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In the entire SMT patch line process, after the placement machine completes the placement process, the next step is the soldering process. The reflow soldering process is the most important process in the entire SMT surface mount technology. Common soldering equipment has crests Soldering, reflow soldering and other equipment, today Topco editor discusses with you the role of the four temperature zones of reflow soldering, which are preheating zone, constant temperature zone, reflow zone and cooling zone, among the four temperature zones Each stage has its important meaning.
SMT reflow soldering preheating zone
The first step of reflow soldering is preheating. The preheating is to activate the solder paste and avoid the preheating behavior caused by the rapid high temperature heating during the immersion tin.
The normal temperature PCB board is evenly heated to achieve Target temperature. In the heating process, the heating rate must be controlled. If it is too fast, it will cause thermal shock, which may cause damage to the circuit board and components; if it is too slow, the solvent will not evaporate sufficiently, which will affect the quality of soldering.
SMT reflow soldering holding area
The second stage-the heat preservation stage, the main purpose is to stabilize the temperature of the PCB board and various components in the reflow oven to keep the component temperature consistent. Due to the different sizes of components, large components require a lot of heat, slow heating up, and small components heat up quickly. Give enough time in the heat preservation area to make the temperature of the larger component catch up with the smaller component, so that the flux is fully volatilized. Avoid bubbles during welding. At the end of the heat preservation section, the oxides on the pads, solder balls and component pins are removed under the action of the flux, and the temperature of the entire circuit board is also balanced. Tips from Topco Editor: All components should have the same temperature at the end of this section, otherwise various bad soldering phenomena will occur in the reflow section due to the uneven temperature of each part.
Reflow soldering reflow area
The temperature of the heater in the reflow soldering area rises to the highest, and the temperature of the component rises rapidly to the highest temperature. In the reflow street section, the peak soldering temperature varies with the solder paste used. The peak temperature is generally 210-230 degree Celsius. The reflow time should not be too long to prevent adverse effects on the components and PCB, which may cause the circuit board to be baked. Jiao etc.
Reflow soldering cooling zone
In the final stage, the temperature is cooled to below the freezing point of the solder paste to solidify the solder joints. The faster the cooling rate, the better the welding effect. If the cooling rate is too slow, it will cause excessive eutectic metal compounds to be produced, and large grain structures are prone to occur at the welded joints, which will lower the strength of the welded joints. The cooling rate in the cooling zone is generally about 4°C/S, and it is cooled to 75°C. .
