Epoxy silicone polymers, properties

The protection of microelectronics from the effects of humidity and corrosive environments presents especially demanding requirements on protective coatings and encapsulants. Silicone polymers, epoxies, and imide resins are among the materials that have been used for the encapsulation of microelectronics. The physiological environment to which implanted medical electronic devices are exposed poses an especially challenging protection problem. In this volume, Troyk et al. outline the demands placed on such systems in medical applications, and discuss the properties of a variety of silicone-based encapsulants. 

Conductive adhesives are generally formulated from base polymers that are low-viscosity, thermosetting resins such as epoxies. Where elastomeric properties are required, silver-filled flexible epoxy and silver-filled silicone rubber systems are commercially available. 

Wang, W.J. Perng, L.H. Hsiue, G.H. Chang, F.C. Characterization and properties of new silicone-containing epoxy resin. Polymer 2000, 47, 6113. 

Adhesives are often referred to by their polymer type. The major polymer types used in electronics packaging are epoxies, silicones, acrylics, polyurethanes, polyimides, and cyanate esters. Some generic properties apply to each type but, in generalizing, one must be careful since there are hundreds of formulations of each type on the market, each with minor or major differences in properties. Even for a specific formulation, an adhesive s processing conditions, such as its cure schedule, can affect the final properties. Chapter 3 provides a discussion of the major polymer types and their properties. 

Reliability of the electrical properties of silicone-based isotropic adhesives has been the major difficulty to overcome and has essentially prevented commercialization. Another problem associated with silicones is that the addition polymerization reaction of silicones must be carefully controlled to prevent cure inhibition from various common chemical contaminants such as amines and sulfides. Other concerns include low-molecular-weight silicone polymer migration onto wirebond pads and very high GTE. There has been some activity in the development of hybrid resins that contain silicone blocks as comonomer with epoxies such that the epoxy processing can be maintained with the added stress reduction property of the silicones [52]. 

A basecoat is a layer on the surface that changes the properties of the surface. Rowed basecoats of polymers on rough surfaces are used to provide a smooth surface for deposition. Basecoat materials of acrylics, poljmrethanes, epoxies, silicones, and siloxanes are available and are very similar to the coating materials that are used for coirformal coatings. In solvent-based... 

Silicone acrylates (Fig. 5) are again lower molecular weight base polymers that contain multiple functional groups. As in epoxy systems, the ratio of PDMS to functional material governs properties of release, anchorage, transfer, cure speed, etc. Radiation induced radical cure can be initiated with either exposure of photo initiators and sensitizers to UV light [22,46,71 ] or by electron beam irradiation of the sample. 

Silane coupling agents may contribute hydrophilic properties to the interface, especially when amino functional silanes, such as epoxies and urethane silanes, are used as primers for reactive polymers. The primer may supply much more amine functionality than can possibly react with the resin at the interphase. Those amines that could not react are hydrophilic and, therefore, responsible for the poor water resistance of bonds. An effective way to use hydrophilic silanes is to blend them with hydrophobic silanes such as phenyltrimethoxysilane. Mixed siloxane primers also have an improved thermal stability, which is typical for aromatic silicones [42]. 

Three classes of polymer encapsulant materials were studied. These are listed in Table I and Include novolac epoxy and silicone-epoxy compounds. A pure silicone formulation served as a control for comparison of the thermal degradion properties... 

Most structural PMCs consist of a relatively soft matrix, such as a thermosetting plastic of polyester, phenolic, or epoxy, sometimes referred to as resin-matrix composites. Some typical polymers used as matrices in PMCs are listed in Table 1.28. The list of metals used in MMCs is much shorter. Aluminum, magnesium, titanium, and iron- and nickel-based alloys are the most common (see Table 1.29). These metals are typically utilized due to their combination of low density and good mechanical properties. Matrix materials for CMCs generally fall into fonr categories glass ceramics like lithium aluminosilicate oxide ceramics like aluminnm oxide (alnmina) and mullite nitride ceramics such as silicon nitride and carbide ceramics such as silicon carbide. 

The most common advanced composites are made of thermosetting resins, such as epoxy polymers (the most popular singlematrix material), polyesters, vinyl esters, polyurethanes, polyimids, cianamids, bismaleimides, silicones, and melamine. Some of the most widely used thermoplastic polymers are polyvinyl chloride (PVC), PPE (poly[phenylene ether]), polypropylene, PEEK (poly [etheretherketone]), and ABS (acrylonitrile-butadiene-styrene). The precise matrix selected for any given product depends primarily on the physical properties desired for that product. Each type of resin has its own characteristic thermal properties (such as melting point... 

Structure II polymers are relatively elastic when polymethylphenylsiloxanes are obtained by the hydrolytic cocondensation only of trifunctional monomers (e.g., methyl- and phenyltrichlorosilanes), there are polymers with low elasticity. Polydimethyl- and polymethylphenylsiloxanes can be modified with organic polymers (polyester, epoxy) or silicone substances, e.g. methyl(phenylaminomethyl)diethoxysilane. The modification of polydimethyl- and polymethylphenylsiloxanes improves some properties of these polymers and varnishes based on them in particular, it considerably increases adhesion and mechanical durability of varnish films. 

Epoxy polymers (including epoxy novolacs) have been designed to meet most of these requirements and are almost universally used in such encap-sulant applications. Epoxy polymers exhibit superior adhesion that in many cases eliminates the need for a barrier or junction coating. They have a low coefScient of thermal expansion low shrinkage and low injection velocity, which means that low transfer or injection pressures can be used. These polymers also possess excellent mechanical properties coupled with low moisture and gas permeability. Above all, they are cheap and readily available. Other transfer-molding materials used to a limited extent include silicones, phenolic materials, and even polyesters. Most molding formulations are highly filled (70-75%) with materials such as quartz, fused silica, short... 

The properties of thermosetting and thermoplastic resin systems are continually improved to meet increasing performance requirements of end users. One way to enhance material properties is to incorporate nano-modifiers, based on elastomeric silicone particles, which are optionally grafted with other (acrylic) polymers to control dispersibility, viscosity, and other parameters. As an example, epoxy resin formulations have been modified with silicone nanospheres to improve low-stress behavior. Table 1 shows the outstanding fracture toughness improvement of silicone coreshell nanospheres, even at very low particle loading levels. 

The first truly synthetic resin was developed by Baekeland in 1911 (phenol-formaldehyde). This was soon followed by a petroleum-derived product called coumarone-indene, which did indeed have the properties of a resin. The first synthetic elastomer was polychloroprene (1931) originated by Nieuwland and later called neoprene. Since then many new types of synthetic polymers have been synthesized, perhaps the most sophisticated of which are nylon and its congeners (polyamides, by Carothers), and the inorganic silicone group (Kipping). Other important types are alkyds, acrylics, aminoplasts, polyvinyl halides, polyester, epoxies, and polyolefins. 

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