What are the temperature zones for reflow soldering? What is the role of each?
SMT reflow soldering preheating zone
The first step of reflow soldering is preheating. Preheating is to activate the solder paste, avoid the preheating behavior caused by rapid high temperature heating when dipping tin, and heat the PCB board at room temperature evenly to achieve target temperature. During the heating process, the heating rate should 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 volatilize enough, which will affect the welding quality.
SMT reflow soldering insulation 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, so that the temperature of the components remains consistent. Due to the different sizes of components, large components require more heat and slow to heat up, while small components heat up quickly. Give enough time in the heat preservation area to make the temperature of the larger components catch up with the smaller components, so that the flux can be fully volatilized. Avoid air bubbles when soldering. At the end of the heat preservation section, the oxides on the pads, solder balls and component pins are removed under the action of flux, and the temperature of the entire circuit board also reaches equilibrium. Tip 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 area
The temperature of the heater in the reflow area rises to the highest, and the temperature of the component rises rapidly to the highest temperature. In the reflow street section, the soldering peak temperature varies with the solder paste used. The peak temperature is generally 210-230°C. The reflow time should not be too long to prevent adverse effects on components and PCB, which may cause the circuit board to be baked Jiao et al.
Reflow Soldering Cooling Zone
In the final stage, the temperature is cooled below the freezing point of the solder paste to solidify the solder joint. The faster the cooling rate, the better the weld. If the cooling rate is too slow, excessive eutectic metal compounds will be produced, and large grain structures will easily occur at the solder joints, which will reduce the strength of the solder joints. The cooling rate in the cooling zone is generally around 4°C/S, and the temperature is cooled to 75°C .
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