http:\/\/dl.dbank.com\/c0jtkba9tb<\/a>\uff09\u6709\u4ec0\u4e48\u60f3\u6cd5\u4e5f\u53ef\u4ee5\u8bc4\u8bba\u4ea4\u6d41\u4e00\u4e0b\u54e6\uff01<\/p>\nElectrically Conductive Adhesives\u00a0 Characteristics and Applications
\nDarryl J. Small, Senior Applications Engineer and
\nBrian Eisenach, Product Manager
\nLoctite Corporation<\/p>\n
Continuing improvements in adhesive technology have enabled adhesives to replace solder in many electronic assembly applications. There are three types of electrically conductive adhesives formulated to provide specific benefits where an electrical interconnect is desired. Isotropic materials, which will conduct electricity along all axes, are able to replace solder on thermally sensitive components, and can also be used on devices that require a ground path. Conductive silicones help protect devices from environmental hazards such as moisture, and shield electromagnetic and radio frequency interference (EMI\/RFI) emissions. Anisotropic conductive polymers, which allow electrical current to flow only along a single axis, provide electrical connectivity and strain relief for flip chip devices. All electrically conductive adhesives have two common qualities: they provide a chemical bond between two surfaces, and they conduct electricity.<\/p>\n
Isotropic Materials<\/p>\n
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Electrically-conductive isotropic epoxies can be used as an electrical interconnect on non-solderable substrates such as glass and ceramic, or to replace solder on thermally-sensitive components that cannot withstand the 200?C processing temperatures of typical solders. Traditional applications for isotropic epoxies include bonding the flex circuit of a glass LCD display to the mating trace on a PCB, or attaching a component lead to a matching land on a thermally-sensitive printed circuit board.<\/p>\n
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Isotropic electrically-conductive epoxies offer non-directional, or all-directional, conductivity by incorporating conductive particles such as silver, nickel, or gold into the adhesive formulation. These particles carry electrical current through the epoxy resin.<\/p>\n
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The most popular filler material is silver due to its moderate cost and superior conductivity.<\/p>\n
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Isotropic silver-filled epoxies are available in heat or room-temperature curing, single- or two-component formulations. Since electrically conductive epoxies require temperatures of only 150?C or less to cure, and room temperature curing is a viable option, isotropic epoxies are an ideal alternative to solder on thermally sensitive parts.<\/p>\n
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Several varieties of silver-filled, isotropic epoxies are available to electronic device manufacturers. General-purpose silver-filled room temperature or heat curing epoxies have been formulated specifically for rework and repair applications on large-pitch interconnects. For example, if the trace on a circuit board\u62af surface gets gouged, selected versions of these epoxies may be used to repair the circuit in place of the original solder. These adhesives can also be used as a solder substitute on through-hole components.\u00a0 Both electrically and thermally conductive, these epoxies develop strong, durable bonds on many different substrates including metals, ceramics, glass, laminates, and molded plastics.<\/p>\n
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Room temperature curing, flexible silver-filled epoxies have been formulated for electronic interconnect applications that require high flexibility, such as assembly and repair of flexible circuits or bonding flexible substrates and connectors.<\/p>\n
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“Gold Bonders” are heat curable epoxies formulated to offer enhanced adhesion to goldplated devices and other high-end metal surfaces such as palladium. These adhesives offer enhanced shear strength, and are formulated to absorb the stress associated with extreme thermal mismatch between dissimilar substrates. Typical adhesives on the market today cure in 30 minutes at 150?C, or in one hour at 125?C.<\/p>\n
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Certain grades of silver-filled epoxies are specifically formulated for direct use on the die, and are aptly referred to as “Die Attach Grades.” These adhesives are formulated to provide an electrical groundpath and transfer heat from the chip to the substrate (Photo 1). A variety of formulations are available, offering benefits such as high purity and low ionic content, a long working life, or snap heat cures of three minutes or less.<\/p>\n
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Descriptions of real-world applications will better illustrate the use of isotropic\u00a0 electrically conductive adhesives:<\/p>\n
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Application: Low Temperature Processing<\/p>\n
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A manufacturer of antennas sold to wireless communications companies was asked by a customer to attach the aluminum plane on the antenna to a flexible coax connector. The speed of the bonding process was not an important issue; however, the cost of the process was critical. Although the manufacturer\u62af first inclination was to solder the two components together, they soon learned that thesensitive components inside the coax connector would not tolerate the temperatures required by soldering. The antenna manufacturer solved the problem by bonding the two components with a room temperature-curing electrically conductive isotropic adhesive. The adhesive provided the electrical interconnect required for the device, reduced processing temperatures, and, as no oven was required for the assembly process, greatly controlled equipment and processing costs.<\/p>\n
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Application: Solder Leaching<\/p>\n
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On an electronic device used in a satellite link control unit, two high-frequency antennas are attached to an FR4 printed circuit board. The manufacturer of the device had considered using solder, but, during testing, experienced significant problems with leaching during the reflow process. Electricity is transmitted through the device at ultrahigh frequencies, and mechanical methods of attachment were not feasible because of the use of fine pitch components. The manufacturer chose a heat-cure electrically conductive isotropic adhesive paste over solder, and thereby eliminated leaching problems and provided a long use life that did not require freezer storage. In addition, the electrically conductive adhesive provided the necessary electrical interconnect for the thermally sensitive components used to manufacture the device.<\/p>\n
