In the SMT entire line process, after the SMT machine completes the mounting process, the next step is the welding process, reflow welding process is the most important process in the entire SMT surface mounting technology. Common welding equipment includes wave welding, reflow welding and other equipment, and the role of reflow welding four temperature zones, respectively, are preheating zone, constant temperature zone, back welding zone and cooling zone. Each of the four temperature zones has its own significance.
SMT reflow preheating area
The first step of reflow welding is preheating, which is to activate the solder paste, avoid the preheating behavior caused by poor welding caused by rapid high temperature heating during tin immersion, and evenly heat the normal temperature PCB board to achieve the target temperature. In the heating process to control the heating rate, too fast will produce thermal shock, may cause damage to the circuit board and components; Too slow, the solvent volatilization is insufficient, affecting the welding quality.
SMT reflow insulation area
The second stage - insulation stage, the main purpose is to make the temperature of the PCB board and the components in the reflow furnace stable, so that the temperature of the components is consistent. Because the size of the components is different, the large components need more heat, the temperature is slow, the small components are heated up fast, and enough time is given in the insulation area to make the temperature of the larger components catch up with the smaller components, so that the flux is fully volatilized to avoid bubbles when welding. At the end of the insulation section, the oxides on the pad, solder ball and component pins are removed under the action of flux, and the temperature of the entire circuit board is also balanced. All components should have the same temperature at the end of this section, otherwise there will be various bad welding phenomena in the reflux section due to the uneven temperature of each part.
Reflow back weld 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 reflux street section, the peak welding temperature varies with the welding paste used, the peak temperature is generally 210-230 ° C, and the reflux time should not be too long to prevent adverse effects on the components and PCB, which may cause the circuit board to be scorched.
Reflow cooling zone
In the final stage, the temperature is cooled below the freezing point temperature of the solder paste to solidify the solder joint. The faster the cooling rate, the better the welding result. If the cooling rate is too slow, it will lead to the generation of excessive eutectic metal compounds, and the large grain structure is easy to occur at the welding point, so that the strength of the welding point is low, and the cooling rate of the cooling zone is generally about 4℃/S, cooling to 75℃.
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