Temperature requirements for SMT reflow soldering
SMT reflow soldering is a unique and important process in patch processing. The four major temperature zones of reflow soldering are preheating zone, constant temperature zone, reflow zone and cooling zone. Each temperature zone heating stage has its own important significance.
1. Preheating zone stage
The PCBA for patch processing reaches the reflow oven heating temperature of 150°C from the indoor temperature, and the heating rate is less than 2°C/s, which is called the preheat stage. The purpose of the preheating stage is to evaporate the lower melting point solvent in the solder paste. The main components of flux in solder paste include rosin, activators, viscosity improvers and solvents. The role of the solvent is mainly as a carrier of rosin and to ensure the storage time of the solder paste. During the preheating stage, excess solvent needs to be evaporated, but the heating rate must be controlled. Too high a heating rate will cause thermal stress impact on the components, damaging the components or reducing component performance and lifespan. The latter is more harmful because the products have already been delivered to customers. Another reason is that a too high heating rate will cause the solder paste to collapse, causing the risk of short circuit, and a too high heating rate will cause the solvent to evaporate too quickly, which can easily cause metal components to splash out and tin beads to appear.
2. Constant temperature zone stage
The entire PCBA board is slowly heated to 170°C to make the circuit board reach a uniform temperature, which is called the constant temperature (soak or equilibrium) stage. The time is generally 70-120s. At this stage, the temperature rises slowly. The setting of the constant temperature stage should mainly refer to the recommendations of the solder paste supplier and the heat capacity of the PCBA board. The constant temperature stage has three functions. One is to make the entire PCBA reach a uniform temperature and reduce the thermal stress impact entering the reflow area, as well as other welding defects such as component warping, cold welding of some large-volume components, etc.; the other important function is That is, the flux in the solder paste begins to undergo an active reaction, increasing the wetting performance of the surface of the weldment, so that the molten solder can well wet the surface of the weldment; the third function is to further volatilize the solvent in the flux. Due to the importance of the heat preservation stage, the heat preservation time and temperature must be well controlled, not only to ensure that the flux can clean the soldering surface well, but also to ensure that the flux is not completely consumed before reaching the reflow stage, and can be activated during the reflow stage. To prevent re-oxidation.
3. Return zone stage
The PCBA board is heated to the melting zone to melt the solder paste. The board reaches the highest temperature, usually 230°C-245°C, which is called the reflow stage. The time above the liquidus line is generally 30-60 seconds. During the reflow stage, the temperature continues to rise across the reflow line, the solder paste melts and a wetting reaction occurs, and an intermetallic compound layer begins to form, eventually reaching the peak temperature. The peak temperature in the reflow zone is determined by the chemical composition of the solder paste, the characteristics of the components and the PCB material. If the peak temperature in the reflow area is too high, the circuit board may be burned or scorched; if the peak temperature is too low, the solder joints will appear gray and grainy. Therefore, the peak temperature in this temperature zone should be high enough to allow the flux to fully function and have good wettability, but it should not be high enough to cause damage, discoloration or scorching of components or circuit boards. The reflow area should also consider that the rising slope of the temperature should not subject the components to thermal shock. The reflow time should be as short as possible while ensuring good soldering of the components. Generally, 30-60s is the best. Excessively long reflow time and high temperature will damage components that are easily affected by temperature, and will also cause the intermetallic compound IMC layer to be too thick, making the solder joints very brittle and reducing the fatigue resistance of the solder joints.
4. Cooling zone stage
The process of temperature drop is called the cooling stage, and the cooling rate is 3-5°C/s. The importance of the cooling phase is often overlooked. A good cooling process also plays a key role in the final result of the weld. Faster cooling rates can refine the microstructure of the solder joints. Change the morphology and distribution of intermetallic compounds and improve the mechanical properties of solder alloys. For lead-free soldering in actual production, increasing the cooling rate can usually reduce defects and improve reliability without adversely affecting components. However, too fast a cooling rate will also cause impact on components, causing stress concentration, causing the solder joints of the product to fail prematurely during use. Therefore, reflow soldering must provide a good cooling curve.
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