What is the Temperature for Soldering PCB

In the PCB manufacturing process, soldering temperature plays a critical role in ensuring solder joint quality and product reliability. The required soldering temperature can vary significantly depending on the soldering method and the type of components being used.
Here is a quick overview of the general soldering temperature ranges for different PCB assembly methods and components:

• Surface-Mount Technology (SMT): Typically between 220°C and 250°C.
• Through-Hole Soldering: Typically between 250°C and 300°C.
• Copper Foil Soldering: Typically between 280°C and 330°C.
• Reflow Soldering:
  • Lead-Based: Preheat temperature 80-120°C, soak temperature 150-180°C, reflow temperature 210-240°C.
  • Lead-Free: Preheat temperature 100-150°C, soak temperature 180-220°C, reflow temperature 240-260°C.
• Manual Soldering: Temperature typically ranges between 240°C and 280°C, with higher temperatures (up to 350-370°C) for larger components.

The specific temperature range for each soldering method is determined by factors such as solder material, component type, and PCB design. Maintaining precise temperature control throughout the soldering process is essential to achieving high-quality solder joints and ensuring the overall reliability of the finished product. 

The Importance of Soldering Temperature for PCB

Now that we have an overview of the typical temperature ranges for different soldering techniques, let's delve deeper into the factors influencing soldering temperature and the specific requirements for each soldering method. Effective temperature control throughout the entire soldering process, from preheating to cooling, is crucial to ensure optimal solder joint quality and to prevent damage to sensitive PCB components.

1. Factors Influencing Soldering Temperature

The required soldering temperature for PCB assembly is influenced by several factors, including the type and composition of the board material, the soldering method, soldering materials, and equipment. Different PCB materials, such as FR-4, CEM series, and specialized materials like metal substrates or Teflon, have different thermal tolerance characteristics, which necessitate adjusting the soldering temperature accordingly.
Moreover, manual soldering and automated soldering (such as SMT, wave soldering, and reflow soldering) have different temperature requirements. Below, we will explore the specific temperature needs for various soldering methods and component types.

2. Soldering Temperature Requirements for Different Methods

2.1 Surface-Mount Technology (SMT)

SMT is a commonly used automated soldering method, with soldering temperatures typically ranging from 220°C to 250°C, depending on the soldering materials and component requirements. SMT processes primarily involve reflow soldering and wave soldering:

• Reflow Soldering: The reflow soldering temperature profile typically includes three stages: preheating, soaking, and reflow. Preheat temperatures are generally 80-150°C, soak temperatures range from 150-220°C, and reflow temperatures typically fall between 210-260°C. For lead-free soldering, higher reflow temperatures are required due to the higher melting points of lead-free solders.
• Wave Soldering: The typical soldering temperature for wave soldering is between 250-280°C, with the wave soldering pot set at 288±5°C. PCB preheating is essential in wave soldering to reduce thermal stress and solder joint defects.

2.2 Through-Hole Soldering

Through-hole soldering, used for larger components or pin connections, generally requires soldering temperatures between 250°C and 300°C, depending on the type and requirements of the soldering materials.

2.3 Copper Foil Soldering

Copper foil soldering, used for multi-layer PCBs or high-power applications, requires higher soldering temperatures, typically between 280°C and 330°C, depending on the soldering materials and copper foil specifications.

2.4 Manual Soldering

Manual soldering for PCBs, which requires human intervention, typically uses soldering iron temperatures between 240°C and 280°C. For large components, the soldering iron temperature can be adjusted between 350°C and 370°C, but should not exceed 390°C. The soldering time should generally be kept within a few seconds to avoid damaging the PCB or components.

3. Key Stages in Soldering Temperature Control for PCB

Temperature precision is critical throughout the soldering process. The entire soldering procedure can be broken down into four key stages: preheating, soaking, reflow, and cooling.

3.1 Preheating Stage

The primary purpose of preheating is to reduce thermal shock during soldering and ensure uniform heating of the PCB and components. The preheating temperature is typically set between 80-120°C for leaded solder and 100-150°C for lead-free solder, with preheating time ranging from 1 to 3 minutes.

3.2 Soaking Stage

The soaking stage ensures that the solder paste fully melts and wets the component leads and PCB pads. Soaking temperatures typically range from 150-180°C for leaded solder or 180-220°C for lead-free solder, with soaking time lasting between 30-60 seconds.

