Parylene Coating: Does Thicker or Thinner Coating Provide Better Performance?

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. A distinctive feature of XY is its capacity to form micron-thin coating layers. This sets it apart from liquid coatings like acrylic (AR), epoxy (ER), silicone (SR), and urethane (UR), which typically require application at least twice as thick, often constraining their versatility. Parylene typically is applied at 0.1 to 25 microns (0.004 – 1 mil), 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 0.13 to 0.30.

Parylene is typically applied in extremely thin layers to provide protection to coated items. However, its thin and optically clear nature can also pose a drawback. 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 functional requirements of a coating after application and during operation.

An illustration is given by example for two cases related to XY’s insulation resistance (IR) and breakdown voltage (Vbd) 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 (Vbd) of most Parylene types – N, C, ParyFree^® and Parylene HT^® – are also a function of the polymer’s applied thickness. An insulator’s Vbd is the minimum quantity of voltage sufficient to cause the insulating material to become electrically conductive. For XY, as with other conformal films, Vbd 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 and ParyFree are superior to N for films under 5 micrometers (0.0002 in).

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 coatings are preferable to thicker.