The cooling section in the reflow soldering process plays an important role, where the lead tin powder in the solder paste has melted and fully wetted the connected surface. Cooling should be carried out as quickly as possible to obtain bright solder joints with good shape and low contact angle. Slow cooling can cause more decomposition of the circuit board into the tin, resulting in gray and rough solder joints. In extreme cases, it can cause poor soldering and weaken the bonding strength of solder joints. The cooling rate of the cooling section is generally 3-10 ℃/s, and cooling to 75 ℃ is sufficient.

As far as the editor knows, a good reflow soldering is achieved by appropriate solder joint size and good and sufficient wetting phenomenon.

The size of the solder joint directly affects its mechanical force and the mechanical impact it can withstand, making it a key quality indicator. Due to the large contact surface, the stress borne by large solder joints can be distributed over a larger area, and there is a longer distance to extend the failure time in case of fracture, resulting in a generally longer lifespan. However, too large solder joints will affect the inspection effect after assembly, such as making it difficult to judge the wetting situation.

Good and sufficient wetting phenomenon. Good wetting indicates the formation of an alloy interface between the two metals on the welding surface. The formation of alloy interface is the most basic requirement for welding. If the compatibility between welding materials is poor, or if pollutants or compounds that hinder the formation of alloys appear at the interface, the alloy layer cannot be formed intact, which is also reflected in the signs of wetting damage. Therefore, a certain degree of quality judgment can be made from the degree of wetting. The judgment of wettability is generally evaluated from two aspects, one is the degree of wetting or diffusion; The other is the final formed wetting angle.