A natural process, corrosion enacts chemical/electrochemical reactions that degrade and gradually destroy materials or components within a functional environment. The outcome can be dangerous and costly to repair.
For printed circuit boards (PCBs) and similar assemblies, corroded electrical contacts can cause potentially life-threatening mechanical malfunction of aerospace/automotive/industrial systems during operation; corroded medical implants may disrupt pacemaker function or lead to blood poisoning. PCBs can suffer electrolytic corrosion, when:
● Electrical contacts within the assembly are subject to water or other moisture trapped between them.
● Applied electrical voltage causes development of unintended electrolytic cells, which can decompose chemical compounds, initiating corrosive reactions.
● However, contaminants such as chemical residue, dirt, oil and salt trapped between the substrate surface and the conformal film can also generate corrosion.
● Although corrosion generally starts from underneath a conformal coating, because of liquid/residue on the substrate surface, rapid changes in temperature can also crack or rupture the coating’s external layer, instigating corrosion response.
● Similarly, 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.
Considering the smaller size of PCBs, some corrosion mechanisms are less visible and therefore difficult to predict, but pits and cracks can develop, leading to wider spread physical disruption of assembly surfaces and interiors. Conformally coating PCBs with the non-critical, non-toxic layer material parylene (XY/poly-para-xylylene), can prevent corrosion in most cases.
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 measures. It has a broad temperature range, can withstand most normal types of abrasion and is chemically inert, making corrosion unlikely. However, one should not think parylene is foolproof. Contamination generated by dirty surfaces can stimulate coating delamination and severe degradation of affected operating systems, as the parylene coating begins to disengage from the surface. 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.
Highly corrosion resistant, parylene coatings nevertheless adhere poorly to metals, a potential problem for use with PCBs; for instance, because of its conductive properties, many PCB manufacturers equip their assemblies with gold components. 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.
However, 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 methods have limited parylene delamination for medical implantables. In a study of XY corrosion protection of coated aluminum sheeting, pre-corrosive film delamination was rectified by deposition of an ultra-thin layer of plasma polymer (50 nm) on the substrate.
● Two-layer (organic silane 174 + parylene) coating of 2 μm successfully protects implant grade stainless steel surfaces against corrosion in body fluid.
● Pre-treatment of medical implants with silane A174 prior to parylene coating supports the film’s reliable corrosion protection, while improving XY’s biocompatibility and ultra-thin/continuous/inert film formation. Unlikely to trigger immune response, clear XY layers are highly resistant to corrosive conditions of bodily fluids, protecting against development/channeling of contaminants.
● Interface engineering (IE) improves parylene C’s corrosion protection of cold-rolled steel (CRS). Adhesion between XYC films and most smooth or nonporous substrates is minima; directly applied parylene C offers little corrosion-support to CRS surfaces. IE processes situate a layer of plasma polymer between XYC/CRS, stimulating interfacial bonding between the two materials, enhancing corrosion protection.
For reliable corrosion protection, pre-CVD treatments begin with cleanliness testing to check for contaminants, followed by thorough cleaning if they are detected. Connectors, electrical components and other keep-out areas need to be masked. Non-porous materials like glass, metal, paper and plastic generally require a pre-CVD application of A-174 silane adhesion promoter to minimize delamination and assure corrosion cannot begin.
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