Comprehensive Guide to PCB Reflow Soldering

Joining two metals together is a complex issue, and common methods such as metal-specific glue are ineffective in the microelectronics realm. In the PCB assembly process, it’s crucial to securely fix components onto a PCB, ensuring not just a simple physical connection but also unimpeded electrical signal transmission between the PCB and component pins.

So, how can this be achieved?

The most common approach today is the use of reflow soldering technology. This technique involves applying solder paste between component pins and the PCB, forming robust solder joints in a high-temperature reflow oven. In this article, TechSparks will provide a detailed reflow soldering guide to help you understand the intricacies of this technology.

Table of Contents

What is Reflow Soldering

reflow process soldering

To securely fasten components onto a PCB, we employ PCB soldering processes, such as using a soldering iron to individually solder pins. This is a labor-intensive process and is clearly impractical for electronic manufacturing. For PCB assemblies with high complexity, the number of components on a PCB can reach several hundred, and a single batch may require thousands to tens of thousands of PCB assemblies. Relying on manual labor for such a large-scale production would be inefficient. Therefore, we need to leverage automation to achieve the automatic soldering of pins.

Reflow soldering is a form of automated soldering technology typically integrated into SMT (Surface Mount Technology) production lines. It comes into action after the solder paste printing and component placement are completed by the pick-and-place machine. The term “reflow soldering” may be derived from the recirculation of nitrogen in the reflow oven, creating high temperatures to achieve the soldering objective.

With the ongoing trend towards miniaturization, there’s a growing need to assemble more SMD components on a PCB. This signifies the increasing importance of reflow soldering technology in modern electronic manufacturing. Advanced equipment is required to accomplish the soldering of components.

Types of Reflow Soldering

Based on the characteristics and functions of the soldering ovens, reflow soldering equipment can be categorized in different ways. TechSparks uses their technical features as a benchmark to introduce and compare several typical types of reflow soldering ovens:

In modern electronic manufacturing, hot air reflow soldering is the most frequently used type. Using a hot air circulation system, it ensures that the hot air is evenly distributed on the surface of the PCBA board. This guarantees a stable heat during the soldering process, avoiding temperature fluctuations, which directly impact the success rate of soldering.

hot air reflow soldering

Another common choice is infrared (IR) reflow soldering ovens, known for their low cost and fast heating. This type is particularly popular in China, and the PCBA company I used to work for utilized this technology. By employing infrared radiation, it achieves the soldering process. The oven’s internal temperature is uniform, and it is well-suited for double-sided PCB assembly due to its larger mesh. However, for large-scale production, this might not be the optimal choice.

infrared reflow soldering

Vapor phase soldering is an older reflow soldering technology that uses a vapor phase reflow soldering oven. It relies on the heat conduction and cooling effects of vapor to complete the soldering process. This technique requires a substance with a high melting point called perfluorocarbon. Condensation pipes above and on the sides of the oven confine the vapor within the heating chamber. When the component to be soldered comes into contact, solvent vapors release latent heat of vaporization, melting the solder paste and soldering the component to the PCB pad. Vapor phase soldering can complete the soldering process more rapidly, and due to the nature of vapor, it can uniformly surround the soldering area, ensuring consistent heating and consistent solder joints. However, due to the environmental unfriendliness of perfluorocarbons and their impact on the ozone layer, this technique is now restricted.

Vapor phase reflow soldering

If you are considering purchasing reflow soldering equipment or selecting a PCBA factory to collaborate with, it’s a good practice to check how many temperature zones the reflow soldering equipment has. In fact, regardless of the technology or equipment used, the working principle of reflow soldering is based on heating the solder paste and then cooling it to solidify, forming robust solder joints. Next, TechSparks will provide you with detailed information on the reflow soldering process in modern electronic manufacturing.

