Moisture Protection of Parylene

Conformal coatings play a vital role in safeguarding the performance of advanced electronics, like printed circuit boards (PCBs), ensuring their functionality under demanding conditions. A key focus of these coatings is shielding PCBs from moisture damage, which can stem from various sources. Exposure to acid rain, harsh solvents, air pollutants, chemicals, fog, high humidity, intermittent submersion, continuous rain, snow, saltwater/mist and wet sprays can all pose challenges, hindering proper assembly operations.

Within a PCB, excess moisture disrupts thermo-mechanical properties, causing development of:

• Component Corrosion
• Decreased interfacial adhesion strength,
• Diminished glass transition temperature (Tg)
• Electrical short-circuiting
• Metal migration

To effectively mitigate performance issues arising from moisture, conformal coating can be applied. This coating serves as a protective barrier between the device’s components and its operating environment.

Vacuum-deposited Parylene (poly-p-xylylene/XY) coatings offer many protective benefits for electronic devices. XY deposits uniformly and pinhole-free, hydrophobically, with a low dielectric constant and excellent biocompatibility. Parylene permeation barriers assure the water vapor transmission rate (WVTR) affecting an assembly is minimal, prohibiting moisture from passing through its protective film to underlying components. As a measure, WTVR provides a numerical value for the level of water vapor migration through thin film barriers, calculated in terms of time required for moisture permeation and total inner assembly area made wet.

Lower numerical values represent better WTVR. Compared to liquid coatings – acrylic, epoxy, silicone and urethane – Parylene WTVR values are the lowest, representing superior moisture barrier provision. XY adheres to the widest range of substrate substances/surface geometries, creating moisture barriers throughout the assembly. Of all XY types, Parylene C has the best water vapor barrier properties. The WVTR XY compared to 3 liquid coatings is provided in Table 1.

All Parylene types out perform liquid coatings as moisture barriers, as measured by WVTR; types C and D offer lowest WVTR-levels.

Numerous studies have verified XY’s ability to provide ongoing protection from moisture incursion.

• In one, ~ 8 microns (μm) of Parylene C film was sufficient to preserve the magnetic properties of a V[TCNE]_x (TCNE = tetracyanoethylene) room-temperature, organic-based magnet, devices which has a tendency to rapidly degrade when exposed to moisture or oxygen. Typically beginning within minutes of exposure, Parylene C coating maintained magnetic capacity for more than 7 hours, which grew to 14 hours when a subsequent overcoating of 2-μm Parylene with a 0.1-μm layer of gold was added..
• A second study investigated the corrosion/moisture protection of Parylene C for cold rolled steel (CRS). XY has previously exhibited poor CRS-adhesion, because the steel’s own surface characteristics were too dense to permit Parylene penetration during chemical vapor deposition (CVD) processing. Addition of a plasma polymer interlayer generated sufficient interfacial bonding to the CRS substrate, significantly enhancing Parylene adhesion. Type C then provided CRS an excellent moisture barrier, as well as additional bulk properties, including electrical insulation and film layer toughness.
• A third report provided additional verification of Type C’s superior moisture barrier performance, following immersion in saline solution of sodium chloride and water, compared to liquid coating materials. At a thickness of 25 μm, Parylene C lasted in excess of 30 days before coating breakdown commenced. Epoxy and urethane coatings 4 times thicker (100+ μm) began to breakdown in just over 6 hours; silicone at the same thickness lasted 58 hours (2 days, 10 hours). Thus, a third demonstration of XY’s superior reliability providing substrates water barrier protection.

Moisture in a PCB can impact its thermo-mechanical properties and operational efficiency. XY coatings excel in nanometer precision, resisting chemicals, corrosion, moisture and solvents. They exhibit minimal thermal expansion, ensuring the PCB’s functionality and performance across various conditions. Per IPC-CC-830B, Parylene meets the Class B specification as a hydrolytically-stable conformal coating, ideal for environments demanding high moisture insulation resistance and humidity-aging tests. Parylene’s reliable barriers prevent moisture intrusion, reducing WVTR. XY’s exceptional moisture management extends to bio-implantable medical devices, where Parylene films outperform liquid coatings in withstanding internal body moisture.