3.3 Reflow Stage

The reflow stage is the critical step where solder paste begins to melt and wet the component leads. Reflow temperatures generally range from 210-240°C for leaded solder and 240-260°C for lead-free solder, with reflow time lasting between 20-40 seconds. If temperatures are too high or the time is too long, components can be damaged or the PCB may warp.

3.4 Cooling Stage

The cooling stage allows the soldered PCB and components to gradually cool down to room temperature, preventing thermal stress caused by rapid cooling. The cooling rate should be controlled between 4-6°C per second.

4. What Are the Risks of High-Temperature Soldering to PCB Inner Layers?

While high-temperature soldering is necessary to achieve reliable joints, it can also introduce potential risks to the inner layers of the PCB, especially in cases where the temperature, time, and process control are not carefully managed. One of the main concerns is whether high temperatures could cause inner layer circuit damage or even breakage.
Several factors can contribute to the risk of inner layer circuit damage during high-temperature soldering:

4.1 Thermal Stress

High temperatures during soldering can introduce thermal stress, especially when the temperature rises and falls rapidly. This can cause the PCB material to expand and contract, potentially applying force to the inner layers, leading to cracks or circuit breakage.

4.2 Inappropriate Material Selection

The choice of PCB material and the materials used in the inner layers is crucial. If the materials' coefficients of thermal expansion (CTE) are mismatched, thermal stress could be generated during the temperature changes, leading to damage in the inner circuitry.

4.3 Thermal Shock

Rapid heating and cooling cycles can cause thermal shock, which could damage the inner layers of the PCB. This is particularly true for multi-layer PCBs or PCBs with components that undergo substantial temperature variation.

4.4 Poor Design

PCB design plays an essential role in ensuring the stability of the inner layers. Poor design choices, such as concentrating traces in specific areas, may make those regions more susceptible to the effects of thermal stress.

4.5 Improper Soldering Process

The soldering process itself—particularly temperature control, duration, and cooling rates—plays a significant role in preventing PCB damage. An improper soldering process can damage the PCB, potentially affecting the integrity of the inner circuits.

Mitigating Risks to PCB Inner Layers During High-Temperature Soldering

To reduce the risk of damage to the inner layers of the PCB during high-temperature soldering, the following measures can be taken:

• Select Appropriate PCB Materials: Ensure that the materials used in the PCB have matching thermal expansion properties to reduce stress during temperature changes.
• Optimize Soldering Process Parameters: Ensure appropriate temperature control, soldering time, and cooling rates to minimize thermal stress.
• Design for Thermal Stability: When designing PCBs, consider the layout of the inner circuits and avoid placing them in areas that may experience high stress.
• Conduct Regular Thermal Stress Testing: Perform routine thermal stress testing and quality checks to ensure the PCB's stability and reliability.

In conclusion, while high-temperature soldering is a necessary part of the PCB assembly process, it can present risks to the inner layer circuits. By carefully selecting materials, optimizing the soldering process, and considering design factors, these risks can be minimized, ensuring that the final PCB is both reliable and durable.

5. Important Considerations in Soldering Temperature Control

5.1 Temperature Uniformity

Soldering temperature must be uniform across the entire surface of the PCB to ensure consistent solder joint quality. Typically, the temperature difference across the PCB should not exceed 2-3°C.

5.2 Risks of Excessive Temperature

Excessively high soldering temperatures can lead to several problems, including:

• Solder joint deformation
• Solder material loss
• Insulation layer damage

These issues can significantly affect the electrical performance and lifespan of the PCB. Therefore, temperature control must be strictly adhered to during the soldering process.

5.3 Risks of Insufficient Temperature

If the soldering temperature is too low, incomplete soldering can occur, leading to weak solder joints and reduced electrical reliability.

Conclusion

PCB soldering temperature control is a vital element in ensuring solder joint quality. Depending on the soldering method, component type, and PCB material, it is essential to adjust soldering temperatures and profiles accordingly. Proper control of the preheating, soaking, reflow, and cooling stages, as well as selecting the appropriate soldering temperature profile (such as triangular profiles, peak temperature profiles, or low-peak profiles), can guarantee the quality and reliability of the soldering process.
We hope this article serves as a valuable reference for your PCB soldering processes, ensuring successful soldering operations and high-quality final products. If you have any questions or requirements related to PCB soldering technology, feel free to contact us anytime.