Polyimide

Polyimides are a family of thermoset and thermoplastic resins characterized by repeating imide linkages. There are four types of aromatic polyimides: (1) con­densation products formed by the reaction of pyromellitic dianhydride (PMDA) and aromatic diamines such as 4,4'-diaminodiphenyl ether; (2) condensation prod­ucts of 3,4,3',4'-benzophenone tetracarboxylic dianhydride (BTDA) and aromatic amines; (3) the reaction of BTDA and a diisocyanate such as 4,4/-methylene-bis(phenylisocyanate); and (4) a polyimide based on diaminophenylindane and a dicarboxylic anhydride such as carbonyldiphthalic anhydride. Thermoset poly-imides are produced from condensation polymers that possess reactive terminal groups capable of subsequent cross-linking through an addition reaction [33]. Polyimides have excellent mechanical and electrical properties and can be used in a wide range of temperatures. They are widely used in electronics for their excel­lent high temperature and chemical resistance, but may not be ideal for use in high temperature curing and high pressure applications.

12.7.3 Polydimethylsiloxane (PDMS)

Polydimethylsiloxane (PDMS) is the most commonly used elastomer in MEMS, especially in microfluidic systems. Although available in an array of formulations, polydimethylsiloxanes have limited mechanical properties, but are balanced by good chemical and thermal degradation resistance. In comparison, polyurethanes can be either mechanically softer or stiffer than PDMS, offering better adhesion and tear resistance. These are available in biocompatible formulations that exhibit excellent chemical resistance. So far, polyurethanes are used as encapsulating structures for chemical sensors and numerous bio-MEMS applications.

Epoxy

Almost all of these encapsulants use epoxies that are not all that different from the original ones developed in 1927. Epoxies react with many kinds of molecules, and those that are useful for producing products are called hardeners. Anhydrides are one of the most important reactants, or hardeners, and are used in many encapsu-lants and underfills. Epoxies are noted for versatility and balanced properties but are rather average in general characteristics. Additives are required to bring their properties to a level where they can function as encapsulants. Flame retardants, especially bromine-containing epoxies, are added to reduce flammability and to meet specifications. The average epoxy coefficient of thermal expansion (CTE) is too high for encapsulation and acceptable levels are achieved by adding low CTE fillers such as silica. Both liquid and solid epoxies must be thoroughly polymerized to be useful for package enclosures. Epoxy molding compounds (EMCs) are actu­ally low melting mixtures of resins and other constituents, and must be polymerized into non-melting structures that have good mechanical strength and thermal stabil­ity to survive the solder assembly process. The same is true for liquid encapsulants. Polymerization, the formation of high molecular weight structures by chemical reac­tions, occurs when heat is applied to a system that contains epoxy resin, hardener, and usually an accelerator to increase the reaction rate. The resulting thermoset is a 3D structure that is highly cross-linked and non-melting. Excessive heating, how­ever, can cause the polymer to degrade by thermal decomposition, a present concern with the increasing temperatures required for many lead-free solders. Epoxies also absorb significant amounts of moisture, and the explosive release of steam, termed popcorning, is exacerbated by higher soldering temperatures [34].

Date: 2015-02-28; view: 996