Common Wave Soldering Defects and Their Causes

In PCB assembly, wave soldering is a very common and efficient soldering method, mainly used for soldering through-hole components, and can also be applied to some surface mount devices. However, if the parameters are not well controlled, the wave soldering process can produce various soldering defects.

It is easy to have production issues without understanding the common types of soldering defects and their causes. Poorly controlled wave soldering may lead to solder defects and even cause repeated rework due to soldering mistakes, affecting the performance and reliability of the product.

This article will provide a detailed introduction to:

•  What is wave soldering

•  Why do soldering defects occur

•  Common soldering defects

•  How to reduce solder joint defects

•  How to stabilize wave soldering through parameter control

Whether you are an engineer, quality professional or PCB designer, understanding wave soldering defects can help you achieve higher yield and reduce problems.

What Is Wave Soldering?

Before discussing the common defects in wave soldering, let's briefly explain what wave soldering is.

Wave soldering is a batch soldering method mainly used to solder electronic components onto PCBS. In the wave soldering process, the PCB passes through a flowing wave of molten tin. The solder liquid will automatically adhere to the exposed pads and component pins, forming a strong electrical and mechanical connection.

Wave soldering is a bulk soldering method mainly used to solder electronic components onto PCBs. In the wave soldering process, the PCB passes through a flowing wave of molten solder. The solder will automatically adhere to the exposed pads and component leads, forming a strong electrical and mechanical connection.

A standard wave soldering process generally includes several steps:

•  Flux application

•  PCB preheating

•  Passing through the molten solder wave

•  Cooling and solidification

Wave soldering is widely used because it is fast, efficient and more stable than manual soldering, and can reduce human-related soldering mistakes.

However, if the temperature, conveyor speed, or other parameters are not properly controlled, various soldering defects can easily occur.

Why Do Wave Soldering Defects Occur?

Although the wave soldering process is automated, it is still possible to produce soldering defects due to a variety of factors.

The following table summarizes the common causes in a clearer way:

Common Wave Soldering Defects (With Causes & Solutions)

Item	Solder Bridges / Shorts	Poor Hole Fill	Pin / Blow Holes	Cold Solder Joint	Lifted Pads	Solder Balls
Pictures
Description	Excess solder connects adjacent pins	Through-hole not fully filled with solder	Small holes or voids visible in solder joints	Dull surface with weak mechanical strength	Copper pad separates from PCB substrate	Small solder spheres scattered on PCB surface
Main Causes	Excessive wave height, slow conveyor speed, small pin spacing, poor flux control	Low solder temperature, insufficient contact time, improper hole-to-lead ratio	Moisture in PCB, excessive flux, insufficient preheating	Low solder temperature, insufficient contact time, oxidized pads	Overheating, mechanical stress, poor PCB quality	Excess flux, rapid heating, contamination
Solutions	Adjust wave height, optimize conveyor angle, improve pad design	Increase solder pot temperature, adjust conveyor speed, improve PCB design	Bake PCB before soldering, optimize preheat profile, control flux quantity	Increase solder temperature, improve flux activation, clean PCB surfaces	Reduce dwell time, improve PCB material grade	Optimize flux density, improve preheat ramp
Impact	Severe solder joint defects, may cause electrical shorts	Typical solder defects, affects mechanical strength	Common types of soldering defects	Common solder joint defects, often seen in bad soldering examples	Serious solder defects, affects long-term reliability	Common soldering mistakes, often caused by unstable wave soldering conditions

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Process Control Parameters for Stable Wave Soldering

To reduce soldering defects, every process parameter must be well controlled. Any fluctuation in temperature, conveyor speed or flux can easily cause solder defects, which affect product quality and reliability.

1. Preheat Temperature

The purpose of the preheat temperature is to raise the PCB temperature and, at the same time, to fully activate the flux. Too low preheat temperature is likely to cause solder joint defects; excessive preheat temperature may damage the PCB or components.

Therefore, the preheat temperature should be reasonably set according to the thickness of the board and the type of flux.

2. Solder Pot Temperature

The temperature of the solder pot is usually controlled at 245-260°C (adjusted according to different alloys). If the temperature is too low, cold joints may occur, and the solder points may be weak. Excessively high temperatures will accelerate oxidation and even cause serious solder defects.

Maintaining a stable temperature is the key to ensuring the quality of soldering.

3. Conveyor Speed

The conveyor speed determines the soldering contact time. Too fast speed will lead to poor hole fill. If the speed is too slow, it will cause bridging or excessive solder.

In the wave soldering process, the conveyor speed must be highly matched with the temperature and wave height.

4. Wave Height

The wave height of solder directly affects whether the solder can fully contact the solder joint. If the wave is too high, it can cause solder shorts; if the height is too low, there will be insufficient filling and repeated soldering defects.

Reasonable adjustment of the solder wave height can enhance the stability of soldering.

5. Flux Control

Flux control includes managing flux density, spray volume, and uniformity. Excessive or uneven flux can result in voids, contamination, and other types of soldering defects.

Controlling the flux well is a fundamental requirement for ensuring the stable operation of wave soldering.

Conclusion

Although wave soldering is a mature and efficient assembly method, it is still susceptible to various soldering defects. From solder bridges and poor hole fill to pin holes and cold joints, it is crucial to have an in-depth understanding of the root causes of these soldering defects.

Most of the solder defects do not occur by chance, but are caused by improper parameter settings, material issues or design errors in the wave soldering process.

By mastering the following key contents:

•  What is wave soldering

•  Complete wave soldering process

•  Common soldering mistakes

•  Ability to identify bad soldering examples

Manufacturers can significantly reduce defects, improve yield, and ensure the long-term reliability of products.

FAQs about Soldering Defects

1. What is the most common wave soldering defect?

Solder bridging and poor hole fill are among the most common soldering defects in wave soldering operations.

2. Can PCB design cause solder joint defects?

Yes. Improper pad spacing, hole size, or layout often leads to solder defects and recurring soldering mistakes.

3. How can I reduce soldering defects in the wave soldering process?

Control temperature

Optimize conveyor speed

Use proper flux

Maintain equipment regularly

4. Are all solder defects visible?

No. Some internal voids or weak solder joint defects may require X-ray or cross-section inspection.