Parylene conformal coatings are highly reliable and are highly sought after in applications such as military sensors to medical implants. Because, parylene coatings are colorless (transparent), thin (micro-scale) and uniformly deposited all over the target surface they are hardly visible to the naked eye. However, there are methods to detect or test the quality of the coatings that are designated by standards (MIL-STD, ASTM). These standards test coatings for the encapsulation properties of Parylene conformal coatings depending on where they will be used. Leakage current and accelerated lifetime tests under different conditions (salty water, temperature, etc.)
Also, taking a closer look into the chemistry and parylene deposition process can explain their superior conformal coating properties.
- Parylene chemistry:
Polymers are formed of repeating and identical units (monomers). Thin films of polymers are therefore formed by these monomers building chemical bonds. The basic parylene molecule is known as the Poly-para-xylylene (Parylene N, see Fig. 1) monomer, which is also highly employed in many applications because it offers room temperature vacuum deposition and relatively cheap precursor material. Poly-para-xylylene was first observed by Szwarc (1947) as a product of vacuum pyrolysis (thermal decomposition) of para-xylene . Later, the process was improved by Gorham (1966), who used di-para-xylylene instead of para-xylene and the yield was improved to 100% . Today, Gorham method is being utilized for vacuum deposition of the Parylene conformal coatings. Gorhams method results in linear parylene thin films. In the case of Parylene-N the dimer is exposed to temperatures above 550 °C and pressures less than 1 Torr resulting in two monomers that are adsorbed onto the target surface at room temperature and spontaneously polymerize.
Figure 1 Polymerization route for poly-para-xylylene (Parylene N - C16H16)
- The Parylene Deposition Process:
The chemical vapor deposition (CVD) process of parylene takes place in three steps (sublimation, pyrolysis, deposition). The final coating is formed on the target surface (Elastomer, glass, metal, paper, plastic and, others). To improve the adhesion on the target surface extra care is taken to clean the surface. Subsequently, the samples surfaces are treated using an adhesion promoter such as a silane (A-174). Adhesion promoter forms a monolayer of molecules in a way to provide a high strength bond interface between the surface and the parylene.
- Sublimation: The coating process takes place once the parylene powder precursor is sublimated,
- Pyrolysis: Above a certain temperature pyrolysis takes place forming the monomers. Pyrolysis of the precursor is defined as the thermal decomposition of materials at elevated temperatures in an inert atmosphere and this reaction is irreversible.
- Deposition: Monomers are deposited as a thin layer in a way to allow for a top layer to grow on them. In the meanwhile, the monomers penetrate to the smallest voids resulting in a uniform, void-free conformal coating. Characterization Techniques and Military Standards:
Today, standardization of any kind of a product is done following set rules. Military Standards for example are used to standardize the products for use in military applications by department of defense and are commonly employed by other industries as well. Also, parts and products used in medical applications are standardized with stringent requirements. The design of the product and test limits to be achieved are established with use of such standards under replicated service conditions. Test methods for standard electronic and electrical component parts are determined by MIL-STD-202 where methods for testing of seal, mechanical, chemical and thermal properties are described . Parylene XY testing for use as an encapsulant for PCBs is also covered by MIL-I-46058 C . In the standards a sequence of tests are specified for the parts to be mechanically and thermally stressed prior to being subjected to a moisture resistance test as follows:
Mechanical impact and failure: Sensors may encounter friction, compression, tensile stresses and bending under service conditions. Protective layers, insulators, dielectric materials used in such systems must possess mechanical properties that can withstand such mechanical forces for the longevity of applications in the field.
Temperature: (i) Abrupt changes in temperature (thermal shock) (ii) high temperatures, and (iii) extreme cold can affect the functions of a protective layers used in electronic parts (eg. dielectric layers, seal).
Humidity /Moisture: in naval operations, medical implants exposure to salty water can damage the circuits of electronics due to corrosion and oxidation of parts.
Chemicals: Oxidative gases, acidic, basic liquids can penetrate through protective layers inevitably leading to the device failure.
Example case and alternative methods:
In medical applications, many times implants or devices that come into contact with bodily fluids are required to be sealed using a conformal biocompatible coating with 100% coverage. This coating is expected to electrically isolate and protect electronic parts. Parylene-C is a biocompatible conformal coating and in a study leakage current tests were used to investigate the encapsulation performance of Parylene-C films, and the results showed hermetic protection as well as long-term (>100 days) stability of the films . In the same study, parylene samples were subjected to accelerated lifetime testing (85 % relative humidity (RH) and 85 ˚C) for 20 days, optical microscopy did not reveal any changes in the structure. Similar methodologies can be used to showcase the conformal coating and excellent protective properties of parylene.
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 M. Szwarc, “Some remarks on the di-para-xylylene molecule,” p. 4.
 “A New, General Synthetic Method for the Preparation of Linear Poly‐p‐xylylenes - Gorham - 1966 - Journal of Polymer Science Part A-1: Polymer Chemistry - Wiley Online Library.”
 J. B. Fortin and T.-M. Lu, “A Model for the Chemical Vapor Deposition of Poly(para-xylylene) (Parylene) Thin Films,” Chem. Mater., vol. 14, no. 5, pp. 1945–1949, May 2002, doi: 10.1021/cm010454a.
 “MIL-STD-202 , Test Method Standard for Electronic and Electrical Component Parts.” [Online]. Available: https://www.document-center.com/standards/show/MIL-STD-202. [Accessed: 18-Dec-2019].
 “MIL-I-46058 C INSULATING COMPOUND ELECTRICAL (FOR COATING PRINTED CIRCUIT ASSEMBLIES).” [Online]. Available: http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-I/MIL-I-46058C_11366/. [Accessed: 18-Dec-2019].
 “Characterization of parylene C as an encapsulation material for implanted neural prostheses - Hassler - 2010 - Journal of Biomedical Materials Research Part B: Applied Biomaterials - Wiley Online Library.”