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Can I Spray Parylene Myself?
The answer is “No!” A detailed discussion on “Why Parylene can not be spray coated?” can be found in the next few paragraphs. Also, differences between various deposition methods are explained. Polymerization type and process: Forparyleneto serve as a conformal coating it has to undergo chemical reactions. This requirement arises from its molecular structure and its polymerization mechanism. Parylene undergoes chain growth type of polymerization [1]. In this type of polymerization, a polymer chain grows by the attachment of a monomer to one of the chain ends. This process takes place one unit at a time. Duringparylene polymerizationthe chain ends are not finalized and the chains that do not react are buried in theparylene film. Later, these unreacted chain ends may react with oxygen once exposed to air. Duringparylene depositionno solvent is used and no by-products are formed. Parylene Deposition Method: Parylene is deposited using chemical vapor deposition (CVD) in the gas phase.Parylene CVDprocess takes place in three steps: sublimation, pyrolysis, deposition. The powdery precursor, dimer, is weighed and inserted into the sublimation chamber using a boat. The coating process takes place once theparylene dimeris sublimized and pyrolised forming the monomers (Fig. 1). Pyrolysis of the precursor is defined as the thermal decomposition of materials at elevated temperatures under vacuum and this reaction is irreversible. Afterwards, monomers are deposited as a thin layer in a way to allow for a top layer to grow on them (chain growth). In the meanwhile, the monomers penetrate to the smallest voids resulting in a uniform, void-freeconformal coating. The deposition chamber needs to be under vacuum as mentioned earlier, this determines whether the deposition will be 100% efficient or not. In the presence of other gases by-products may form decreasing the process efficiency [1]. The temperature and pressure are extremely important and requires precise control for the bestquality conformal coating. A professional service provider such as Diamond MT can help you with your needs in achieving the best results. Figure 1 Polymerization route forpoly-para-xylylene(Parylene N - C16H16) Differences between various conformal coating deposition methods: The parylene CVD method (Gorham[2]) makes use of reactive monomers that are produced through sublimation and pyrolysis of parylene dimers. These highly reactive monomers subsequently get adsorbed onto the substrate and polymerize. Spray, dip and brush conformal coating processes all take place in the liquid state. Their commonality is that the coating material is dissolved in an appropriate solvent and applied onto the surface of the substrate. The removal of the solvent results in the expected conformal coating. In Table 1, relative comparison of conformal coating systems is listed [3]. Solution based conformal coating can coat arbitrarily shaped structures with ease, however one must keep in mind that these methods can not fill in the crevices in the sub-micron level while Parylene CVD can. The coating method is selected depending on the viscosity of the silicone, acrylic, epoxy and polyurethane that is being used. Table 1 Relative Comparison ofConformal Coating Systems[3] In the case of spray coating, the coating is deposited using a spraying tool (eg. Spray can, Inert gas filled nozzle or ultrasonic atomization). Formed droplets land on the substrate and form a conformal coating. Thickness and surface roughness is hard to control compared to CVD method. Depending on the type of polymer used post-processing processes such as a heat treatment may be required. A good wetting, edge coverage and film adhesion can be achieved by substrate pre-treatment, controlling the precursor viscosity and spraying process variables. If process parameters are not well controlled subsequent heat treatment may lead to cracks and delamination. Viscosity of the solvent is of high importance, too low viscosity (high solvent concentration) reduces the edge coverage and improves the smoothness of the thin film. There is a compromise between the two. In order to achieve the best results time for droplet landing, droplet diameter and viscosity must be optimized. In some cases, selective spraying or maskless spraying is also possible. Precision control can be done in advanced spraying systems via nozzle flow rate control and small footprint. Dip coating can be used for large-scale coating applications as well as PCBs. The substrate is basically dipped into the solution that will be coated onto the substrate. When done manually, the professional immerses the substrate or PCB in a tank of coating solution. If masking is required components are simply masked by use of a tape. After the substrate is treated the mask is removed. The substrate cleanliness, roughness, presence of patterns and structures are all evaluated and the required time for dipping velocity is decided for optimal coating quality. Brush coating is simply done using a brush, manually. PCB’s can be painted and the coating process can be relatively precise in the macro scale and selective depending on the person applying the film. Among all these four methods, parylene CVD conformal coatings offer the best uniformity, smoothness and hermeticity. To learn more about parylene, download our whitepaper:https://www.paryleneconformalcoating.com/
What Happens to Parylene when the Dielectric Voltage is Exceeded?
Poly(para-xylylene) derivatives (parylenes) are used asconformal coatingsin a wide range of applications in the automotive, medical, electronics, military and semiconductor industries. They are inert, transparent and have excellent barrier properties as dielectric thin films. Because their deposition takes placeunder vacuum sub-micron range crevices can be coated leading to excellent barrier properties (void free) and they have extraordinary purity that is of great importance in electronic applications. Not all parylene derivatives show same dielectric properties. It is also important to note that dielectric properties of parylenes depend on their thickness thus their %crystallinity which is explained below. Before, diving intoparylenessome general definitions would be helpful to explain what dielectric strength and breakdown voltage is. Dielectric strength of a polymeric material can be defined as the measure of dielectric breakdown resistance under an applied voltage (Dielectric strength is calculated by dividing the breakdown voltage by the thickness of the sample.). It is expressed as Volts per unit thickness. The higher the value, the more electrically insulating a material is. Technically, insulators do not conduct electricity but if the electric field is large enough, the insulator breaks down and conducts electricity [1]. Once the breakdown voltage is exceeded physically the polymer may burn or punctures can occur that will conduct electricity[2]. It must be kept in mind that the thickness of an insulating material plays a role in determining its breakdown voltage and it is not linear. Parylene derivativesare partially crystalline [3], [4]. Deposition rate, thickness, post-deposition thermal treatment affects the crystallinity of Parylenes [3], [5]. As reported by Kahouli, the dielectric constant ofParylene Cincreases as a power law according to film thickness ε′ = dβ with β = 0.042 ± 0.04 for d < 1000 nm and 0.0138 ± 0.02 for d > 1000. This was attributed to the decreasing number of voids and discontinuities of the material with increasing film thickness. The significance of this study is that normally the electrical properties of thin films are speculated from their bulk samples. This study reports the dielectric properties of the Parylene C films on substrates with film thicknesses of 40–83000 nm. It was reported that the polymer becomes semi-crystalline above 40 nm thickness. Critical film thickness was measured as 1000 nm and below this critical thickness the dielectric constant decreases below the bulk value [3]. Figure 1 shows the thickness vs dielectric constant dependency. Figure 1 Dielectric constant ofparylene-C as a function of thickness [3] In another study, Parylene C was measured for its electrical breakdown voltage on a 2 mm × 2 mm area capacitor structure where it acted as a dielectric layer as illustrated in Figure 2. Results have shown that Parylene C has excellent electrical properties, particularly in terms of its dielectric strength. Its breakdown voltage was reported to remain constant across the entire capacitor area, which is typical for uniform and pinhole-free layers. The above results show that thin Parylene C films are good candidates for the gate insulating material in microelectronic applications[5]. In a different study, breakdown voltage was studied for Parylene C and they showed that the breakage results in deformation of theparylene layerforming bubbles in the film (Figure 3) [6] Figure 2 Thin Parylene C layers breakdown voltage as a function of their thickness [5]. Figure 3 Sample after measurement: close up of a defect in the Parylene C layer where the breakdown appeared (Parylene thickness of 1.9 µm) [6]. To discover more about parylene, download our whitepaper now:https://www.paryleneconformalcoating.com/
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