As the electrical components used to power printed circuit boards (PCBs) grow smaller, conventional conformal films become less effective for coating them. Ongoing development of microelectricalmechanical systems (MEMS) and nano technology (NT), has little room for the thicker conformal films provided by liquid materials, such as acrylic, epoxy, silicone and urethane. Nanocoats (NCs) are increasing in prominence, frequently surpassing micro-thin parylene (XY) for many MEMS/NT purposes.
This is surprising, since XY’s micron-thin coating layers typically range between 0.1 to 50 microns (0.004 -2 mils), with film thicknesses controllable to less than a single micron (1 μm). Applied via a chemical vapor deposition (CVD) process, XY until recently generated the materially-finest coating layers available. And it remains true that parylene’s capacity for providing effective, pinhole-free conformal protection with micro-level coating layers unavailable to liquid materials offer a significantly wider range of product/process applications, in comparison to conventional liquid film materials.
Nanocoats are another story.
Nanocoat Properties in Comparison to Other Conformal Films
Evolving NT deploys individual atoms as working units, engineering functional systems on a molecular scale. Incredibly minute, 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 nms thick. In addition to being far smaller, nano devices offer additional advantages:
- substantially lighter in weight,
- enhanced chemical reactivity/strength compared to larger-scale structures, with
- better control of the light spectrum.
- Some NT devices possess the mechanical complexity of machines.
Nanocoatings provide reliable conformal protection at far finer film layers than any liquid coatings or parylene, often replacing XY for projects only it was suitable for, in the recent past. NCs prevention of corrosive substances access to electronic components aligns them with conventional conformal coatings, but far more effectively at MEMS/NT levels. NC flexibility and nano-thickness permits excellent, uniform coverage of complicated 3D-structures, deposited essentially anywhere regardless of a component’s size, with minimal impact on performance. They provide uniform, pinhole-free protective films with excellent dielectric/insulative properties, able to conform to virtually any substrate configuration. In addition, NCs are:
- scratch-resistant, limiting surface chipping/scratching,
- ultra-hydrophobic, repelling water/moisture, and
- barrier-resistant, protecting substrates from intrusive elements.
NC Application: Similarities and Differences
NC-application can use liquid methods; both immersion (dip) and spray procedures are acceptable. Brush procedures are unsatisfactory for NCs; nanocoatings’ minute and comparatively delicate structures are simply overwhelmed by brush procedures. With liquid processes, nano-particles suspended in solvent are applied to the PCB, and allowed to either bake in an oven or air dry. In the first case, temperatures need to be precise, because nano-particles can melt into a glassy substrate if the oven is too hot. Nanocoats’ ultra-thin film consistency makes them susceptible to abrasion, although amenable to timely rework.
Nanocoat somewhat resembles parylene because it can also be deposited using non-curing, single-step plasma deposition techniques, similar to XY’s (CVD) methodology. Nano-plasma technology (NPT) transforms matter from solid-into-liquid-into-gas-into-plasma, in a manner parallel to parylene; with CVD, chemically inert, powdered-but-solid parylene dimer is transformed into a vapor at the molecular level, in a vacuum and at ambient temperature. Like nanocoating, parylene uniformly covers virtually any board topography.
NPT creates a stable plasma through electromagnetic discharge of gas at low pressure/temperature; molecules decompose into a mix-composition of neutral and charged particles, which interact with exposed substrate surfaces. The result is coating through plasma particle-interaction with internal surfaces.
XY coatings are ultra-thin, measuring 0.1 torr (0.000133322 bar), very useful for MEMS/NT applications. While these levels are far thinner than liquid coatings, they can only approximate those operational for NC. Assessing NC-thicknesses in relation to parylene compares nanometers with centimeters; 1 cm = 10,000,000 nm. The 0.1 cm common to XY molecular path-separation equals 1,000,000 nm. By any estimation, the difference is remarkable, especially in consideration of the fact that some nanocoats are effective at 1 nm.!!
Parylene can provide reliable conformal protection for many MEMS/NT applications, and does share a similarity with some NC application processes. It is not a nanocoat in the strictest sense, however, primarily due to NC’s exceptional film layer thinness, compared to all other conformal materials. Parylene does come closest, and further technological development will improve its performance compared to NC. At this juncture is process evolution, XY does provide a generally stronger conformal film than NC, for many MEMS/NT purposes.
Table I offers a comparison between parylene and nanocoating, for selected characteristics.
Table I: Comparing Parylene with Nanocoat
Material Process Uses Pros Cons
Parylene CVD Antimicrobial, Coats almost Expensive, high
chemical/dust/elec- everything/where, temperature, masking,
trical/moisture/oil excellent adhesion, potential contamina-
resistance abrasion/chemical tion during
Nanocoat Liquid (dip/ Chemical/dust/ Thinnest coating Poor abrasion,
spray), NPT moisture resistance available, applied long cure time, new
virtually everywhere tech/properties vary
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