What are the roles of flux and solder?

In the soldering process, flux and solder are common soldering materials, so what are their respective functions? What is the role of flux and solder? Flux and solder play different roles in the soldering process: 1. The role of solder

1.Achieving Electrical Connections In the assembly of electronic circuits, the electrical connection of solder is very important. Many electronic components, such as resistors, capacitors, chips, etc., need to be connected by circuits in electronic devices to function. Solder is a high-quality conductive material that connects the pins of each electronic component to the corresponding pads on the printed circuit board (PCB) to create a reliable electrical path. For example, on complex computer motherboards, thousands of components are connected by solder to form a complete circuit system, allowing data and signals to be accurately transmitted between components. Solder is a filler for soldering, which melts and wets the soldering surface to fill the gaps between the welded joints to form a weld seam and achieve a soldering connection.

2.Provide mechanical fixation In addition to the function of electrical connection, solder also plays the role of mechanical fixation. When the solder is melted and then cooled and solidified, the pins of the electronic components are firmly attached to the PCB. For some small surface-mount components (SMDs), such as resistors or capacitors in the 0402 package, solder prevents them from loosening or shifting due to external forces such as vibration and collision during the use of the device. For larger plug-in components, such as electrolytic capacitors or transformers, the mechanical fixation of the solder also ensures that they are stably mounted on the PCB and that the entire electronic device is structurally intact. Solder has a low melting point and can melt at relatively low temperatures, which reduces the thermal impact on the soldered substrate and avoids melting or deformation of the substrate.

3.Building a heat dissipation channel In some high-power electronic devices, solder can be used as part of the heat dissipation channel. Electronic components generate heat when they work, and they need to be effectively dissipated to avoid overheating and damage. Solder is attached between the component pins and the PCB, and because of its thermal conductivity, it conducts the heat generated by the component to the heat dissipation area of the PCB or to the heat sink. For example, in a power amplifier circuit, a power transistor is connected to the PCB by solder, and the heat can be conducted through the solder to the large copper foil heat dissipation area of the PCB, keeping the components within the appropriate operating temperature range.

4.Solder can form a layer of metal protection during the welding process, protect the welded joint from oxidation and pollution, and improve the stability and reliability of the welded joint.

2、  The role of flux

1. Removal of oxides When soldering, metal surfaces, such as the copper pads of PCBs and the pins of electronic components, are prone to oxides. These oxides can prevent the solder from bonding well to the metal surface, resulting in a decrease in the quality of the solder, such as virtual soldering. The active ingredient in the flux can chemically react with the oxide to remove it. For example, for copper oxide on the surface of copper, the organic acid components in the flux can react with it to form compounds that are soluble in the flux, thereby exposing the clean metal surface and creating favorable conditions for the bonding of solder to metal. The flux removes oxides, dirt and impurities from the solder surface, keeping the solder surface clean and providing good soldering conditions.

2. Reduce surface tension Flux can reduce the surface tension of solder. Solder has a certain surface tension when in the liquid state, which makes it difficult to spread on metal surfaces. The flux changes the interfacial tension between the solder and the metal surface, making it easier for the solder to flow and fill on the surface being soldered. In surface mount technology (SMT), this role of flux is especially important for tiny pads and pins. It ensures that the solder is evenly distributed in the soldering area, improving the success rate and quality of the soldering. Flux reduces the surface tension of the soldering surface, making it easier for the solder to wet and expand, and improving the wettability and contact properties of the solder.

3. Prevent oxidation During the soldering process, the flux will form a protective film in the soldering area. This film prevents the metal surface from being oxidized again during the welding heating process. Once the metal surface is oxidized again, it will affect the quality of the solder-metal bond. The protective film of the flux can last for a period of time after the soldering is completed, which protects the solder joint and prolongs the service life of the solder joint. The flux forms a protective film during the soldering process, preventing oxidation of the soldering surface and reducing the oxygen in contact with the solder, thereby reducing the occurrence of soldering defects.

4. Auxiliary heat conduction The flux has a certain thermal conductivity, which can assist in the transfer of heat in the soldering area. When soldering, the heat from the tip needs to be quickly transferred to the part to be soldered, so that the solder melts and a good soldering is achieved. The flux fills the tiny void between the soldering iron tip, the solder and the surface being soldered, improving the efficiency of heat transfer and making the soldering process faster and more stable. Solder is mainly responsible for electrical connection, mechanical fixation and heat dissipation, while flux is mainly responsible for removing oxides, reducing surface tension, preventing oxidation and assisting heat transfer during the soldering process.

5. In order to understand the role of flux and solder in practical applications, let's look at some specific examples. In the welding of electronic devices in the aerospace sector, the welding quality requirements are extremely high. Due to the complex environment of space, the equipment faces a variety of challenges such as extreme temperatures and radiation. In this case, the electrical connection of the solder must be absolutely reliable, as a small electrical connection failure can cause the entire device to malfunction. For example, the connections between the various tiny electronic components in a satellite navigation system rely on solder, which ensures that the signal can be reliably transmitted during long-term space flight. The role of flux is also important to ignore, as it prevents false soldering due to problems such as oxides or surface tension when soldering these delicate components. In such extreme environments, the protective film formed by the flux prevents oxidation of the solder joints for a long time, thus ensuring the stability of the equipment.

7. In the welding of automotive electronic systems, the mechanical fixation of solder is particularly important. When the car is constantly vibrating while driving, the solder joints of electronic components in the engine compartment, such as the engine control unit (ECU), need to withstand the vibration and not loosen. Solder securely holds each component to the circuit board and prevents it from shifting or falling off. Fluxes make the soldering stronger by removing oxides and reducing surface tension when soldering these components. In addition, the flux's auxiliary heat transfer function can also speed up the soldering process, improve production efficiency, and reduce thermal damage to automotive electronic components caused by prolonged heating. In the manufacture of consumer electronic devices such as smartphones, surface mount technology (SMT) is widely used in the miniaturization of electronic products. In this case, the flux plays a key role in the spread of the solder on the tiny pads and pins. It ensures that the solder is evenly distributed and that the electrical connection is precise. The heat dissipation channel function of solder is indispensable for some high-performance smartphone chips, which generate a lot of heat when the chip is running, and solder can conduct heat into the heat dissipation structure of the mobile phone to prevent the chip from overheating and frequency reduction affecting performance. From the above references and analysis, it can be seen that whether it is in the high-end aerospace field, complex automotive electronic systems, or mass consumer smartphone manufacturing, the role of flux and solder is very critical, and their interaction is the key to ensuring high-quality soldering. With the continuous development of science and technology, the requirements for soldering quality will become higher and higher, and the performance of flux and solder needs to be continuously optimized to adapt to different application scenarios.