Nanocoatings for Electronics

The significance of polymeric conformal coatings in safeguarding printed circuit boards (PCBs) from functional impediments like dust, corrosion, moisture, and temperature fluctuations is widely recognized. What may be less apparent is that as electrical components in PCBs shrink, conventional conformal films are proportionally less effective for certain coating purposes. With the emergence of microelectromechanical systems (MEMS) and nanotechnologies, nanocoatings are gaining prominence, often surpassing even micro-thin Parylene coatings in usefulness for MEMS/nano applications.

The development of low pressure plasma polymerized coating technology now supports precise deposit of nanocoatings on substrate materials. Appropriate plasma chemistry and technology can render materials permanently hydrophilic, super-hydrophobic and/or super-oleophobic, aiding conformal purposes. Nanocoating systems are increasingly implemented for mass production of electronic devices, including printed circuit boards (PCBs).

Evolving nanotechnology — the engineering of functional systems at the molecular scale — deploys individual atoms as working units, some complex as machines. Incredibly small, one nanometer (nm) equals one-billionth of a meter (10^-9 of a meter) so that one inch = 25,400,000 nanometers; more illustratively, a sheet of newspaper is 100,000 nm thick. Despite their small size, nano devices boast significantly lighter weight and heightened chemical reactivity and strength compared to larger structures. They also provide superior control over the light spectrum.

Traditional liquid materials – acrylic, epoxy, silicone and urethane – have many uses as conformal coatings for electrical assemblies, but due to their material properties, they must be applied in layers far too thick to do anything but encase (pot) nanotech electronics. Nanocoatings are functional at far finer film layers than competing liquid coatings. This is true even for non-liquid Parylene, which has previously offered the materially-finest coating layers available, with film thicknesses controllable to less than a single micron (1 μm). Nanocoatings match or surpass Parylene’s performance, offering conformal film layers so fine, they can be deposited virtually anywhere regardless of a component’s size.

Nanocoatings do resemble traditional conformal coatings, safeguarding PCBs through their ultra-hydrophobic properties, repelling liquid water and blocking moisture, thus preventing corrosive ions access to PCBs’ surfaces. Coating flexibility and nano thickness permits excellent, uniform coverage of complicated 3D-structures, with minimal impact on performance.

Successful development of nanowires and nanotubes for use in PCB transistors has generated wires with diameters as small as 1 nm (0.001 μm) and tubes six times stronger than steel. Durable nanocoatings resist scratches, prevent surface chipping, repel water and moisture and offer dependable protection against external elements. Depending on the technology used, application methods resemble both liquid coatings and Parylene:

• Nanocoatings mimic wet coatings to the extent that they can be applied by dip (immersion) and spray procedures.
• Using single-step plasma deposition techniques (without curing), nanocoatings can resemble Parylene’s chemical vapor deposition (CVD) methodology.

These similarities are not extensive. Using a brush for applying liquid coatings is ineffective when dealing with nanocoatings due to their exceptional fineness compared to traditional liquid materials. Affordable and easy-to-apply acrylics that cure rapidly provide moisture and dust resistance, as well as mechanical reinforcement to assemblies. However, they are flammable and soften under heat, making them susceptible to biological infestation and chemical damage, limiting their applications. On the other hand, extremely durable epoxy offers exceptional protection against barriers and security threats, along with strong resistance to chemicals and heat. Yet, it can be brittle and challenging to rework or remove. Urethane, with good hydrophobic and oleophobic properties, resembles a nanocoating but may face significant adherence challenges. Similarly, thickly-applied silicone is hydrophobic, heat-resistant and chemically inert. While it shares these properties with urethane, it also encounters adherence issues like urethane when compared to nanocoating.

Plasma polymerized coatings create a stable plasma through electromagnetic discharge of gas at low pressure/temperature. Adding energy transforms matter from solid into liquid into gas into plasma, where:

• Molecules are decomposed into a mixture of neutral and charged particles
• The molecules interact with a targeted material’s exposed surfaces
• Open-cell structured materials experience plasma particle-interaction with internal surfaces

Plasma coating resembles Parylene chemical vapor deposition (CVD), wherein chemically inert, powdered Parylene dimer (a solid) is transformed into a gaseous state at ambient temperature and at the molecular level, in a vacuum, subsequently polymerizing onto the substrate upon entering the deposition chamber. Parylene CVD results in consistently pinhole-free conformal films that penetrate even the smallest surface crevices on a molecule-by-molecule basis. Like nanocoating, Parylene uniformly covers virtually any board topography.

Deposited from the vapor phase, Parylene polymers measure 0.1 torr (0.000133322 bar); the smallest path between the molecules averages 0.1 centimeter (cm), making them very useful for MEMS/nano-tech. What is instructive for examining thicknesses of nanocoats in relation to Parylene coating is comparing nanometers to centimeters; 1 cm = 10,000,000 nm. The 0.1 cm molecule path separation cited above still equals 1,000,000 nm, a considerable difference by any calculation, since some nanocoatings are effective at 1 nm.

Resistant to heat, chemical corrosion and biological infestation, Parylene remains a good choice for a wide range of conformal film projects. Traditional wet coatings similarly retain their value as conformal coatings. And nanocoatings are not without disadvantages:

• Their ultra-thin design makes them susceptible to abrasion.
• Curing-oven exposure can melt nano-particles into a glassy substrate if wet application methods are used.
• Nanocoatings cannot always completely prevent corrosion.
• Extensive masking can cause a decline in surface flexibility after application.

Although there are limitations, nanocoating proves versatile in various conformal coating applications across aerospace, automotive, consumer, defense, and medical sectors, especially those associated with MEMS/nano products.