Parylene (XY) conformal coatings are known and recommended because of their many beneficial performance characteristics. They provide uniform, pinhole-free protective films with excellent barrier/dielectric/insulative properties, able to conform to virtually any substrate configuration. One property in particular – micron-thin coating layers – distinguishes XY from liquid coating materials such as acrylic (AR), epoxy (ER), silicone (SR) and urethane (UR), which need to be applied at least twice as thick in most cases and frequently more, limiting their range of uses. Parylene typically is applied at 0.1 to 50 microns (0.004 -2 mils), while the thicknesses of liquid coatings generally range from 25 to 250 microns (1-10 mils). Compared to liquid processes, gravity and surface tension generate negligible impact with parylene, eliminating film bridging, pinholes, puddling, run-off, sagging or thin-out during application. XY’s coefficient of friction coefficient can be as low as 0.25 to 0.30.
More likely to be applied in exceptionally thin layers, parylene protects the items it coats. However, being thin and being clear can also be a disadvantage. For instance, SR’s thicker coating level adds an additional layer of cushioning and shock protection to the printed circuit board (PCB) or assembly it covers. Parylene requires several layers of coating to provide even similar impact resistance. In general, thinner is better for XY but, as with all uses for all conformal coatings, much depends upon the specific coating project and its functional requirements, post-application, after operation commences.
This condition can be illustrated for parylene by two cases, concerning XY’s insulation resistance (IR) and breakdown voltage (BV) performance:
- Insulation resistance is the alternating-current resistance between two electrical conductors or two systems of conductors separated by an insulating material, such as XY conformal coating. IR is generally greater with thicker, rather than thinner, parylene layers. However, in all cases parylene IR-values exceed the prescribed specification for successful IR by approximately one order magnitude, regardless of XY coating thickness.
- Breakdown voltages of most parylene types – N, C, D, F – are also a function of the polymer’s applied thickness. An insulator’s BV is the minimum quantity of voltage sufficient to cause the insulating material to become electrically conductive. For XY, as for other conformal films, BV defines the maximum voltage difference that can be applied across the material before it conducts. These circumstances also differ by XY type. For instance, parylene C is superior to N for films under 5 micrometers (0.0002 in). Regardless of type, parylenes display:
- excellent dielectric withstanding voltages,
- even below the 1 micrometer thickness range;
- the voltage breakdown per-unit thickness generally increases with decreasing thickness,
- indicating more current is required to breakdown XY’s insulative power
- at thinner, rather than thicker, film layers --
- thus, BV capacities increase as XY layers diminish in size.
This condition is opposite than that for parylene IR, where thicker layers offer greater protection.
However, despite certain operational cases where thicker parylene coatings offer better performance, thinner XY coating layers represent one of the materials greatest functional advantages. Thinner coatings respond optimally to proliferating development of microelectromechanical systems (MEMS) and nanotechnology (NT) applications, which can have operating components in the micrometer (0.001 mm)/nanometer (a billionth [10-9] of a meter) functional range. MEMS/NT require the same protections from contaminants as larger components; of conventional coatings, only XY can provide coating functional at these minute levels. Parylene’s use for biocompatible implants is also best-served by thinner coating levels, which work effectively internally, where operating clearance can be too small for effective liquid coating.
Parylene also offers excellent dry-film lubricity for implant and surgical instruments, providing wear and abrasion resistance. XY’s low surface energy and hydrophobic properties generate reliable solutions to the friction/stiction problems affecting biomedical and MEMS/NT applications. In most cases, thinner XY coats are preferable to thicker.
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