Reflow soldering machine is divided into four temperature zones: preheating zone, soaking zone, reflow soldering zone, and cooling zone. If these four temperature zones are not controlled properly, it can easily lead to a large number of defective products. Here, Shandong Puhui Electric will explain the key points of controlling these four main temperature zones of the reflow soldering machine.
I. Key points of controlling the preheating zone of the reflow soldering machine
The purpose of this area is to quickly heat the room-temperature PCB to achieve two specific goals, but the heating rate should be controlled within an appropriate range. If it is too fast, thermal shock may occur, damaging both the circuit board and the components. If it is too slow, the solvent volatilization may be insufficient, affecting the soldering quality. Due to the fast heating speed, there is a large temperature difference in the SMA (surface mount assembly) in the later stage of the temperature zone. To prevent component damage from thermal shock, the maximum speed is generally set to 4°C/s. However, the typical rise rate is set at 1-3°C/s, with a typical rise rate of 2°C/s.
II. Key points of controlling the soaking zone of the reflow soldering machine
The soaking zone refers to the area where the temperature rises from 120°C to 150°C, which is the solder paste melting point. Its main purpose is to stabilize the temperature of each component in the SMA and minimize temperature differences. Sufficient time should be given in this area to bring the temperature of larger components in line with smaller components and ensure that the flux in the solder paste is fully volatilized. At the end of the soaking zone, the oxide on the pads, solder balls, and component leads is removed, and the temperature of the entire circuit board reaches equilibrium. It should be noted that all components on the SMA should have the same temperature at the end of this stage; otherwise, various soldering defects may occur when entering the reflow stage due to uneven temperatures.
III. Key points of controlling the reflow soldering zone of the reflow soldering machine
In this area, the temperature of the heaters is set high to quickly raise the temperature of the components to the peak temperature. In the reflow zone, the peak soldering temperature varies depending on the solder paste used. It is generally recommended to be 20-40°C above the melting point of the solder paste used. For solder paste with a melting point of 183°C (63Sn/37Pb) and 179°C (Sn62/Pb36/Ag2), the peak temperature is generally set at 210-230°C. The reflow time should not be too long to avoid adverse effects on the SMA. The ideal temperature curve has a small area covered by the "end zone" above the solder melting point.
IV. Key points of controlling the cooling zone of the reflow soldering machine
In this stage, the lead-tin powder in the solder paste has melted and fully wetted the mating surface. It should be cooled as quickly as possible to achieve bright solder joints with good appearance and low contact angles. Slow cooling can cause more decomposition to enter the tin from the circuit board, resulting in dark and rough solder joints. In severe cases, it can cause poor soldering and weaken the bond strength of the solder joints. The cooling zone's temperature reduction rate is generally 3-10°C/s, and cooling to 75°C is sufficient.