Parylene Process: 5 Keys to Success

Parylene Process:

The basic Parylene variant is Parylene N (poly-para-xylylene). Additional variants of Parylene can be achieved via chemical modification to Parylene N main-chain phenyl ring and its aliphatic carbon bonds. Parylene N’s modification by a functional group such as Chlorine and Fluorine leads to Parylene C (poly (2-chloro-para-xylylene)) and Parylene F, respectively. Derivatization results in a set of new material properties: %crystallinity, melting temperature, resistivity, mechanical and electrical properties. Other more complex modifications to the dimer synthesis process result in Parylene HT^® and ParyFree^®.

Conformal coating using Parylene derivatives is achieved by the vapor deposition polymerization (VDP) process also named as the Gorham process (after the scientist who achieved 100% yield under vacuum deposition conditions). VDP processing results in the formation of polymers with high molecular weight. The Parylene film formation process takes place by the polymerization mechanism (chain growth type) [1], [2]. VDP process is done under vacuum and is achieved in three steps in different parts of the VDP instrument:

• Sublimation: The granular precursor, called dimer, is weighed and inserted into the sublimation chamber using a boat.
• Pyrolysis: Heating the dimer cleaves the dimer molecules into reactive monomers.
• Deposition: Monomers travel to the deposition chamber and are deposited onto surfaces and into microscopic topographies and polymerize as a thin film, resulting in a uniform, void-free conformal coating.

Applications areas:

Parylene thin conformal coatings are used in numerous products and processes across all markets. Application areas include:

• PCB and electronic circuit encapsulation layers: Parylenes exhibit excellent sealing properties and chemical durability. They comply and surpass the requirements of the MIL-STD-302 for electronic components when used as an encapsulation material. Proven results make them useful as an encapsulation material for PCB’s [3]–[5],
• Intermediate bonding materials: Parylene is vapor deposited onto substrate surfaces from a solid dimer to a solid long-chain polymer, providing intimate coverage, eliminating air gaps. Thin films of Parylene are void-free and are highly uniform in terms of thickness across surfaces, which is crucial for the overall uniformity of wafer produced components [6],
• Thin film membranes in sensors and actuators: Due to their high level of flexibility and low residual stress [7], [8] Parylenes are used as membranes,
• Gate dielectric layers in electronics: Their relatively low dielectric constants in the 2-3 range make them useful as a low-K candidate as a gate dielectric in transistors [9],
• Microfluidic channel layer for chemical and bio-sensors: Parylenes have been recognized by the FDA as biocompatible. They additionally show outstanding chemical durability which makes them ideal candidate materials for use in microfluidic devices and are well researched for patterning of the microfluidic channels [10].

While there are diverse types of applications, Parylene conformal coatings are mostly used for PCB and electronic device encapsulation/sealing purposes. IPC-833 and MIL-I-46058 are standards that cover Parylenes and their testing for use as a protective layer on PCB’s. The required thicknesses are 0.0006 ± 0.0001 inch (15.24 ± 2.5 μm) [11].

Keys for Success:

1. Expectations from the conformal coating must be in alignment with the area of application. Properties of the Parylene variant used must align with the standards of the application. To work with the team of experts at SCS to evaluate conformal coatings for your potential application, click here.
2. Masking of the substrates may be required before the Parylene coating process. Removal of deposited Parylene can be difficult, and may harm the whole device if the masking areas are not defined before the process. It is vital for the customer to clearly indicate keep-out areas, which is most effectively done on a drawing. Masking assures selected assembly components are not covered by the applied Parylene film, which would inhibit their functionality.
3. Substrate surface cleanliness: The surface where the interface between the conformal coating and the substrate will be formed is of high importance. The cleanliness of this surface has a great impact on the results of the conformal coating process and the coatings durability. Organic residues and dust, resulting in either uncoated areas or delamination of the coatings, can alter the surface energy.
4. Trained professional operators: SCS applications engineers will assess the masking and coating process for your parts, from simple to complex structures. We can work on your topographical substrates to ensure the best coverage and complete coating trial runs before working on the final product.
5. Collaboration: Our team of experts will  guide you through the selection of coating materials and make sure the specific coating-needs for your application are met.

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]        T. Marszalek, M. Gazicki-Lipman, and J. Ulanski, “Parylene C as a versatile dielectric material for organic field-effect transistors,” Beilstein J. Nanotechnol., vol. 8, no. 1, pp. 1532–1545, Jul. 2017, doi: 10.3762/bjnano.8.155.
[3]        R. Olson, “Parylene conformal coatings for printed circuit board applications,” in 1985 EIC 17th Electrical/Electronics Insulation Conference, Boston MA, USA, 1985, pp. 288–290, doi: 10.1109/EIC.1985.7458626.
[4]        “MIL-STD-202 , Test Method Standard for Electronic and Electrical Component Parts.” [Online]. Available: https://www.document-center.com/standards/show/MIL-STD-202. [Accessed: 18-Dec-2019].
[5]        “Coating Materials for Electronic Applications | ScienceDirect.” [Online]. Available: https://www.sciencedirect.com/book/9780815514923/coating-materials-for-electronic-applications. [Accessed: 18-Dec-2019].
[6]        H. Kim and K. Najafi, “Characterization of low-temperature wafer bonding using thin-film parylene,” J. Microelectromechanical Syst., vol. 14, no. 6, pp. 1347–1355, Dec. 2005, doi: 10.1109/JMEMS.2005.859102.
[7]        S. Satyanarayana, D. T. McCormick, and A. Majumdar, “Parylene micro membrane capacitive sensor array for chemical and biological sensing,” Sens. Actuators B Chem., vol. 115, no. 1, pp. 494–502, May 2006, doi: 10.1016/j.snb.2005.10.013.
[8]        Cheol-Hyun Han and Eun Sok Kim, “Parylene-diaphragm piezoelectric acoustic transducers,” in Proceedings IEEE Thirteenth Annual International Conference on Micro Electro Mechanical Systems (Cat. No.00CH36308), 2000, pp. 148–152, doi: 10.1109/MEMSYS.2000.838506.
[9]        J. Jakabovič et al., “Preparation and properties of thin parylene layers as the gate dielectrics for organic field effect transistors,” Microelectron. J., vol. 40, no. 3, pp. 595–597, Mar. 2009, doi: 10.1016/j.mejo.2008.06.029.
[10]      E. Meng and Yu-Chong Tai, “Parylene etching techniques for microfluidics and bioMEMS,” in 18th IEEE International Conference on Micro Electro Mechanical Systems, 2005. MEMS 2005., 2005, pp. 568–571, doi: 10.1109/MEMSYS.2005.1453993.
[11]      “MIL-I-46058 C INSULATING COMPOUND ELECTRICAL (FOR COATING PRINTED CIRCUIT ASSEMBLIES).” [Online]. Available: http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-I/MIL-I-46058C_11366/. [Accessed: 18-Dec-2019].