SMT reflow oven temperature process curve setting
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With the advancement of science and technology and the improvement of people's living standards, people's requirements for smart electronic products are "light, thin, small, high-performance and multifunctional". The miniaturization and integration of electronic products have become the mainstream direction of their development; surface mount technology SMT, as the fourth generation packaging technology, is known as one of the world's top ten new technologies in the 1990s. It is widely used in microelectronic circuits for its low cost, high integration, light weight of electronic components, and easy automation. In the SMT process, electronic components are rigidly connected to the PCB printed circuit board through the solder paste on the solder pad through reflow soldering to form components. To optimize the soldering effect of SMT reflow soldering, people all know that the key is to set the furnace temperature curve of the reflow oven. The furnace temperature curve of the reflow oven has been reported in many professional articles. But facing a new full hot air reflow oven, how to set the reflow oven temperature curve as soon as possible? This requires us to first have a comprehensive understanding of the metal composition, melting point, active temperature and other characteristics of the solder paste used, and a comprehensive understanding of the structure of the full hot air reflow oven, including the number of heating temperature zones, hot air system, heater size and temperature control accuracy, effective length of the heating zone, cooling zone characteristics, transmission system, etc., as well as the size of the soldering object - surface mount components (SMD), component size and distribution. It is not difficult to see that reflow soldering is a complex and critical part of the SMT process, which involves knowledge of materials, equipment, heat conduction, welding and other aspects. 1. SMT reflow soldering temperature curve setting and process flow: The influence of SMT reflow soldering process parameters on the key indicators of reflow soldering temperature curve provides a reference for the setting and adjustment of reflow soldering process parameters; The reflow soldering temperature curve of SMT surface mounting technology includes four parts: preheating, wetting, reflow and cooling. The following is collected and sorted by individuals on the Internet. If there are any errors or deviations, please feel free to advise. SMT reflow oven soldering in the electronics manufacturing industry is an important process in the PCBA electronic circuit board assembly operation. If it is not well mastered, not only will many "temporary failures" occur, but it will also directly affect the life of the solder joints. Almost all SMT reflow oven thermometers are available, but many users do not perform temperature measurement certification and adjust temperature settings for all products; some users use temperature measurement but do not master the key points of welding process and cannot optimize the process; in this way, a lot of electricity is wasted and product quality cannot be well guaranteed! Correctly setting the reflow oven temperature curve is energy-saving and environmentally friendly: II. Cost strategy of SMT reflow process ![]() Esamber gave an example, as shown in the figure above, in the reflow process, the average input cost of a single solder point reflow in Chinese factories is 4 times that of Germany, 3 times that of the United States, and more than 2 times that of Japan (this calculation does not take into account China's lower labor costs, and the labor cost calculation of various countries is consistent). At the same time, China also has many SMT manufacturing companies with the best reflow soldering technology and cost control in the world, such as Huawei, ZTE, Delta, Valeo China SMT Factory, Dell (China) and other companies. They are constantly pursuing advanced control of process technology in reflow soldering, wave soldering, selective soldering and other processes. Starting from zero defects in welding, they promote unmanned closed-loop control, regular comprehensive physical examinations to prevent problems, reasonable planning and maintenance, regular inspections of processes and equipment, rapid fault location, advanced process detection technology, and cultivating a group of process experts. A series of means have achieved high-performance operation of equipment, high utilization rate operation, long-life operation and minimum labor input, and ultimately minimized comprehensive costs. 3. How to correctly set the reflow soldering temperature curve: First, we need to understand several key areas of reflow soldering, the temperature zoning and the types of reflow soldering. The key areas that affect the furnace temperature are: 1: Temperature setting value of each temperature zone 2: Temperature difference of each heating motor 3: Speed of chain and mesh belt 4: Composition of solder paste [color=rgb(51, 51, 5: PCB thickness, component size and density 6: Number of heating zones and reflow length 7: Effective length of heating zone and cooling characteristics The working principle of the temperature zone of the SMT reflow oven is that when the assembled PCB board is on a metal mesh or double-track conveyor belt, it passes through the hot and cold journey of each temperature zone of the reflow oven (for example, a large machine with 8 hot and 2 cold, a lead-free reflow oven with a total length of 5-6m) to achieve the purpose of melting the solder paste and cooling it to form a solder joint. , 51, 51)] SMT reflow soldering zoning: 1: Preheating zone (also known as: heating zone) 2: Constant temperature zone (insulation zone / active zone) 3: Reflow zone 4: Cooling zone There are also several other undesirable phenomena that are related to the temperature rise in the preheating zone. They are explained one by one below: 1. Collapse: This mainly occurs in the paste stage before the solder paste melts. The viscosity of the solder paste will decrease as the temperature rises. This is because the increase in temperature causes the molecules in the material to vibrate more violently due to heat; in addition, the rapid temperature rise will prevent the solvent from volatilizing properly, causing the viscosity to drop more rapidly. To be precise, the temperature rise will cause the solvent to evaporate and increase the viscosity, but the amount of solvent evaporated is proportional to both time and temperature. That is to say, for a certain temperature rise, the longer the time, the more solvent evaporates. Therefore, the viscosity of the solder paste that heats up slowly will be higher than that of the solder paste that heats up quickly, and the solder paste will be less likely to collapse. [color=rgb(51, 51,2.Solder beads: The rapidly evaporating gas will carry the solder paste out together, forming separate solder paste blocks under parts with small gaps. During reflow, the separated solder paste blocks will melt and emerge from under the parts to form solder beads. 