How to Improve Parylene Adhesion to Noble Metals

Characteristics of Noble Metals

Selecting the appropriate pre-treatment procedures is a key factor to this success of Parylene adhesion to any substance. Procedures can vary and are dictated by the substrate to be coated. Chemically inert surfaces like gold, silver and other noble metals, and nonpolar thermoplastics such as Parylene, are extremely difficult to bond and can require additional surface treatments beyond surface cleaning.

Managing noble metal adhesion is further complicated by the lack of covalent bonding between the substrate and Parylene. This is particularly true when the substrate/coating interface is subjected to sliding friction or comparable lateral forces applied to the surface.

This is less the case where a metal substrate has a high surface roughness (RA). Higher RA values signify a sufficient quantity of surface cavities (flaws and fissures) to allow Parylene to create improved mechanical bonding, holding it to the surface, prompting acceptable levels of adhesion and diminished tendency toward delamination. However, once refined, the RA of most noble metal surfaces is low. The minimal micro-porosity of these surfaces is absent the required quantity fissures or flaws for generating longer-term Parylene adhesion, mandating application of adhesion promotion techniques.

Improving Surface Energy

Without appropriate adhesion, any conformal coating will fail, whether due to friction, pressure, heat or thermal cycling.

The importance of surface cleanliness prior to Parylene deposition cannot be overstressed. In addition to this, improved adhesion can be facilitated through the use of adhesion promotion technologies including A-174, plasma processing and the SCS AdPro Family of adhesion technologies.  The use of A-174 silane has been used for decades to improve otherwise questionable adhesion for most applications. Adhering chemically to the substrate surface, A-174 silane provides precisely the kind of uneven, flawed surface that stimulates Parylene attachment, helping it to bond far more conformally to surfaces during the VDP process.

Application of A-174 is completed via dipping, spraying or vapor phase techniques. Spraying is recommended when only selected portions of the substrate require treatment; soaking or vapor phase are suggested modalities for treating an entire assembly or component. Appropriate operational caution needs to be observed during processing of A-174 silane adhesion promoter; it is a moderate skin, respiratory, and eye irritant. Although not overly flammable, it is combustible, and should be handled with care. While A-174 silane is favorably endorsed throughout the industry as a surface treatment for noble metals requiring the benefits of Parylene conformal coating, other treatment methods offer advantages of their own.

SCS AdPro Adhesion Technologies

While improved adhesion of Parylene to a wide variety of substrates is commonly achieved by a treatment with A-174 silane prior to Parylene coating, it often fails to meet the highest standards on many of today’s difficult substrates (e.g., highly polished stainless steel, titanium, exotic alloys and polyimides, etc.). SCS’ AdPro family of technologies increase adhesion between Parylene coatings and historically challenging substrates.

AdPro Plus^® and AdPro Poly^® are biocompatible and biostable. Additionally, they have demonstrated stability at elevated temperatures, making them excellent adhesion promotion tools for critical components and harsh environment applications. The AdPro family of adhesion technologies is available to SCS commercial coating service customers.

Surface Treatment Alternatives

Plasma polymerization processing can improve the adhesion and barrier properties of conformal films deposited on such noble metals as platinum. Although capital intensive and costly, this approach has been successful as a pre-treatment for MEMs and nano- applications. Research has demonstrated chemical oxygen plasma pretreatments characterized by microscopic and surface-sensitive techniques could increase barrier hydrophilicity and surface energy for metal implant coatings, improving their overall biocompatibility and performance. Mechanical abrasion processes treat noble metal surfaces with a very fine industrial grit. The objective is roughening the smooth surfaces by scraping them with the grit through tumbling or similar industrial processes, lightly abrading the designated substrates. Mechanical abrasion can be a potential solution for simple components that lend themselves to surface roughening techniques.