Comprehensive introduction of wave soldering process

As a PCBA company, FS Technology understand the importance of precise component assembly and soldering techniques in ensuring the high quality of your project. We employ both SMT and THT to accommodate different component types. In these two technologies, wave soldering and reflow soldering are involved. In this article, we will focus on the wave soldering process, which is a vital step in the overall THT assembly process. To help you better understand this process, we recommend watching a YouTube video that provides a visual demonstration.

What is Wave Soldering

Wave soldering is a highly efficient soldering method developed by Albert Hanson, a German inventor, to streamline the assembly process of hundreds of components on a Printed Circuit Board (PCB). In this process, the PCB is passed over a pan of molten solder via a conveyor belt. A pump positioned beneath the conveyor belt generates a wave-like motion of the molten solder, resembling a wave crashing onto the surface of the circuit board. It is worth noting that soldering has been in practice since the discovery of tin, which remains a critical component of soldering paste.

The concept of a multi-layered flat structure, comprising an insulating layer and foil conductors, was pioneered by Albert Hanson. He also introduced the idea of drilling holes in the board, similar to the through-hole assembly technique employed for mounting DIP components today. As a result, companies began manufacturing components in through-hole DIP packages. It became more convenient to apply solder paste to the entire board at once instead of soldering each individual electronic component using a soldering iron. This led to the invention of wave soldering, where the entire board is exposed to a wave of solder.

Modern wave soldering machines have automated the soldering process, ensuring precise and consistent results. In the following sections, FS Technology will provide a detailed explanation of how wave soldering works.

Wave Soldering Process Steps

There are five basic steps involved in the wave soldering process: solder melting, component cleaning, PCB placement, soldering, and cleaning. Next, FS Technology will explain each step in detail step by step.

Step 1: Solder Melting

The wave soldering machine incorporates a solder reservoir that is precisely controlled and maintained at temperatures ranging from 180 to 450 °C, depending on the solder type and machine model. This critical step is paramount as it ensures the solder is melted to its optimal state, guaranteeing purity and effectiveness. Accurate temperature control is of utmost importance in achieving consistency, smoothness, and superior results throughout the soldering process.

Step 2: Components Cleaning

During this crucial step, the components destined for placement on the PCBA undergo a meticulous cleaning process. The cleaning procedure involves thorough inspection for any presence of oxide layers, which must be addressed promptly. Moreover, proactive measures are implemented to prevent the formation of oxide layers altogether. To facilitate this cleaning process and enhance solder wetting, a specialized chemical called flux is utilized. PCB flux is a well-engineered blend of base materials and activators, specifically formulated to remove oxides from metal surfaces and promote superior solder wetting.

Step 3: PCBA Board Placement

In this pivotal step, the components are meticulously positioned and secured onto the circuit board, preparing the PCBA for integration into the wave soldering machine. The assembled boards are then carefully placed within the machine, ensuring precise alignment with the conveyer belt at a 0° angle. To ensure stability and prevent any unintended movement, the wave soldering machine is equipped with sturdy metal clasps specifically designed to firmly hold the circuit board in place throughout the soldering process. This ensures optimal soldering results by maintaining the board’s fixed position and alignment.

Step 4: Soldering

This crucial phase represents the core of the soldering process, where our objective of effectively soldering the PCB components onto the circuit board is accomplished. As the conveyer belt steadily propels the board forward, it progressively approaches the solder pan. To facilitate optimal soldering results, we meticulously control the speed of the conveyer belt, ensuring a slow and deliberate movement that allows any excess solder to gracefully flow back into the solder bath. This deliberate pace promotes thorough solder coverage, ensuring the solder fully and securely settles within the joints, guaranteeing reliable and robust connections.

Step 5: Cleaning

This step can be regarded as a crucial aspect of the process, focusing on the final cleaning and refinement of the product. Here, we employ a meticulous cleaning procedure to ensure the circuit board is free from any residual flux. Multiple solvents and deionized water are utilized to thoroughly wash the board, effectively removing any remaining flux residues. Among the various solvents available for this task, isopropyl alcohol is commonly employed due to its affordability and non-toxic nature, making it a preferred choice in comparison to other solvents.

Working Principle of Wave Soldering Systems

The wave soldering machine is an essential piece of equipment required for successful PCBA projects. As different projects may have varying requirements, it is crucial to assess the equipment capabilities of the chosen PCBA company. When selecting a PCBA service provider, it is advisable to evaluate their equipment portfolio based on the specific needs of your project.

