Setting and measurement of temperature curve of standard reflow soldering

2022-10-24 54

Abstract: Reflow soldering is a unique process in surface mount technology (SMT). This paper mainly introduces the setting method and measuring method of reflow temperature curve of solder paste process from Mangangquan SMT. Key words: temperature curve, reflow soldering, temperature zone, setting of temperature curve, introduction: Since 1980s, electronic products have been developing towards thin, short and high performance at an alarming speed. In this process, the popularization and application of surface mount technology (SMT) has played a key role. At present, the printing and pasting equipment and technology in the industry are similar, so the quality of reflow soldering technology is very important for the quality and reliability of the final product. Therefore, in-depth research on reflow soldering process and development of reasonable reflow soldering temperature curve are important links to ensure the surface assembly quality. Setting and Measurement of Temperature Curve of Standard Reflow Welding in 2015 1. Development of Reflow Welding Equipment: In the electronic industry, a large number of surface mount components (SMA) are welded by reflow soldering. At present, the types of reflow soldering equipment are divided into infrared, all-hot air and infrared heated air by heat transfer. Infrared ray: Infrared reflow soldering is a kind of soldering method that realizes the heating of the components to be soldered by infrared radiation. It has the characteristics of fast heating, energy saving and stable operation. However, the thermal absorptivity of infrared radiation is very different between printed circuit boards and various components due to different materials and colors, which leads to the phenomenon of uneven temperature among various components on printed circuit boards and among different areas of the same component. All-hot air: All-hot air reflow soldering is a kind of welding method that uses convection jet nozzle or heat-resistant fan to force the hot air flow in the furnace to circulate, thus realizing the heating of the components to be welded. In this heating mode, the temperature of the components on the printed circuit board is close to the gas temperature in the set heating temperature zone, which completely overcomes the temperature difference and shielding effect of infrared reflow soldering. However, the convection speed of the circulating gas is very important in all-hot air reflow soldering equipment. In order to ensure that the circulating gas in the furnace can act on every area on the printed circuit board, the air flow must have sufficient speed, which to some extent easily causes the jitter of the printed circuit board and the displacement of the components. In addition, this heating method has poor efficiency and high energy consumption in terms of heat exchange. Infrared heating wind: Infrared heating wind reflow soldering is a kind of welding method that adds hot air circulation on the basis of infrared heating, and realizes the heating of welded components through the double effects of infrared and hot air. This heating method makes the temperature in the furnace more uniform, makes full use of the strong penetrating power of infrared rays, and has the characteristics of high thermal efficiency and low energy consumption. At the same time, it effectively overcomes the temperature difference and shielding effect of the infrared heating method, and makes up for the adverse effects caused by the excessive requirement of the hot air heating method on the gas flow speed. Second, the temperature curve The temperature curve is a function Y=F(T) of the temperature applied to the assembled components against time, which is a curve of the temperature of a given point on the printed circuit board changing with time in the reflow process. As shown in Figure 1 (Figure 1 shows a typical RSS temperature curve). In the figure, the horizontal axis is time, the vertical axis is temperature, and the curve Y is a curve in which the temperature keeps changing with the increase of time. The area enclosed by the curve Y in OtT coordinate system is the sum of the energy received by the measured point in the whole reflow soldering process. The temperature curve function expressed by the concept of energy is Y=∫ d(T). Temperature curve can be divided into RSS curve and RTS curve. RSS curve: (Figure 2) is a temperature curve composed of four temperature intervals: heating, heat preservation, reflux and cooling. Each temperature range plays a different role in the whole reflow soldering process. Heating zone: the printed circuit board is heated from room temperature to 135-170℃(SN63/PB37) by slow heating, and the heating rate is generally 1-3℃/s. Heat preservation zone: the soldering flux in the solder paste can function and be properly distributed by maintaining a relatively stable temperature. Reflow zone: The temperature in the furnace reaches the highest point, so that the solder paste is liquefied, and an alloy is formed between the pad of the printed circuit board and the welding electrode of the component, thus completing the welding process. Cooling zone: cooling the soldered printed circuit board. RTS curve: (Figure 3) is a temperature curve from temperature rise to reflux. It can be divided into heating zone and cooling zone. Temperature rising zone: accounting for 2/3 of the whole reflow soldering process, with a gentle speed of 0.5-1.5℃/s. The temperature of the printed circuit board is increased from room temperature to the peak temperature. Cooling zone: cooling the soldered printed circuit board. Comparison between RSS curve and RTS curve: RSS curve: attaches importance to the combination of temperature and time, and the interval of the curve is finely divided, with high production efficiency and general adaptability. It is suitable for products with small size of printed circuit board, small volume of components on board and few types. RTS curve: Pay attention to the heating rate, the interval division of the curve is fuzzy, the production efficiency is not high, and the adaptability is strong. It is suitable for products with large size of printed circuit board, large volume of components on board and many kinds. Setting the temperature curve The temperature curve is the result of the joint action of several parameters of the reflow oven, among which the two decisive parameters are the conveyor belt speed and the temperature setting of the temperature zone. The speed of the conveyor belt determines the duration of exposure of the printed circuit board to each temperature zone. Increasing the duration can make the temperature of the components on the printed circuit board closer to the set temperature of the temperature zone. The sum of the duration of each temperature zone determines the treatment time of the whole reflux process. The temperature setting of each temperature zone affects the temperature of the printed circuit board when it passes through the temperature