different solid-liquid phase temperature zones, which can meet the requirements of different welding equipment and welding processes, and obtain welding quality with good mechanical properties, firm solder joints and high reliability at a lower welding temperature.

[0024] In a second aspect, an embodiment of the present invention further provides a method for preparing a photovoltaic solder, comprising the following steps: Step a, weighing: weigh tin, lead, bismuth and silver respectively according to the proportions and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250-270° C. to melt the raw materials, stirring for 25-35 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing a second molten liquid: adding metallic bismuth into the first molten liquid, and covering the surface of the molten metal with mica sheets and / or quartz sand, stirring for 40 to 60 minutes until all the metal is melted to obtain a second molten liquid, and cooling the tin furnace to 220 to 230° C.; Step d, preparing photovoltaic solder: adding weighed silver particles into the second molten liquid, and continuing to fully stir for 30 to 40 minutes to obtain a finished photovoltaic solder.

[0025] In a third aspect, an embodiment of the present invention further provides a method for preparing a photovoltaic solder, comprising the following steps: Step a, weighing: weigh tin, lead and bismuth respectively according to the ratio and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250-270° C. to melt the raw materials, stirring for 25-35 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing photovoltaic solder: adding metallic bismuth into the first molten liquid, and covering the metal liquid surface with mica sheets and / or quartz sand, stirring for 40 to 60 minutes until all the metal is melted, to obtain a finished photovoltaic solder.

[0026] It should be noted that the mica sheet and quartz sand in step c can prevent the solder from oxidizing and ensure the quality of the solder.

[0027] In a fourth aspect, an embodiment of the present invention further provides a busbar-free photovoltaic welding ribbon, comprising a conductive substrate and a photovoltaic solder coated on a surface of the conductive substrate; The conductive substrate includes a wire or strip made of copper, aluminum, or a copper-aluminum composite material; The photovoltaic solder is the photovoltaic solder prepared according to the second aspect or the third aspect.

[0028] In a fifth aspect, an embodiment of the present invention further provides a method for preparing a busbar-free photovoltaic welding ribbon, which is obtained by coating the photovoltaic solder prepared in the second aspect or the third aspect on the surface of a conductive substrate, and specifically comprises the following steps: Step S1, preparing solder: putting photovoltaic solder into the tin furnace of the tin plating production line, and setting the temperature of the tin furnace to 190-220° C.; Step S2, preparing a conductive substrate: processing the conductive substrate by a wire drawing process and / or a calendering process; Step S3, pre-treating the conductive substrate: annealing and softening the conductive substrate prepared in step S2, and then surface treating the conductive substrate with a low-temperature soldering flux; Step S4, coating solder: passing the pretreated conductive substrate in step S3 through the tin furnace in step S1, setting a pressure air knife at the tin outlet to control the thickness of the photovoltaic solder layer on the surface of the conductive substrate, and obtaining a finished busbar-free photovoltaic solder strip after cooling.

[0029] Further, in step S2, the diameter of the conductive substrate is 0.16-0.2 mm, for example, 0.16 mm, 0.17 mm, 0.18 mm, 0.19 mm, 0.2 mm, etc.; And / or, in step S3, the current of the annealing treatment is 9-14A, for example, 9A, 10A, 11A, 12A, 12.5A, 13A, 13.5A, 14A, etc.; And / or, in step S3, the running speed of the conductive substrate during annealing is 250-350 m / min; and / or, in step S3, the yield strength of the conductive substrate after annealing is 55 to 75 MPa; and / or, in step S3, the elongation of the conductive substrate after annealing is 20-30%; And / or, in step S3, the activity range of the low temperature soldering flux is 150-240°C, for example, 150°C, 170°C, 190°C, 200°C, 220°C, 240°C, etc.; And / or, in step S3, the running speed of the conductive substrate when coating the photovoltaic solder layer is 250-350 m / min; And / or, in step S4, the thickness of the photovoltaic solder layer is 5 to 15 μm, for example, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, 15 μm, etc.

[0030] And / or, in step S4, the diameter of the busbar-free photovoltaic welding ribbon is 0.18-0.22 mm, for example, it can be 0.18 mm, 0.19 mm, 0.2 mm, 0.21 mm, 0.22 mm, etc.

[0031] Furthermore, the pressure air knife is filled with protective gas, which is an inert gas including nitrogen, and the protective gas can prevent the solder from being oxidized.

