Epoxy adhesives volume resistivity

Silver-filled epoxies and other electrically conductive adhesives are widely used to electrically connect chip devices or packaged components to interconnect substrates or printed-circuit boards. Chip capacitors, resistors, transistors, diodes, and magnetic components may be attached with silver-filled epoxies whose volume resistivities range from 1 x 10 " to 3 x 10 " ohm-cm or with gold-filled epoxies whose volume resistivities are approximately 8 x 10 ohm-cm. Conductive adhesives are also finding use as replacements for solder balls in flip-chip devices. In all cases, to achieve reliable connections, initially low-contact resistances or volume resistivities must remain low on aging and on exposure to operational stress conditions, such as humidity, temperature, vibration, shock, and power. 

For polyamine-cured resin, recycle of presoaked epoxy also increased both impact strength and heat deflection temperature. For polyamide-cured resin, recycle also increased volume resistivity. Pre-soaked recycle dramatically increased adhesion to aluminum, especially in the anhydride system. 

Other effects were more selective. While recycle usually lowered impact strength and heat deflection temperat ire, pre-soaking polyamine recycle surprisingly improved both of these properties. While recycle usually lowered volume resistivity, polyamide recycle improved it. Finally, adhesion of epoxy formulations to the aluminum mold, in spi/ e of wax and silicone mold release agents, was dramatically increased by the use of presoaked recycle, especially in the ai ydrlde system, suggesting unexpected usefulness in epoxy adhesive formulations. 

To function as electrical conductors, epoxies and other polymer resins, because they are inherently insulators, must be filled with electrically conductive particles such as metals. The selection of electrically conductive or insulative adhesives is based largely on their conductivities or, conversely, on their volume resistivities. Electrically conductive adhesives should have low resistivities initially and retain those values on aging, moisture exposure, thermal cycling, and other operating and test conditions. The resistivities of metal-filled epoxy adhesives can range from... 

Table 2.3 Volume resistivity versus cure conditions for a silver-filled epoxy adhesive ...

Figure 2.12 Volume resistivity versus temperature for silver-filled epoxy paste adhesive.

Table 2.4 Volume resistivities for silver-filled epoxy adhesives (ohm-cm)...

For electrically conductive metal-filled adhesives, dielectric constants and dissipation factors are not meaningful parameters, but volume resistivities and contact resistances are significant. The volume resistivities for the best silver-filled epoxies range from 1 x 10 to 8 x 10 " ohm-cm. 

Table 2.3. Volume Resistivity vs Cure Conditions for a Silver-filled Epoxy Adhesive ...

A typical thermally conductive epoxy system used as an adhesive, as well as for other purposes, has a thermal conductivity of 0.0026 cal/cm/sec/°C and a volume resistivity of 1.5 x 10 ohm.cm (1.5 x 10 ohm.m). Fillers include alumina (aluminum oxide), beryllia (beryllium oxide), other unspecified inorganic oxides, boron nitride, and silica. Boron nitride is an excellent choice as a thermally conductive filler except that its content reaches a maximum at about 40% by weight in epoxy resins. The resultant products are always thixotropic pastes. BerylUa powder has excellent thermal conductivity by itself, but when mixed with a resin binder its conductivity drops drastically. It is also highly toxic and high in cost. Alumina is a commonly used filler to impart thermal conductivity in resins. ... 

Some applications call for an electrically conductive adhesive. Masterbond offers EP21TDC/S Medical which is a two-component, electrically conductive, silver-filled tough epoxy adhesive with a high peel strength as well as low volume resistivity. 

According to their chemical composition, conductive adhesive formulations have to he preserved either at low temperature, typically - 40°C for one-part epoxies, or at room temperature. For example, IP 680 polyimide or two-part epoxies can he kept at 20°C for at least 6 months, whereas one-part epoxies have a shelf life of 2 months at - 10°C and 6 months at - 40°C. The minimum shelf life requirement is 6 months at the temperature indicated hy the supplier. Material inspection includes the measurement of the pot life, corrosivity, volume resistivity at 25°C, and lap shear strength at 25°C. Typically pot life, bond strength, and volume resistivity have been used by adhesive manufacturers to determine the shelf life of the formulated materials. However, other methods such as DSC can predict the remaining shelf life by a quantitative measurement of the heat of reaction [58],... 

A process for chip-on-glass bonding of bumped integrated circuits to ITO conductors has been developed by using a two-part isotropic epoxy adhesive loaded with silver flakes [122]. The first observation is that the volume resistivity and contact resistance depend on the type of flakes and the percentage by weight of particles dispersed in the adhesive composition. According to the kind of flakes, the volume resistivity of adhesives loaded with 75-80 wt% silver varies... 

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