Conformal Coatings Thickness: Comparing Parylene with Liquid Coatings

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° 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 type of conformal coating material boasts a unique set of performance characteristics that dictate its application. A significant challenge faced by conformal films is the improper application of the film at thicknesses that do not align with the assembly’s intended use, leading to a breakdown in coating functionality and assembly performance. The ideal coating thickness necessary for peak performance varies depending on the film material employed, with each material specifying a particular depth of coating layer from the substrate surface that best supports assembly functionality. While one coating may require a specific film thickness, it might not be suitable for another coating, although there could be some crossover for certain product applications.

For optimal performance, AR, ER and UR work best within a film thickness range of 0.025 to 0.127 mm (0.001 to 0.005 in). SR is most effective at thicknesses almost double this, between 0.051 to 0.203 mm (0.002 to 0.008 in). Parylene coatings, notably thinner, fall within 0.013 to 0.051 mm (0.0005 to 0.002 in). These conformal coating thickness standards align with IPC-610 and MIL-I-46058C guidelines. The IPC’s J-STD-001 benchmark ensures consistent film thickness levels for conformal coatings, offering a reliable measure across different materials.

Parylene’s thin coating layers provide a distinct advantage for microelectromechanical systems (MEMS) and nanotechnology devices. MEMS/NT components, often consolidated onto a single micro/nano-chip, operate within micrometer (0.001 mm)/nanometer (a billionth [10^-9] of a meter) functional ranges. Traditional monolayer, wet coating materials fall short in meeting MEMS/nano coating needs due to their thickness and density, which exceed acceptable levels for MEMS/nanotechnology. Parylene films excel with operating thicknesses as low as 0.1 micron, making them ideal for MEMS/nanotechnology applications.

Parylene’s remarkable capability to offer efficient, pinhole-free conformal protection through micro-level coating layers, not achievable with liquid materials, expands the scope of applications significantly into the foreseeable future.