Parylene Coating Blog by Diamond-MT

Can I Spray Parylene Myself?

Posted by Sean Horn on Fri, Apr 17, 2020 @ 08:00 AM

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: For parylene to 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. During parylene polymerization the chain ends are not finalized and the chains that do not react are buried in the parylene film. Later, these unreacted chain ends may react with oxygen once exposed to air. During parylene deposition no 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 CVD process 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 the parylene dimer is 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-free conformal 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 best quality 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 for poly-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 of Conformal 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:

Download our guide  on Parylene 101

 

References:

[1]          J. B. Fortin and T.-M. Lu, Chemical Vapor Deposition Polymerization: The Growth and Properties of Parylene Thin Films. Springer Science & Business Media, 2003.

[2]          “A New, General Synthetic Method for the Preparation of Linear Poly‐p‐xylylenes - Gorham - 1966 - Journal of Polymer Science Part A-1: Polymer Chemistry - Wiley Online Library.”

[3]          M. A. Reighard and N. A. Barendt, “ADVANCEMENTS IN CONFORMAL COATING PROCESS CONTROLS,” p. 10, 2000.

 

Tags: parylene conformal coating, parylene coating process, parylene properties