[0032] In the following specific examples, the operations involved, if the conditions are not specified, are all carried out under conventional conditions or the conditions recommended by the manufacturer. All raw materials, if the manufacturers and specifications are not specified, are conventional products that can be obtained through commercial purchase.

[0033] Example 1 A method for preparing a photovoltaic solder comprises the following steps: Step a, weighing: 100 kg of ingredients, weigh various metals according to the mass ratio of tin, lead, bismuth and silver of 38:36:25:1, and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 240° C. to melt the raw materials, stirring for 30 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing a second molten liquid: adding metallic bismuth into the first molten liquid, and covering the surface of the molten metal with a mica sheet, stirring for 50 minutes until all the metal is melted, obtaining a second molten liquid, and cooling the tin furnace to 225° C.; Step d, preparing photovoltaic solder: adding weighed silver particles into the second molten liquid, and continuing to stir fully for 35 minutes to obtain a finished photovoltaic solder.

[0034] The solidus and liquidus phase of the photovoltaic solder prepared in Example 1 is 128-142°C.

[0035] Example 2 A method for preparing a photovoltaic solder comprises the following steps: Step a, weighing: 100 kg of ingredients, weigh various metals according to the mass ratio of tin, lead, bismuth and silver of 37:38:24.5:0.5, and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250° C. to melt the raw materials, stirring for 30 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing a second molten liquid: adding metallic bismuth into the first molten liquid, and covering the surface of the molten metal with a mica sheet, stirring for 50 minutes until all the metal is melted, obtaining a second molten liquid, and cooling the tin furnace to 225° C.; Step d, preparing photovoltaic solder: adding weighed silver particles into the second molten liquid, and continuing to stir fully for 35 minutes to obtain photovoltaic solder.

[0036] The solidus and liquidus phase of the photovoltaic solder prepared in Example 2 is 129-143°C.

[0037] Example 3 A method for preparing a photovoltaic solder comprises the following steps: Step a, weighing: 100 kg of ingredients, weigh various metals according to the mass ratio of tin, lead and bismuth of 40:41:19, and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250° C. to melt the raw materials, stirring for 30 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing photovoltaic solder: adding metallic bismuth into the first molten liquid, and covering the surface of the molten metal with a mica sheet, stirring for 50 minutes until all the metal is melted, to obtain photovoltaic solder.

[0038] The solidus and liquidus phase of the photovoltaic solder prepared in Example 3 is 138-155°C.

[0039] Example 4 A method for preparing a busbar-free photovoltaic ribbon comprises the following steps: Step S1, preparing solder: putting the photovoltaic solder prepared in Example 1 into a tin furnace of a tin plating production line, and setting the temperature of the tin furnace to 190° C.; Step S2, preparing a conductive substrate: processing pure copper into a copper wire with a diameter of 0.16 mm using a wire drawing process; Step S3, pre-treating the conductive substrate: subjecting the copper wire to short-circuit annealing treatment and then to surface treatment with a low-temperature soldering flux, setting the annealing current to 10A, the running speed of the copper wire during annealing to 300m / min, the yield strength of the copper wire after annealing to 55MPa, the elongation to 20%, and the activity range of the low-temperature soldering flux to 150-200°C; Step S4, solder coating: the copper wire pretreated in step S3 passes through the tin furnace in step S1 at a running speed of 300m / min, and a pressure air knife is set at the outlet of the tin furnace to control the thickness of the photovoltaic solder layer to 12μm, to obtain a finished main grid-free photovoltaic welding strip with a diameter of 0.18mm.

[0040] Example 5 A method for preparing a busbar-free photovoltaic ribbon comprises the following steps: Step S1, preparing solder: putting the photovoltaic solder prepared in Example 2 into a tin furnace of a tin plating production line, and setting the temperature of the tin furnace to 200° C.; Step S2, preparing a conductive substrate: using a rolling process to process pure copper into a copper wire with a diameter of 0.18 mm; Step S3, pre-treating the conductive substrate: subjecting the copper wire to short-circuit annealing treatment and then to surface treatment with a low-temperature soldering flux, setting the annealing current to 12A, the running speed of the copper wire during annealing to 300m / min, the yield strength of the copper wire after annealing to 65MPa, the elongation to 25%, and the activity range of the low-temperature soldering flux to 150-200°C; Step S4, solder coating: the copper wire pretreated in step S3 passes through the tin furnace in step S1 at a running speed of 300m / min, and a pressure air knife is set at the outlet of the tin furnace to control the thickness of the photovoltaic solder layer to 13.5μm, to obtain a finished main grid-free photovoltaic welding strip with a diameter of 0.20mm.

