SURFACE ENGINEERING

Optimizing surface preparation to ensure adhesion of Parylene-based coating to the substrate

Coating sensitive and high performance products often require a special attention to their surfaces. Whatever the materials properties, best performances and controlled reliability can only be ensured through optimized coating adhesion to the substrate. 

As the surface varies not only with the nature of the materials but also according to their manufacturing process, an adapted surface preparation is essential. Different techniques have been developed at Comelec to optimize coating adhesion, with regards to the following objectives: 

Cleaning

Cleaning methods are mainly required to remove large contaminations. Residues of grease and oil, soldering flux (PCBs), detergent or demolding agent are the most common examples of chemicals that can be harmful for coating adhesion.

For such cleaning processes, solvents are needed. Large volume productions require economically and environmentally sustainable process.

Isopropyl alcohol (possibly combined with DI water) is a highly effective, general use cleaning solvent that is used to clean a variety of substrates and remove a variety of handling soils. It dissolves a wide range of polar and ionic compounds, evaporates quickly, leaves almost no oil traces and is relatively non-toxic, compared to alternative solvents.

Alkoxy propanols (also known as modified alcohols) are solvents presenting well-balanced properties between polar and nonpolar components.  That gives superior capacity to dissolve both polar and nonpolar contaminants while protecting against corrosion. 

Cleaning using modified alcohol can be achieved using a vacuum automatic cleaning & degreasing systems. Parts are in contact with liquid or vapor phase solvents at 80-90°C. Ultrasonic energy can be used to further increase cleaning efficiency while vacuum drying ensures reliable results.

Other solvent based cleaning solutions are available on request at smaller scale production, especially for medical applications.

Comelec Cleaning

Plasma surface activation

Plasma activation consists in modifying the surface properties of a product thanks to unique chemical reactions. At Comelec, this approach is mainly used to improve adhesion of Parylene or Parylene-based multilayer coatings to the substrate.

Plasma processing creates functional groups that react with molecules used for coating, enabling strong and reliable bonding. It can also have the effect of ultra-fine cleaning by removing organic contaminants.  Metal oxide reduction, polymer oxidation, surface roughening and UV cross-linking are other examples of the unlimited surface reactions that can be achieved.

For Parylene coatings, oxygen, hydrogen and fluorinated molecules are the most common gases used to modify surface properties and optimize coating adhesion. Plasma processing can be applied directly in the Parylene deposition reactor or through remote plasma equipment.

Most advanced Comelec deposition platforms include different plasma source configurations to enlarge application windows through the use of complementary physico-chemical mechanisms.

Plasma processing is a low temperature process (typically close to room temperature) suited to the most sensitive parts. Comelec especially integrates remote plasma sources compatible with ESD sensitive components, and for PCB applications.

Besides improving adhesion, plasma processing might also be used to control a wide range of surface functionalities. In particular, a post-treatment might be required to improve glueability of the Parylene or enhance oleophobic behavior.

Comelec - Plasma surface activation

Applying adhesion promotors / adhesion layers

Parylene-based coatings do not naturally form strong links to surfaces. To ensure reliable encapsulation, the use of adhesion promotors and/or adhesion layers is, in general, required.

While most industrial applications are fulfilled using the well-known Silane A-174 adhesion promotor (wet or dry process), Comelec developed unique solutions to meet state of the art performances on very challenging product configurations. Thus, processes ensuring reliable adhesion on noble metals or low surface energy polymers have been successfully demonstrated following intense developments.

In general, organosilane molecules (such as A-174) contain two functional groups, one reacting with inorganic surfaces (or -OH groups) and the other reacting with organic materials (i.e. Parylene). This mechanism allows the formation of reliable bridges between coating and product.  

When the use of an adhesion promotor does not allow sufficient adhesion, introducing special surface modification processes or intermediate coatings (adhesion layers) are an alternative solution Comelec can provide.

Comelec equipments include versatile functions allowing several approaches to ensure reliable and stable adhesion results.

Comelec Silane

Silane A-174 or 3 (Trimethoxysilyl) propyl methacrylate, common adhesion promotor for Parylene coatings