Conductive epoxy adhesives

Flip-chip devices have solder bumps, other metal bumps, or even conductive adhesive bumps on the face of the device for I/O connections. During assembly, the devices are flipped face down, then mated and bonded to corresponding solder or metal pads on the package or interconnect substrate. In the quest to eliminate tin-lead solders, electrically conductive epoxy adhesives are beginning to be used for the bumps. 

Pandiri SM. The Behavior of Silver Flakes in Conductive Epoxy Adhesives. Adhesives Age. Oct. 1987. 

Some of the first commercial conductive epoxy adhesives were simply based on silver powder dispersed in a liquid epoxy resin [e.g., diglycidyl ether of bisphenol A (DGEBA)] with an aliphatic amine [e.g., triethylene tetramine (TETA)] as a curing agent. Although capable of room-temperature cure, commercialization of this type of system was hampered by severe mix ratio disparity (typically, 50 1 by weight), problematic mixing due to viscosity differences, short pot life, and safety concerns. See Table 3 for typical properties of this type of formulation. 

P. Savolainen and J. Kivilahti, A Solder Alloy Filled Z-Axis Conductive Epoxy Adhesive, J. Adhes., Vol 49, 1995, p 187-196... 

The resins, solvents and curing agents used in modem conductive epoxy adhesives are chosen to minimize residual vapors which can be released after cure. Modem epoxy adhesives generally consist of high purity Bis-A epoxies cured with a phenolic novolac resin. These mixtures do not contain boron or fluorides, and generate less ammonia or other corrosive vapors than epoxies available before 1982. 

Ionic impurities such as Cl, Na", and can cause corrosion of A1 bond pads and other metallization, loss in oxide dielectric strength in FET circuits, and other problems in high reliability (high rel) ICs. Until recently, most conductive epoxy adhesives contained very high levels of extractable chloride ions and other ionic impurities. The adhesives of Table 3, for example, would typically yeild over 600 ppm Cl and over 200 ppm Na, after a 24 hour extraction in neutral water at 100 C. 

A conductive epoxy adhesive is screen printed over the connecting pads of the wiring board on top of which the integrated circuit carrying gold stud bumps is flip... 

Pandiri, S.M. The behavior of silver flakes in conductive epoxy adhesives. Adhes. Age, 1987 31-35. Gunther, B. Schafer, H. Porous metal powders for conductive adhesives. Proceedings of the 2nd International Conference on Adhesive Joining and Coating Technology in Electronics Manufacturing, Stockholm, Sweden, June 1996 55-59. 

Fig. 8.5 Epoxy curing through resistive heating of nanocarbons dispersed in the matrix (a) shows a schematic representation of the process (b) experimentally obtained curing cycle and (c) repair of a structural composite panel using the conductive epoxy as resistively curable adhesive [36]. With kind permission from Elsevier (2013).

Titanium or beryllium oxide also provides a degree of improvement in thermal conductivity to epoxy systems. Magnesium oxide and aluminum oxide have also been commonly used for this purpose, although the degree of improvement is not as great as with the fillers discussed above. The effect of various fillers on the thermal conductivity of cured adhesive is shown in Fig. 9.6. The incorporation of metal fibers with metal powders has been shown to provide synergistic improvement to the thermal conductivity of adhesive systems,... 

Nonconductive fillers are employed with electrical-grade epoxy adhesive formulations to provide assembled components with specific electrical properties. Metallic fillers generally degrade electrical resistance values, although they could be used to provide a degree of conductivity as discussed above. 

Aluminum powder, in particular, is frequently employed at relatively high concentrations in high-temperature epoxy adhesive formulations. The filler provides improvement in both tensile strength and heat resistance, and it increases the thermal conductivity of the adhesive. Aluminum powder fillers also reduce undercut corrosion and, hence, improve adhesion and durability of epoxy adhesive between bare steel substrates. It is believed that this is accomplished by the aluminum filler providing a sacrificial electrochemical mechanism.27... 

These processes have an advantage in that the heat penetrates deeply into the joint and into the epoxy material itself. With conventional thermal energy processes, the heat must be conducted into the mass of the epoxy adhesive from outside the joint. This is hindered by the presence of the substrates, the substrate geometry, and the relatively low thermal conductivity of the epoxy itself. 

Heating elements can be anything that conducts current and can be heated through Joule heating. This includes nichrome wire, carbon fiber, woven graphite fabric, and stainless steel foil. Implant materials should be compatible with the epoxy adhesive and the intended application, since they will remain in the bond line for the life of the product. 

When the epoxy adhesive cannot be made flexible enough, the thermal conductivity and thermal expansion coefficient are controlled by appropriate fillers. General-purpose room temperature cured epoxy-polyamide adhesive systems can be made serviceable at low temperatures by the addition of appropriate fillers to control thermal expansion. 

Relatively low thermal expansion coefficient and high thermal conductivity compared to most epoxy adhesives... 

A third difficulty in bonding metal surfaces is that they have a higher thermal coefficient of expansion and thermal conductivity than most epoxy adhesive systems. As explained in other chapters of this book, the difference in rates of thermal expansion results in internal stresses in the adhesive joint, especially when the adhesive bond is cured at elevated temperatures or when it is exposed to low temperatures or repeated thermal cycling. 

Adhesive testing can also be classified as to the condition of the epoxy adhesive at the time of test. Tests are conducted on the... 

Dielectric test methods are used to measure the cure of epoxy adhesives between two conducting electrodes. This method is especially appropriate for metal-to-metal joints because the substrates themselves can be used as the electrode. The adhesive is treated as a capacitor during the test. Its response (dielectric constant, dissipation factor, etc.) over a range of electrical frequencies is measured as a function of curing time. 

Inorganic fillers such as clays, CaCC>3, talc, silica, titanates, A1 and asbestos are commonly used in epoxy adhesives, as they are cheap and readily available. Conducting epoxies can be formulated with powdered copper metal or a mixture of a blend of Sn-Pb-Bi. 

Thermal conductivity and expansion are important properties of adhesives used in electronics. Both properties influence the performance of computer chips. Generally, the chip has a protective cover which is attached by an adhesive. The adhesive bond must be maintained during thermally induced movement in the chip. The chip is bonded to its base with an adhesive which must also take thermal movement and, in addition, transfer heat from the chip. Two epoxy adhesives were used in the study silica filled epoxy (65 and 75 wt% SiO2 epoxy) and epoxy containing 70 wt% Ag. Figure 15.6 shows their thermal conductivities. The behavior of both adhesives is completely different. The silver filled adhesive had a maximum conductivity at about 6()"C whereas the maximum for SiOz filled adhesive was 120"C. The Tg of both adhesives was 50 and 160 C, respectively. Below its Tg, the thermal conductivity of the adhesive increases at the expense of increased segmental motions in the chain molecules. Above the Tg the velocity of photons rapidly decreases with increasing temperature and the thermal conductivity also decreases rapidly. 

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