Comprehensive Introduction to Wave Soldering Process

In order to enhance the market efficiency of electronic products, Fry’s Metal, a British company, developed an automated soldering device in 1956 known as the wave soldering machine. This technology marked a new era, transitioning from manual point-by-point soldering with soldering irons to large-scale and automated machine soldering. The introduction of wave soldering machines significantly reduced PCB soldering defects, boosting the reliability of electronic products, and simultaneously lowering production costs, contributing immensely to the electronics manufacturing industry. This article from TechSparks will delve into the concept, applications, and working principles of wave soldering, aiding readers in gaining a more comprehensive understanding of this critical technology.

Wave soldering production line

Table of Contents

What is Wave Soldering

Wave soldering is a specialized PCB soldering process designed for through-hole components. In this process, components are inserted into the holes on the PCB, with the entire board positioned at an angle, and the component body located above the board while the pins are underneath. Using a pump and pressure, molten solder material is formed into a distinctive liquid wave shape on the surface. This wave makes contact with the bottom pins, ultimately creating robust solder joints.

wave soldering

In modern automated soldering, two main methods exist: wave soldering and reflow soldering. Wave soldering is primarily suitable for through-hole components, with the heat application point focused on the bottom of the board, having no adverse impact on the encapsulation of the component body. Conversely, for surface-mount components, as the pins are flat, they are not conducive to wave soldering and are better suited for reflow soldering. In reflow soldering, heating is typically done using a hot air oven or infrared radiation, providing easier temperature control.

Pros and Cons of Wave Soldering

The most significant advantage of wave soldering lies in its efficiency. By immersing the entire PCB at once, multiple pin soldering can be completed simultaneously, and the entire soldering process can be finished within 2-4 seconds, greatly improving production efficiency, especially beneficial for large-scale projects.

Secondly, wave soldering exhibits high soldering precision. Operators set soldering parameters such as temperature, time, and flow based on project specifics to control the soldering process, ensuring stable soldering quality and preventing soldering defects. Moreover, these parameters are input into the equipment at the beginning stage, ensuring consistency across every PCBA board on a production line.

Most importantly, wave soldering significantly reduces production costs. This is evident in various aspects, such as the avoidance of labor investment through machine operation; easier batch production and inventory management for through-hole components; prevention of rework costs due to high soldering quality.

While the wave soldering process has many advantages, it’s essential to be aware of its objective drawbacks before use. The most crucial point is that wave soldering is only suitable for through-hole components and cannot be used for soldering surface-mount components. The flat pins of these components cannot be immersed in liquid solder.

Additionally, for manufacturers, using wave soldering requires some additional processes. On the one hand, there is the PCB cleaning process, as wave soldering covers the entire bottom surface, leading to residues of flux and other chemicals. On the other hand, wave soldering machines require specialized technicians for regular maintenance and cleaning.

Wave Soldering Process Flow

Step 1: Preparations

Before starting the wave soldering process, ensure the accuracy of previous steps such as coating with solder paste, curing of solder paste, and component insertion. Pay attention to the soldering surfaces of component pinholes and gold fingers, ensuring the application of solder resist or using heat-resistant tape. These steps are aimed at preventing solder from blocking through-holes after wave soldering. For larger slots and holes, use heat-resistant tape to cover them to prevent solder from flowing onto the upper surface of the PCB during wave soldering.

For water-soluble flux, it is recommended to use liquid solder resist. After coating, wait for 30 minutes or bake under a heat lamp for 15 minutes before inserting components. This helps facilitate direct water cleaning after soldering. To ensure the proper density of the flux, a densitometer can be used for measurement. If the density is too high, consider dilution with a solvent.

For traditional foaming flux, pour the flux into the flux tank to ensure smooth progress of the wave soldering process and improve soldering quality. During the operation, carefully inspect and use coating and adhesive materials appropriately, ensure the correct selection of flux types, and effectively avoid potential issues.

Step 2: Power On and Setup

Once the PCB is confirmed to be error-free, proceed with the setup of the wave soldering machine. First, adjust the width of the wave soldering machine conveyor based on the PCB size. Then, power on the wave soldering machine and the exhaust fan. Initiate the setup based on the specific project requirements:

Adjust the foaming airflow or flux spraying pressure based on the contact between the flux and the bottom surface of the PCB. Ensure a small amount of flux penetrates upward through the through-holes onto the pad on the top surface of the through-hole, without affecting the main body of the components.

Set the preheat temperature according to the actual conditions in the preheating zone of the wave soldering machine, typically ranging from 90 to 130°C. For thicker or larger-sized PCB, a higher upper limit may be chosen.

