Oxide filled epoxy adhesives

Aiming at the CSP application market, Merix Corporation and Auburn University collaborated to develop anisotropic conductive adhesives called Area Bonding Conductive (ABC) adhesives. ABC adhesives are two-region thermoset adhesives with electrically conductive adhesive pads surrounded by a continuous oxide-filled dielectrical adhesive to form a total area bond. Both regions are solvent-free, B-staged, nontacky epoxies supplied on a Mylar carrier release film. In contrast to conventional ACAs, conductive areas of ABC adhesives are located only at bond pad locations. The assembly process for using ABC adhesives is shown in Fig. 9. The ABC adhesives... 

Thermal-transfer adhesives that are electrically insulating also exhibit wide ranges of thermal conductivities, depending on the filler type and amount. The thermal conductivities of epoxies filled with boron nitride or diamond are approximately 4 W/m K and 12 W/m K, respectively, while those of the more common aluminum-oxide-filled adhesives range from 1 to 2 W/m K. 

Lin and Chiu [55] studied the effects of silver or copper particle composition (silver coated or uncoated copper), on particle shape (flake or spherical), particle size and oxidation temperature on the electrical properties of copper-filled epoxy resin electrically conductive adhesives. They also studied pressure dependent conduction behaviour of compressed copper particles. The silver-coated copper particles showed significantly greater oxidation resistance than un-coated copper particles because the... 

Oxide fillers are electrically nonconductive but are used to provide thermal conductivity. Alumina, the most commonly used filler, is fairly inexpensive. It can be added in high concentrations to epoxies and silicones without significantly increasing the viscosity of the uncured material. Minimum bond line thicknesses are desirable for thermally conductive adhesives because heat flow is proportional to the ratio of thermal conductivity to bond thickness. Alumina-filled epoxies contain up to 75 wt % filler and have thermal conductivities ranging from 1.38 to 1.73 W m K. ... 

The best current 100% solids epoxy adhesives contain about 70% aluminum oxide by weight and give thermal conductivities in the range of 0.8-1 in the English units shown in Table 2. For convenience, a conversion chart is included in Table 2 to permit conversion to any other set of units. The k values for the best alumina-filled epoxies are 10-12 times greater than for unfilled epoxy resins, but are still much lower than for pure metals or solders. Nevertheless, heat flow is adequate for bonding most components. For example, an adhesive with a thermal conductivity of 0.91 and a bond thickness of 3 mils would be able to transfer about 20 W/cm of surface area, with a AT only about 10 C above the heat sink temperatures ... 

Another way of automating assembly is to use the epoxy adhesive in the front of a silver filled or oxide filled tape. These tapes are made by impregnating the epoxy adhesive into a glass mesh support, B-staging to form a tack-free film, and then slitting the tape into rolls or diecutting the tape into custom-made preforms. In production, the user places a preform under the part to be bonded, or uses a machine to chop or punch preforms from a roll of tape. In either case, the tape provides precise control of bond thickness, eliminates voids, and avoids an excessive fillet. The tapes can be cured very quickly, at temperatures over 160°C, and the controlled thickness of the bond reduces thermal stresses due to thermal expansion mismatch between bonded part and the substrate. 

The so called "flexiblized" epoxy is most commonly used for filling such joints. Sponge rod is not used in food plants due to possible subsurface contamination and bacterial growth. Several years ago, a series of tests were made and it was found that silicones are better for strong oxidizing chemical resistance, such as nitric acid. Unfortunately they lose adhesion in wet or submerged exposures. They require very dry clean surfaces and a silicone primer. A test installation is recommended before you use silicone in a project. 

In formulating epoxy resin via-fill ink, one must consider the adhesion between the fill material and the copper plating in the hole barrel. Normally it may seem suitable to use a material that does not include an acrylic component. However, some supphers have found it more beneficial to add an acryhc component because of its resistance to moisture absorption. They determined that it would be preferable to improve the ink s adhesion to copper by pretreatment with a black oxide or chemical etching process.This is similar to the approach taken by many dielectric suppliers. 

Electrically conductive adhesives are used for such applications as bonding lead wire to electrodes, solder paste replacement in the assembly of SMD, die attachment, and repair of conducting paths. Adhesives such as epoxies or acrylics are highly filled (typically 80%) with conductive metal particles. Metals such as nickel and copper cannot be used because of their susceptibility to oxidation. Precious... 

In general, conductive adhesive pastes are formulated by mixing polymeric resins (such as epoxies, silicones, and polyimides) and highly conductive metallic fillers The most popular combination has been silver (Ag) particles in epoxy resin [2,3,4,5]. The formulas and preparation techniques for conductive adhesives are still imder development. Nickel, with its lower cost than silver, and its comparatively less oxidation and better thermal stability than copper, offers an appropriate alternative to silver for ICA s, while the copper-filled systems may be unstable after exposure to elevated temperatures due to oxide growth on the particle surfaces [6]. 

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