3.Solder balls: When the temperature rises too quickly, the solvent gas will quickly evaporate from the tin and cause solder paste to splash. Slowing down the rate of temperature rise can effectively control the generation of solder balls. However, heating too slowly can also cause excessive oxidation and reduce the activity of the flux. 4. Lamp wick siphon phenomenon: This phenomenon is that after the solder wets the pin, the solder climbs up from the solder joint area along the pin, resulting in insufficient solder or empty soldering at the solder joint. The possible reason is that when the solder paste is melting, the temperature of the component pin is higher than the temperature of the PCB pad. The problem can be improved by increasing the temperature of the bottom of the PCB or extending the time that the solder paste is near the melting point. It is best to achieve temperature balance between the component pin and the pad before the solder wets. Once the solder has wetted the pad, the shape of the solder is difficult to change and is no longer affected by the temperature rise rate. 5. Poor wetting: Generally, poor wetting is caused by excessive oxidation of the tin powder during the soldering process, which can be improved by reducing the amount of heat absorbed by the solder paste during preheating. The ideal reflow time should be as short as possible. If there are other factors that prevent the heating time from being shortened, it is recommended to use a linear temperature from room temperature to the melting point of the solder paste, so that the possibility of tin powder oxidation can be reduced during reflow. 6. Head-In-Pillow: The main cause of hollow solder joints may be due to the wick siphon phenomenon or non-wetting. The wick siphon phenomenon can refer to the solution to the wick siphon phenomenon. If it is a non-wetting problem, that is, the pillow effect, this phenomenon is that the part feet have been immersed in the solder, but no real alloy or wetting has been formed. This problem can usually be improved by reducing oxidation. You can refer to the solution to poor wetting. 7. Tombstoning and skew: This is caused by uneven wetting at both ends of the component, similar to the siphon phenomenon of a lamp wick. It can be improved by extending the time that the solder paste stays near the melting point, or by reducing the rate of heating so that the temperature at both ends of the component reaches equilibrium before the solder paste melts. Another thing to note is the design of the PCB pads. If there are obvious differences in size, asymmetry, or one pad is grounded but no thermal resistance is designed while the other pad is not grounded, it is easy to cause different temperatures at both ends of the pad. When one pad melts first, the surface tension will pull the parts upright (tombstone) and tilt them. 8. Voids: This is mainly caused by the rapid oxidation of the solvent or water vapor in the flux, and the failure to escape immediately before the solder solidifies. Wetting zone is also called active zone. In the constant temperature zone, the temperature is usually maintained at 150℃±10. At this time, the solder paste is on the eve of melting, the volatiles in the solder paste are further removed, the activator starts to start, and effectively removes the oxides on the soldering surface. The surface temperature of the PCB is affected by the hot air convection. The temperature of components of different sizes and textures can be kept uniform, and the board temperature difference △T is close to the minimum value. The curve shape is close to horizontal. It is also a window for evaluating the reflow oven process. Choosing a furnace that can maintain a flat active temperature curve will improve the welding effect, especially prevent the occurrence of tombstone defects. Usually the constant temperature zone is between the 2nd and 3rd zones of the furnace, and the maintenance time is about 60~120s. If the time is too long, it will also cause the problem of solder paste oxidation, resulting in an increase in flying beads after welding. 4. Tips for setting the temperature curve of SMT reflow soldering 1. Preliminary SMT reflow soldering furnace temperature setting: 1） Look at the type of solder paste, lead or lead-free? The characteristics of solder paste should also be considered. Solder paste is a paste made by evenly mixing alloy powder and paste flux. The flux in solder paste (click on the characteristics of solder flux) is mainly composed of four types of raw materials: solvent, rosin or synthetic resin, activator and anti-sag agent. The solvent determines the drying time required for the solder paste. In order to increase the viscosity of the solder paste and make it have good rheological properties, synthetic resin or rosin is added. The activator is used to remove the oxides produced by the alloy to clean the board pads. The addition of anti-sag agent helps the alloy powder to be suspended in the solder paste to avoid sedimentation. There are many factors to measure the quality of solder paste. In actual production, the following solder paste characteristics should be considered. (1) The activity and alloy content of solder paste shall be determined according to the requirements of circuit board surface cleanliness; (2) The viscosity, rheology and collapse characteristics of solder paste