[0041] Example 6 A method for preparing a busbar-free photovoltaic ribbon comprises the following steps: Step S1, preparing solder: putting the photovoltaic solder prepared in Example 3 into a tin furnace of a tin plating production line, and setting the temperature of the tin furnace to 220° C.; Step S2, preparing a conductive substrate: processing pure copper into a copper wire with a diameter of 0.19 mm using a wire drawing process; Step S3, pre-treating the conductive substrate: subjecting the copper wire to short-circuit annealing treatment and then to surface treatment with a low-temperature soldering flux, setting the annealing current to 13A, the running speed of the copper wire during annealing to 300m / min, the yield strength of the copper wire after annealing to 75MPa, the elongation to 30%, and the activity range of the low-temperature soldering flux to 150-200°C; Step S4, solder coating: the copper wire pretreated in step S3 passes through the tin furnace in step S1 at a running speed of 300m / min, and a pressure air knife is set at the outlet of the tin furnace to control the thickness of the photovoltaic solder layer to 17.5μm, to obtain a finished main grid-free photovoltaic welding strip with a diameter of 0.22mm.

[0042] Comparative Example 1 A method for preparing a photovoltaic solder comprises the following steps: Step a, weighing: 100 kg of ingredients, weigh various metals according to the mass ratio of tin to bismuth of 42:58, and set aside; Step b, preparing photovoltaic solder: tin and bismuth are put into a tin furnace in sequence, and the temperature of the tin furnace is raised to 250° C. to melt the raw materials, and a mica sheet is covered on the metal liquid surface. After stirring for 50 minutes, the tin slag on the liquid surface is cleaned to obtain photovoltaic solder; The solidus and liquidus phase of the photovoltaic solder in Comparative Example 1 is 138°C.

[0043] Comparative Example 2 A method for preparing a photovoltaic solder comprises the following steps: Step a, weighing: 100 kg of ingredients, weigh various metals according to the mass ratio of tin, lead and bismuth of 35:35:30, and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250° C. to melt the raw materials, stirring for 30 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing photovoltaic solder: adding metallic bismuth into the first molten liquid, and covering the surface of the molten metal with a mica sheet, stirring for 50 minutes until all the metal is melted, to obtain photovoltaic solder.

[0044] The solidus and liquidus phase of the photovoltaic solder in Comparative Example 2 is 123-131°C.

[0045] Comparative Example 3 A method for preparing a photovoltaic solder comprises the following steps: Step a, weighing: 100 kg of ingredients, weigh various metals according to the mass ratio of tin, lead and bismuth of 40:40:20, and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250° C. to melt the raw materials, stirring for 30 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing photovoltaic solder: adding metallic bismuth into the first molten liquid, and covering the surface of the molten metal with a mica sheet, stirring for 50 minutes until all the metal is melted, to obtain photovoltaic solder.

[0046] The solid-liquid phase line of the photovoltaic solder in Comparative Example 3 is 137-154°C.

[0047] Comparative Example 4 A method for preparing a photovoltaic solder comprises the following steps: Step a, weighing: 100 kg of ingredients, weigh various metals according to the mass ratio of tin, lead and bismuth of 38:38:24, and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250° C. to melt the raw materials, stirring for 30 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing photovoltaic solder: adding metallic bismuth into the first molten liquid, and covering the surface of the molten metal with a mica sheet, stirring for 50 minutes until all the metal is melted, to obtain photovoltaic solder.

[0048] The solid-liquid phase line of the photovoltaic solder in Comparative Example 4 is 131-146°C.

[0049] Comparative Example 5 A method for preparing a photovoltaic solder comprises the following steps: Step a, weighing: 100 kg of ingredients, weigh various metals according to the mass ratio of tin, lead and bismuth of 43:43:14, and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250° C. to melt the raw materials, stirring for 30 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing photovoltaic solder: adding metallic bismuth into the first molten liquid, and covering the surface of the molten metal with a mica sheet, stirring for 50 minutes until all the metal is melted, to obtain photovoltaic solder.

