The engineering of functional systems at the molecular scale, nanotechnology encompasses management of individual atoms, combined into effective working units, often complex as machines.
The engineering of functional systems at the molecular scale, nanotechnology encompasses management of individual atoms, combined into effective working units, often complex as machines. Yielding advantages like enhanced chemical reactivity and strength than larger-scale structures, they offer greater control of the light spectrum and weigh significantly less. Incredibly small, one nanometer is a billionth of a meter (10-9 of a meter) -- one inch equals 25,400,000 nanometers; more illustratively, a sheet of newspaper is 100,000 nanometers thick.
The Development of Nano Coatings
Already an interdisciplinary field, nanotechnology continues to cultivate an extensive range of applications. In addition to building machines at the subatomic level, nanotechnology has been adapted for use as a protective conformal coating. Nanowires and nanotubes are being developed for use in transistors for printed circuit boards (PCBs) and associated electronic assemblies. The wires can have a diameter as small as 1 nanometer; tubes can be six times stronger than steel, with half the weight. Liquid crystal displays (LCDs), bio-nanobatteries, capacitators and microprocessors (Intel) are several other items benefitting from nanotechnology. Simply because of their size, nano coatings are the most appropriate protective covering for these devices.
Nano conformal films have already proven useful as scratch-resistant coatings, created by combining aluminum silicate nanoparticles to scratch-resistant polymer coatings. The resultant product better resists surface chipping and scratching for an extensive list of current products, ranging from automobiles to eyeglass lenses. In another case, a fabric coating has been developed (Nanorepel/First Choice Armor) for bullet-proof vests: a thin layer of organic nanomolecules on the surface of each fiber freezes up upon impact, locking the sturdy strands in place, markedly limiting the projectile’s destructive effect.
Like traditional conformal coatings, nano coatings protect PCBs through their ultra hydrophobic properties, which repel liquid water and block moisture, preventing its capacity for transporting corrosive ions onto boards’ surfaces. Some of nano’s specialized uses go beyond the normal applications attributed to conformal coatings for aerospace, automotive, consumer, defense and medical purposes, particularly those aligned with MEMS/nano products.
Examples of nano coatings’ utility include:
Nano Coatings vs Traditional Conformal Coatings
A conformal coating is applied to an assembly requiring protection; this coating covers and seals the device’s working components, delivering reliable protection against intrusive elements. Nano coatings are emerging as viable alternatives to traditional conformal coatings for protecting PCBs. This is especially true for mobile electronics, including biomedical devices. In addition to hydrophobicity, nano coatings repel oils (oleophobicity), with low viscosity and solids’ content.
Nano coatings are functional at far finer film layers than competing coatings. They respond well to biomedical concerns of personal safety and environmental protection because they are far more benign than such solvent based wet conformal coatings as acrylic, epoxy, silicone and urethane; curing is generally unnecessary, or minimal, even when applied with an atomized spray applicator.
To the extent that nano coatings are applied by spray procedures, they resemble traditional wet conformal coatings -- acrylic, epoxy, silicone, urethane –which use this similar liquid application approach (as well as brushing, dipping, etc.). However, nano coatings are increasingly applied via single-step plasma deposition techniques, without curing, closer to parylene’s chemical vapor deposition (CVD) methodology.
Because they are nanotechnology-based, nano coverings are better suited for MEMS/nano applications than most conformal coatings. They are stronger, lighter-weight, and far more amenable to the confined special requirements of microscopic technologies. This makes them a better choice for much biomedical technology, particularly for devices implanted within the human body, which must operate continually, and in many case without fail, to assure the patient’s health. Nano superiority for these many biomedical functions does not diminish their value as coating for agricultural, automotive, consumer goods/appliances, industrial metals, and marine coatings’ purposes, among many other applications.
Although its basic component is remarkably small – with 25,400,000 nanometers included in just one inch(!!) -- nanotechnology encompasses a growing, interdisciplinary field with an unlimited future. Nanowires and nanotubes are used in transistors for printed circuit boards (PCBs) and associated electronic assemblies. Bio-nanobatteries, capacitators, LCDs, and microprocessors represent just a few nano-applications, which include uses for aerospace, agricultural, automotive, consumer, industrial, medical, military and oceanic products.
