Different Coatings for Electronics
Posted by Sean Horn
Friday, January 26, 2018 7:18
@ 7:18 AM
The value of polymeric conformal coatings for protecting printed circuit boards (PCBs) from functional retardants like dust, corrosion, moisture, and temperature fluctuations is well-known. What may be less known is, that as the electrical components used in PCBs become smaller, traditional conformal films are commensurately less effective for certain coating purposes. With the rise of microelectricalmechanical systems (MEMS) and nano technology, nanocoats are increasing in prominence, in many cases surpassing even micro-thin parylene not-liquid coatings in utility for MEMS/nano applications.
The development of low pressure plasma 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 nms thick. In addition to being far smaller, nano devices
- weigh substantially less,
- offering enhanced chemical reactivity and strength than larger-scale structures,
- with better control of 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, 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 nanometer (0.001 μm), and tubes six times stronger than steel. Scratch-resistant, conformal nanocoatings limit surface chipping and scratching, simultaneously repelling water and moisture, delivering reliable protection against intrusive elements. Depending on the technology used, application methods resemble both liquid coatings and parylene:
- · Nanocoatings echo wet coatings, to the extent that they can be applied by dip (immersion) and spray procedures.
- · Also using single-step plasma deposition techniques (without curing), nanocoatings can resemble parylene’s chemical vapor deposition (CVD) methodology.
These similarities are not extensive. Brush procedures acceptable for liquid coatings are a non-starter for nanocoat, due to nanocoatings’ exceptional fineness in comparison to liquid materials. Inexpensive, easy-to-apply/rework, rapid-cure acrylics are moisture/dust resistant and offer mechanical reinforcement to assemblies, but are flammable, softening with sufficient heat; susceptibility to biological infestation and chemical damage limits their uses. Extremely hard epoxy offers exceptional barrier/security protection and chemical/thermal resistance, but can be brittle and difficult to rework/remove. With good hydrophobic/oleophobic properties, urethane resembles nanocoating, but can exhibit considerable adherence problems. Thickly-applied silicone is similarly hydro/oleophobic, heat resistant and chemically inert; it displays the same adherence issues as urethane, compared to nanocoat.
Nano-plasma technology (NPT) creates 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,
- · that interact with a targeted material’s exposed surfaces.
- · Open-cell structured materials experience plasma particle-interaction with internal surfaces.
NPT 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. Consistently pinhole-free conformal films that penetrate even the smallest surface crevices on a molecule-by-molecule basis result from CVD. 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 nanocoats 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:
- · Ultra-thin coatings make them susceptible to abrasion.
- · Curing-oven exposure can melt nano-particles into a glassy substrate if wet application methods are used.
- · Nanocoat cannot always completely prevent corrosion.
- · Often extensive masking causes a decline in surface flexibility after application.
Despite these problems, nanocoating is adaptable to numerous conformal coating applications for aerospace, automotive, consumer, defense and medical purposes, particularly those aligned with MEMS/nano products.