What is the difference between reflow soldering and wave soldering?

Depth analysis and comparison of differences between reflow soldering and wave soldering

In the electronics manufacturing industry, soldering technology is the core link to achieve reliable connections between electronic components and printed circuit boards (PCBs), among which reflow soldering and wave soldering are two extremely widely used soldering processes. With their unique principles and characteristics, they play an irreplaceable role in different production scenarios.

1. The core principle and process characteristics of reflow soldering

The core principle of reflow soldering is to melt the solder paste pre-coated on the PCB pad by heating it, enabling the electrical interconnection of the pre-mounted surface mount device (SMD) pins or solder ends to the pads. The entire soldering process relies on the action of hot airflow, and the glue-like flux undergoes a series of physical reactions in a specific high-temperature airflow environment to finally complete the soldering of SMD. It is called "reflow soldering" because during the soldering process, the gas continuously circulates inside the soldering machine, creating a stable high-temperature environment that provides continuous and uniform heat for the melting of the solder paste.

From the perspective of process structure, reflow soldering equipment is mainly composed of three parts: preheating zone, heating zone and cooling zone. The PCB board passes through these three areas in turn driven by the conveyor mechanism: in the preheating area, the solvent in the paste solder gradually volatilizes, and at the same time makes the PCB board and components gradually heat up to avoid the thermal shock caused by the subsequent high temperature; After entering the heating zone, the temperature rises rapidly above the melting point of the solder paste, melting the solder paste and fully wetting the pads and component pins to form a reliable solder joint; Finally, in the cooling area, the molten solder solidifies quickly, the solder joint is fixed, and the entire soldering process is completed.

2. The working mechanism and equipment composition of wave soldering

Wave soldering is another important soldering process, which is to pass molten soft brazing solder (usually lead-tin alloy) through an electric pump or electromagnetic pump to form a solder crest that meets the design requirements. When the printed board preloaded with components passes through this solder peak, the solder end or pin of the component and the pad of the printed board will be soaked in the molten solder, thus realizing the mechanical and electrical connection.

The structure of the wave soldering machine mainly includes a transport belt, a flux addition area, a preheating area and a wave tin furnace. The conveyor belt is responsible for smoothly transporting the PCB board to various work areas; In the flux addition area, the flux is evenly coated on the pad of the PCB board, which removes oxides from the pads and component pins and enhances the wettability of the solder. The function of the preheating zone is similar to that of reflow soldering, mainly to remove the solvent in the flux and preheat the PCB board and components at the same time, reducing thermal stress during soldering. The wave tin furnace is the core part of the wave soldering equipment, which contains molten solder and forms a stable solder crest through the action of the pump to complete the soldering operation. The main material used in wave soldering is solder strips, which are heated and melted in a corsage furnace to continuously replenish the solder consumed during the soldering process.

3. Key differences between reflow soldering and wave soldering

1. Applicable component types

Reflow soldering is mainly suitable for SMD electronic components, which are small in size and light in weight, and the pins or solder ends are distributed at the bottom or side of the device, which is suitable for soldering by melting solder paste. Wave soldering, on the other hand, is more suitable for prong-pin electronic components, which have long pins that need to pass through the through-holes in the PCB board to complete the soldering through the infiltration of solder crest crests.

2. Equipment structure design

The structural design of the reflow soldering equipment revolves around the circulation of hot air flow, and the layout of the preheating zone, heating zone and cooling zone is designed to achieve the gradual melting and solidification of the solder paste. At its core, it ensures that the solder paste completes the soldering reaction at the right temperature by precisely controlling the temperature and flow velocity of the hot gas flow.

The structure of the wave soldering equipment is to achieve the stable formation of the solder crest and the smooth passage of the PCB board. The transport belt, flux addition zone, preheating zone, and solder furnace work together to ensure uniform flux coating, adequate preheating of the PCB board, and effective infiltration of solder peaks into the pins. The design of the equipment focuses on the shape control and stability maintenance of the solder crest to meet the soldering needs of different prongs.

3. The essence of the welding method

Reflow soldering uses a soldering method in which high-temperature hot air forms a reflux to melt the solder. The hot air flow circulates inside the device, transferring heat evenly to the solder paste on the PCB board, melting the paste solder with heat, thereby realizing the connection between the components and the pads. This welding method is non-contact heating, with uniform heat transfer and less thermal damage to components.

Wave soldering uses molten solder to form a solder crest to solder the components. As the PCB board passes through the solder peak, the molten solder climbs up along the component's pins, filling the gap between the pins and the vias to form solder joints. This soldering method is contact soldering, where the solder is in direct contact with the component pins and pads, which can ensure the mechanical strength of the soldering.

