Лако-красочные материалы — производство

Conductive adhesives include both electrically and thermally conductive materials. Most adhesive fillers that result in electrical conductivity in a material also contribute to its thermal conductivity. However, thermally conductive adhesives that are elec­trically insulative are also available.

The majority of applications involve the use of silver, either in flake or powder form. This filler is preferred to gold because of its lower cost and its lower volume resistivity (that is, higher electrical conductivity). The best silver-filled epoxies have a volume resistivity of 0.001 Q cm. With silver-filled adhesives, migration of silver to the surface under conditions of high humidity and direct current can occur. Neither gold — nor silver-coated copper fillers in various adhesives have this migration problem. Epoxies with up to 85 wt % silver filler are available, but adhesives with lower filler loadings generally have better strengths.

Other fillers that provide electrical conductivity include copper and aluminum, although oxide formation on the surfaces of these fillers can occur. This results in lower conductivity because of decreased particle-to-particle contact.

Recently, conductive epoxy resins and polyimides have been used as die-attach adhesives. The chloride ion contents of polyimides are inherently lower than those of epoxies. However, these adhesives are more expensive, have lower bond strength, and are more difficult to cure. Epoxies with low contents of ionic impurities, including chloride anions and sodium and potassium cations, have been available since 1981. These materials are fast curing and have excellent high-temperature strength and a glass transition temperature of up to 200 °С.

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 nrfi1 K_1.

Other thermally conductive fillers include beryllium oxide, which is expensive and toxic, and boron nitride, which has a loading limitation of ca. 40 wt % in epoxies. Other inorganic oxides, including silica, are also used to provide thermal conductivity.

The chemical families that are used most often to provide electrical and/or thermal conductivity include epoxies, polyurethanes, silicones, and polyimides. Epoxies are the most widely used.