Of the five most commonly used conformal coatings, four – acrylic (AR), epoxy (ER), silicone (SR) and urethane (UR) – are classified as wet materials, meaning they are applied to substrates by three basic types of liquid-based technology:
- Brush application is recommended for smaller-batch acrylic/urethane coating projects; manual brush application is slow and subject to operator error. Extreme masking is generally necessary for epoxy and silicone brush application, and focuses on touchup of imperfect film surfaces.
- Dip methods immerse components in a bath of liquid coating material, either manually or with automated equipment suitable for larger scale production; for instance, large product-batches of epoxy respond very well to machine dipping, but all liquid coatings can efficiently employ dip-immersion processes.
- Compared to brush/dip methods, very cost-effective automated spray procedures generate superior coating surface-quality for high-volume coating assignments; all liquid materials are adaptable for spray application. Typically, coating material is diluted with solvents to achieve a predetermined viscosity; manual or aerosol spraying requires application from all four quadrants at a 45-degree angle.
A fifth conformal coating material, parylene (XY), is not applied as a wet substance. Rather, a unique chemical vapor deposition (CVD) method transforms solid, powdered parylene dimer to a gas which permeates substrate surfaces, providing an under-, as well as an over-layer, of conformal protection. The process allows uniform conformal film application to virtually any surface topography and material, including ceramics, ferrite, glass, metal, paper, plastics, resin, and silicone. These capabilities far exceed those of liquid coatings.
Coating application methods substantially impact the thickness of film deposited on substrates.
Coating Materials and Film Thickness
Each conformal coating material displays a specialized collection of performance attributes governing use. A major problem for conformal films is applying the film at thicknesses unsuitable for the assembly and its operational purpose; this failure mechanism disrupts coating function and assembly performance. The coating-thickness required to provide optimal performance varies with film material used; each material has a specified coating-layer depth measured from the substrate surface that best supports assembly operation. Appropriate film-thickness for one coating may be either too wide or narrow for effective use with another, although there can be some overlap for product purposes.
More precisely, AR, ER and UR are effective within a film thickness range of 0.025 – 0.127 mm. (0.001 to 0.005 in.); the other liquid material – SR -- is most efficient at levels nearly twice as thick, between 0.051 – 0.203 mm. (0.002 to 0.008 in). CVD parylene coats are considerably thinner (half or more in depth), ranging between 0.013 – 0.051 mm. (0.0005 to 0.002 in.). These general standards of conformal coating thickness are supported by IPC-610 and MIL-I-46058C guidelines. The IPC created the J-STD-001 benchmark to regulate and standardize appropriate film thickness levels for conformal coatings, providing the most reliable measure for each film material.
Parylene’s thinner coating layers offer a clear advantage for use with microelectromechanical systems (MEMS) and nanotechnology (NT) devices/structures. Often integrating their functions onto a single micro/nano-chip, MEMS/NT operating components are in the micrometer (0.001 mm)/nanometer (a billionth [10-9] of a meter) functional range. Monolayer, wet coating materials are simply unable to accommodate MEMS/nano coating requirements; each operates in a range significantly thicker and more physically dense than is acceptable for MEMS/NT. Parylene films can be successful at 0.1-micron operating thicknesses, enhancing their use for MEMS/NT components.
Parylene’s ability to provide effective, pinhole-free conformal protection with micro-level coating layers unavailable to liquid materials generates a significantly wider range of applications into the foreseeable future.
To discover more about parylene's thickness, download our whitepaper now: