Flux is removed from a PCB primarily to prevent corrosion, ensure electrical reliability, and improve the board's appearance.
During the soldering process, flux is used to clean metal surfaces by removing oxides and allowing the solder to flow properly. While essential for good solder joints, flux residues left on electronic circuit boards are acidic. If these residues aren't removed with a cleaning process, the residues can draw in ambient moisture from the air and lead to corrosion of component leads and PCB contacts. This corrosion can degrade performance and eventually cause component or board failure.
Key Reasons for Flux Removal
Removing flux residue is a critical step in PCB manufacturing and rework processes. Beyond preventing corrosion, other significant reasons include:
1. Preventing Corrosion
As highlighted by industry standards and practices, the acidic nature of flux residues, especially those containing halides, makes them hygroscopic. This means they readily absorb moisture from the surrounding environment. The combination of acidic residue, absorbed moisture, and potential electrical bias can accelerate electrochemical migration, leading to metal corrosion on solder joints, component pins, and traces. This corrosion degrades the integrity of the connection and can lead to intermittent or complete circuit failure.
2. Ensuring Electrical Reliability
Flux residue, particularly non-rosin or synthetic types, can be conductive or become conductive in the presence of moisture. Leaving these residues on the board can create unwanted electrical pathways between traces or component pins. This can lead to:
- Reduced insulation resistance
- Leakage currents
- Short circuits
- Signal integrity issues, especially in high-frequency or high-impedance circuits
Removing the residue ensures that current flows only through the intended paths, maintaining the circuit's designed performance.
3. Facilitating Inspection and Testing
Clean boards are much easier to visually inspect for soldering defects like opens, shorts, or cold joints. Flux residue can obscure solder connections, making thorough inspection difficult or impossible. Automated optical inspection (AOI) systems and in-circuit testers also perform better on clean boards, reducing the risk of false failures or missed defects.
4. Improving Aesthetics and Adhesion
A clean PCB looks professional. Flux residue can be sticky and leave a dull or discolored film on the board surface. Beyond aesthetics, a clean surface is necessary for subsequent processes such as:
- Conformal coating application: Residues can prevent proper adhesion of conformal coatings, which are applied to protect boards from environmental factors like moisture, dust, and chemicals.
- Underfill application: In advanced packaging techniques like BGA assembly, underfill is applied beneath components to improve reliability. Residues can interfere with underfill flow and adhesion.
- Potting or encapsulation: Similar to conformal coating, residues can compromise the effectiveness of potting compounds used for protection.
Summary Table: Why Clean Flux?
| Reason | Impact |
|---|---|
| Prevent Corrosion | Acidic residues + moisture = metal degradation on components and PCB contacts. |
| Ensure Electrical Reliability | Conductive residues can cause leakage currents, shorts, and signal integrity issues. |
| Facilitate Inspection | Residues hide defects, making visual and automated inspection/testing difficult. |
| Improve Aesthetics | Boards look cleaner and more professional. |
| Aid Subsequent Processes | Ensures proper adhesion of conformal coatings, underfill, and potting materials for protection and reliability. |
Practical Considerations
The need to remove flux depends largely on the type of flux used:
- Rosin Flux (R, RA, RMA): Generally considered less corrosive, especially after the soldering heat has driven off activators. "No-clean" versions are designed to leave benign residues that don't require removal in many applications.
- Water-Soluble Flux (OA): These are highly active and corrosive. Their residues are hygroscopic and must be cleaned thoroughly with water.
- Synthetic Activated Flux (SA): Highly active and often require cleaning, though some "no-clean" versions exist.
For high-reliability applications, medical devices, automotive electronics, or products exposed to harsh environments, cleaning flux residue is often mandatory regardless of the flux type, ensuring long-term performance and preventing field failures caused by contamination.
