How to Prevent PCB Substrate Burning During Laser Soldering?

How to Prevent PCB Substrate Burning During Laser Soldering?

What methods can prevent laser-induced substrate burning during PCB soldering? 

The substrate refers to copper-clad laminate (CCL), which serves as the fundamental material for manufacturing PCB circuit boards due to its three key functions: conductivity, insulation, and structural support. With the development and advancement of electronic technology, new requirements continue to emerge for PCB substrate materials, driving the ongoing evolution of copper-clad laminate standards. Soldering remains an essential process in the complete circuit board manufacturing process.

In substrate soldering, laser soldering is currently a mature technology. However, even with mature technology, it's challenging to guarantee 100% soldering yield. It's important to understand that laser soldering is a high-efficiency precision welding method that uses high-energy-density laser beams as a heat source. During batch soldering of hundreds or thousands of pieces, laser soldering machines may occasionally encounter issues with burning the substrate. So how can we prevent this problem?

Issue 1: Thermal Expansion and Separation
High-energy localized rapid heating causes thermal expansion deformation of solder joint copper foil, leading to separation from the substrate.

Solution:
Implement gradual temperature ramping from low to high. For example:

• Required soldering temperature: 350°C

• Heating profile: Ramp from 280°C to 350°C over 0.3 seconds
       This controlled heating typically resolves the separation issue.

Issue 2: Improper Power Mode Application
The laser soldering controller operates in power mode without professional laser engineer calibration.

Explanation of Power Mode:

• Delivers constant output power according to preset values

• Does not account for actual soldering temperature

• Only suitable for solder joints with rapid heat dissipation

Solution: Current laser soldering systems now employ closed-loop control, and temperature mode is generally adopted for non-special solder joints. 

Issue 3: Temperature Overload in Temperature Mode of Laser Soldering Machine

The temperature overload occurs when the actual soldering temperature exceeds the preset value. As we know, modern laser soldering machines adopt closed-loop control systems where the controller automatically calculates the required output power after setting the temperature. This power calculation relies on collecting real-time temperature data, making temperature acquisition a crucial part of the closed-loop control in laser soldering.

The root cause of temperature overload lies in the delayed temperature feedback. When the controller fails to receive timely temperature feedback, it continues to increase the output energy, leading to excessive temperature rise beyond the set parameters.

Solution 1: Utilize laser soldering modules with more responsive temperature feedback.
  Songsheng Optoelectronics' constant-temperature laser system employs high-speed response continuous semiconductor lasers (15μs response time) to match its temperature control speed (20μs). Only such a rapid-response constant-temperature system can ensure optimal soldering quality.

Solution 2: Implement power limitation in temperature mode.

  As previously explained, temperature overload in temperature mode results from delayed feedback. By analyzing the temperature curve of the temperature-controlled laser soldering machine, we can identify the power output levels at overload points and accordingly set power limits, effectively resolving overload issues.

Additional Recommendations:
  Prior to mass production, conduct comprehensive process testing and optimization:

1. Perform soldering tests on sample PCBs to observe substrate      heating patterns and joint quality

2. Adjust laser parameters (power, duration, etc.) based on test      results until achieving satisfactory outcomes

3. Ensure operators possess proper training and experience - they      must:
        • Fully understand equipment capabilities and operating procedures
        • Strictly follow standardized welding protocols