The main function of the reflow oven is the process of melting tin
Five temperature zones are typically placed in a box reflow oven to simulate the five temperature zones of a chain conveyor reflow reflow oven. In order to ensure the different requirements of the SMT PCB board, the temperature points and corresponding time of each temperature segment are designed. In order to better illustrate the requirements and effects of the various temperature zones, the temperature zones are now described separately.
1: The purpose and function of the preheating section
The purpose is to heat the component PCB board to about 120-150 °C. At this temperature, the moisture of the PCB board can be fully evaporated, and at the same time, the stress inside the PCB board and some residual gas can be eliminated, and the heating section is heated in advance. The preheating period is generally controlled in 1-5 minutes. The specific situation depends on the size of the board and the number of components.
2: Purpose and function of the heating section
Through the preheating section of the PCB board, the flux in the solder paste is activated during the heating section, and the oxide inside the solder paste and the surface of the component is removed under the action of the flux. Prepare for the welding process. In this stage, the temperature of Sn63%-Pb37% tin-lead formula with lead medium temperature alloy solder and Sn95%-Ag3%-Cu2% tin-silver-copper formula precious metal lead-free alloy solder is usually set at 180-230 °C. between. The time is controlled in 1-3 minutes. The goal is to fully activate the flux and remove the pad and solder oxide well.
Low-temperature lead-free solder of Sn42%-Bi58% tin-bismuth formula in low-temperature solder formulations with melting point below 160 °C, Sn43%-Pb43%-Bi14% tin-lead-bismuth formula has lead low temperature Solder. Sn48%-In52% tin-indium formula lead-free low-temperature solder, the temperature of the heating section can be set at about 120-180. The time is controlled in 1-3 minutes.
Medium temperature lead alloy solder is generally set at 180-220 ° C. High temperature lead-free alloy solders are typically placed between 220 and 250 °C. If you have solder and solder paste data on hand, the temperature of the heating section can be set to about 10 °C below the melting temperature of the solder paste.
3: Purpose and function of the welding section
The main purpose of the welding section is to complete the welding process of SMT. Since this stage is the highest temperature section in the whole reflow process, it is very easy to damage the components that cannot meet the temperature requirements. This process is also a perfect process of reflow, the physical and chemical changes of the solder are the largest, and the molten solder is easily oxidized in high temperature air. At this stage, the melting temperature provided by the tin slurry data is generally about 30-50 °C. Regardless of lead or lead-free solder, we generally divide it into low temperature solder (150-180 ° C), medium temperature solder (190-220 ° C), high temperature solder (230-260 ° C). The lead-free solders commonly used today are high-temperature solders. The low-temperature solders are generally lead-free solders of precious metals and special low-temperature lead solders. They are rare in general-purpose electronic products and are used in electronic equipment with special requirements. . The lead medium temperature solder has excellent electrical properties, physical and mechanical properties, thermal shock resistance and oxidation resistance. These properties are currently not replaceable with various lead-free solders, so they are also widely used in general-purpose electronic products.
The time of this phase is generally set according to the following requirements. Solder is shown as liquid after melting at high temperature. All SMT components will float on the surface of the liquid solder. Under the action of flux and liquid surface tension, the floating components will move to the center of the pad and will be automatically corrected. effect. In addition, the solder will form an alloy layer with the surface metal of the pre-case under the flux of the flux. Infiltrate into the structure of the component structure to form an ideal brazed structure. The time is generally set at about 10-30s. The PCB board with large area and large component shading surface should be set for a long time; small area PCB board with few parts Generally, the setup time is shorter. In order to ensure that the quality of the reflow is as short as possible in this phase, this will help protect the components.
4: Purpose and function of the insulation section
The function of the heat preservation section is to solidify the high-temperature liquid solder into a solid solder joint. The quality of the solidification directly affects the crystal structure and mechanical properties of the solder. The too fast solidification time will cause the solder to form crystal roughness, and the solder joint is not smooth. Physical performance is degraded. The solder joints are prone to cracking under high temperature and mechanical impact, losing mechanical connection and electrical connection, and the durability of the product is lowered. We used to stop heating and keep warm with residual temperature for a short time. Let the solder solidify and crystallize well during the slow temperature drop. This temperature point is generally set at about 10-20 °C lower than the solder melting point. With the natural cooling time setting, it can enter the cooling section after falling to this temperature point.
5: Purpose and function of the cooling section
The effect of the cooling section is relatively simple, usually it is cooled to a temperature that will not burn people. However, in order to speed up the operation process, it is also possible to end the process when it falls below 150 °C. However, when removing the soldered PCB board, use a tool or a hand strap and heat-resistant gloves to prevent burns.
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