[0050] The solidus and liquidus phase of the photovoltaic solder in Comparative Example 5 is 143-163°C.

[0051] Comparative Example 6 A method for preparing a photovoltaic welding ribbon comprises the following steps: Step S1, preparing solder: putting the photovoltaic solder in Comparative Example 1 into a tin furnace of a tin plating production line, and setting the temperature of the tin furnace to 190° C.; Step S2, preparing a conductive substrate: processing pure copper into a copper wire with a diameter of 0.18 mm using a wire drawing process; Step S3, pre-treating the conductive substrate: subjecting the copper wire to short-circuit annealing treatment and then to surface treatment with a low-temperature soldering flux, setting the annealing current to 10A, the running speed of the copper wire during annealing to 300m / min, the yield strength of the copper wire after annealing to 55MPa, the elongation to 20%, and the activity range of the low-temperature soldering flux to 150-200°C; Step S4, solder coating: the copper wire pretreated in step S3 is passed through the tin furnace in step S1 at a running speed of 300m / min, and a pressure air knife is set at the outlet of the tin furnace to control the thickness of the photovoltaic solder layer to 12μm, so as to obtain a photovoltaic solder strip with a diameter of 0.18mm.

[0052] Comparative Example 7 A method for preparing a photovoltaic welding ribbon comprises the following steps: Step S1, preparing solder: putting the photovoltaic solder in Comparative Example 2 into a tin furnace of a tin plating production line, and setting the temperature of the tin furnace to 190° C.; Step S2, preparing a conductive substrate: processing pure copper into a copper wire with a diameter of 0.18 mm using a wire drawing process; Step S3, pre-treating the conductive substrate: subjecting the copper wire to short-circuit annealing treatment and then to surface treatment with a low-temperature soldering flux, setting the annealing current to 10A, the running speed of the copper wire during annealing to 300m / min, the yield strength of the copper wire after annealing to 55MPa, the elongation to 20%, and the activity range of the low-temperature soldering flux to 150-200°C; Step S4, solder coating: the copper wire pretreated in step S3 is passed through the tin furnace in step S1 at a running speed of 300m / min, and a pressure air knife is set at the outlet of the tin furnace to control the thickness of the photovoltaic solder layer to 12μm, so as to obtain a photovoltaic solder strip with a diameter of 0.18mm.

[0053] Comparative Example 8 A method for preparing a photovoltaic welding ribbon comprises the following steps: Step S1, preparing solder: putting the photovoltaic solder in Comparative Example 3 into a tin furnace of a tin plating production line, and setting the temperature of the tin furnace to 190° C.; Step S2, preparing a conductive substrate: processing pure copper into a copper wire with a diameter of 0.18 mm using a wire drawing process; Step S3, pre-treating the conductive substrate: subjecting the copper wire to short-circuit annealing treatment and then to surface treatment with a low-temperature soldering flux, setting the annealing current to 10A, the running speed of the copper wire during annealing to 300m / min, the yield strength of the copper wire after annealing to 55MPa, the elongation to 20%, and the activity range of the low-temperature soldering flux to 150-200°C; Step S4, solder coating: the copper wire pretreated in step S3 is passed through the tin furnace in step S1 at a running speed of 300m / min, and a pressure air knife is set at the outlet of the tin furnace to control the thickness of the photovoltaic solder layer to 12μm, so as to obtain a photovoltaic solder strip with a diameter of 0.18mm.

[0054] Comparative Example 9 A method for preparing a photovoltaic welding ribbon comprises the following steps: Step S1, preparing solder: putting the photovoltaic solder in Comparative Example 4 into a tin furnace of a tin plating production line, and setting the temperature of the tin furnace to 220° C.; Step S2, preparing a conductive substrate: processing pure copper into a copper wire with a diameter of 0.19 mm using a wire drawing process; Step S3, pre-treating the conductive substrate: subjecting the copper wire to short-circuit annealing treatment and then to surface treatment with a low-temperature soldering flux, setting the annealing current to 13A, the running speed of the copper wire during annealing to 300m / min, the yield strength of the copper wire after annealing to 75MPa, the elongation to 30%, and the activity range of the low-temperature soldering flux to 150-200°C; Step S4, solder coating: pass the pretreated copper wire in step S3 through the tin furnace in step S1 at a running speed of 300m / min, set a pressure air knife at the outlet of the tin furnace to control the thickness of the photovoltaic solder layer to 17.5μm, and obtain a photovoltaic solder strip with a diameter of 0.22mm.

