Introduction

In the complex world of printed circuit board (PCB) manufacturing, achieving robust and dependable electrical connections is paramount. Wave soldering is one essential step in the process.

This technique ensures that through-hole components adhere seamlessly to PCBs, anchoring them securely for their intended electronic applications.

Within PCBA production, the wave soldering process involves complicated steps, each integral to the overall outcome. From the meticulous preparation of the PCB and the precise application of flux to controlling solder temperature and wave height, this method holds secrets to creating reliable solder joints.

In this exploration, we delve into the world of wave soldering. We will let you know the details of preheating, solder wave creation, soldering, cooling, and the significance of the final inspection. 

What is Wave Soldering?

Wave soldering is a prominent method for soldering in bulk during the production of printed circuit boards. In this procedure, we move a circuit board over a container filled with liquid solder. 

Inside this container, a pump device creates a rising wave of solder that resembles a stationary wave. As the circuit board interfaces with this solder wave, it facilitates the soldering of its components to the board. 

While wave soldering primarily finds its application in the soldering of through-hole components, it can also be employed for surface mounting in specific scenarios.

The fundamental equipment utilized in the wave soldering PCB process encompasses a conveyor transporting the printed circuit board through various sequential zones. 

The solder bath is pivotal in the soldering procedure, and a pump generates the essential solder wave. Additionally, this system includes integral components like a flux sprayer and a preheating pad.

It is worth noting that diverse types of wave solder machines are available, but their core components and operational principles remain consistent. 

Wave Soldering Machine

The wave soldering machine comprises a heated solder pot to maintain the necessary temperature. Within this container, we create a solder wave, and the printed circuit board traverses it, ensuring that only the board's underside makes contact with the solder wave.

It's essential to regulate the height of the wave to prevent solder from seeping into unwanted areas on the top surface of the board. Usually, we securely mount the boards on a conveyor tray that can endure high temperatures. In electronics manufacturing facilities, you'll commonly find bimodal or electromagnetic pump wave soldering machines in use. Further, the wave soldering price varies depending on the model, features, and capabilities.

Steps Involved in the Wave Soldering Process

Here are the critical steps involved in the wave soldering PCB process:

PCB Preparation

Start by preparing the PCB, ensuring it is clean and free from contaminants or residue. This step may involve cleaning the PCB surface to optimize solder adhesion.

Flux Application

Flux plays a crucial role in the wave soldering process, primarily for cleansing the components soldered, particularly addressing any oxide layers that may have formed. 

There are two different kinds of flux used: corrosive and non-corrosive. Non-corrosive fluxes necessitate pre-cleaning and are employed when low acidity is a requirement, while corrosive fluxes, being faster and requiring less pre-cleaning, possess a higher acidity level. 

We need to put the flux on the bottom of the board we want to solder. It's essential to use just the right amount. Too little can lead to weak solder joints, and too much can leave extra flux on the board. The right amount is essential for good soldering.

In the wave soldering PCB process, we utilize two primary techniques for applying flux.

1. Spray Flux Application: This method involves spraying a fine mist of flux onto the underside of the board that is going for soldering. In some systems, a compressed air jet may remove excess flux, ensuring precise application.

2. Foam Flux Application: In this approach, the electronic printed circuit board goes over a cascading head of flux foam. We make the foam by putting flux in a tank. Inside the tank, there's a plastic cylinder with tiny holes. A metal chimney covers the cylinder, and operators push air through it, causing the flux foam to rise inside the vent.

These two application methods enable the controlled and effective flux deposition on the PCB, a crucial step in ensuring reliable soldering.

Preheating

During the wave soldering process, high temperatures, much higher than those experienced in hand soldering, expose PCB boards and components. This sudden temperature change, called thermal shock, can increase the risk of failure. 

Therefore, it is necessary to preheat the boards gradually, allowing them to reach the desired temperature and thus minimizing thermal shock, which will address this issue. In the preheating zone, we commonly use hot air heaters. 

Users require blowing warm air onto the PCBs as they move through the wave soldering machine. An infrared heater may ensure even heating in cases where the board is dense, avoiding any shadowed areas.

Setting the preheating temperature varies based on the type of assembly or PCB:

• For single-sided wave soldering, set the temperature to 90 to 100 degrees.
• For double-sided wave soldering and double-sided reflow soldering, maintain the temperature within the 100 to 110-degree range.
• For multilayered PCB wave soldering and multilayered PCB reflow soldering, adjust the temperature to between 115 and 125 degrees.

Solder Wave Creation & Soldering Process

A solder pot or tray within the machine holds the molten solder in the wave soldering process. A pump creates a controlled upwelling of the solder, forming a distinct standing wave. It's crucial to maintain this wave at the correct height and temperature to facilitate the soldering process effectively.

Once we have preheated the printed circuit board (PCB), we carefully move it over the wave of molten solder. As the PCB comes into contact with the solder wave, the solder bonds with the exposed metal pads and component lead. This interaction results in the formation of solid and reliable solder joints.

