The reflow oven is used to solder the SMT chip components to the circuit board in the SMT process soldering production equipment. The reflow oven relies on the hot air flow in the furnace to brush the solder paste on the solder joints of the solder paste circuit board, so that the solder paste is re-melted into liquid tin so that the SMT chip components and the circuit board are welded and welded, and then reflow soldering The furnace is cooled to form solder joints, and the colloidal solder paste undergoes a physical reaction under a certain high temperature airflow to achieve the soldering effect of the SMT process.
The soldering in the reflow oven is divided into four processes. The circuit boards with smt components are transported through the reflow oven guide rails through the preheating zone, heat preservation zone, soldering zone, and cooling zone of the reflow oven respectively, and then after reflow soldering. The four temperature zones of the furnace form a complete welding point. Next, Guangshengde reflow soldering will explain the principles of the four temperature zones of the reflow oven respectively.
Preheating is to activate the solder paste, and to avoid the rapid high temperature heating during the immersion of tin, which is a heating action performed to cause defective parts. The goal of this area is to heat the PCB at room temperature as soon as possible, but the heating rate should be controlled within an appropriate range. If it is too fast, thermal shock will occur, and the circuit board and components may be damaged. If it is too slow, the solvent will not evaporate sufficiently. Welding quality. Due to the faster heating speed, the temperature difference in the reflow furnace is larger in the latter part of the temperature zone. In order to prevent thermal shock from damaging the components, the maximum heating rate is generally specified as 4℃/S, and the rising rate is usually set at 1~3℃/S.
The main purpose of the heat preservation stage is to stabilize the temperature of each component in the reflow furnace and minimize the temperature difference. Give enough time in this area to make the temperature of the larger component catch up with the smaller component, and to ensure that the flux in the solder paste is fully volatilized. At the end of the heat preservation section, the oxides on the pads, solder balls and component pins are removed under the action of the flux, and the temperature of the entire circuit board is also balanced. It should be noted that all components on the SMA should have the same temperature at the end of this section, otherwise, entering the reflow section will cause various bad soldering phenomena due to the uneven temperature of each part.
When the PCB enters the reflow zone, the temperature rises rapidly so that the solder paste reaches a molten state. The melting point of the lead solder paste 63sn37pb is 183°C, and the melting point of the lead solder paste 96.5Sn3Ag0.5Cu is 217°C. In this area, the heater temperature is set high, so that the temperature of the component rises quickly to the value temperature. The value temperature of the reflow curve is usually determined by the melting point temperature of the solder and the heat resistance temperature of the assembled substrate and components. In the reflow section, the soldering temperature varies depending on the solder paste used. Generally, the high temperature of lead is 230-250℃, and the temperature of lead is 210-230℃. If the temperature is too low, it is easy to produce cold joints and insufficient wetting; if the temperature is too high, coking and delamination of the epoxy resin substrate and plastic parts are likely to occur, and excessive eutectic metal compounds will form, which will lead to brittle solder joints, which will affect the welding strength . In the reflow soldering area, pay special attention to the reflow time not to be too long, to prevent damage to the reflow furnace, it may also cause poor functions of the electronic components or cause the circuit board to be burnt.
At this stage, the temperature is cooled to below the solid phase temperature to solidify the solder joints. The cooling rate will affect the strength of the solder joint. If the cooling rate is too slow, it will cause excessive eutectic metal compounds to be produced, and large grain structures are prone to occur at the solder joints, which will lower the strength of the solder joints. The cooling rate in the cooling zone is generally about 4℃/S, and the cooling rate is 75℃. can.
After brushing the solder paste and mounting the smt chip components, the circuit board is transported through the guide rail of the reflow soldering furnace, and after the action of the four temperature zones above the reflow soldering furnace, a complete soldered circuit board is formed. This is the whole working principle of the reflow oven.
Post time: Jul-29-2020