[0055] Comparative Example 10 A method for preparing a photovoltaic welding ribbon comprises the following steps: Step S1, preparing solder: putting the photovoltaic solder in Comparative Example 5 into a tin furnace of a tin plating production line, and setting the temperature of the tin furnace to 220° C.; Step S2, preparing a conductive substrate: processing pure copper into a copper wire with a diameter of 0.19 mm using a wire drawing process; Step S3, pre-treating the conductive substrate: subjecting the copper wire to short-circuit annealing treatment and then to surface treatment with a low-temperature soldering flux, setting the annealing current to 13A, the running speed of the copper wire during annealing to 300m / min, the yield strength after annealing to 75MPa, the elongation to 30%, and the activity range of the low-temperature soldering flux to 150-200°C; Step S4, solder coating: pass the pretreated copper wire in step S3 through the tin furnace in step S1 at a running speed of 300m / min, set a pressure air knife at the outlet of the tin furnace to control the thickness of the photovoltaic solder layer to 17.5μm, and obtain a photovoltaic solder strip with a diameter of 0.22mm.

[0056] Application Examples The main grid-free photovoltaic welding strips prepared in Examples 4-5 and the photovoltaic welding strips prepared in Comparative Examples 6-8 are assembled through a coating + lamination welding process to obtain a photovoltaic module. Specifically, the sample is passed through a laminating machine, and the photovoltaic welding strips are fixed on the battery cell grid line through an adhesive film. The battery strings are arranged and welded with the busbars. Then they enter a laminator. The temperature of the first temperature zone of the laminator is set to 120°C, and the second temperature zone is set to 140°C. Lamination welding is performed for 15 minutes to obtain a photovoltaic module. The performance parameters of the photovoltaic module are shown in Table 1.

[0057] The main grid-free photovoltaic ribbon prepared in Example 6 and the photovoltaic ribbon prepared in Comparative Examples 9-10 were assembled by a string welding process to obtain a photovoltaic module. Specifically, the photovoltaic ribbon was passed through a string welding machine, the heating temperature was set to 180°C, and the welding was heated for 1s. After the battery string was made, a lamination process was performed to obtain a photovoltaic module. The performance parameters of the photovoltaic module are shown in Table 2. During the lamination process, the temperature of the first temperature zone of the laminator was set to 120°C, the second temperature zone was set to 140°C, and the lamination welding was 15 minutes.

[0058] The photovoltaic modules made of the main-grid-free photovoltaic welding strips prepared in Examples 4-6 of the present application have all passed the EL power-on test, the DH2000 test, and the TC200 test. Among them, the main-grid-free photovoltaic welding strips prepared in Examples 4 and 5 are welded at the lamination welding temperature, and there is no tin bead phenomenon after welding, and there are few cold solder joints. After the reliability test (the reliability test includes a damp heat test, a temperature shock test, a cold and hot cycle test, etc., and the EL test is required to verify the result after the reliability test is completed, that is, the EL test is used to check the changes in the components after the reliability test is completed), the cold solder joints are much less than those of the conventional high-bismuth low-temperature welding strips. The solder joint peeling rate is very low; the busbar-free photovoltaic welding strip prepared in Example 6 is welded by a series welding process. Compared with the laminated welding method, the solid-liquid phase line of the series welding process is slightly higher, the welding quality is better, and the welding strength is higher. The melting point of the photovoltaic solder in Example 3 corresponding to Example 6 avoids the lamination temperature, so there will be no loss of the original welding effect due to melting twice during laminated welding, but the welding temperature is nearly 30°C lower than that of conventional tin-lead solder (Sn60Pb40, liquidus temperature is 187°C, welding temperature is 210-220°C), which can significantly reduce the problems of broken grids and hidden cracks in battery segments, and can be applied to busbar-free photovoltaic modules.