Nano coatings can also be applied to a wide variety of substrates, like ceramics, glass, metals, polymers, and even other nano coatings. They impart corrosion resistance and hydrophobic/oleophobic properties that enhance the PCB’s conductive/insulative properties.
Uses of Nano Coatings
Conformal coatings provide PCBs, and a vast spectrum of components/devices, with sufficient protection from moisture and contaminants to assure their functionality under stressful operating conditions. In comparison to liquid coatings and parylene, nano coatings represent a viable alternative for safeguarding these devices.
Like other electronic assemblies, nano components require protection during operation and storage. Because of their microscopic size, nano coatings are, at first glance, the optimal protective covering choice for nano devices. That is particularly the case in comparison to liquid conformal coatings – acrylic, epoxy, silicone, and urethane. The film thickness required for wet coatings to provide effective protection is too great for nano devices, essentially defeating the advantages of their exceptionally minute proportions by interfering with their function.
In Comparison to Parylene
To be effective, coating thicknesses/coverage for both nano and parylene coatings need to demonstrate the following properties:
Unlike liquid coatings, parylene has been shown effective for most MEMS/nano applications, even though its conformal films are considerably thicker than those generated by nano technology. Nano coatings dry to a thickness of 100 – 5,000 nanometers (0 – 0.0002 inches). In contrast, parylene coatings typically measure 0.000394 - 0.00197 inches. Statistically, the parylene measurements are significantly larger; parylene’s smallest coating thickness (0.000394 in.) is nearly twice as substantial as the largest nano coating (0.0002 in.). However, the thicknesses of both coatings are miniscule; parylene’s remain sufficiently small for effective MEMS/nano conformal coating.
Also, parylene’s utility is more apparent for larger-scale PCBs and related products. Depending on the particular application – its function and size – parylene easily matches nano’s uses for a wide range aerospace, automotive, consumer, defense and medical purposes. And while certain nano coatings can generate no-mask solutions that deliver the benefits of conformal protection against water/moisture, they often lack the impact-resistance and corrosion defense of parylene. This can be a primary production consideration. Nano’s superior capacity for coating rework is largely negated by its comparative lack of resilience and strength under the same range of operating circumstances common to parylene coatings.
Other production considerations involve application. Nano coatings can be applied through wet dip or atomized-spray methodologies. Wet applications are cost-effective, since they can be employed:
Unlike liquid coatings, nano post-application curing is generally limited to air drying, when it needs to be enacted at all.
While nano coatings can now be readily applied by conventional liquid methods, their application via chemical vapor deposition (CVD) and single-step plasma deposition techniques is very like parylene. Parylene CVD applies gaseous parylene deep into substrate surfaces in a specialized deposition chamber. The process is expensive, and production batches small compared to liquid methods, but the resultant films offer significant advantages:
CVD-applied nano coatings offer a similar range of advantages.
Nano plasma deposition uses no solvents and requires no curing. It utilizes a single-step process to apply a thin, uniform nano film, often negating masking requirements for connectors and contacts, lowering production cost/time.
Liquid application of nano coatings are faster and more economical than use of plasma or CVD methods. In such cases, nano coatings are also produced more cheaply and quickly than parylene.
Nano coating can be applied over parylene and other coatings to obtain additional performance and protection. However, doing so is neither always beneficial nor recommended. The typical value-proposition with nano coating relies on its cost effectiveness, ease of application and scalability replacing a parylene coating, developments that do not always occur; in those situations, doubling down (applying nano coating on-top parylene) is more effective if extra coverage is required. It should also be remembered that unless nano coatings are CVD or plasma applied, their spray or other liquid applications may not combine favorably to CVD-applied parylene, potentially causing adhesion issues.
