Corrosion Protection with Parylene

Corrosion is a natural occurrence that involves chemical and electrochemical reactions that slowly deteriorate materials or components in their operating environment. The resulting damage can pose risks and incur significant repair costs.

Corroded electrical contacts in printed circuit boards (PCBs) and similar assemblies pose a serious risk, potentially causing life-threatening mechanical failures in aerospace, automotive, and industrial systems during operation. In medical implants, corrosion can disrupt pacemaker function or result in blood poisoning. PCBs are prone to electrolytic corrosion under the following conditions:

• The electrical contacts in the assembly may be affected by water or trapped moisture between them.
• The application of electrical voltage leads to the formation of unintended electrolytic cells. These cells can break down chemical compounds, triggering corrosive reactions.
• Contaminants like chemical residue, dirt, oil, and salt that are caught beneath the conformal film on the substrate’s surface can lead to corrosion.
• While corrosion typically initiates beneath a conformal coating due to liquid or residue on the substrate, abrupt temperature shifts can also cause cracking or rupturing of the external layer of the coating, triggering a corrosion reaction.
• Metal components within a PCB can produce oxides or salts within the operating environment, leading to corrosive dysfunction.
• Other assembly materials, like ceramics or plastics, can also suffer corrosion and subsequent degradation of their useful properties, performance expectations and structural integrity.

Due to the compact size of PCBs, certain corrosion mechanisms may be less apparent and challenging to anticipate. The formation of pits and cracks could occur, resulting in extensive physical damage to assembly surfaces and interiors. By applying a conformal coating of Parylene (XY/poly-para-xylylene), a non-critical and non-toxic material, corrosion can typically be prevented.

Parylene provides substrates with ultra-thin, pinhole-free conformal protection characterized by excellent moisture barrier properties, as well as surface resilience/strength and insulation. Unlike wet coatings – acrylic, epoxy, silicone or urethane – which are applied by brushing/spraying the wet substance onto an assembly, or immersing it in a bath of liquid coating, Parylene uses a chemical vapor deposition (CVD) application method. With CVD, a powdered poly-para-xylylene dimer is subjected to intense heat, transforming it into a gas, which penetrates the targeted surface internally, while also forming an external layer that conforms precisely to virtually any assembly shape. XY synthesizes in-process, basically growing on the deposition surface one molecule at a time. It does not require curing after application, as liquid coatings do.

Parylene outperforms wet coatings in most instances. It has a broad temperature range, can withstand most normal types of abrasion and is chemically inert, making corrosion unlikely. Despite its benefits, Parylene is not foolproof. An unclean surface can hinder its adhesion and corrosion barrier performance. Contamination from dirty surfaces may lead to coating delamination and severe degradation of systems, causing the Parylene coating to detach. To assure reliable XY-adherence to substrates, contaminants of any kind – chemicals, dust, oils, organic compounds, process residue, wax – must be removed, negating development of mechanical stress between coating/substrate.

Although Parylene coatings offer high corrosion resistance, they exhibit poor adhesion to metals, posing a challenge for their application with PCBs. Due to the conductivity of gold components, numerous PCB manufacturers outfit their assemblies it. Advanced adhesion promotion technologies such as SCS AdProPlus® and AdProPoly® offer a distinctive market solution for metal adhesion. These technologies have been specially designed to address the challenges of promoting adhesion between difficult substrates, including titanium, stainless steel, gold, chromium, solder mask and many polymeric materials. With XY coatings for metallic medical implants, formation of OH-dot radicals on the implant’s metallic surface may result from the body’s inflammatory response. In these cases, degradation processes start at metal/polymer interface and progress towards the outer, Parylene surface.

Adhesion to metal surfaces and subsequent corrosion resistance can be vastly improved by addition of a silane layer (A-174 silane) at levels of 2 μm to the Parylene. A-174’s molecules form a unique chemical bond with the substrate surface, improving XY’s mechanical adhesion. Silane application is achieved by immersion, manual-spray or vapor-phase processing, forming a chemical bond with the surface. In addition to metal, materials benefiting from A-174 silane treatment prior to CVD implementation include elastomer, glass, paper and plastic.

Research has repeatedly substantiated the corrosion resistant powers of appropriately treated Parylene:

• Plasma surface treatment techniques have effectively reduced Parylene delamination in medical implants. A study on the corrosion protection of aluminum sheets revealed that pre-corrosive film detachment was resolved by applying a minute layer of plasma polymer (50 nm) onto the substrate.
• A dual-layer coating (composed of organic silane A-174 and Parylene) with a thickness of 2 μm effectively shields implant-grade stainless steel surfaces from corrosion in bodily fluids.
• Treating medical implants with silane A-174 before applying Parylene coating enhances the film’s corrosion protection, boosts XY’s biocompatibility and ensures the formation of thin, continuous and inert films. These clear XY layers are resistant to bodily fluid corrosion, reducing the risk of immune responses and contamination.
• Interface engineering (IE) enhances the corrosion protection of cold-rolled steel (CRS) by Parylene C. While the adhesion between Parylene C films and smooth or nonporous substrates is minimal, directly applying Parylene C provides limited corrosion support to CRS surfaces. By utilizing IE processes that introduce a plasma polymer layer between Parylene C and CRS, it promotes interfacial bonding between the materials, thus improving corrosion resistance.

To ensure effective corrosion protection, the pre-chemical vapor deposition (CVD) process begins with a cleanliness assessment to identify any impurities. If contaminants are present, a thorough cleaning is conducted. It is crucial to mask connectors, electrical parts, and other restricted zones. Prior to CVD, impermeable materials such as glass, metal, paper and plastic typically necessitate the application of A-174 silane, AdProPoly, or AdProPlus adhesion promoters to reduce delamination risks and prevent corrosion initiation.