Poor parylene adhesion negates many of the coating’s most-valued functional properties, including dielectric strength, and resistance to the effects of chemicals, corrosive agents, and moisture. Surface treatments that amplify the interface adhesion between the deposited parylene and the coated substrate are therefore highly desirable. These treatments entail depositing parylene on a clean hydrophobic surface before its chemical vapor deposition (CVD) process is enacted.
Parylene is applied to substrates at ambient temperatures within a specialized vacuum, conducted at pressures of around 0.1 torr. To assure complete impingement of the parylene monomer, uniformly encapsulating the substrate, provision of appropriate surface support prior to CVD limits subsequent factors of peeling force, soaking undercut rate, and vertical attack bubble density (VABD). that can lead to lack of coating adhesion and delamination. A truly conformal coating, parylene provides superior, uniform barrier protection on almost any surface geometry or topography. However, any contaminants present on a substrate surface prior to CVD will inevitably have a negative impact on parylene adhesion. Chemicals, dust, oils, organic compounds, process residue, wax – contaminants of any kind – need to be thoroughly removed, leaving the substrate surface entirely devoid of their presence; if unattended, issues such as mechanical stress can develop. 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.
Cleanliness Inspection and Testing
Thorough surface inspection is the first step to delivering a substrate surface suitable to parylene adherence. Identifying contaminants significantly lowers the risk of incomplete surface cleansing, while informing selection of task-appropriate materials and methods.
Costly cleaning and rework issues can emerge if thorough surface-inspection is overlooked at any stage during the production/coating process. Poor inspection fails to detect and identify contaminants, leading to delamination, exposed surfaces and component dysfunction. In such cases, it is not uncommon for leakage of non-organic, electrically-conductive sediments beneath the parylene to interfere with and ultimately wreck the performance of electrical components.
Useful surface inspection techniques for organic contaminants include Gas Chromatography (GC) and Fourier Transform Infrared Spectroscopy (FTIR). Sometimes used in conjunction with mass spectroscopy, GC splits unidentified organic chemical mixtures into their distinct components, specifying their discrete properties. FTIR identifies specific organic contaminants by comparing evidence from spectrum analysis to those of known substances; contaminants such as silicon oils and mold-release agents are identified with FTIR. Valuable for determining the presence of inorganic contaminants like chloride, fluoride, potassium, or sodium, Ionic Exchange Chromatography (IOC) uses electrical-charges to separate the compounds’ ions and polar molecules.
Parylene Surface Cleaning Agents
A variety of nonhazardous cleaning agents can be effectively applied to substrates, according to their precise identification. Regular detergent cleaning is suggested for soluble contaminants. Less soluble contaminants require use of biodegradable, multi-faceted, solvent-strength solutions like deionized water, isopropyl, and methyl ethyl.
Cleansing methods are also dependent on the composition of both the identified contaminants and surface materials, to achieve satisfactory levels of substrate neutralization. Solvent immersion, surface-spraying, substrate-tumbling, or vapor-degreasing are primary disinfectant procedures. However, the substrate surface may also require manual, hand-cleaning, or application of batch, inline, or ultrasonic methods.
Integral to surface preparation, the masking process is implemented to assure designated components of a PCB or similar electrical assembly are protected from the effects of the parylene itself, which can interfere with expected functionality. Some of parylene's key properties can be both desirable and detrimental to an assembly, if applied to the wrong areas. For instance, parylene’s excellent dielectric properties simultaneously disable a PCB's contacts, rendering it inoperable, even as they safeguard the substrate surface from electrical interference.
Masking the contacts resolves this issue, coating only those PCB-parts that won’t be negatively impacted by conformal protection. In this way masking preserves an assembly’s operational integrity and performance. This critical pre-phase of the parylene coating process can be exceptionally labor-intensive. Considerable operator attention to the task is necessary to ensure effective masking of each connector, sealing it from penetration by gaseous parylene molecules during deposition. All tape, or other covering materials, must thoroughly shelter the keep-out regions, without gaps, crevices or other openings, to ensure connector function is retained after coating.
A-174 silane adhesion promoter chemically bonds with the substrate surface to stimulate resolute parylene adhesion. Manual-spray, soaking, or vapor-phase processing methods are used to apply A-174 to the substrate after the masking-operation, forming a chemical bond with the surface. Substrates responding well to treatment with A-174 silane prior to implementation of parylene coating processes include those made of elastomer, glass, metal, paper and plastic.
A-174’s molecules form a unique chemical bond with the substrate surface, sufficient to improve parylene’s mechanical adhesion. However, not all substrate materials benefit from A-174. In its place:
- Plasma-surface treatment methods have limited parylene delamination for medical implantables.
- Silicon substrates roughened with xenon difluoride gas demonstrate enhanced parylene adhesion.
Researchers continue to seek additional cleansing/adherence agents to improve parylene's conformal utility for these purposes.
The diversity of adhesion promotion methods requires a similarly diverse list of raw materials and techniques. Surface treatments prior to CVD begin with cleanliness-testing and cleaning to remove surface contaminants, followed by masking of connectors and electrical components. Materials such as glass, metal, paper and plastic benefit from application of A-174 silane adhesion promoter for necessary, pre-CVD surface modification. Establishing best-adhesion practices and strict adherence-standards is critical to maintaining quality conformal coatings and minimizing delamination.
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