Comprehensive Guide to Preventing PCB Corrosion

PCBs are fundamental components in modern electronics, providing the backbone for connecting and supporting electronic elements in various devices. However, corrosion is a major threat to PCBs, potentially leading to malfunction or failure. Understanding the causes and prevention strategies for PCB corrosion is essential for maintaining device longevity and functionality. This guide explores the causes, types, cleaning methods, and preventive measures for circuit board corrosion, offering valuable insights for electronics professionals.

Understanding PCB Corrosion

Corrosion is an oxidation process where oxygen bonds with metal, causing rust and degradation. Given that PCBs consist largely of metal and are exposed to oxygen, they are susceptible to corrosion over time.

Metals and Corrosion Resistance

Metals used in PCBs vary in their resistance to corrosion. Some metals are highly resistant, while others are more prone to corrosion. Here’s a breakdown:

Noble Metals (Highly Resistant)

• Graphite
• Gold
• Silver
• Copper-nickel alloy

Base Metals (Highly Susceptible)

• Plated tin
• Lead
• Copper
• Plated nickel

Types of PCB Corrosion: Detailed Analysis and Solutions

Understanding the different types of corrosion that can affect Printed Circuit Boards (PCBs) is crucial for effective prevention, maintenance, and long-term reliability of electronic devices. Below, we delve into various forms of circuit board corrosion, their causes, implications, and strategies for mitigation.

Atmospheric Corrosion

Atmospheric corrosion occurs when metal surfaces on a PCB are exposed to moisture containing oxygen, leading to the formation of metal oxides, particularly on copper. This type of corrosion is exacerbated by environmental pollutants such as sulfur dioxide (SO₂) and chlorine, which can form acids upon contact with moisture, further accelerating the oxidation process. Corroded copper significantly loses its electrical conductivity, affecting signal integrity and overall device performance. Mitigation strategies include:

• Environmental Control: Implementing humidity and temperature controls in manufacturing and storage environments to maintain optimal conditions.
• Protective Coatings: Applying conformal coatings such as acrylics, polyurethanes, or parylene, which provide a barrier against moisture and contaminants. Using corrosion-resistant surface finishes like Immersion Silver (ImAg), Electroless Nickel Immersion Gold (ENIG), or Organic Solderability Preservatives (OSP) to protect exposed copper.
• Regular Maintenance: Conducting periodic inspections and cleaning using isopropyl alcohol or specialized PCB cleaners to remove any corrosive residues that may have accumulated over time.

Galvanic Corrosion

Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte, forming a galvanic cell. This causes the more anodic metal (less noble) to corrode at an accelerated rate, while the cathodic metal (more noble) remains protected. For instance, when gold-plated contacts are in proximity to tin-lead solder joints, the tin-lead alloy corrodes faster due to its anodic nature compared to gold. Key strategies to prevent galvanic corrosion include:

• Material Selection: Choosing metals with similar galvanic potentials to minimize potential differences. For example, using nickel instead of tin in conjunction with gold.
• Barrier Layers: Applying dielectric coatings or using insulating materials like polyimide films to prevent direct metal-to-metal contact.
• Design Optimization: Ensuring proper drainage and avoiding crevices where electrolytes can accumulate, thereby reducing the formation of galvanic cells. Utilizing design techniques such as via tenting to cover exposed copper.

Electrolytic Corrosion

Electrolytic corrosion, also known as dendritic growth, involves the formation of conductive metal filaments (dendrites) between adjacent PCB traces due to the presence of ionic contaminants and moisture. This process can be driven by bias voltages applied across the traces, causing metal ions (e.g., silver, copper) to migrate and form bridges that lead to short circuits. Preventive measures include:

• Clean Manufacturing Processes: Ensuring strict cleanliness standards during PCB fabrication to eliminate ionic contaminants. This includes using deionized water for rinsing and maintaining a cleanroom environment.
• Enhanced Cleaning: Implementing thorough post-assembly cleaning processes using ultrasonic cleaners and specialized flux removers to eliminate any residual flux and other contaminants.
• Conformal Coating: Applying conformal coatings like silicone, epoxy, or urethane to create a protective barrier against moisture ingress and ionic contamination. Using thicker coatings or multiple layers in high-density interconnect (HDI) PCBs.