[0059] This application embodiment conducts appearance, hidden crack rate, cold soldering rate, EL power-on test, DH1000 and TC200 related tests on photovoltaic modules. The EL power-on test is performed on the welding strip according to the method provided in the "NB / T 11080-2023 Photovoltaic Module Electroluminescence (EL) Detection Technical Specification" standard, and the DH1000 and TC200 related tests are performed on the welding strip according to the "IEC61215-2 Module Test Standard" standard. The specific test results are shown in Tables 1 and 2 below.

[0060] Table 1 Performance parameters of photovoltaic modules in Example 4 and Comparative Examples 6-8

[0061] In Table 1, the bismuth content in the photovoltaic solder of Comparative Example 2 is greater than 25%, the solidus temperature of the solder is lower than the temperature of the first temperature zone of lamination (120°C), and the liquidus temperature is lower than the lamination welding temperature (140°C). Therefore, for the solder strip of Comparative Example 7 based on the solder of Comparative Example 2, during the lamination process, the solder has high fluidity and will gather to the end of the solder strip along the direction of the solder strip to form tin beads, resulting in an increase in the hidden crack rate of the battery cell. In addition, since the excessively high bismuth content reduces the mechanical properties of the solder, after the reliability test, the power attenuation is more obvious, far exceeding the solder strip in Example 4.

[0062] Comparative Example 1 uses conventional tin-bismuth eutectic low-temperature solder, whose solidus temperature is the same as the liquidus temperature, which is consistent with the lamination temperature. The solder strip of Comparative Example 6 based on the solder of Comparative Example 1 has a normal appearance after lamination, but due to the excessively high bismuth content, its weldability is significantly lower than that of the solder strip of Example 4, which is manifested in a high false soldering rate after lamination, and due to the high bismuth content, it is brittle, has poor mechanical properties, and poor oxidation resistance. After DH2000 and TC200 tests, the surface of the solder strip is seriously yellowed, severe desoldering occurs, and the power attenuation is very obvious. (See TC200 attenuation in the table).

[0063] In Comparative Example 3, the bismuth content in the photovoltaic solder is less than 23%, and the liquidus temperature of the solder is higher than that in Example 1. At this time, the liquidus temperature of the solder is greater than the lamination welding temperature of 140°C. In the solder strip of Comparative Example 8 based on the solder of Comparative Example 3, most of the solder is in a solid state during lamination welding and is not liquefied. Therefore, the welding effect is not good, and the false solder joint rate will be significantly increased after lamination. Due to the insufficient strength of the solder joints, obvious desoldering will occur after the TC200 test, resulting in a high attenuation rate (see TC200 attenuation in the table).

[0064] Table 2 Performance parameters of photovoltaic modules in Example 3 and Comparative Examples 4-5

[0065] In Table 2, the bismuth content in the photovoltaic solder of Comparative Example 4 is greater than 21%, the liquidus temperature of the solder is lower than the liquidus temperature in Example 3, and the solder weldability is relatively weak. The solder strip of Comparative Example 9 based on the solder of Comparative Example 4 has a higher EL test false solder rate after welding than that of Example 6, and the solder strip will melt twice during lamination, resulting in a certain increase in the false solder rate after lamination; the bismuth content in the photovoltaic solder of Comparative Example 5 is less than 19%, and the liquidus temperature of the solder is higher than the liquidus temperature in Example 3. The solder strip of Comparative Example 10 based on the solder of Comparative Example 5, at the same welding temperature (180°C), due to insufficient heating temperature and too short heating time, the solder cannot be completely liquefied, and some positions cannot form an alloy layer with the silver paste of the battery grid line, and the false solder rate after welding is significantly improved. At this time, if the false solder rate is to be reduced, the welding temperature must be increased, and the increase in welding temperature will cause an increase in the rate of hidden cracks and broken grids.

[0066] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A photovoltaic solder, characterized in that: At least tin, lead and bismuth are included, wherein the content of bismuth does not exceed 25% by mass.

2. The photovoltaic solder according to claim 1, characterized in that: The photovoltaic solder comprises the following components by mass fraction: 39% to 41% tin, 39% to 41% lead, 19% to 21% bismuth, and 0% to 0.5% silver. The solid-liquid phase line of the photovoltaic solder is 132 to 154°C.

3. The photovoltaic solder according to claim 1, characterized in that: The photovoltaic solder comprises the following components by mass fraction: 36% to 38% tin, 36% to 38% lead, 23% to 25% bismuth, and 0 to 1% silver. The solid-liquid phase line of the photovoltaic solder is 125 to 145°C.

