Parylene Barrier Properties

Permeation barriers for electronic devices are essential to assure their ongoing performance through a wide range of operational environments. Polymer flexible conformal coatings provide good barrier protection, shielding device substrates from unwanted incursion by solid contaminants, chemicals, gaseous permeation and liquid water or vaporous forms of moisture. Permeability reduction improves with enhanced coating adhesion, minimizing the surface’s;

• Threshold displacement energy, to the extent no element of the conformal film can be permanently dislocated from the coating barrier
• Water vapor transmission rate (WVTR), the passage of water vapor through barrier film, measured in terms of area and time

Disrupting the thermo-mechanical properties of printed circuit boards (PCBs) and related electrical assemblies, excess moisture caused by uncontrolled WVTR may lead to component corrosion, differential swelling/hygroscopic stress, electrical shorting and compromised surface adhesion.

Conformal protection mitigates performance issues that can result from moisture, the success of which depends upon the specific coating material used.  Parylene (XY) provides reliable surface adhesion and barrier protection, sufficient to withstand permeation by most substances under a wide range of operating circumstances.

Parylene vapor deposition polymerization (VDP) generates an ultra-thin conformal coating with excellent coverage, suitable for most uses. XY typically provides effective barrier protection at film thicknesses ranging from 0.2 µm to 50 µm but with coating thicknesses controllable to less than a single micron. The physics of the gas-phase coating of Parylene results in exceptional uniformity of pinhole-free film, preventing leakage and subsequent performance degradation.

The impact of VDP processes on the surface morphology and molecular structure of both deposited Parylene and the substrate influence the coating’s conformal barrier properties. Operating in a vacuum, the VDP process transforms solid Parylene dimer into a gas, which cracks into individual monomer molecules and penetrates within substrate surfaces as well as forming the exterior encapsulating film. The process effectively generates an internal, as well as external protective layer, maximizing XY’s barrier protection. VDP creates a conformal film unmatched by liquid processes of such coating materials as acrylic, epoxy, silicone and urethane.

Relevant VDP process parameters include the time duration of the deposition process. its growth rate, and both pyrolysis and sublimation temperatures. Appropriate control of these factors develops measured growth of barrier layer thickness across a span of tens of nanometers to the 20+ μm range, generating reliable, longer-term film barrier stability. The coating’s electrical properties are also significantly improved by the VDP process; this is particularly the case for pyrolysis temperature modification. Optimizing VDP process control improves film reproducibly and compatibility for a wider range of application domains.

Other examples of XY barrier superiority include:

• Parylene coatings are commonly used as a strengthening and protective conformal layer for PCBs, solar cells, and light‐emitting diodes (LEDs). Other applications include aerospace/defense, automotive, biomedical, commercial/consumer and industrial uses, both standard and ruggedized.
• For solar cell applications, encapsulated Parylene interlayers efficiently diminish oxygen and water vapor substrate permeation. Electric calcium testing shows WVTR to be approximately 2.5 × 10⁻⁷ (g · m⁻²) d⁻¹ after 75 days. In one test, the WVTR value for Parylene treated solar cells remained about 2.1 × 10⁻⁶ (g · m⁻²) d⁻¹, through continual use; the conformal film was flexed 5,000 times and remained appropriately functional, demonstrating XY’s exceptional value as a multilayer barrier structure for flexible solar cell and organic LED applications.
• Regarding VDP generation of Parylene membranes for gas-separation, films with a thickness less than 200 nm can provide high-level protection. Parylene conformal membranes offer gas permeability of up to hundreds of barrer, a commonly used non-SI standard measure of gaseous permeability for coating/membrane technologies. These gas separation properties can be dependably reproduced with Parylene VDP, in addition to reliable, highly transparent (∼80% in the visible region) conformal film. Low WVTR can be produced with similarly low gas permeability and excellent working mechanical flexibility.

Parylene coatings are uniform at controllable, pinhole-free thickness. XY remains adherent and intact, preserving dielectric/insulation properties, at thicknesses greater than 0.5µm., completely penetrating the narrowest of spaces. Unlike liquid coatings, Parylene’s VDP penetration coats all aspects of an assembly, regardless of surface topography, engendering a truly conformal barrier coating, both above and below component surface.

Parylene resists chemicals, corrosives, moisture and solvents, with minimal thermal expansion, covering virtually any board topography, while ensuring PCB/assembly function/performance through most operating conditions. In addition, chemically inert Parylene has outstanding chemical resistance — sustaining PCB function in harsh environments, including atmospheric pollutants and aggressive solutions.