The Working Principle Of the SMT Reflow Oven
The reflow oven is a soldering equipment used in the SMT process for soldering SMT chip components to the circuit board. The reflow oven relies on the hot air convection in the furnace chamber to increase the solder paste temperature on the circuit board, so that the solder paste is re-melted into liquid tin, so that the SMT chip components and the circuit board are soldered together, and then cooled to form a solder joint, and the colloidal solder paste undergoes a physical reaction under a fixed high-temperature air flow to achieve the soldering effect of the SMT process.
The soldering process in the reflow oven is divided into four action processes. The circuit board with smt components mounted on it is transported through the guide rail of the reflow furnace through the preheating zone, heat preservation zone, welding zone and cooling zone of the reflow furnace. The four temperature zones of the furnace form a complete welding point. Below we will explain the principles of the four temperature zones of the reflow soldering furnace separately.
The working principle of the preheating furnace preheating zone:
The preheating is to activate the solder paste, and to avoid the heating behavior of the defective parts caused by the rapid high temperature heating during the tin immersion. 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, it will cause thermal shock, and the circuit board and components may be damaged. If it is too slow, the solvent will not evaporate enough, affecting Welding quality. Due to the fast heating speed, the temperature difference in the reflow furnace in the rear of the temperature zone is large. In order to prevent damage to the components caused by thermal shock, the general temperature increase rate is 4°C/S, and the rising rate is usually set to 1~3°C/S.
The working principle of the insulation area of the reflow oven:
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. Allow enough time in this area to allow the temperature of the larger component to catch up with the smaller component, and ensure that the flux in the solder paste is fully evaporated. By the end of the insulation section, the oxides on the pads, solder balls and component pins are removed by 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 period, otherwise entering the reflow section will cause various bad soldering phenomena due to the uneven temperature of each part.
The working principle of the reflow soldering zone of the reflow oven:
When the PCB enters the reflow zone, the temperature rises rapidly and the solder paste is melted. 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 temperature of the heater is set high, so that the temperature of the component rises rapidly. The temperature of the reflow soldering curve is usually determined by the melting temperature of the solder and the heat resistance temperature of the assembled substrate and components. In the reflow section, the soldering temperature depends on the solder paste used. Generally, the high temperature of lead is 230～250℃, and the lead temperature is 210～230℃. If the temperature is too low, it is easy to produce cold junctions and insufficient wetting; if it is too high, coking and delamination of the epoxy resin substrate and the plastic part are likely to occur, and excessive eutectic metal compounds will form, resulting in brittle solder joints and affecting the welding strength . Special attention should be paid to the reflow time in the reflow soldering area to prevent damage to the reflow furnace, which may also cause adverse effects on the electronic components or the circuit board to be burnt.
Working principle of the reflow oven cooling zone:
At this stage, the temperature is cooled below the solid phase temperature, solidifying the solder joints. The cooling rate will affect the strength of the solder joint. If the cooling rate is too slow, excessive eutectic metal compounds will be generated, and large grain structure will easily occur at the welding point, which will reduce the strength of the welding point. The cooling rate of the cooling zone is generally about 4℃/S. can.
The circuit board with the solder paste mounted and smt chip components mounted on it is transported through the guide rails of the reflow soldering furnace and passes through the four temperature zones above the reflow soldering furnace to form a complete soldered circuit board. This is the entire working principle of the reflow oven.
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