Parylene Barrier Properties
Posted by Sean Horn
Friday, June 1, 2018 7:30
@ 7:30 AM
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, protecting 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, and also its
- 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 leads to:
- component corrosion,
- differential swelling/hygroscopic stress,
- diminished glass transition temperature (Tg),
- electrical shorting, and
- lower surface adhesion.
Conformal protection controls moisture-originated performance issues; success-level depends on the film-material used. Parylene (XY) generates reliable surface adhesion and barrier protection, sufficient to withstand permeation by most substances under a wide range of operating circumstances.
Parylene chemical vapor deposition (CVD) generates the thinnest available conformal coatings for most uses, with excellent substrate coverage. XY typically provides effective barrier protection at film thicknesses ranging from 0.013 – 0.051 mm. (0.0005 to 0.002 in.), but coatings are controllable to less than a single micron (1 μm/1,000 nms). The exceptional uniformity of pinhole-free XY prevents leakage; vaporous coatings seep deep into substrate surfaces during application, providing additional moisture protection.
The impact of CVD 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, CVD transforms solid parylene dimer into a gas, which 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. CVD creates a conformal film unmatched by liquid processes of such coating materials as acrylic, epoxy, silicone and urethane.
Relevant CVD 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 CVD-processing; this is particularly the case for pyrolysis temperature modification. Optimizing CVD process control improves film reproducibly and compatibility for a wider range of application domains.
Other examples of XY barrier superiority include:
- Parylene organic buffers are commonly used as a smoothing, strengthening and defect-decoupling 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, thermal-treated 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 CVD synthesis of parylene membranes for gas-separation, films with thickness less than 200 nm. can provide high-level protection. Parylene conformal membranes offer gas permeability of up to hundreds of barrer, a standard measure of gaseous permeability for coating/membrane technologies. These gas-separation properties can be dependably reproduced with parylene CVD, 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 spaces narrow as 0.01mm. Unlike liquid coatings, parylene’s CVD 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, characterized by atmospheric pollutants and aggressive solutions – better than liquid coatings. Heat resistance is also very dependable; XY’s dielectric properties are high, well protected by its ongoing barrier strength.
To learn more about the properties of parylene, download our whitepaper now: