Conformal coatings primary purpose is protecting the performance of highly sophisticated electronics such as printed circuit boards (PCBs), sustaining their functionality through often unfriendly operating conditions. Among the most important coating-requirement is safeguarding PCBs from the negative impact of moisture incursion. Sources are many. Liquidized obstacles to appropriate assembly function can result from unwanted contact with acid rain, aggressive solvents, atmosphere pollutants, chemicals, fog, high humidity, intermittent immersion, persistent rain, snow, salt water/mist and wet sprays of any kind.Read More
Parylene Coating Blog by Diamond-MT
Chemically inert parylene (Poly-para-xylylene/XY) conformal film is often selected because its micron-thin protective films generate precise coating uniformity, regardless of substrate topography. To this extent, XY far exceeds the capacities of liquid materials – resins of acrylic, epoxy, silicone or urethane – for a wide range of coating assignments. It is true that pre-synthesized liquid coatings are easier to apply. However, their conformal films are dimensionally thicker, making them difficult to position in constricted operating spaces. Liquids are also generally less resistant to contaminant incursion and other problems that interfere with reliable performance of printed circuit boards (PCBs), and most other contemporary electronics, including biomedical implants.Read More
A specialized chemical vapor deposition (CVD) process attaches conformal coatings composed parylene (XY) to substrates. CVD uniformly encapsulates all exposed substrate surfaces as a gaseous monomer; completely eliminating wet coatings’ liquid phase and need for post-deposition curing. Synthesizing in-process, CVD polymerization requires careful monitoring of temperature levels throughout.
Beneficial thermal properties of XY protective coatings include reliable performance through an exceptional range of temperatures. Parylene is available in variety of material formats, prominently Types C, N, F, D and AH-4. Each has a particular range of properties that determine its optimal uses. Types C and N exhibit faster deposition rates than other parylenes, making them useful for a wider range of coating functions. However, operating temperature is a significant determinant of use: Much depends on chemical composition.
- Used more frequently than other XY varietals, Parylene C is a poly-monochoro para-xylene. It is a carbon-hydrogen combination material, with one chlorine group per repeat-unit on its main-chain phenyl ring. In oxygen-dominated atmospheres, C conformal films regularly provide reliable assembly security at temperatures of 100° C (212° F/water’s boiling point) for 100,000 hours (approximately 10 years). C is suggested for use in operating environments reflecting these temperature conditions. Chemical, corrosive gas, moisture, and vapor permeability remain consistently low. C generates exceptional vacuum stability, registering only 0.12% total weight-loss (TWL) at 49.4° C/10-6 torr (1 torr = 1/760 SAP (standard atmospheric pressure, 1 mm Hg). C can also be effective at temperatures below zero, to -165º C.
- With a completely linear chemical format, Parylene N is the most naturally-occurring of the parylene series. Used less regularly than Type C, N is highly crystalline; each molecule consists of a carbon-hydrogen combination. N’s melting point of 420° C is greater than most other XY types. Vacuum stability is high, registering TWL-levels of 0.30% at 49.4° C, and 10-6 torr. These properties encourage higher temperature applications. Compared to other XY varietals, N’s low dielectric constant/dissipation values also recommend uses with assemblies and parts subjected to higher levels of unit vibration during operation. N’s electrical/physical properties are not noticeably impacted by cycling from -270º C to room temperature, adding to its versatility.
- Parylene F has fluorine atoms on its aromatic ring. Possessing aliphatic -CH2- chemistry, F’s superior thermal stability is attributed to this aliphatic C-F bond, compared to Type C’s C-C bond. Better thermal stability, and reduced electrical charge/dielectric constant expand its use for ILD (inner layer dielectric) applications, such as those for ULSI (ultra large-scale integration), where a single chip can incorporate a million or more circuit elements. F is a good choice for many microelectromechanical systems (MEMS)/nanotech (NT) solutions.
- Originating from the same monomer as Type C, Parylene D’s chemical composition contains two atoms of chlorine in place of two hydrogen atoms. Like Type C, D conformal films can perform at 134° C (273° F), dependably securing assembly performance in oxygen-dominated environs for 10 years, at a constant 100° C. Parylene F resists higher operating temperatures and UV light better than C or N.
- Parylene AF-4’s melting point is greater than 500° C. It survives at higher temperatures/UV-exposure better than other parylenes for long durations because it possesses CF2 units, situated between its polymer-chain rings.
Protecting printed circuit boards (PCBs) and similar electronics from the incursion of water is an essential responsibility of parylene (XY) conformal coating. Suitable XY permeation barriers assure no form of liquid passes through to underlying components and that the water vapor transmission rate (WVTR) is minimal. WVTR measures the level of water vapor migration through the applied barrier film, in terms of area and time. Optimal WTVR ratings are represented by lower numerical values. In comparison to liquid coatings, parylene typically provides lowest-level values, indicating better moisture barrier provision.
Acrylic, epoxy, silicone and urethane coatings can be more quickly affected by water, its vapor, and other sources of moisture, such as:
- acid rain,
- mists of other airborne pollutants,
- salt-air and
- chaotic weather.
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.Read More
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Unlike liquid coatings – acrylic, epoxy, silicone and urethane – parylene (XY) does not use wet method application. It can neither be brushed or sprayed onto substrate surfaces, nor will immersion – soaking the substrate in a bath of coating material – work. In addition, XY’s:Read More
Although parylene (XY) is a well-recognized and often used conformal coating, misconceptions about what it is and can do are common. These mistaken beliefs interfere with true understanding of parylene’s uses. Five of the most consistent misconceptions – and appropriate corrective information – should clear things up.Read More
Permeation barriers for electronic devices are essential to assure their ongoing performance through a wide range of operational environments. Polymer flexible conformal coatings provide good barrier protection, protecting device substrates from unwanted incursion by solid contaminants, chemicals, gaseous permeation and liquid water or vaporous forms of moisture. Permeability reduction improves with enhanced coating adhesion, minimizing the surface’sRead More
Conformal coatings are used to protect printed circuit boards (PCBs) from dust, humidity/moisture, mildew/mold, temperature extremes, and other elements whose prolonged contact might interfere with assembly function. Coatings also enhance electrical clearance-tolerance, while safeguarding PCB components from contamination (particulate or otherwise), corrosive materials, and mechanical stress.Read More