Acrylic (PMMA)

Poly(methyl methacrylate) (PMMA) is a thermoplastic and transparent plastic. Chemically, it is the synthetic polymer of methyl methacrylate. Acrylic coatings have numerous benefits from a high volume and cost effectiveness perspective. Acrylics tend to be hard, rigid, and extremely tough. Combined with excellent electrical properties, acrylics can be considered for commercial high volume appli­cations. As opposed to epoxy, acrylics exhibit little shrinkage during the curing process. They have excellent moisture resistance and can endure weak acids and bases. Decreased dielectric constants with increasing frequencies make them good candidates for high frequency applications. However, acrylics offer poor abrasion resistance and can be attacked by strong solvents and acids.

12.7.7 Parylene

Parylene is the tradename for a variety of chemical vapor deposited poly(p-xylylene) polymers used as moisture barriers and electrical insulators. Among them, Parylene C is the most popular due to its combination of barrier properties, cost, and other manufacturing advantages. Parylene's low permeability to moisture and gases and hydrophobic nature makes it advantageous in electrical applications. Parylene is often used as a conformal coating in critical applications. Parylene C is an inert, hydrophobic, optically clear biocompatible polymer coating material used in a wide variety of industries, and, because it is deposited by vapor deposition, Parylene pro­vides a conformal coating on virtually any substrate which is not achievable by other means. Its lubricity is close to that of Teflon but can be applied uniformly in much thinner coatings, making them ideal for MEMS and bio-MEMS-based appli­cations. Parylene's pinhole free conformal coating protects coated substrates from moisture, chemicals, and electric charge. While Parylene is typically used to coat circuit boards or immobilize particles, it can also be removed from the original surface and used as a microstamp allowing MEMS patterns to be reproducibly trans­ferred to polymers with various properties (Parylene properties are available from the manufacturer at www.scscookson.com/parylene_knowledge/index.cfm).

12.7.8 Liquid Crystal Polymer

Liquid crystal polymers (LCPs) are a class of aromatic polyester polymers. They are unreactive, inert, and highly resistant to fire. LCP has been identified as an excel­lent candidate to control the humidity in nonhermetic packaging and is used in RF MEMS. LCPs have the following advantages [36]:

Near-hermetic permeability to moisture keeping the MEMS dry for months even in humid environments;

Low RF loss properties allowing excellent RF performance after packaging;

Excellent moldability allowing high-speed glob-top encapsulation.

Date: 2015-02-28; view: 986