The function of reflow welding in SMT process

Reflow soldering is the most widely used surface component welding method in SMT industry. The other welding method is wave soldering. Reflow soldering is suitable for chip components, while wave soldering is suitable for pin electronic components.

Reflow soldering is also a reflow soldering process. Its principle is to print or inject an appropriate amount of solder paste on the PCB pad and paste the corresponding SMT patch processing components, then use the hot air convection heating of the reflow furnace to melt the solder paste, and finally form a reliable solder joint through cooling. Connect the components with the PCB pad to play the role of mechanical connection and electrical connection. Generally speaking, reflow soldering is divided into four stages: preheating, constant temperature, reflow and cooling.


1. Preheating zone

Preheating zone: it is the initial heating stage of the product. Its purpose is to quickly heat the product at room temperature and activate the solder paste flux. At the same time, it is also a necessary heating method to avoid the poor heat loss of components caused by high-temperature rapid heating during subsequent tin immersion. Therefore, the influence of temperature rise rate on the product is very important and must be controlled within a reasonable range. If it is too fast, it will produce thermal shock, PCB and components will be affected by thermal stress and cause damage. At the same time, the solvent in the solder paste will volatilize rapidly due to rapid heating, resulting in splashing and formation of solder beads. If it is too slow, the solder paste solvent will not volatilize fully and affect the welding quality.


2. Constant temperature zone

Constant temperature zone: its purpose is to stabilize the temperature of each element on the PCB and reach an agreement as far as possible to reduce the temperature difference between each element. At this stage, the heating time of each component is relatively long, because small components will reach balance first due to less heat absorption, and large components need enough time to catch up with small components due to large heat absorption, and ensure that the flux in the solder paste is fully volatilized. At this stage, under the action of flux, the oxide on the pad, solder ball and component pin will be removed. At the same time, the flux will also remove the oil stain on the surface of the component and pad, increase the welding area and prevent the component from being oxidized again. After this stage, all components shall maintain the same or similar temperature, otherwise poor welding may occur due to excessive temperature difference.

The temperature and time of constant temperature depend on the complexity of PCB design, the difference of component types and the number of components. It is usually selected between 120-170 ℃. If the PCB is particularly complex, the temperature of constant temperature zone should be determined with rosin softening temperature as a reference, in order to reduce the welding time of reflow zone in the later section. The constant temperature zone of our company is generally selected at 160 ℃.


3. Reflux area

The purpose of the reflow zone is to make the solder paste melt and wet the pad on the surface of the element to be welded.

When the PCB board enters the reflow zone, the temperature will rise rapidly to make the solder paste reach the melting state. The melting point of lead solder paste SN: 63 / Pb: 37 is 183 ℃, and the lead-free solder paste SN: 96.5/ag: 3 / Cu: 0. The melting point of 5 is 217 ℃. In this section, the heater provides the most heat, and the furnace temperature will be set to the highest, so that the solder paste temperature will rise rapidly to the peak temperature.

The peak temperature of reflow soldering curve is generally determined by the melting point of solder paste, PCB board and the heat-resistant temperature of the component itself. The peak temperature of products in the reflow area varies according to the type of solder paste used. Generally speaking, the maximum peak temperature of lead-free solder paste is generally 230 ~ 250 ℃, and that of lead solder paste is generally 210 ~ 230 ℃. If the peak temperature is too low, it is easy to produce cold welding and insufficient wetting of solder joints; If it is too high, the epoxy resin type substrate and plastic parts are prone to coking, PCB foaming and delamination, and will also lead to the formation of excessive eutectic metal compounds, making the solder joint brittle and the welding strength weak, affecting the mechanical properties of the product.

It should be emphasized that the flux in the solder paste in the reflow area is helpful to promote the wetting between the solder paste and the component welding end and reduce the surface tension of the solder paste at this time, but the promotion of the flux will be restrained due to the residual oxygen and metal surface oxides in the reflow furnace.

Generally, a good furnace temperature curve must meet that the peak temperature of each point on the PCB should be consistent as far as possible, and the difference should not exceed 10 degrees. Only in this way can we ensure that all welding actions have been completed smoothly when the product enters the cooling area.


4. Cooling area

The purpose of the cooling zone is to rapidly cool the melted solder paste particles and quickly form bright solder joints with slow radian and full amount of tin. Therefore, many factories will control the cooling area well, because it is conducive to solder joint forming. Generally speaking, too fast cooling rate will make it too late for the molten solder paste to cool and buffer, resulting in tailing, sharpening and even burrs of the formed solder joint. Too low cooling rate will make the base material of PCB pad surface integrate into the solder paste, making the solder joint rough, empty welding and dark solder joint. What’s more, all metal magazines at the component solder end will melt at the solder joint position, resulting in wet refusal or poor welding at the component solder end, It affects the welding quality, so a good cooling rate is very important for solder joint forming. Generally speaking, the solder paste supplier will recommend the solder joint cooling rate ≥ 3 ℃ / s.

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Post time: Apr-09-2022