PCB Soldering Temperature: The Key Factor for Soldering Quality

In the soldering process for PCBs (Printed Circuit Boards), the soldering temperature is a core factor that directly influences the quality of the solder joints. Think of it as the fine-tuning knob on a precision instrument—a small deviation can significantly compromise the overall soldering result.

Why Soldering Temperature Is So Important

The proper soldering temperature ensures that the solder completely melts, forming a sound metallurgical bond between the PCB pads and component leads. It’s much like constructing a bridge: only when the concrete is properly mixed and set can the bridge be sturdy. If the temperature is too low, the solder won’t melt fully, leading to cold or weak joints that may break later, thereby affecting the performance and reliability of the electronic product. Conversely, if the temperature is too high, it can damage the components on the PCB—especially those that are temperature-sensitive, such as some plastic-packaged chips—and even cause the PCB to delaminate or the pads to lift, potentially rendering the entire board unusable.

What Is the Temperature for Wave Soldering?

1. Wave Soldering Preheat Temperature:

• A. Preheat Temperature Setting:
  Typically set between 90-110°C, where the “temperature” refers to the actual heat experienced at the soldering surface of the PCB after preheating—not the displayed temperature. If the preheat temperature falls short, issues like excessive solder residue, solder balls, or solder pull can occur.
• B. Factors Affecting Preheat Temperature:
  • B1. PCB Thickness:
    The thickness affects heat absorption and conduction. Thin PCBs heat up quickly, so if components are not tolerant of thermal shock, the preheat temperature should be lowered. Conversely, thicker PCBs take longer to transfer heat to the component side and can withstand higher preheat temperatures.
  • B2. Board Speed:
    Generally, a board speed of about 1.1-1.2 meters per minute is recommended. Adjustments in board speed should be paired with changes in preheat temperature. For instance, if you speed up the board, you might need to raise the preheat temperature to ensure the soldering surface reaches the desired temperature.
  • B3. Preheat Zone Length:
    A longer preheat zone allows the PCB surface to approach the desired temperature more accurately. If the preheat zone is short, the set temperature should be increased accordingly.

2. Wave Soldering Oven Temperature:

Taking 63/37 leaded solder as an example, the solder liquid temperature should generally be adjusted to around 245–255°C, and it should not exceed 260°C. Temperatures above 260°C accelerate the formation of oxides in the new solder. The conveyor belt speed is set according to the specific wave soldering machine and the PCB being soldered, usually between 0.8–1.92 m/min.

1. Reflow Soldering Temperature Profile

Reflow soldering is the core of the SMT process, and its temperature profile is usually divided into four stages:

• Preheat Zone:
  Temperature ramps at a rate of 1–3°C/s to 140–170°C and holds for 40–120 seconds to uniformly heat the PCB and activate the flux.
• Activation Zone (Soak):
  The temperature is maintained at 140–170°C to further remove oxides and ensure proper wetting of the solder.
• Reflow Zone:
  Temperature quickly rises to the peak (for lead-free solder, typically 205–230°C) and is held for 10–20 seconds to fully melt the solder and form the alloy layer.
• Cooling Zone:
  The temperature drops at a rate of 2–4°C/s. Cooling too fast may cause stress cracking, while cooling too slow increases the thickness of intermetallic compounds (IMCs), potentially shortening joint life.

2. BGA Rework Station Temperature Profile

For BGA rework, the temperature profile must precisely match the solder paste characteristics. A typical profile includes:

• Preheat Zone:
  1–3°C/s ramp-up to 140–170°C for 40–120 seconds.
• Reflow Zone:
  The peak temperature is set 10–20°C above the solder’s melting point (for 63Sn/37Pb solder, approximately 200–210°C).
• Cooling Zone:
  A cooling rate of about 4°C/s is maintained to balance joint structure and stress relief.

3. Common Issues and Their Solutions

Oxidation and Lack of Wetting

In high-temperature environments, PCB pads can oxidize, reducing wettability. Solutions include:

• Controlling Workshop Temperature and Humidity:
  Maintain around 24-26°C with 40%-60% humidity to prevent condensation.
• Avoid Handling by Bare Hands:
  Use gloves, and use the PCB promptly after unpacking.
• Prefer Leaded Hot Air Solder Leveling:
  Leaded solder typically offers better oxidation resistance than lead-free processes.

IMC Layer Thickening

Slow cooling can accelerate IMC formation, reducing mechanical strength. Ensure rapid cooling near the solidification point (220–200°C), but avoid cooling rates that exceed what the component’s thermal expansion can handle.

Thermal Stress Damage

Rapid temperature changes or large thermal differences can crack components or deform the PCB. For BGA rework, use multi-zone heating to reduce localized thermal shock.

4. Optimization Strategies and Practical Recommendations

1. Solder Alloy Selection:
   Choose the appropriate solder alloy based on product reliability requirements. Lead-free solders (e.g., SAC305) require higher soldering temperatures (240–250°C), but be mindful of controlling IMC thickness.
2. Environmental Control:
   Use temperature and humidity sensors to continuously monitor the workshop environment and prevent exposure of PCBs to high humidity or acidic conditions.
3. Process Verification:
   Use thermocouples to test the actual temperature profile, and adjust equipment parameters to ensure the real temperature curve matches the theoretical profile. Test a small batch of boards (5–10 pieces) before mass production to fine-tune settings.
4. Manual Soldering Tips:
   For hand soldering, it is recommended to set the soldering iron temperature to 380–410°C. First, secure diagonal leads, then evenly pull the solder to avoid prolonged heating at a single point.

Conclusion

Precise control of PCB soldering temperature is crucial for ensuring the long-term reliability of electronic products. By carefully designing the temperature profile, optimizing process parameters, and managing the environment, manufacturers can significantly enhance soldering quality and reduce defect rates. In practical production, dynamic adjustments based on specific materials, equipment characteristics, and product requirements are essential to ensure that every stage of temperature control is scientific and stable.