Fretting Corrosion

Fretting corrosion occurs due to repeated micro-motion between contacting surfaces, such as solder-plated switches or connector interfaces. This motion wears away protective oxide layers, exposing the underlying metal to oxidative damage. Over time, this can lead to the formation of non-conductive oxides, increased contact resistance, and eventual failure of the mechanical switches and connectors. To mitigate fretting corrosion:

• Use of Lubricants: Applying anti-corrosive lubricants, such as conductive greases or oils, to reduce friction and wear. These lubricants also provide a protective layer against oxidation.
• Material Hardening: Using harder materials or surface treatments, such as electroplating with harder metals like gold or palladium, to increase resistance to wear and reduce fretting.
• Design Modifications: Minimizing relative motion in the design by using secure mechanical fastenings and ensuring robust mating cycles. Implementing compliant pin designs to absorb vibrations and reduce micro-motion.

Cleaning Corroded PCBs

If a PCB becomes corroded, cleaning it can restore its functionality. This process requires basic household items and a fundamental understanding of electronics to avoid damaging the board.

Required Materials

• Baking soda: Effective for neutralizing corrosion due to its alkalinity and abrasiveness.
• Deionized water: Prevents further damage due to its lack of ions.
• Household cleaner: A phosphate-free cleaner is ideal for removing dirt and corrosion.
• Soft-bristle brush: A toothbrush or paintbrush works well for scrubbing delicate components.
• Lint-free towel: Microfiber or other lint-free fabrics dry the board without leaving particles.
• Oven: Used to dry the PCB with indirect heat.

Cleaning Steps

1. Record the PCB’s Layout: Take detailed notes or pictures for proper reassembly.
2. Disassemble the PCB: Separate the board from cables and chips for thorough cleaning.
3. Apply Baking Soda Paste: Mix baking soda with deionized water to form a paste. Scrub the corroded areas with the brush and allow the board to dry before rinsing.
4. Use Household Cleaner: Spray the corroded parts with cleaner, scrub again, and pat dry.
5. Dry in the Oven: Preheat the oven to 170°F, turn it off, and place the PCB inside to dry.
6. Reassemble and Test: Reassemble the board and test its functionality. Use an eraser to remove any remaining corrosion if necessary.

Preventing PCB Corrosion

Preventing corrosion is key to extending the lifespan of PCBs. Although it is impossible to avoid corrosion entirely, certain measures can delay it significantly.

Key Preventive Measures

• Avoid Ionic Contamination: Keep PCBs and their components clean and dry to prevent exposure to ionic moisture.
• Manufacturing Practices: Fabricators should avoid fingerprints or liquids on PCBs and manufacture them in controlled environments.
• Design Considerations: Prevent moisture buildup on exposed conductors, especially in environments prone to liquid exposure.

Additional Protective Measures

• Conformal Coating: Applying a protective coating shields PCBs from environmental contaminants.
• Encapsulation: Encasing PCBs in protective materials provides extra defense against moisture and corrosive agents.
• Environmental Controls: Maintain controlled environments during PCB manufacturing and storage to minimize corrosion risks.

Conclusion

Understanding and mitigating PCB corrosion is essential for ensuring the longevity and reliability of electronic devices. Implementing advanced preventative measures such as environmental controls, protective coatings, material selection, and maintaining stringent cleanliness standards during manufacturing can significantly reduce the risk of corrosion-related failures. Regular maintenance, periodic inspections, and adopting best design practices further ensure the sustained performance of PCBs under various operating conditions. By addressing these challenges with specialized knowledge and techniques, the integrity and functionality of electronic devices can be preserved over extended lifetimes.