Recognition of parylene's excellence as a conformal coating for many product uses has grown along with its application. However, issues of barrier failure, current leakage, poor processing, and cost limit its further development and use.
Challenges to Wider Adaption of Parylene Conformal Coatings
Parylene's deserved reputation as a superior conformal coating for innumerable components and products is not without exception. Problems do emerge for parylene, interfering with wider product adaption. Among the most prominent challenges for parylene are:
- Improving moisture barrier performance: Parylene moisture barriers are susceptible to failure after prolonged exposure at higher temperatures. Coatings with lower than 5 micron thickness frequently demonstrate poor response when situated in operating environments characterized by persistent heat, within high electric field. Under such conditions, resistance to corrosion can decline due to contaminants or particles trapped inside the coating-film. This failure has been attributed to inadequate sustained moisture diffusion, when tested under accelerated soak conditions. Evidence suggests updated assessment parameters and procedures should be devised for parylene conformal coatings applied to specified aeronautic and biomedical functions, to assure appropriately functioning moisture barriers are retained (http://ecst.ecsdl.org/content/11/18/1.full.pdf).
- Minimizing leakage of current during operation: Although parylene coatings are well known for complete substrate encapsulation and superior transference of electrical impulses, leakage of current during operation can be an issue. Where it exists, current leakage tends to increase during a product's life, culminating at times with component dysfunction after as few as 120 days. The problems can become more severe if surface topography is complex, or forces of micromotion during operation remain consistent through the component's operation (http://scitation.aip.org/content/aip/journal/apl/101/9/10.1063/1.4748322). Challenge conditions 1 and 2 coincide with another problem limiting persistently superior parylene performance, cited directly below.
- Bubbles and delamination within coatings: Although parylene's reputation rests largely upon its pinhole-free, completely conformal covering, results are not entirely perfect. These generally uncommon imperfections are a primary source of parylene failure; lack of homogeneity, resulting from heterogeneous composition and structure among parylene coating layers, can cause this problem, limiting the conformal effectiveness not only between parylene layers, but also between parylene and the substrate. Inappropriate processing in the vacuum chamber can interfere with continuous polymerization of parylene molecules during deposition, inhibiting homogeneous interface between parylene, other bilayer materials or substrate surfaces. During deposition, difficult-to-repair breakdown holes can be induced, that can ultimately leave substrates exposed and unprotected. Inspection by scanning-electron microscopy (SEM) or energy-dispersive X-ray spectroscopy (EDS) can help discern breakdown regions in the conformal film, to focus repair efforts (http://ecst.ecsdl.org/content/11/18/1.full.pdf).
- Loss of coating resiliency under constant thermal treatment: Parylene coatings exhibit dependable consistency for many applications where exposure to ongoing thermal pressure is the rule. However, cases have emerged where parylene films covering component substrates have become brittle and stiff from persistent thermal exposure. These cases have particularly presented themselves for biomedical applications implanted within the body where malfunction may be life threatening (http://onlinelibrary.wiley.com/doi/10.1002/jbm.b.31584/abstract;jsessionid=1D087A8668E783BE6800A43963BC9F23.f03t03).
- Difficulty reworking/repairing problem areas: Because it deposits as a vapor, parylene forms a truly conformal substrate coat. An advantage in most cases, this property can be a major challenge to coating-repair. Although acceptable mechanical, microabrasive and thermal methods of parylene removal exist, they are costly and time-consuming. Parylene's ability to permeate even the smallest crevices on the coated item generate typically strong adhesion. Since parylene dimer has to be vaporized to coat the item, simply peeling off the substrate coating can be problematic. Reliably strong and hard, parylene coatings are exceptionally difficult to chip away or otherwise mechanically compromise; all covered surfaces are typically resistant when reworking is necessary. Thermal processes often require temperatures as high 350°C (higher in a vacuum), which can also melt the underlying substrate (http://www.paryleneconformalcoating.com/parylene-removal-rework-and-repair).
- Higher costs for parylene can outweigh its conformal benefits: Although no conformal coating is as reliably protective for as many materials and products as parylene, its high cost limits its larger-scale application. Useful for MEMS and similar specialized technology, expenditures for vapor deposition and basic raw materials exceed those for other conformal coatings. Techniques for streamlining the parylene deposition process, including growing current batch-size capabilities, represent a major challenge to wider implementation of parylene encapsulation.
- Parylene's conformal, protective qualities vary significantly among substrate materials: Although parylene is exceptionally adaptable as a coating surface for an extensive range of materials, certain metals -- for instance, gold, silver, and stainless steel – adhesion to noble metals can represent a significant challenge. Ensuring desired levels of conformal protection for these substances with parylene can be both costly and time-intensive. (http://www.ncbi.nlm.nih.gov/pubmed/24771981).
Developing solutions to these challenges should improve parylene's utility for a wider range of products and purposes.