Reflow Soldering Process

reflow soldering temperature profile

Preheating Zone

When the PCB enters the reflow soldering oven, it first goes through the preheating zone, where it gradually heats up from room temperature to about 150°C. During this stage, the applied solder paste on the PCB undergoes changes:

  • Volatile components evaporate, purifying the solder paste.
  • Flux in the solder paste wets various parts that need soldering.
  • Non-volatile components harden, collectively forming a sealing layer that effectively prevents the entry of oxygen and inhibits oxidation reactions, thus enhancing the quality of solder joints.

The key in the preheating zone is to control the temperature ramp-up. The preheating time is typically set between 60-90 seconds, and the temperature ramp-up is controlled between 2-4°C/second. Proper preheating time and ramp-up ensure a uniform soldering process, avoiding uneven soldering results caused by excessive temperature gradients. Too rapid heating can lead to issues such as volatile components rapidly evaporating, solder splattering, intense thermal stress causing component damage, and excessive flow of solder paste leading to collapse.

Soaking Zone

After entering the soaking zone, the PCBA board goes through a stage with a relatively constant temperature. The main purpose is to ensure the entire board is sufficiently heated, preventing faults due to sudden temperature changes during soldering. In this stage, it is crucial to set an appropriate soaking time.

Insufficient soaking time: Incomplete reaction between activators and oxides may occur, failing to effectively remove oxides. Additionally, solvents in the solder paste may boil, leading to the formation of solder balls. If the solder paste does not completely melt, the wetting effect may be inadequate, resulting in issues like voids and false solder joints.

Excessive soaking time: Activators may be overly consumed, eventually being unable to effectively remove residues of oxides and low-temperature carbonization of flux in low-temperature areas. This can lead to issues like dull or false solder joints.

Reflow Zone

The reflow zone is the main stage of reflow soldering. In this stage, a rapid temperature increase causes the solder paste to fully melt, completing the wetting, diffusion, or flow process and forming the solder joints. The maximum temperature in the reflow zone depends on the solder paste’s melting point, usually ranging from 235-250°C. During this process, the solder paste plays a significant role:

  • Resin or rosin hardens, forming a protective film that effectively isolates oxygen.
  • Surface activators are activated, reducing the surface tension between the solder material and the soldered surface, enhancing the wetting ability of the liquid solder material.
  • Activators react with oxides, continually clearing oxides produced at low temperatures, and provide some fluidity to carbonized residues, until complete decomposition and volatilization without leaving any residues.
  • High-boiling-point solvents gradually evaporate and completely disappear at the end of the reflow soldering.

Stabilizers are uniformly distributed on the metal surface and solder joint surfaces, effectively maintaining the solder joints without succumbing to oxidation. Solder powder transforms from a solid to a liquid state on the coated surface, accompanied by the wetting and diffusion of the flux, achieving various chemical reactions and physical changes during the soldering process.

Cooling Zone

The cooling zone is the final stage of reflow soldering. As the temperature inside the oven gradually decreases, the liquid solder paste begins to solidify, ultimately forming robust solder joints connecting the PCB and components. Quality changes of the solidified solder joints as the liquid solder material cools below the liquidus line are challenging to observe with the naked eye in a short period, many factories overlook the setting of the cooling zone.

However, the actual cooling speed of the solder joints directly influences their long-term reliability. Therefore, it is crucial to focus on and control the cooling speed to ensure the quality of the solder joints while not compromising the durability of the board and components. According to TechSparks recommendation, the minimum cooling speed should be maintained at no less than 2.5°C/second, preferably exceeding 3°C/second.

Ending

In summary, reflow soldering stands as a crucial stage in SMT assembly. Reflow soldering ovens are integrated into SMT production lines to achieve large-scale manufacturing. To ensure the quality of soldering, it is essential to understand the characteristics of solder paste and components. Factors such as the melting and cooling temperatures of solder paste, as well as the sensitivity of components to temperature, should be taken into account.

Having previously worked at a PCBA company, our organization emphasized the use of a 10-zone reflow soldering process, highlighting it as one of our competitive advantages. This led me to delve into the intricacies of reflow soldering and document my findings in this article. The content is based on training received at my former company and extensive online research. If there are any factual inaccuracies in the article, please let me know. Thank you!

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