In the welding process of crystalline silicon solar cells, in addition to the quality factors of the cell itself, the main factors that affect the welding effect are the following aspects: welding process parameters (welding temperature, welding time, etc.), selection of flux, and ribbon The selection of solder and the operating specifications of the operator, etc.
(1) Welding temperature. Generally, the soldering temperature is 25℃~60℃ higher than the melting point of the solder. It should not be too high, otherwise it will deform the cell and cause defects due to local overheating.
(2) Welding time. Depending on the intensity of the interaction between the solder and the battery sheet electrode, a proper soldering time is conducive to the mutual diffusion and wetting between the solder and the battery sheet to form a firm contact.
The welding process parameters depend on the solder and the paste used in the cell electrode, and the soldering temperature and soldering time have the greatest influence. In single welding and series welding, the welding temperature directly affects the welding quality of solar cell modules. The cells are placed on the welding panel for operation. The welding panel is generally maintained at about 50°C, which plays a role in heat transfer and uniform heating of the cells. Avoid local heating. During the soldering process, due to the high temperature of the soldering iron, a certain temperature difference is formed on the battery, which has a thermal shock. If the soldering temperature is low, on the one hand, the oxide layer on the soldering surface is not easy to remove, and rough pits like sand will appear. Moreover, when the main grid line is below a certain temperature value, it cannot form a good ohmic contact with tin. It is welded, but it is not an alloy connection in the true sense, forming a virtual weld, which also leads to low operating efficiency; and high welding temperature will cause deformation of the cell due to thermal stress, leading to cracks and fragments The production. At the same time, whether the solder contains lead in the soldering process also determines the soldering temperature. The melting point of 100% tin is 232℃. A certain percentage of lead content will reduce its melting point. Currently, the most used lead-containing solder is SnPbAg62/36/2. The melting point is 190°C, and the melting point of lead-free solder SnAg6,5/3,5 is 221°C. The melting point of conventional lead-free solder is 30°C higher than that of lead-containing solder, so the use of lead-free solder will increase soldering The welding temperature in the process is more prone to cracks. At present, the melting point of Sn-Bi system eutectic solder is l38°C, but its reliability is not strong. The melting point of Sn-Bi solder can be controlled by adjusting the content of Bi to make it close to tin-lead solder, which is the focus of lead-free solder research.
For manual soldering, in addition to considering the melting point of the solder, it is also necessary to consider that after the soldering iron tip contacts the tin-coated tape during the soldering process, it needs to transfer heat to the tin-coated solder tape to increase its temperature, which requires heat; the solder changes from solid to solid. The liquid also needs to absorb heat; the volatilization of the flux also requires heat; if the molten solder can smoothly flow into the base (here, the main busbar) and form an alloy with the base material to be welded, the busbar must also have a certain temperature, otherwise The molten solder can not form alloy when poured on the cold solder base. Based on the above factors, the current general single crystal single soldering temperature is 320℃-330℃, the polycrystalline temperature is 330℃~350℃, and the string soldering temperature is 330℃-360℃, depending on the quality of the solder ribbon cell. It differs from the substrate temperature.
In the selection of solder, the solder with good wettability with the grid line of the front electrode of the solar cell should be selected, which can increase the contact area between the solder and the electrode, and improve the adhesion and reliability after soldering. The role of solder and different silver paste electrodes is shown in Figure 1.
Through the above, we can know the welding process parameters of solar cells. If you want to better understand solar cell welding, there are What is a laser scribing machine and how to use it?, Tin-coated solder tape for solar cell welding, Solar cell welding operation method and post-welding inspection method and Welding equipment for solar photovoltaic modules-electric soldering iron and other articles