How the first preheating zone works
Preheating is a heating action performed to activate the solder paste and avoid parts failure caused by rapid high temperature heating during 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, thermal shock will occur, and the circuit board and components may be damaged. Welding quality. Due to the fast heating speed, the temperature difference in the reflow furnace chamber in the latter part of the temperature zone is large. In order to prevent damage to the components due to thermal shock, the maximum temperature rise rate is generally specified as 4°C/S, and the usual rise rate is set at 1~3°C/S.
How the second insulation zone works
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 for the temperature of the larger components to catch up with the smaller components and to ensure that the flux in the solder paste is fully evaporated. By the end of the holding period, 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 has reached equilibrium. 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.
How the third reflow soldering zone works
When the PCB enters the reflow zone, the temperature rises rapidly and the solder paste reaches a molten state. The melting point of leaded solder paste 63sn37pb is 183℃, and the melting point of lead-free solder paste 96.5Sn3Ag0.5Cu is 217℃. The heater is set to the highest temperature in this area, allowing the component temperature to rise rapidly to the peak temperature. The peak temperature of the reflow profile 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 peak temperature of soldering varies depending on the solder paste used. Generally, the highest temperature of lead-free is 230-250℃, and that of lead is 210-230℃. If the peak temperature is too low, it is easy to produce cold junctions and insufficient wetting; if the peak 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 be formed, which will lead to brittle solder joints and affect the solder strength. . In the reflow soldering area, special attention should be paid to the reflow time not being too long, in order to prevent damage to the reflow soldering furnace, which may also cause poor function of electronic components or cause adverse effects such as scorching of circuit boards.
How the fourth cooling zone works
At this stage, the temperature is cooled below the solid phase temperature to solidify the solder joint. The cooling rate will have an effect on the strength of the solder joint. If the cooling rate is too slow, excessive eutectic metal compounds will be produced, and large grain structures will easily occur at the solder joints, which will reduce the strength of the solder joints. That's it.
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