Solder Material Choice

The choice of solder material is critical. Traditionally, the most widely used solder alloy is Sn63Pb37, which consists of 63% tin and 37% lead. This combination offers excellent soldering properties, making it famous for electronics manufacturing.

However, lead is a hazardous substance that can harm human health and the environment. Its presence in soldering materials poses a risk to workers and can lead to pollution. 

Manufacturers have introduced lead-free solder alloys as a safer alternative to address these concerns. These lead-free solders often consist of different materials, and two common choices are Cu6Sn5 and Ni3Sn4.

Solder Temperature 

In the wave soldering process, solder temperature is a pivotal factor influencing the quality of solder joints. It is crucial to carefully manage the temperature of both the solder pot and the printed circuit board (PCB) for successful wave soldering. Below are recommended temperature ranges and considerations for an effective wave soldering operation:

• Top-Side Temperature: For lead-free wave soldering, the typical top-side temperature falls within 180 to 225°C. However, the specific temperature may vary based on the type of flux we use.
• Wave Solder Pot Temperature: In the case of lead-free soldering, operators should ideally maintain the wave solder pot temperature in the range of 265 to 270°C.
• Underside PCB Temperature: The temperature on the underside of the PCB at the last preheat zone should be approximately 150°C.
• Tin Point Temperature: Operators should closely control the tin point temperature range at 245±10℃ during soldering.
• Temperature Between CHIP and WAVE: It's vital to ensure that the temperature in the zone between the CHIP and WAVE does not fall below 180℃.
• Preheating Temperature Ramp Rate: Operators should maintain the temperature ramp rate of the PCB board's bottom during preheating at a rate less than or equal to 5°C per second.
• PCB Outlet Temperature: At the outlet of the furnace, we should carefully control the temperature of the PCB board to ensure it remains below 100 degrees.

Effective management of these temperature parameters is essential to achieve reliable solder joints and ensure the overall success of the wave soldering process.

Solder Wave Height

When configuring the wave soldering process, one critical parameter to consider is the height of the solder wave. The contact duration between the solder wave and the soldered components typically lasts 2 to 4 seconds. 

Two key machine settings regulate this contact time: the conveyor speed and the wave height. Altering either of these parameters will directly impact the contact time. We usually increase or decrease the pump speed on the machine accordingly to adjust the wave height. 

Further, we can use a tempered glass plate for a more precise evaluation to assess any changes in wave height. When a fixture is in place, it allows for the digital recording of the contact time, wave height, and speed for comprehensive process control.

Cooling Process

It is crucial to maintain an appropriate cooling rate for the printed circuit board (PCB) post-soldering. Rapid cooling can lead to PCB warping and potential damage to the solder joints.

Conversely, if the PCB cools too slowly, it can become brittle, risking damage to some components due to prolonged exposure to high temperatures. It is advisable to employ methods like water-cooling or air-cooling, which help regulate the cooling process and minimize potential harm to the PCB for the right balance.

Trimming and Cleanup

Sometimes, you might have too much solder that needs fixing for a neat and robust connection. To do this, you may need to remove extra solder or solder balls from the PCB to make things clean and reliable.

Final Inspection

A final inspection of the printed circuit board (PCB) ensures that all solder joints meet the necessary quality standards and that the PCB is free from any faults. This step is vital to confirm the board's integrity and soldering quality.

Basic Equipment Used in Wave Soldering Process

The essential equipment used in the wave soldering process includes:

• Conveyor belt: The conveyor belt facilitates the movement of the printed circuit board, guiding it through different sections of the soldering machine.

• Solder tray: It serves as the core of the soldering process, containing the molten solder required for the operation.

• Pump: The pump generates distinctive solder waves resembling standing waves, enabling effective soldering.

• Flux sprayer: The flux sprayer dispenses flux onto the underside of the PCB, preparing it for preheating and submersion in the liquid solder.

• Preheat pad: This element preheats the PCB, ensuring optimal soldering conditions.

Safety Precautions in the Wave Soldering Process

When conducting wave soldering, it's crucial to observe various precautions to safeguard against potential risks and ensure the final product's quality. Some of these precautions include:

Machine Inspection

Before starting, it's crucial to thoroughly check the wave soldering machine, which involves examining the condition of the furnace surface, ensuring the tin slag is in good shape, and verifying the pressure air supply.

Machine Maintenance

Regular maintenance is essential to avoid potential hazards, such as loose screws, slow chain speed, and rusted drive shafts. These issues can lead to board detachment and poor quality after soldering.

Preventing Component Lift

The heat generated during soldering can weaken the adhesion of copper, making the board fragile. It's crucial to handle the board carefully when removing it from the soldering machine to prevent components from lifting.

