The importance of rework cleaning in electronic manufacturing

The need for PCB rework, which involves repairing or altering existing PCBs, can arise for assorted reasons, including part defects, design changes, and quality control adjustments. Given the complexities of modern electronics, rework cleaning requires careful consideration of key factors.

Why rework cleaning matters

Rework cleaning plays a vital role in maintaining the functionality and reliability of electronic assemblies. It is the process of removing contaminants that accumulate during rework, ensuring that electronic components perform as intended. Here are the primary reasons why rework cleaning is essential:

• Removing contaminants: During rework, contamination like flux residues, solder splashes, dust, and oils from handling can build up on PCBs. These contaminants can lead to corrosion, electrical shorts, and other issues compromising performance. Effective cleaning removes these contaminants, ensuring the board's performance and long-term reliability.
• Ensuring reliable solder joints: Soldering needs clean surfaces for adequate bonding. Residual flux or debris can prevent proper adhesion, resulting in weak or cold solder joints. Post-rework cleaning ensures strong connections, reducing the risk of failure.
• Preventing corrosion: Flux residues can be corrosive, posing a long-term risk to the PCB's components. Proper cleaning minimises the potential for corrosion, extending the lifespan of electronic assemblies.
• Avoiding electrical shorts: Contaminants can cause unintended electrical connections, leading to shorts. Thorough cleaning mitigates this risk, ensuring the safety and reliability of the PCB.
• Improving inspection and testing: A clean PCB allows for more exact inspection and testing, making identifying defects or other rework-related issues easier. This step is crucial for quality assurance and troubleshooting.
• Meeting industry standards: Industries such as aerospace, automotive, and medical device manufacturing demand strict compliance with cleanliness standards. Proper cleaning helps meet these standards, ensuring regulatory compliance and product safety.

Cleaning methods for rework cleaning

Choosing the correct cleaning method for the rework of electronic devices involves balancing multiple factors, including effectiveness, cost, throughput, and compatibility. Two common methods are benchtop cleaning and vapour degreasing.

Benchtop cleaning

Benchtop or manual cleaning is often used during assembly, post-reflow touch-ups, rework, and repair. Common techniques include dip and brush cleaning, aerosol spray cleaning, and the wet, scrub, rinse, and dry approach.

• Dip and brush cleaning: This method uses a low-cost cleaning fluid, usually Isopropyl Alcohol (IPA), dispensed from a pump bottle and an acid brush to clean specific areas of a PCB. Although cost-effective, it has drawbacks such as cross-contamination and inconsistent process control. The cleaning fluid can easily become contaminated from reusing the dirty brush, and success depends on the technician's skill and experience.
• Aerosol spray cleaning: This approach uses aerosolised cleaning fluids to achieve more consistent results. There are add-on straw attachments that can help control the flow and direction of the fluid. But any cleaning power relies merely on the pressure in the straw to blow out trapped contaminants from under low-mounted components. It avoids cross-contamination but can be messy and costly due to high fluid consumption. Process control stays challenging, and fume exposure can be a safety concern.
• Wet, scrub, rinse, and dry cleaning: This four-step process is dependable, consistent, and controlled. It involves applying cleaning fluid, scrubbing with a brush, rinsing with fresh fluid, and drying with a lint-free wipe. This approach addresses cross-contamination and process control issues, ensuring thorough cleaning.

A controlled dispensing system can also reduce cleaning fluid waste. Technicians typically use up to 60% less cleaning fluid when they use these attachments. The dispenser hose connects to the can of cleaning fluid for better control of where the cleaning fluid goes and how much gets used. This allows for improved precision and reduces cleaning fluid waste, resulting in cost savings.

Vapour degreasing

Vapour degreasing uses a closed-loop system with fluid immersion, vapour rinsing, and drying

Vapour degreasing is a more sophisticated cleaning method using a closed-loop system with fluid immersion, vapour rinsing, and drying. It is highly effective in removing contaminants and is commonly used for PCBs with stringent cleanliness requirements.

The vapour degreaser consists of two chambers holding cleaning fluid. The first chamber heats a carefully engineered fluid to create a vapour cloud, which rises to meet cooling coils. The condensed vapours return to liquid form, which is then used for rinsing. Soiled parts are immersed in the continuously filtered cleaning fluid to remove contaminants. The process concludes with a vapour rinse and drying, leaving PCBs clean and spot-free.

Modern vapour degreasing fluids are specifically engineered to ensure high solvency while still being gentle enough for sensitive components. The cleaning fluid and method must not damage sensitive components, substrates, or conformal coatings. These fluids have low boiling temperatures and are compatible with various PCB materials. Additionally, they are ultra-pure, leaving no residue behind. Vapour degreasing is particularly useful for complex assemblies with tight spacing and low stand-offs, allowing thorough cleaning even in hard-to-reach areas.

Testing for compatibility

Whether using a benchtop cleaning method or vapour degreasing, PCB manufacturers must ensure that their cleaning fluids are compatible with the specific materials in their PCB assemblies. Conducting compatibility tests is crucial to avoid potential damage and ensure reliable cleaning. It is recommended to conduct a limited test clean before moving to full-scale production. Test cleaning can be performed in-house if your facility has the capabilities, but outsourcing is also a common choice for PCB cleanliness assurance. Specialised companies can conduct comprehensive testing and cleaning trials, adhering to the standards specified by you or your customer. These tests can range from simple visual inspections to rigorous IPC-610 requirement testing.

Adhering to industry standards

Following accepted industry standards and specifications is key to supporting quality and cleanliness in electronics manufacturing. Standards such as the IPC guidelines provide a framework for ensuring proper cleaning and quality assurance. By adhering to these standards, manufacturers can ensure that their rework cleaning processes meet industry expectations to ensure product reliability and performance.

Prioritise rework cleaning

Rework cleaning plays a vital role in maintaining the functionality and reliability of electronic assemblies.

Rework cleaning is becoming more challenging in electronics manufacturing, especially as components become smaller and assemblies more complex. Manufacturers can achieve effective and reliable rework cleaning by carefully selecting the correct cleaning methods and fluids and addressing key considerations like material compatibility and industry standards. This ensures the quality and longevity of electronic assemblies and contributes to safer and more reliable end products. As technology continues to evolve, keeping exacting standards in rework cleaning will be essential for meeting the demands of modern electronics manufacturing.

This article was contributed by Elizabeth Norwood, Senior Chemist at MicroCare, LLC.