shall be determined according to the solder paste printing equipment and production environment; "]（3）Determine the melting point of the solder paste based on the process requirements and the temperature that the component can withstand; （4）Determine the particle size of the solder paste alloy powder based on the minimum foot spacing of the pads. 2) What is the thickness of the PCB board? At this time, combined with the above 1 and 2 points, you can have a preliminary furnace temperature based on experience; 3) Look at the PCB board material and set the furnace temperature of the reflow zone in detail; 4) Look at the various components on the PCB board, consider the different sizes of components, special components, special components required by the manufacturer, etc., and then carefully set the furnace temperature; 5) Also consider the heating efficiency of the furnace, because there are many kinds of confluence furnaces today, and their heating efficiency is different, so this point should not be ignored; Combining the above 5 aspects, you can set the preliminary furnace temperature. Second, the selection of test points, generally at least three points, can represent the test points of temperature changes on the PCB components (can reflect the temperature changes of high, medium and low temperature parts on the PCB components); Under normal circumstances, the highest temperature part is at the center of the edge of the PCB perpendicular to the transmission direction without components, and the lowest temperature is at the half-field terminal of the large component near the center of the PCB (PLCC.QFP, etc.). In addition, there must be test points on the surface of parts with poor heat resistance, as well as specific requirements of customers. The peak temperature of SMT reflow soldering usually depends on the melting point of the solder and the temperature that the assembled parts can withstand. First of all, you need to consider whether your components and PCB board can withstand this temperature, because the composition of lead solder paste and lead-free solder paste is different. The composition of lead solder paste is: tin: 63%, lead: 37%, the melting point is generally 183. The peak value is 205-230, while the lead-free solder paste is: tin 96.5%; silver: 3%; copper: 0.5%, so the high melting point is generally: 217, the peak value is: 245-250; the general peak temperature should be about 25~30°C higher than the normal melting point of solder paste to successfully complete the welding operation. If it is lower than this temperature, it is very likely to cause the disadvantages of cold welding and poor wetting. 3. The relationship between reflow soldering defects and poor temperature curve: The following only lists the reflow soldering defects caused by poor temperature curve. Other factors that affect the quality of reflow soldering include the quality of silk screen printing, the accuracy and pressure of the patch, the quality of solder paste and environmental control, which are not elaborated in this article. , BlinkMacSystemFont, "] 1) The preheating temperature of the insulation section is insufficient; 2) The preheating temperature of the insulation section is insufficient; 3) The preheating temperature of the insulation section is insufficient; 4) The preheating temperature of the insulation section is insufficient; 5) The preheating temperature of the insulation section is insufficient; 6) The preheating temperature of the insulation section is insufficient; 7) The preheating temperature of the insulation section is insufficient; 8) The preheating temperature of the insulation section is insufficient; 9) The preheating temperature of the insulation section is insufficient; 10) The preheating temperature of the insulation section is insufficient; 11) The preheating temperature of the insulation section is insufficient; 12) The preheating temperature of the insulation section is insufficient; 13) The preheating temperature of the insulation section is insufficient; 14) The preheating temperature of the insulation section is insufficient; 15) The preheating temperature of the insulation section is insufficient; 16) The preheating temperature of the insulation section is insufficient; 17) The preheating temperature of the insulation section is insufficient; 18) The preheating temperature of the insulation section is insufficient; 19) The preheating temperature of the insulation section is insufficient; 20) The preheating temperature of the insulation section is insufficient; 21) The preheating temperature of the insulation section is insufficient; 22) The preheating temperature of the insulation section is insufficient; 23) The preheating temperature of the insulation section is insufficient; 24) The preheating temperature of the insulation section is insufficient; 25) The preheating temperature of the insulation section is insufficient; 26) The preheating temperature of the insulation section is insufficient; 27) The preheating temperature of the insulation section is insufficient; 28) The preheating temperature of the insulation section is insufficient; 29) The preheating temperature of the insulation section is insufficient; 30) BlinkMacSystemFont, "]2) The temperature in the insulation section rises too quickly. The solder joint is gray and the cooling speed of the cooling section is too slow. No tinning 1) The fusion welding temperature of the welding section is low; [font=-apple-system-font, 2) The insulation period of the insulation section is too long; 3) The temperature of the insulation section is too high. The insulation period of the insulation section is short after welding. Solder beads 1) The temperature of the insulation section rises too fast; 2) The temperature of the insulation section is low; 3) The heat preservation period is short. Void 1) The temperature of the insulation section is low; 2) The insulation period is short. Raw welding 1) The fusion welding temperature of the welding section is low; 2) The fusion welding cycle of the welding section is short. The board or component changes color 1) The welding temperature of the welding section is too high; 2) The welding cycle of the welding section is too long.
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