A tin-plated copper strip is generally used for the connection between crystalline silicon solar cells. This copper strip is divided into interconnection strips and bus bars according to different functions, which are collectively referred to as tin-coated solder strips. The interconnection bar is mainly used for the connection between single cells, and the bus bar is mainly used for the interconnection between the battery strings and the connection of the internal circuit of the junction box. The welding strip is generally based on copper with a purity of more than 99.9%, and the surface is plated with a layer of SnPb alloy of 10~25μm to ensure good welding performance.
According to the different copper substrates, the welding strip can be divided into pure copper (99.9%), oxygen-free copper (99.95%) welding strip; according to different coatings, it can be divided into tin-lead welding strip (60%Sn, 40%Pb), lead Silver-containing tin-coated solder tape (62%Sn, 36%Pb, 2%Ag), lead-free environment-friendly tin-coated solder tape (96.5%Sn, 3.5%Ag), pure tin solder tape, etc.; can be divided according to yield strength For ordinary type, soft type, super soft type and so on.
Because the output current of crystalline silicon solar cells is relatively large, the electrical conductivity of the ribbon has a great influence on the output power of the module, so most photovoltaic ribbons use more than 99.95% oxygen-free copper to achieve the minimum resistivity and reduce the series resistance. resulting power loss. The welding ribbon also needs to have excellent welding performance. In the welding process, not only should the welding be reliable, no virtual welding or over welding, but also the warpage and damage of the battery should be avoided to the greatest extent. Therefore, Sn60Pb40 alloy with a lower melting point is generally used as the coating. If a silver-containing coating is used, the melting point of the ribbon will be reduced by 5 °C, which is more conducive to improving the welding performance, but it is usually not used due to the high cost. Reducing the yield strength of the welding ribbon can improve the reliability of the welding and connection of the components, especially the stress release in the thermal cycle, but this puts forward higher requirements for the welding ribbon manufacturing process. The strength is controlled below 75MPa. The yield strength of the early welding ribbon was too high, resulting in too low tensile strength and elongation, resulting in more component failures caused by the welding ribbon problem in actual use. Table 1 lists the main technical indicators of general-purpose welding tape.
|Serial number||Project||Technical parameter|
|1||Dimensions (including plating)||According to each specification|
|2||Coating composition and thickness||Coating composition: Sn60Pb40, deviation 5% /|
Single-sided coating 0.025mm±0.005mm
|7||Elongation after break/%||≥20|
Ribbon width and thickness are selected based on component design or customized to specific needs. Usually, the width of the interconnecting strip is mainly determined according to the width of the busbars of the battery, and the width ranges from 1.5 to 0.9mm. For example, 3 busbar batteries generally use 1.5mm wide welding tape, and 5 busbar batteries use 0.9mm wide. Ribbon. The thickness of the substrate is generally 0.1~0.2mm, and the thickness of the coating is 0.025mm. The bus bar is determined according to the current load requirements of the components. The thickness of the substrate is generally 0.1~0.25mm, and the width is 4~8mm. At present, the development of multi-busbar assemblies has brought new challenges to the processing of the ribbon, because the multi-busbar needs to use a round ribbon, which generally requires a diameter of 0.3~0.5mm.
Welding ribbon has an important influence on the power and service life of photovoltaic modules. At present, various welding ribbon manufacturers and component manufacturers have optimized from many aspects such as electricity and optics, and designed various welding methods with low resistivity, different surface coatings, and different surface structures. At the same time, the optical utilization rate and output power of the module can be further improved. For example, a light-trapping structure can be formed on the surface of the ribbon by means of calendering, as shown in Figure 1(a), or a light-trapping structure can be attached to the surface of the ribbon. film layer, etc. For the surface coating technology, the coating on the surface of the ribbon using the ordinary hot-dip process is uneven, as shown in Figure 1(b). thickness, thereby reducing the resistance; a special process can also be used to form a coating on the uneven surface with a light trapping structure on the surface.
The new low temperature welding process is an important development direction in the future. Traditional welding ribbons need to be alloyed at high temperature to complete the welding process, but high temperature can cause warping of the battery, causing cracks and even fragments, affecting the production yield of modules, and may affect the power output of modules, such as heterojunction cells (HIT) , the amorphous layer contained in its structure is very sensitive to temperature, and excessive temperature will cause the cell efficiency to decrease. Therefore, the traditional tin-coated solder tape needs to be further improved in terms of environmental protection, low temperature, optics, electricity, mechanics, etc., in order to achieve high power and long life of components.