5 Keys to Parylene Process

Parylene Process:

Diamond MT is specialized in xylylene (parylene derivatives) deposition for use in all the industries where it is applicable. The basic parylene monomer is Parylene N (poly-para-xylylene). The derivatization of new varieties can be achieved by the addition of functional groups to Paryelene 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.

Conformal coatings of parylene derivatives is achieved by the CVD process also named as Gorham after the scientist who achieved 100% yield under vacuum deposition conditions.  CVD process 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].  CVD process is done under vacuum and is achieved in 3 steps in different parts of the CVD instrument:

• Sublimation: The powdery precursor, dimer, is weighed and inserted into the sublimation chamber using a boat.
• Pyrolysis: Heating the dimer results in the formation of monomers.
• Deposition: Monomers travel to the deposition chamber and 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.

Applications areas:

Parylene thin films find applications in numerous products and processes. Some of the application areas can be listed as follows:

• PCB and electronic circuit encapsulation layers: Parylene derivatives 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 on them. Proven results make them useful as an encapsulation material for PCB’s  [3]–[5],
• Intermediate bonding materials: Parylene is vapor deposited from a solid powder onto the substrate surfaces providing a seamless (eliminates air gaps) interface. Thus, thin films of parylene are void-free and are highly uniform in terms of thickness across the wafers diameter which is crucial for the overall uniformity of chips produced on the substrate [6],
• Thin film membranes in sensors and actuators: Due to their large elastic compliance and low residual stress [7], [8] parylene derivatives 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: Parylene C is an FDA approved biocompatible material which shows a very good chemical durability which makes it an interesting material for use in microfluidic devices, also it is well researched for patterning of the microfluidic channels [10]

While there is diverse types of applications parylene conformal coatings are mostly used for PCB and electronic device encapsulation/sealing purposes. MIL-I-46058 is a military standard that covers 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. The type of parylene derivative used must comply with the standards of the application and it may or may not conform to all the standards out of this application area. Therefore, we offer to discuss your requirements with a conformal coating professional at Diamond MT.
2. Masking of the substrates must be done before the parylene coating process. Removal of parylene is relatively hard and may harm the whole device if the masking areas are not defined before the process. It is vital for our clients to indicate the masking areas once they contact our professionals at Diamond MT. Masking assures selected assembly components are not covered by the applied parylene film, which would inhibit their functionality. For most applications, use of conventional masking materials and techniques obstructs parylene deposition on designated PCB keep-out regions. However, masking for MEMS/nano medical devices is more challenging and requires advanced masking solutions.
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 final results of the conformal coating process and the coatings durability. The surface energy is changed by organic residues and dusts resulting in either uncoated areas or delamination of the coatings. At Diamond MT we provide professional surface cleaning services ensuring the long lasting results for your components. Alternatively, the surface can be cleaned by the client before handing over the substrates. However, this approach may result in organic deposition (carbon residues) and we suggest surface cleaning and preparation to take place just before the coating process.
4. Trained professional operators: Diamond MT ensure optimal masking and coating process for complex structures. We can work on your topographical substrates to ensure the best coverage and do trials before working on the final product.
5. Cooperation: We understand that some organizations have highly trained professionals who are experts in conformal coatings. If you are one of these lucky organizations we will be happy to coat your substrates and fully cooperate with the professional. If you do not have a conformal coating professional our experts are in your services and will be happy to guide your through the selection of materials and coating processes till the final product is obtained.

 

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].