zone. The heating rate of printed circuit board in the whole reflow soldering process is the result of the joint action of two parameters: conveyor belt speed and temperature setting of each temperature zone. Therefore, an ideal furnace temperature curve can be obtained only by reasonably setting furnace temperature parameters. Now take the most commonly used RSS curve as an example to introduce the setting method of furnace temperature curve. Setting of chain speed: The first parameter to be considered when setting the temperature curve is the speed setting of the conveyor belt, which will determine the time it takes for the printed circuit board to pass through the heating channel. The setting of conveyor belt speed can be obtained by calculation. Here, an index, load factor, is introduced. Load factor: F=L/(L+s) L= the length of the substrate, and S= the spacing between substrates. The load factor determines the degree of influence of the printed circuit board in the furnace on the furnace temperature in the production process. The greater the value of the load factor, the more unstable the temperature in the furnace will be. Generally, the value is between 0.5 and 0.9. After weighing the efficiency and the stability of the furnace temperature, the recommended value is 0.7-0.8. After knowing the board length and takt time of production, the conveying speed (slowest value) of the conveyor belt can be calculated. Transmission speed (slowest value) = printed circuit board length /0.8/ takt time. The transmission speed (the fastest value) is determined by the characteristics of solder paste, and most solder paste requires that the time from the temperature rise to the peak temperature in the furnace should be no less than 180 seconds. In this way, the conveying speed (maximum value) = the length of the heating zone in the furnace /180S can be obtained. After two limit speeds are obtained, the appropriate conveying speed can be selected according to the difficulty of the actual production of products. Generally, the intermediate value can be taken. Setting of temperature zones: A complete RSS furnace temperature curve includes four temperature zones (as shown in Figure 2). Respectively: preheating zone: its purpose is to raise the temperature of printed circuit board from room temperature to the active temperature of 135℃ required by soldering flux in solder paste. The heating rate of the warm zone should be controlled at 1~3℃ per second. Too fast temperature rise will cause some defects, such as tiny cracks of ceramic capacitors. Heat preservation zone: its purpose is to maintain the printed circuit board in a certain temperature range for a certain period of time, so that the components in various areas on the printed circuit board have the same temperature, reduce their relative temperature difference, and make the soldering flux inside the solder paste fully play its role to remove the oxides on the surface of the component electrodes and pads, thus improving the soldering quality. Generally, the active temperature range is 135-170℃ (taking SN63PB37 as an example), and the active time is set at 60-90 seconds. If the active temperature is set too high, the soldering flux will lose its decontamination function prematurely, while if the temperature is too low, the soldering flux will not play its decontamination role. If the activation time is set too long, the flux in the solder paste will volatilize excessively, resulting in the lack of flux participation during welding, which makes the solder joints prone to oxidation and poor wetting ability. If the activation time is set too short, too much flux will participate in the welding, which may lead to poor welding such as solder balls and beads. Thereby affecting the welding quality. Reflow zone: Its purpose is to raise the temperature of printed circuit board to above the melting point of solder paste and maintain a certain welding time, so that it can form an alloy and complete the welding of component electrodes and pads. The temperature of this zone is set above 183℃ for 30-90 seconds. (take SN63PB37 as an example) the peak value should not exceed 230℃, and the time above 200℃ is 20-30 seconds. If the temperature is lower than 183℃, the alloy can't be formed and the welding can't be realized. If the temperature is higher than 230℃, the components will be damaged and the deformation of the printed circuit board will be aggravated. If the time is insufficient, the alloy layer will be thinner, and the strength of solder joints will be insufficient; if the time is longer, the alloy layer will be thicker and the solder joints will be brittle. Cooling zone: Its purpose is to cool the printed circuit board, usually set at 3-4℃ per second. If the speed is too high, the solder joints will crack; if it is too slow, the solder joints will be oxidized. The ideal cooling curve should be a mirror relationship with the reflow curve. The closer it is to this mirror relationship, the tighter the solid structure of the solder joint is, the higher the quality of the obtained solder joint and the better the bonding integrity. Knowing the characteristics of each temperature zone of the temperature curve, you can set the temperature of each temperature zone of the reflow oven according to the characteristics of the product. However, after the temperature of the first temperature zone is set, the heat capacity in the reflow oven is determined. In the production process, products passing through the furnace will continuously absorb heat, and with the increasing number of products in the furnace, the absorbed heat is also increasing. If the heat that can be replenished by the reflow oven is less than the heat absorbed by the product, the quality of the product cannot be guaranteed. However, in actual production, it is impossible to update the furnace temperature in real time, so it requires that the set temperature curve has certain adaptability or set different temperature curves for different product types. For example, for products with small size of printed circuit board and small volume of components, the heating rate of components itself is relatively fast due to the small absorption of heat, so the heating rate of curve heating zone can be appropriately increased, and the heat preservation time of heat preservation zone can be relatively shortened. However, for products with larger printed circuit boards and larger components, the requirements for heat absorption are higher, and the internal and external temperature difference of the components is larger, so the heating rate of the heating zone should be reduced, and the heat preservation time of the heat preservation zone should be lengthened to ensure the minimum temperature difference between various components and each part of the components on the board. Typical interval temperature setting (SN63PB37) Interval temperature time rate is increased from room temperature to 135℃ > 30S 1~3℃/S, kept at 135 ~ 170℃ for 60-90s 0.5 ~ 1℃/s, refluxed at 183 ~ 183℃ for 60 ~ 90s 1 ~ 3℃/s and cooled for 1 ~ 4℃/s.