Specialty Coating Systems Inc. (SCS) explains that corrosion of electrical contacts within printed circuit boards (PCBs) and similar devices can create critical safety issues, potentially resulting in catastrophic failures during the operation of aerospace, automotive, or industrial equipment.
In medical applications, corrosion may interfere with pacemaker performance or introduce toxic substances into the bloodstream. PCBs are particularly susceptible to electrolytic corrosion when the following conditions are present:
- Moisture or water may become trapped between electrical contact points.
- When voltage is applied, unintended electrolytic cells may form, breaking down compounds and initiating corrosive processes.
- Residues such as salt, dirt, oil, or chemicals caught under a conformal coating on the substrate may provoke corrosion.
- Corrosion often begins under the conformal layer due to liquids or residues, but sudden temperature changes can cause cracks in the coating itself, further exposing the surface to corrosion.
- Metals used in PCBs can form oxides or salts during use, contributing to corrosion and performance issues.
- Non-metallic materials like plastics or ceramics can also experience chemical degradation, reducing mechanical integrity and functional performance.
Due to the small scale of PCBs, corrosion mechanisms can be difficult to detect or predict. Pitting and cracking can cause substantial physical damage to both exterior surfaces and internal areas. Applying a conformal layer of Parylene (XY/poly-para-xylylene)—a non-toxic and non-critical material—can generally prevent corrosion.
Benefits of Parylene Coatings
Parylene delivers an ultra-thin, seamless, and pinhole-free protective coating with strong barrier properties against moisture, offering mechanical strength and electrical insulation. Unlike wet coating methods—such as acrylic, epoxy, silicone, or urethane—that are applied through spraying, brushing, or dipping, Parylene is deposited using a chemical vapor deposition (CVD) process.
In CVD, a solid dimer of poly-para-xylylene is heated to a high temperature, converting it into a vapor that penetrates the component surfaces and forms a uniform coating, both inside and out. The Parylene film develops directly on the substrate one molecule at a time. No curing step is necessary, which distinguishes it from traditional liquid coatings.
In most use cases, Parylene outperforms wet coatings. It has excellent thermal stability, resists abrasion, and is chemically inert, making it highly resistant to corrosion.
However, Parylene is not without limitations. If the substrate is not properly cleaned, adhesion may suffer, reducing the coating’s protective capabilities. Contamination from residues such as oils, dust, process byproducts, or organic material can cause the coating to peel away and expose the surface. To ensure strong adhesion of XY to the substrate, all contaminants must be removed to prevent mechanical stress between the surface and coating.
Although Parylene coatings offer superior corrosion resistance, they often adhere poorly to metal surfaces, which presents challenges for PCB applications. Many PCBs include gold components due to their conductivity. Technologies like SCS AdProPlus® and AdProPoly® are designed to enhance adhesion to difficult surfaces such as gold, titanium, stainless steel, chromium, solder masks, and various polymers. In medical implants, reactions to the body’s immune system can generate OH-dot radicals at the metal interface, leading to degradation that begins at the junction of the metal and Parylene layer.
Adding a silane layer, such as A-174 silane, at a thickness of 2 μm can significantly improve both adhesion and corrosion resistance. A-174 chemically bonds with the surface, strengthening the mechanical attachment of the XY layer. Silane can be applied by immersion, vapor-phase, or spray methods, creating a molecular bond with the surface. This treatment is effective not only for metal surfaces but also for elastomers, plastics, paper, and glass before CVD coating is applied.
Numerous studies have supported the effectiveness of treated Parylene in resisting corrosion:
- Plasma surface treatments have been shown to reduce delamination of Parylene in medical implants. In one study, aluminum sheets coated with a 50 nm plasma polymer layer experienced improved corrosion resistance and coating stability.
- A two-layer coating system using both A-174 silane and Parylene, with a total thickness of 2 μm, provides strong protection for medical-grade stainless steel in bodily fluids.
- Medical implants that undergo A-174 silane treatment before Parylene application benefit from improved film continuity and inertness, offering resistance to body fluid-induced corrosion and lowering the risk of immune system reactions.
Interface engineering (IE) further enhances the performance of Parylene C on cold-rolled steel (CRS). On smooth or nonporous surfaces, Parylene C tends to have weak adhesion and limited corrosion protection. However, when an intermediate plasma polymer layer is introduced through IE, it promotes a stronger bond at the interface, improving the overall corrosion resistance.
For optimal results, corrosion protection begins with pre-CVD surface inspection to detect contaminants. If impurities are present, they must be removed. Components such as connectors and electrical elements should be masked during coating.
Surfaces like metal, glass, paper, and plastic typically require pretreatment with adhesion promoters such as A-174 silane, AdProPoly, or AdProPlus to avoid delamination and reduce the likelihood of corrosion onset.