For projects involving SMT assembly, it is important to consider the reflow equipment used by the service provider. Pay attention to the number of temperature zones available in their reflow machines. When dealing with through-hole components, it is essential to ascertain whether the PCBA company offers fully automatic assembly capabilities.

Flux Sprayer

A spray fluxing system performs two crucial actions in the PCB assembly process. Firstly, it accurately applies wave soldering flux to the designated areas, specifically targeting the plated holes of the circuit board. Secondly, it ensures the precise amount of flux is applied. A high-quality fluxing system is designed to excel in achieving these two objectives, regardless of the board’s width, thickness, or hole diameter.

The primary focus of a reliable fluxing system is to ensure that flux is effectively delivered into the plated holes of the circuit board. This step is vital for promoting proper solder wetting and ensuring strong and reliable solder joints throughout the assembly.

Preheating Pad

Although wave soldering can be done without preheating, the soldering process will take comparatively more time. In The Handbook of Machine Soldering SMT and THT, Woodgate mentions, “During some experimental work with preheating, a board was soldered at 12 ft. per minute with the normal preheat applied. Without preheating it could not be soldered faster than 2.5 ft. per minute.” FS Technology understands the importance of preheating in the wave soldering process and would like to provide you with four compelling reasons why preheating is essential:

Enhanced Flux Activity: Preheating facilitates the evaporation of the solvent component in the flux. This controlled application of heat increases the activity of the flux, ensuring optimal solder wetting and promoting reliable solder joint formation.
Prevention of Solder Ball Formation: By drying the flux solvent through preheating, the formation of solder balls is effectively minimized. Solder balls can cause defects and lead to circuit board short if they are present during the wave soldering process. Preheating prevents these solder balls from spitting out when they encounter the solder waves, resulting in a cleaner and more reliable soldering outcome.
Thermal Impact Control: Preheating plays a vital role in minimizing the thermal impact of the wave solder process on the PCB. By gradually raising the temperature of the board prior to entering the solder wave, preheating helps control the risk of warpage and other potential failures that can arise from sudden temperature changes.
Facilitating DIP Soldering: Another crucial advantage of preheating is that it heats the components, boards, and connectors, enabling faster and smoother soldering of DIP components. By adequately preheating the assembly, the temperature is uniformly distributed, allowing for efficient and reliable solder flow during the wave soldering process.

Pump

The linear motor pump in our wave soldering device incorporates multiple pumping channels that are arranged in a parallel and spaced configuration. To ensure optimal performance, our pump includes a stator with numerous segments, allowing each station to be allocated a dedicated pumping channel. This design ensures a uniform pumping performance and a consistent distribution of pumping pressure across a wide range, resulting in a homogeneous solder wave.

One significant advantage of this pumping system is the achievement of a stable and controlled solder wave. With the positioning of the pumping channels and the innovative stator design, the occurrence of chaotic whirling of the solder is virtually eliminated. This precise configuration and alignment of the pumping channels contribute to the generation of a smooth and controlled solder wave, enhancing the quality and reliability of the soldering process.

Pros and Cons of Wave Soldering

Advantages

Wave soldering technology is very suitable for high-quality lead-free PCB assembly requirements, ensuring RoHS compliance while improving production efficiency.
With selective soldering, there is no need to mask off areas of the circuit board that do not require soldering, saving time in the soldering process.
Wave soldering typically offers cost advantages compared to other soldering methods.
Advanced equipment allows parameters to be adjusted for precise results.
Stepper motor-driven spraying, adjustable tin spraying mouth design, automatic positioning spraying and tin spraying, 25% less power, and auxiliary material consumption year over year.
Closed-loop control of the transport system, precise preheating, and welding time control.
Very little tin dross oxidation occurs near the crest of a flat-flow wave, and a humped laminar flow wave tends to produce flawless solder joints.

Disadvantages

SMD components are designed with a high pin density, allowing for more compact and efficient circuit board layouts. Wave soldering, on the other hand, is better suited for through-hole components with larger lead spacing, making it difficult to achieve reliable solder connections with SMD components.

Many SMD components, particularly temperature-sensitive IC, can be susceptible to damage when exposed to high temperatures during the wave soldering process. The heat generated by the molten solder may exceed the component’s specified temperature limits, leading to potential functionality issues or even permanent damage.

As the trend towards miniaturization continues, the size of components used in electronic devices is shrinking. This further reinforces the dominance of SMT in the electronics manufacturing industry. Due to the smaller size and high pin density of SMD components, the automation process becomes more feasible and efficient. Conversely, wave soldering is often impractical for these smaller components, resulting in limited investment by PCBA companies.

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