The function of tinned solder tape is to realize the electrical connection between solar cells, and the function of flux is to assist the completion of the welding work and ensure the quality of welding. This task is mainly to let students understand the basic knowledge of tinned soldering tape and flux, and lay a foundation for the future selection and use of tinned soldering tape and flux.
The interconnection bars and bus bars used to connect the electrical properties of the cells in the photovoltaic module are all tinned solder tape. As shown in Figure 1, the tin-coated soldering tape uses pure copper as the base material, and a tin layer is coated on its surface. On the one hand, it can prevent the copper-based material from oxidizing and discoloring, and on the other hand, it is also convenient to weld the material to the grid line of the solar cell. . There are two types of tin-coated solder tape: lead-containing and lead-free, as shown in Table 1. Among them, lead-free tin-coated soldering tape has become the development direction of tin-coated soldering tape because of its good soldering performance and non-toxicity. Lead-free tin-coated soldering strips are composited with special copper and tin alloy coatings with excellent electrical conductivity and excellent processing ductility. The smaller the thickness of the tin-coated solder strip, the thinner the coating; conversely, the greater the thickness, the thicker the coating.
The tin-coated solder tape for photovoltaic modules should have the following characteristics:
(1) Good solderability.
(2) Good corrosion resistance.
(3) In the case of thermal vibration of -40°C- +100°C (synchronized with the use environment of the solar cell), the long-term work will not fall off.
| Tinned solder tape | Lead | No error |
| Interconnection bar | SnPb40/Cu/SnPb40 | SnAg3.5/Cu/SnAg3.5 |
| Bus bar | SnPb40/ Cu/SnPb40 | Sn/Cu/Sn, SnCu/Cu/SnCu |
The requirements for tinned solder tape include the following aspects:
(1) It has lower resistivity.
(2) It has a lower melting point.
(3) It has excellent elongation.
(4) It has excellent weather resistance.
(5) It has good solderability.
(6) It has good corrosion resistance.
Specifications and selection of tinned solder tape
The tin-coated soldering tape uses pure copper as the base material and is coated with a tin layer on its surface. The higher the purity of copper, the lower the resistivity, the greater the load-bearing capacity, and the better the plasticity. There will be no microcracks during rolling. The coated tin material is the same as copper. The higher the purity, the lower the resistivity and the better the conductivity. For the selection of tin-coated soldering tape, it is necessary to consider the requirements of the solar cell’s own characteristics and the electrical performance of the photovoltaic module, as shown in Figure 2.
When choosing the tin-coated copper strip, the thickness of the tin-coated copper strip is determined according to the thickness of the battery and the amount of short-circuit current. The width of the tin-coated copper strip is required to be consistent with the width of the battery’s main grid line. The hardness of the tin-coated copper strip generally depends on the battery Sheet thickness and welding tools. Manual soldering requires the state of the solder ribbon to be relatively soft, and the soft solder ribbon will come into contact with the cell well after the soldering iron passes, forming a good silver-tin alloy, and its solderability meets the requirements; at the same time during the soldering process The stress generated is very small, which can reduce the fragmentation rate, but the tensile strength and elongation of the too soft ribbon will be reduced, and it is easy to break.
In view of the electrical performance requirements of photovoltaic modules, it is hoped that the electrical loss is the lowest, and the resistance of the tin-coated solder tape is R=ρι/S (ρ is the resistivity, S is the storage area, and ι is the length of the sample), because the resistivity is inherent to the metal Property, it does not change with the change of the cross-sectional length of the metal, so for the output electrical performance of the component, the cross-sectional area should be appropriately increased to reduce the internal resistance of the component and increase the output power. The larger the cross-sectional area of the tin-coated copper tape substrate, the smaller the resistance, and the smaller the series resistance of the component. There are two ways to increase the cross-sectional area of the tin-coated copper tape substrate. Under the same material, one is to increase the substrate The thickness is to increase the width of the substrate. But no matter which method is adopted, increasing the cross-sectional area will inevitably affect the softness of the tin-coated copper tape and also affect the welding damage rate. As for the specifications to be adopted, it needs to be tested according to the actual situation. The purpose is to increase the cross-sectional area of the tin-coated copper strip, the increase in component power and the welding fragmentation rate under the premise of ensuring the welding damage rate. The data reflects the overall improvement effect brought about by changing the specifications of tin-coated copper strip. Therefore, when selecting the specifications of the welding strip, one should not only consider the resistance of the welding strip or the welding damage rate. It is necessary to combine the two to consider. Deviation to the two extremes is not worth the gain. Cells of different sizes have different currents. Generally, there are several specifications of tin-coated tapes for cells of several sizes. In addition, the width of the bus bar of the cell must be taken into consideration to ensure that the width of the tin-coated tape cannot exceed the width of the bus bar.
By increasing the cross-sectional area of the tin-coated copper strip, the power of the module increases, but the rate of fragmentation during welding also increases slightly, but the average power output of all modules increases by more than 1%. However, due to the increase in the thickness of the tin-coated copper strip, there is a mismatch between the stress of the tin-coated copper strip and the battery electrode material during welding, and the curvature of the battery sheet after welding also changes greatly. Therefore, it is necessary to ensure the welding fragmentation rate. Under the premise, the selection of the best tin-coated copper strip and its size specifications is of great help to increase the output power of the module. At the same time, the test of IEC61215 requires the use of new specifications of tin-coated copper strip to produce components for related environmental tests, including thermal Cycle and damp heat test to verify whether the reliability of tin-coated copper strip meets the requirements.
As an example, here is a selection experience of tinned solder tape. According to the carrying current of the interconnection bar, the cross-sectional area of copper in the interconnection bar is selected to be 0.3~0.4mm2, which can carry 3.75A current (for reference). When selecting the specification of tinned copper strip, the 0.2×1.6 interconnection bar is connected in series with 72 pieces of 125mm x 125mm cell, the output power is 170 ~ 180W; the bus belt is 0.2×5, 0.25×5, 0.25×4.5, 0.35×3.5 and other specifications. Among them, 0.2×5 is the most used, and the 0.2×2 interconnection bar is connected in series with 54 156mm x 156mm cells, the output power is 250W, and the bus bar uses 0.25×7. It is suggested that thick and narrow interconnection strips should be preferred when the power of the solar panels is the same. For example, two battery panels assembled with 0.2×1.5 and 0.16×2 interconnect bars, the former is compared with the latter, and the power of each module increases by 1.5-2.5W.