4. Process steps

The process of wave soldering is more complex, first it needs to coat the pad of the PCB board, then go through the preheating area for preheating, then complete the soldering through the solder crest of the wave solder, and finally enter the cooling area to solidify the solder joint. Throughout the process, the solder is provided by the wave during soldering, and the PCB board is not pre-coated with solder before entering the wave soldering equipment.

The process of reflow soldering is relatively simple, and the PCB board has been coated with solder paste on the pad through printing and other methods before entering the reflow soldering equipment, and the components have been accurately mounted on the pad. During the reflow soldering process, the PCB board passes through the preheating zone, heating zone and cooling zone in turn, and the solder paste is melted in the heating zone, and then solidifies in the cooling zone to complete the soldering. The whole process does not require additional solder, just melting and solidifying the pre-coated solder paste.

5. Details of the operation process

The operation process of reflow soldering begins with the application of solder paste, which accurately applies a small amount of tin-lead (SN/PB) solder paste to the pad of the printed board, and then uses a placement machine to accurately place the chip components on the surface of the printed board. After that, the printed plate with the components is placed on the conveyor belt of the reflow soldering equipment, and the series of soldering process of drying, preheating, melting and cooling the solder paste is completed in about 5-6 minutes from the furnace entrance to the outlet.

The process of wave soldering begins with the preparation of the PCB board, ensuring that the through-holes on the PCB board are free of blockages and the pads are clean. The pins of the prongs are then threaded through the through-holes of the PCB board for initial fixation. Next, the PCB board is transferred to the flux addition area of the wave soldering equipment to complete the flux coating, and then after the preheating zone, it enters the wave tin furnace to complete the soldering through the solder crest to complete the soldering, and finally cools through the cooling area.

6. Welding efficiency performance

Reflow soldering has a high welding efficiency, and the entire soldering process takes about 5-6 minutes, which can quickly and accurately complete the soldering of a large number of chip electronic components. Due to its automated production method, it is suitable for large-scale mass production and has obvious efficiency advantages for high-density and miniaturized PCB board soldering.

The soldering speed of wave soldering is relatively slow, because the soldering of prongs electronic components involves multiple links such as the coverage of solder peaks on the pins, the climbing and solidification of the solder, and for complex printed boards, meticulous operations and parameter adjustments are also required to ensure that each pin can achieve a good connection with the pad. Therefore, the efficiency of wave soldering is acceptable when dealing with simple pin components, but the efficiency is compromised when dealing with complex products.

7. Welding quality control

When soldering chip components, reflow soldering can achieve uniform melting and solidification of solder paste by precisely controlling the temperature and time of hot gas flow, and can achieve more accurate soldering for small solder joints. Because the amount of solder paste can be accurately controlled, the size and shape of the solder joints are relatively consistent, the soldering quality is relatively stable and the solder joint quality is high, and the incidence of defects such as virtual soldering and continuous soldering is low.

When soldering prongs in wave soldering, the soldering quality is greatly affected by parameters such as the height of the solder crest, flow rate, temperature, and the transmission speed of the PCB board. If these parameters are not properly controlled, problems such as virtual soldering, continuous soldering, and insufficient solder are prone to occur. However, by reasonably adjusting equipment parameters, such as optimizing the peak height, controlling the solder temperature and the transmission speed of the PCB board, wave soldering can also achieve better soldering quality requirements and meet the needs of product use.

8. Cost considerations

Due to its process characteristics, reflow soldering equipment needs to accurately control the temperature, flow rate and other parameters of the hot gas flow, and the complexity of the equipment is relatively high, so the initial investment cost may be slightly higher. However, in the large-scale production of SMD components, its efficient welding speed and stable welding quality can significantly improve production efficiency and reduce the defective product rate, thereby making up for the investment in equipment costs to a certain extent.

The structure of wave soldering equipment is relatively simple, mainly composed of a transport belt, flux addition area, preheating area and wave tin furnace, and the initial equipment cost may be relatively low. However, when dealing with some special prongs or having extremely high requirements for soldering quality, additional equipment or processes may be required, such as adding flux recovery devices and optimizing the crest shape, which will increase the cost investment. In addition, the solder rod used in wave soldering consumes relatively large amounts and will also incur certain material costs in the long run.

Through the analysis of the principle, structure and differences between reflow soldering and wave soldering, it can be seen that these two welding processes have their own scope of application, advantages and disadvantages. In actual production, the appropriate welding process should be selected according to factors such as the type of components, product structure and quality requirements to improve production efficiency and product quality.