Avoiding Solder Shorts

Solder shorts occur when the solder fails to separate from two or more leads before solidifying. To prevent this issue, reducing the length and size of the pad can help minimize the amount of solder that adheres to the board. These measures contribute to a safer and higher-quality wave soldering process.

Proper Hole Fill

It's crucial to maintain a steady solder flow to prevent insufficient hole fill, which may result from slow solder drainage. Slow drainage can lead to excessive solder on the lead, causing it to drain unevenly and create solder trails on the board. Timely removal of the board from the solder wave can also help avoid this issue.

Managing Solder Quantity

Excessive solder can lead to problems like solder bridging, where solder unintentionally connects two or more leads. Preventing this requires applying a protective coating that hinders solder from bridging connections.

Preventing Solder Balling

Solder balling, which involves the formation of small solder balls on the board, can be reduced by using the appropriate amount of flux and ensuring the board's cleanliness before soldering.

Avoiding False Soldering

False soldering and the occurrence of air bubbles can be a result of a wet hole. It's crucial to confirm that the hole is dry before proceeding.

Monitoring Tin Composition

Regularly checking Tin composition is essential to prevent issues such as poor-quality outcomes after the furnace and potential horizontal deformation of the tracks.

It's worth noting that these precautions serve as general guidelines, and the specific cautions may vary depending on the type of wave soldering machine and the materials in use. Always follow the manufacturer's instructions and safety guidelines before operating a wave soldering machine.

Common Defects Found in the Wave Soldering Process

During the wave soldering process, several defects can affect the final product's quality. Here are some common defects, their possible causes, effects, and solutions:

Pinhole or Blowhole Defects

These defects are because of the thickness of copper plating. Moisture within the board moves out via either the thin plating or gaps.

The possible causes of these defects are poor-quality copper plating, flux, and solder. The effects of these defects are poor electrical conductivity and poor mechanical strength. The solution to this problem is to use high-quality copper plating, flux, and solder.

Insufficient Hole Fill

This problem occurs on printed circuit boards with pre-drilled holes for mounting components onto the board. The possible causes of this defect are low temperature, contaminated solder, surfaces too heavily oxidized for the flux getting used, contaminated flux, and poor application of flux.
 
The effects of this defect are poor electrical conductivity and poor mechanical strength. The solution to this problem is to investigate each possible cause and correct suspected discrepancies one at a time until the restoration of solderability.

Lifted Components

This defect occurs when the user does not properly solder the components to the board. Possible causes of this issue include subpar flux, low-quality solder, and inadequate preheating. 

The consequences of this defect manifest as compromised electrical conductivity and diminished mechanical strength. The remedy for this problem involves utilizing high-quality flux, solder, and preheating techniques.

Excessive Solder

This defect arises when excessive solder is applied to the board. Potential factors contributing to this issue encompass substandard flux, inferior solder, and insufficient preheating. 

The repercussions of this defect result in diminished electrical conductivity and weakened mechanical strength. It is essential to employ high-quality flux, solder, and preheating methods to address this problem.

Solder Balling

This defect occurs when small solder balls form on the board. Contributing factors to this problem may include suboptimal flux, lower-grade solder, and inadequate preheating. 

The effects of this defect include reduced electrical conductivity and weakened mechanical strength. It is advisable to implement high-quality flux, solder, and preheating procedures to overcome this issue. 

Wave Soldering vs Reflow Soldering

Reflow and wave soldering are different methods of producing printed circuit boards (PCBs). Reflow soldering primarily assembles surface mount technology (SMT) components, which are particularly well-suited for smaller components. 

In contrast, wave soldering is the method of choice for through-hole technology (THT) or dual in-line package (DIP) assembly, typically accommodating larger components.

Reflow soldering enjoys greater popularity within the PCB industry, offering cost-effective advantages. In contrast, wave soldering excels in speed and efficiency, making it an ideal choice for high-volume manufacturing scenarios. 

Both wave and reflow soldering have unique advantages and limitations, and the choice of which one to employ depends on the specific requirements of the PCB.

Let's take a look at the advantages and disadvantages of each technique:

Reflow Soldering Pros:

• Ideal for small components
• Creates high-quality solder joints
• Works well for small-scale production
• Cost-effective

Reflow Soldering Cons:

• Not suitable for significant components
• Demands precise temperature control
• Less effective for high-volume production

Wave Soldering Pros:

• Suited for large components
• Fast and efficient
• Excellent for high-volume manufacturing
• Requires less exacting temperature control

Wave Soldering Cons:

• Unsuitable for small components
• Calls for skilled operation
• It may lead to issues like lifted components and excess solder, primarily if not handled with expertise.

Conclusion

In conclusion, mastering wave soldering is not just an industry necessity; it's an art form that combines precision, technology, and environmental responsibility. 

By understanding its complexity, manufacturers can ensure the production of PCBs that meet accurate quality standards while minimizing their eco-friendly footprint. 

Wave soldering remains a fundamental of PCB production, paving the way for the next generation of electronic innovations.