4. The photovoltaic solder according to claim 2, characterized in that: The photovoltaic solder is suitable for a connection process of dispensing and welding, and the welding temperature is 175-185°C.

5. The photovoltaic solder according to claim 3, characterized in that: The photovoltaic solder is suitable for the dispensing / film coating + lamination welding process route, which specifically includes the following steps: (1) The soldering tape is attached to the battery cell by adhesive or film fixing. Then, during the lamination packaging process, the solder on the surface of the soldering tape is melted and the soldering tape and the gate line are welded under vacuum pressure to achieve electrical connection; The lamination packaging process is divided into two stages of heating, the first temperature zone is at 110-120°C, and the second temperature zone is at 135-145°C.

6. The method for preparing the photovoltaic solder according to any one of claims 1 to 5, characterized in that: The following steps are involved: Step a, weighing: weigh tin, lead, bismuth and silver respectively according to the proportions and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250-270° C. to melt the raw materials, stirring for 25-35 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing a second molten liquid: adding metallic bismuth into the first molten liquid, and covering the surface of the molten metal with mica sheets and / or quartz sand, stirring for 40 to 60 minutes until all the metal is melted to obtain a second molten liquid, and cooling the tin furnace to 220 to 230° C.; Step d, preparing photovoltaic solder: adding weighed silver particles into the second molten liquid, and continuing to fully stir for 30 to 40 minutes to obtain a finished photovoltaic solder.

7. The method for preparing the photovoltaic solder according to any one of claims 1 to 5, characterized in that: The following steps are involved: Step a, weighing: weigh tin, lead and bismuth respectively according to the ratio and set aside; Step b, preparing a first molten liquid: putting tin and lead into a tin furnace in sequence, and heating the tin furnace to 250-270° C. to melt the raw materials, stirring for 25-35 minutes, and cleaning the tin slag on the liquid surface to obtain a first molten liquid; Step c, preparing photovoltaic solder: adding metallic bismuth into the first molten liquid, and covering the metal liquid surface with mica sheets and / or quartz sand, stirring for 40 to 60 minutes until all the metal is melted, to obtain a finished photovoltaic solder.

8. A busbar-free photovoltaic welding strip, characterized in that: It includes a conductive substrate and a photovoltaic solder coated on the surface of the conductive substrate; The conductive substrate includes a wire or strip made of copper, aluminum, or a copper-aluminum composite material; The photovoltaic solder is the photovoltaic solder according to claim 6 or 7.

9. A method for preparing a busbar-free photovoltaic ribbon, characterized in that: The photovoltaic solder obtained by the preparation method described in claim 6 or 7 is coated on the surface of a conductive substrate, specifically comprising the following steps: Step S1, preparing solder: putting photovoltaic solder into the tin furnace of the tin plating production line, and setting the temperature of the tin furnace to 190-220° C.; Step S2, preparing a conductive substrate: processing the conductive substrate by a wire drawing process and / or a calendering process; Step S3, pre-treating the conductive substrate: annealing and softening the conductive substrate prepared in step S2, and then surface treating the conductive substrate with a low-temperature soldering flux; Step S4, coating solder: passing the pretreated conductive substrate in step S3 through the tin furnace in step S1, setting a pressure air knife at the tin outlet to control the thickness of the photovoltaic solder layer on the surface of the conductive substrate, and obtaining the main grid-free photovoltaic solder strip after cooling.

10. The method for preparing a busbar-free photovoltaic ribbon according to claim 9, characterized in that: In step S2, the diameter of the conductive substrate is 0.16-0.2 mm; And / or, in step S3, the current of the annealing treatment is 9 to 14A; And / or, in step S3, the running speed of the conductive substrate during annealing is 250-350 m / min; and / or, in step S3, the yield strength of the conductive substrate after annealing is 55 to 75 MPa; and / or, in step S3, the elongation of the conductive substrate after annealing is 20-30%; And / or, in step S3, the activity range of the low temperature soldering flux is 150 to 240°C; And / or, in step S4, the running speed of the conductive substrate when coating the photovoltaic solder layer is 250-350 m / min; And / or, in step S4, the thickness of the photovoltaic solder layer is 5 to 15 μm; And / or, in step S4, the diameter of the busbar-free photovoltaic welding ribbon is 0.18-0.22 mm.

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