Characteristics of Noble Metals
Selecting the appropriate pre-treatment procedures is a key factor to this success of parylene adhesion to any substance. Procedures vary quite considerably, according to the materials designated for conformal coating and substrate. Chemically inert surfaces like gold, silver and other noble metals, and nonpolar thermoplastics such as parylene, are extremely difficult to bond; they require additional surface treatments besides cleaning.
Managing noble metal adhesion is further complicated because parylene sticks to itself, rather than the substrate surface, raising difficulties for suitable attachment on the smooth surfaces typical of assembly components formulated of noble metals. Thus, in usage where the substrate bond is subjected to sliding friction or comparable forces applied perpendicular to the surface, the parylene coating may break away. Similar adhesion difficulties emerge for parylene concerning most functional applications where it is applied as a conformal coating for noble metal substrates.
This is less the case where a metal substrate has a high RA, the arithmetic average of a particular substrate’s roughness parameters; higher RA signifies a sufficient quantity of surface cavities (flaws and fissures) to capture parylene, 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 treatment, fundamental limitations inherent to parylene can render it unsuitable for applications exposed to friction, pressure, heat or thermal cycling. Parylene is inert, very soft and exhibits poor “creep” attributes during untreated application to noble metals. A general absence of active sites for forming intra-molecular bonds results in poor adhesion, a propensity to dislodge from the substrate; delamination and similar non-adhesion problems can occur if the surface is unmodified prior to CVD.
The importance of cleanliness-processing and masking pre-CVD to improve parylene adhesion have already been discussed, as has the use of Methacryloxypropyltrimethoxysilane. This A-174 silane compound is the surface treatment of choice for most applications where developing a reliable, longer lasting adhesive bond between parylene and a noble metal substrate is desired. Adhering chemically to the metal, the introduction of A-174 silane provides precisely the kind of uneven, flawed surface that stimulates parylene attachment, helping it to bond far more conformally to the generated surface cavities and fissures during CVD.
Application of A-174 is implemented through soaking, 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.
Surface Treatment Alternatives to A-174 Silane
Plasma polymerization processing can improve the adhesion and barrier properties of conformal films deposited on such noble metals as platinum; systematic approaches tailored to specific coating requirements are suggested. This approach has been successful as a pre-treatment for MEMs and nano- applications. Recommended for use in aggressive, harsh biomedical environments, plasma activation technology positively energizes the surfaces of many noble metal substrates, and is implemented immediately prior to CVD processing. In this regard, recent research demonstrated chemical oxygen plasma insertion 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. This evidence suggests further development of parylene coating functionality, based on applications of plasma surface treatments for parylene coatings intended for noble metal substrates.
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. Parylene coated wire mandrels used in manufacturing biocompatible metal tubing for medical devices can be pre-CVD treated with abrasion processes before parylene application.
Nevertheless, chemical bonding pre-treatment with A-174 -- applied either through soaking, spraying or vapor deposition -- remains the most commonly used pre-treatment to promote parylene adhesion with noble metals. The consequent chemical surface-bond is significantly improved by this mechanically-induced method. Where pretreatment procedures are appropriately implemented, they forestall coating delamination and enhance the effectiveness of conformal parylene corrosive barriers for noble metal substrates and devices.
To learn more about parylene adhesion, download our whitepaper: