Each conformal coating material exhibits a range of unique performance properties that determine its product uses. Relevant factors include the required coating-thickness necessary to assure reliable performance. Like other coating types, parylene (XY) layer thickness is largely a function of several factors: (1) substrate material, (2) the kind of assembly being covered, and (3) its operational purpose. Chemically inert parylene is effective at far-thinner application thickness than liquid-applied materials for coating printed circuit boards (PCBs) and related electro assemblies:
- Acrylic, urethane, epoxy — 0.025 – 0.127 millimeter (mm. -- 0.001 to 0.005 inches).
- Silicone — 0.051 – 0.203 mm. (0.002 to 0.008 in).
- Parylene — 0.013 – 0.051 mm. (0.0005 to 0.002 in.).
XY’s chemical vapor deposition (CVD) process is a primary reason for its thickness advantage compared to wet substances. Consistently pinhole-free conformal films that penetrate even the smallest surface crevices on a molecule-by-molecule basis provide XY a unique advantage in this regard. Gaseous XY seeps deep into substrate surfaces, effectively adding a layer of protection beneath the assembly surface, as well as above, on the component’s exterior. With operating thicknesses in the 0.1 micron range, XY is exceptionally adaptable for microelectromechanical systems (MEMS)/nano-tech components, expanding their applications far beyond those of liquid coatings. To illustrate the scale involved,
- One nanometer (nm.) equals one-billionth of a meter (10-9 of a meter).
- One inch = 25,400,000 nanometers.
- By these standards. a sheet of newsprint is 100,000 nms. thick.
CVD parylene polymer film thicknesses are controllable to less than a single micron (1 μm.), or 1,000 nms., supporting XY’s adaptation to functional systems operating at the molecular scale.
Parylene coatings are flexible, uniform at controllable, pinhole-free thickness. XY-coating remains adherent and intact, preserving dielectric/insulation properties, at thicknesses greater than 0.5µm., completely penetrating spaces narrow as 0.01mm. Parylene resists chemicals, corrosives, moisture and solvents, with minimal thermal expansion, covering virtually any board topography, while ensuring PCB/assembly function/performance through most operating conditions.
However, each parylene type exhibits a collection of unique performance properties that determine appropriate applied thickness and subsequent product uses. As with other conformal materials, coating thickness is determined by assignment specifics – substrate material, assembly type, operational requirements.
- Often, these are stipulated by the client, whose end-item instructions designate in writing or through drawings precise XY coating thicknesses. Depending on PCB topography, these may vary within the assignment requirements. Parylene applicators are advised to review written/drawn specifications, and compare these to the physical assembly. The objective is accurately determining consistency between the two and identifying potential issues, so the applied parylene thickness reflects the assembly’s physical character and functional properties.
- Considerations of the assignment’s required dielectric strength also influence XY coating thickness. Higher dielectric strength necessitates a thicker coat of parylene; thinner XY films provide lesser dielectric performance.
- Additionally, clearance requirements affect coating thicknesses. This is less of an issue for enclosed PCBs, but even one mm. over-thickness can cause mechanical abrasion sufficient to damage the parylene, rendering the coating and assembly less effective.
- Balancing dielectric strength issues with those attendant to clearance requirements always factor into determining a film thickness level that supports optimal assembly performance.
- Ruggedization or other specialized assignment requirements also impact XY coating thickness.
Coating thicknesses must meet quality specifications. Parylene’s exceptionally thin coatings distinguish it from liquid coating materials, but also require a similarly specific connection between the coating’s final thickness and the functional requirements of the component during operation. As with all conformal coatings, applying XY films too thin eventually renders the PCB vulnerable to a wide range of environmental/performance threats, negating the coating’s protective function. Applied too thick, conformal coatings interfere with an assembly’s operation, particularly for those with moving parts.
Accurately deposited in a single coating-run between 0.5 microns through 50+ microns (2 mm.), typical XY thickness standards are in the 15 micron range, according to IPC CC 830 and MIL-I-46058C guidelines. Here again, coating purpose largely determines its ultimate thickness:
- Typical thickness for a barrier layer ranges from 5 to 20 microns, while a
- dry lubricity layer on silicone generally requires less than one micron of parylene.
Operationally, the coating process runs until all the dimer is vaporized and deposited. Final XY coating thickness is further determined by the quantity of parylene dimer vaporized relative to the assignment’s overall load surface area. Thus, dimer quantity needs to be carefully calculated and controlled, based on the surface area of the load in the deposition chamber. Because the association between dimer quantity and surface area is so critical to the success of each coating assignment, the most consistent coating thicknesses are achieved with similar size production lots. Odd lots, or those of divergent sizes, require additional focus to project specifics to ensure appropriate coating thickness.
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