How to remove flux residues during laser soldering of PCBs?

How to remove flux residues during laser soldering of PCBs?       

Cleaning Methods for Flux Residues in Laser Soldering of PCB Circuit Boards

Soldering is a crucial industrial material used to connect electronic components in circuit assemblies. In PCB soldering processes, common techniques include immersion soldering, reflow oven soldering, wave soldering, machine soldering, and manual soldering with an iron. Regardless of the method used, some residue inevitably remains on the PCB after soldering.

These residues may consist of solder dross, rosin-based flux, or other flux byproducts. Cleaning is an essential post-soldering step to remove excess residues. While flux residues are unavoidable in any soldering process, selective laser soldering offers an advantage over traditional methods: it produces no solder spatter or solder balls—only flux residue—making post-soldering PCB cleaning easier. Below, we outline several practical methods for removing flux residues from circuit boards.

Hazards of Flux Residues:
On one hand, flux residues can corrode the PCB surface, compromising circuit reliability. On the other hand, they may degrade electrical performance, affecting device functionality. Therefore, addressing flux residue is critical. The primary cleaning methods typically fall into the following five categories:

1. Aqueous Cleaning
Aqueous cleaning is a common method that uses purified or deionized water to remove flux residues.

Process: Pre-rinse → Water-based detergent spray/ultrasonic cleaning → Pure water rinse → Drying.

Advantages: No VOC emissions, compliant with RoHS/REACH.

Challenges: Requires thorough drying to prevent electrochemical migration.

2. Solvent Cleaning
Solvent cleaning is relatively simple, using the same solvent for both cleaning and rinsing. Due to the high volatility of solvent cleaners, no dedicated drying process is needed.

Principle: Utilizes organic solvents to dissolve/remove residues.

Recommended Solvents:

Alcohols (Isopropyl Alcohol, IPA): Low cost, effective for rosin-based residues.

Hydrocarbon solvents (eco-friendly): Low toxicity, suitable for precision cleaning.

Modified alcohol / ether solvents (e.g., glycol ether): Strong penetration for stubborn residues.

3. No-Clean Process
The no-clean process relies on strict quality and process control of raw materials (PCBs and components) to eliminate the need for post-soldering cleaning. It features low retrofitting costs, reduced operational expenses, and environmental friendliness.

Best suited for:

Highly automated, large-scale production.

Applications where post-soldering reliability requirements are not extremely stringent.

Benefits:

Eliminates costs for cleaning equipment and chemicals.

Significantly lowers operational expenses.

4. Ultrasonic Cleaning
Ultrasonic cleaning is an effective method for removing flux residues. The PCB is immersed in a cleaning solution, and ultrasonic waves generate vibrations and cavitation to thoroughly clean residues.

Ideal for:

High-density PCBs.

Residues under bottom-filled components.

5. Plasma Cleaning (High-End Applications)
Principle: Low-temperature plasma decomposes organic residues.

Best for: Ultra-precision boards where solvent contact is undesirable (e.g., MEMS sensors).

Limitations: High equipment costs, requires a vacuum environment.

Conclusion
The above methods, compiled by Songsheng Optoelectronics, outline effective approaches for removing solder dross and flux residues. Always follow safety protocols and select the appropriate method based on specific requirements. Before cleaning, review the flux and cleaner instructions and conduct compatibility tests to avoid surface damage.