Contributing to good performance for internal medical appliances, lubricity is a conformal coating’s ability to lower operational friction that might retard its function and endanger patient health. Lubricious coatings offer essential protection for appliances like cardiac-assist devices (CADs), catheters, elastomers, guidewires, and stents. Compared to an uncoated device, lubricious films can reduce frictional forces by more than 90%, dramatically decreasing potential harm caused by excessive insertion-force or internal puncture damage. This relative ease of use is important for implants and similar devices that require navigation throughout the patient’s vascular system or other internal structure; otherwise, patients can suffer from abrasion generated between the device surface and blood vessel walls.
Coefficient of Surface Friction
The degree of physical resistance a device demonstrates is numerically expressed by a coating’s coefficient of friction (µ), which quantifies:
Static friction (µs) occurs when an object moves across a stationary surface; kinetic friction (µk) results for two objects simultaneously in motion, moving across each other. Conformal coatings are used in both circumstances, especially for medical implants with moving MEMS/nano-tech components.
Where higher-level surface lubricity is sought, lower µ-values are the objective; they signify lessened frictional resistance, minimizing non-release, dry-sticking challenges that interfere with devices’ performance. For instance, a µ-value of 1 indicates an equal quantity of force is needed to either lift an object, or slide it across a level surface; these calculations compare an object’s weight to the total force required to make it move. Most everyday objects and materials have a coefficient between 0 and 1; values closer to 1 are not feasible for medical purposes. For medical devices, a µ-value:
Appropriate safety standards also need to be met.
Much depends on the materials comprising the touching surfaces. Conformal coatings like Teflon (PTFE) and parylene, which provide high-level lubricity, maintain that level for a prolonged operational duration, making them very useful for specialized medical applications.
Properties of Reliable Coating Lubricity
Lubricated surfaces have lower levels of friction. Wet hydrophilic coatings amass water as a source of lubricity, applied by liquid methods such as dipping or spraying the film substance onto substrates. Applied to catheters or guidewires, they temporarily minimize development of thrombosis. However, their lubricious function decreases with time, dissociating or dissolving from the matrix surface, leaving particulates in tissue or the bloodstream, endangering patient health. Thus, they are less reliable long-term than hydrophobic coatings.
Materials like PTFE and parylene generate dry-film, hydrophobic lubricity, which offers numerous advantages to coating surfaces/substrates. In addition to extended performance life for both the coating and the substrate, benefits include decreased retention of dirt, improved corrosion/moisture resistance, and self-cleaning. Greater resistance to corrosion and moisture makes them less likely to dissociate/dissolve during use, enhancing better patient safety.
Parylene Dry Film Lubricity
Parylene film provides excellent dry lubricant protection, abrasion and wear resistance for surgical instruments and other medical devices. Hydrophobic PTFE has a generally lesser µ-value (0.2-0.3) than parylene (0.25-0.4), but its wet-technology is more particulate-disposed, prone to chipping and flaking more readily than parylene. In comparison to PTFE, parylene’s chemical vapor deposition (CVD) method of film-application generates a coating typically more resilient, pinhole-free and protective, particularly under harsh operating conditions.
Except in the case of coiled guidewires, parylene lubricity enhances the entry and removal of medical implements within the body. Lubricity is also aided by the micro-thin coating layers parylene can produce, minimizing the actual physical dimensions of an implant, offering lower kinetic resistance when in contact with internal surfaces. Hydrophobic parylene offers a superior option for improved lubricity for metal substrates, selected elastomers like silicone rubber, tubing and wire, without risk of particulates.
Of the parylenes, type AF-4 has the lowest coefficient of friction. However, most parylene types provide reliable dry-film lubricity for medical appliances, improving their operational flexibility.
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:
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:
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
|Parylene||CVD||Antimicrobial, chemical, dust, electrical, moisture & oil resistance||Coats almost everything, excellent adhesion, abrasion/chemical resistance||Expensive, high temperature, masking, potential contamination during processing|
|Nanocoat||Liquid (dip/spray), NPT||Chemical/dust, moisture resistance||Thinnest coating available, applies virtually everywhere||Poor abrasion, long cure time, new tech/properties vary across project|
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
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:
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
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:
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.