Basic knowledge of reflow soldering

Reflow soldering technology is not new in the field of electronic manufacturing. The components on various boards used in our computers are soldered to the circuit board through this process. There is a heating circuit inside this device, which uses air or nitrogen After heating to a high enough temperature, it is blown to the circuit board on which the components have been pasted, so that the solder on both sides of the components melts and bonds with the main board. The advantage of this process is that the temperature is easy to control, oxidation can be avoided during the welding process, and the manufacturing cost is easier to control.

In the whole line process of SMT placement, after the placement machine completes the placement process, the next step is the welding process, and the reflow soldering process is the most important process in the entire SMT surface mount technology. Common welding and welding equipment has wave crests Soldering, reflow soldering and other equipment.

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. 

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 .