Parylene Adhesion & Cleaning
Posted by Sean Horn
Friday, May 8, 2015 8:12
@ 8:12 AM
Parylene Surface Protection
Parylene conformal coatings provide effective chemical, electrical, moisture, and vapor protection for complex electronic circuit assemblies made to maintain performance throughout demanding operational circumstances. Parylene’s effective adherence to the substrate’s surface is imperative if its many benefits are to be realized. Mechanical methods of adhesion promotion must be employed to ensure parylene appropriately adheres to substrate surfaces.
While the objective is achieving the highest levels of uninterrupted performance under the harshest operational conditions, virtually all of parylene’s positive qualities are diminished by inadequate adhesion. Dominating the prospect of appropriate adhesion is surface cleanliness. While short-term effects can be minor, this is especially true for longer-term performance; contamination generated by dirty surfaces can lead to severe degradation of affected operating systems, as the parylene coating begins to disengage from the surface.
Unique Application Process Requires Superior Surface Cleaning
Most conformal coatings are applied by either dipping the substrate into a liquid bath of the coating material, or spraying the material directly onto the substrate surface. ParlyIene’s application process is more complex, requiring a vapor-phase, chemical-vacuum deposition process. There is no intermediate liquid phase. The powdered parylene dimer is first converted to gaseous form at the molecular level, through cross-link polymerization, and then applied directly to targeted surfaces as a transparent, polymerized film.
Dirty surfaces prohibit parylene from penetrating the substrate’s smallest surface pores. Thus, it is unable to bond with both the material being covered and itself after deposition. These conditions reduce its capacity to generate superior conformal coating.
Thorough cleansing of the surface beforehand is necessary to ensure the absolute conformal adhesion that distinguishes parylene from other substrate coatings. If trace organic elements — chemicals, dust, oil, waxes, or other residue left during manufacture, handling or transportation — remain on the surface, the bond between the underlying surface and the applied parylene can be subjected to mechanical stress or thermal cycling. Inorganic compounds similarly disrupt the prospect of surface adhesion.
Inspection to Detect and Identify Potential Contaminants
These conditions can eventually nullify the coating’s beneficial attributes, as delamination occurs, eventually exposing the surface. Electrically conductive, non-organic deposits beneath the parylene can leak current during usage of electronic components, distressing their functionality. Costly cleaning issues can emerge if thorough inspection of surfaces is overlooked at any stage during the production/coating process. Inappropriate inspection for contaminants may compel reworking covered surfaces and similar production delays. Recommended methods of inspecting surfaces include:
- Fourier Transform Infrared Spectroscopy (FTIR): Enacted by comparing the results of spectrum analysis to values for known substances, FTIR detects and identifies specific organic contaminants, such as mold-release agents and silicon oils for appropriate cleansing.
- Gas Chromatography (GC): Also useful in the detection/identification of organic contaminants, GC can isolate separate components of unknown complex organic chemical mixtures into their individual elements, detailing their character. GC is sometimes used in conjunction with mass spectroscopy.
- Ionic Exchange Chromatography (IOC): Separating ions and polar molecules based on electrical charge, IOC detects/identifies such inorganic contaminants as chloride, fluoride, potassium and sodium.
The objective in all cases is determining the optimal solvent and cleaning system suitable for eradicating the contaminant.
After precise identification of surface contaminants, numerous cleaning agents can be effectively applied. Using nonhazardous cleaning materials is advised in all cases. Soluble contaminants generally respond to regular detergent cleaning. Multi-faceted, solvent-strength solutions are necessary to assure surfaces are free from less soluble substances. Safe, biodegradable solvents available for these purposes include deionized water, isopropyl, and methyl ethyl, among others developed in recent years. Depending on the contaminant substances and the requirements for acceptable levels of cleaning, such methods as solvent immersion, spray, tumbling, or vapor-degreasing can be applied; in some cases even cleaning by hand may be the best approach to assure surfaces are clear for parylene adhesion.
A174 Silane Adhesion Promoter
Production and material considerations often recommend the use of A174 silane adhesion promoter for optimal substrate surface-cleansing and subsequent parylene adhesion. Substrates responding well to treatment with A174 silane prior to implementation of parylene coating processes include elastomer, glass, metal, paper and plastic, among many other materials. Application of A174 silane after the masking-operation generates consistently reliable results, either through manual-spray, soaking, or vapor-phase processing. Silane’s chemical-bonding with the substrate surface promotes resolute parylene adhesion.
Despite its value as a promoter of enhanced parylene adhesion, A174 is not effective for all materials or uses. Researchers seek additional cleansing/adherence agents to improve parylene’s conformal utility for these purposes. For instance, parylene delamination problems attendant to medical devices have been reduced through application of plasma surface-treatment methods. Development of additional cleansing/adherence solutions and strategies will be a primary focus of the parylene community through the near future.