Set the conveyor speed based on the characteristics of the PCB and the temperature of the wave soldering machine to ensure successful soldering, typically ranging from 0.8 to 1.92 m/min.

Ensure that the actual temperature of the solder wave surging over the PCB during the wave soldering process is within the range of 250°C ± 5°C (260°C ± 10°C for lead-free applications). Note that due to the temperature sensor being located inside the solder bath, the displayed temperature on the sensor or LCD may be relatively higher by approximately 5-10°C than the actual wave temperature.

Adjust the wave height to exceed the bottom surface of the PCB and measure it at 1/2 to 2/3 of the PCB thickness. This ensures that the height distribution of the solder wave as it surges over the PCB is at an appropriate position relative to the PCB thickness, enhancing soldering uniformity and quality.

Step 3: Production

Before starting mass production, initiate the production and testing of the first piece. Place the PCB on the loader, and then, along with the conveyor, move it to the wave soldering machine. The entire process is automated, including flux spraying, drying, preheating, wave soldering, cooling, and other operations.

After completing the production of the first piece, perform a quality inspection based on the IPC-A-610E standard. Adjust the soldering parameters step by step based on the results of the first piece soldering until the soldering quality meets the requirements. This process ensures that the quality of the first piece meets acceptable standards before continuous mass production.

The process for continuous mass production is consistent with the first piece production, but after mass production, the PCB is sent to the AOI inspection for comprehensive inspection of solder joint quality. This step ensures the consistency and stability of soldering quality throughout the entire production process. Through first-piece inspection and AOI inspection, the production process is optimized, while ensuring the high quality of the final product.

Step 4: Shutdown

After confirming the faultlessness of PCB soldering quality, the shutdown operation can be performed. As a high-power, high-temperature device, the wave soldering machine’s shutdown sequence needs to be executed carefully. First, turn off the tin pot heating power and stop the flux spraying system. Next, unscrew the nozzle cap and soak it in an alcohol cup. When the temperature drops below 150°C, turn off the equipment’s main power. Finally, wipe off the residual flux on the workbench, clean the floor, and turn off the main power. It is essential to note that due to the high performance of the wave soldering machine, which comes with a higher price, regular maintenance and cleaning are indispensable.

Working Principle of Wave Soldering

How wave soldering works

Spray Flux

The first stage of wave soldering involves spraying flux, a chemical substance that enhances soldering quality by improving the wetting properties of the solder while preventing oxidation reactions between the metal surfaces in the soldering area and oxygen in the air. As the PCB enters the wave soldering machine, it is detected by sensors, and the nozzle below moves back and forth at a constant speed along the starting position of the fixture, uniformly applying flux to the exposed pads, through-hole pads, and component pin surfaces on the circuit board.

Preheat Zone

After spraying flux, the PCB is conveyed to the preheat zone, which may be divided into three regions depending on the equipment: preheat 1, preheat 2, and thermal compensation. During this process, the PCB undergoes a slow temperature rise to reach the wetting temperature, achieving the following objectives:

Heating the soldering area to the wetting temperature helps improve soldering quality, ensuring even distribution of solder between the pad, solder wire, and component pins.

The gradual temperature rise allows the PCB and components to adapt to the high-temperature environment, effectively avoiding large thermal shocks when immersed in molten solder.

Facilitates the evaporation of solvents in the flux, reducing gas generation during soldering and preventing the formation of bubbles or other soldering defects.

Initiates the decomposition and activation of rosin and activators in the flux, removing oxide films and contaminants from the pad and pin surfaces, providing protection against secondary oxidation.

Double Wave Soldering

The first wave soldering is commonly referred to as the turbulent wave. In this stage, a narrow nozzle generates a strong jet force, resulting in a faster flow of solder. The main purpose of this stage is to remove gases from the flux, reducing solder bridging and inadequate vertical filling. Additionally, the turbulent wave effectively penetrates areas with shadowing effects, ensuring uniform wetting.

The second wave soldering is known as the smooth wave. In this stage, the flow velocity of the solder is slower. The primary objective is to remove excess solder from the solder terminals, ensuring that all soldering surfaces receive good wetting. Moreover, the smooth wave corrects issues such as solder spiking and bridging that may arise from the first wave.

Double Wave Soldering​

Cooling

The cooling system is employed to rapidly cool the solder joints after soldering, significantly improving potential issues like voids and pad delamination, especially when using lead-free solder. Swift temperature reduction helps solidify the solder joints’ structure, enhancing their reliability.

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