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Application: Stress Cracking<\/p>\n
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An electronics manufacturer was experiencing problems attaching surface mount components to a flex circuit. Since solder cannot be bent to 360? without stress cracking, the use of flexible electrically conductive adhesives made this application possible. The adhesive was first stenciled onto the polyimide flex circuit, the surface mount component was placed onto the polyimide substrate, and a heat cure process bonded the two components together. The polyimide could then be easily flexed without breaking the circuit.<\/p>\n
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Application: Thermal Mismatch<\/p>\n
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In a flip chip in BGA application, a manufacturer of computer components mounts a bumped die directly to a base substrate, and underfills the die to protect the solder joints from thermal and physical shock. To further protect the chip, a metal cap is placed over the chip and bonded in place using a flexible isotropic electrically conductive adhesive. The adhesive provides three benefits in this application: the material flexibility allows it to withstand the differing rates of thermal expansion (CTE) between the metal cap and the silicon chip and to provide bond strength between the parts; the adhesive acts as a ground circuit between the chip and the cover; and the material is thermally conductive to help transfer heat from the chip to the cover, which acts as a secondary heat sink.<\/p>\n
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Conductive Silicones<\/p>\n
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New conductive silicone materials allow device manufacturers to shield EMI\/RFI emissions and seal electronic enclosures, protecting them from environmental hazards. Conductive silicones can be applied directly to areas that require environmental sealing and EMI\/RFI shielding, and cured-in-place to form a customized gasket that eliminates the need to inventory cut gaskets.<\/p>\n
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There are a number of conductive fillers that can be incorporated into gasketing formulations. These materials are available with varying levels of attenuation and different cure methods including heat cure and room temperature vulcanization.<\/p>\n
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In addition to environmental protection and EMI\/RFI shielding, conductive silicones can also be used to provide ground paths for electronic devices that require high flexibility. For example, on video touch screens, silicone conductive adhesives are used to bond wire leads to a ground strip on the back of the screen, providing an electrical interconnect that communicates the request for information from the touch screen to the circuits inside the computer. Whereas solder and most other adhesives would be too rigid to hold up against the constant contact required by the touch screen, highly flexible and resilient silver-filled conductive silicones easily survive high impact applications.<\/p>\n
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Anisotropic Conductive Polymers<\/p>\n
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Electrically conductive anisotropic polymers offer structural integrity as well as electrical interconnection. Because these adhesives are designed to provide electrical interconnection only at planned sites where particles come into direct physical contact with conductive substrates, anisotropic adhesives can be used to provide structural strength without an electrical connection on other areas of the device.<\/p>\n
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Anisotropic polymers allow electrical current to travel uni-directionally, or along only one axis. The adhesive matrix contains a concentration of electrically-conductive metal particles that is below isotropicity ?in other words, the concentration of conductive particles is limited to allow electricity to travel only in the Z-direction, and not on the XY plane. (Figure 2) Particles are isolated from one another so that electricity travels in only one direction; if conductive particles do make contact, electrical current will travel along the X and Y axis as well as the Z-axis.<\/p>\n
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Anisotropic adhesives are currently available in two forms \u2014 single-component, heat-curable liquids and pastes, and heat curable thermosetting or thermoplastic adhesive films. In the global marketplace, 90 percent of all anisotropic polymers are currently sold as films used for LCD production, particularly flexible circuit connection to glass. In addition to LCD assembly, other emerging applications for anisotropic conductive adhesives include flip-chip-onglass, smart cards, and flip-chip-on-board. The common characteristic linking all of these applications is the inability to solder one or more of the substrates that make up the device because of thermal sensitivity of the substrate, or temperature constraints imposed by the process.<\/p>\n
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During assembly, manufacturers using anisotropic polymers must carefully control the placement of the chip. Whereas the surface tension of molten solder will correct for inexact chip placement, for anisotropic materials to establish a working electrical connection the chip must be placed exactly in-line with the Z-axis, and heat and pressure must be maintained on the chip until the adhesive has cured.<\/p>\n
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Currently available anisotropic adhesive tape products offer a random distribution of conductive particles ?this means that particles are not distributed in an organized manner and may come into contact with one another or be absent in areas where they are needed. Contact between a number of particles can cause short circuits in the X-Y plane; voids, or areas where particles are absent at a critical interconnect site, result in open circuits. For high-end electronic applications that require fine pitch capabilities, some adhesive manufacturers are currently working to develop anisotropic adhesive films with ordered particle distribution, which would guarantee only z-axis conductivity and overcome problems with open and short circuits.<\/p>\n
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The majority of anisotropic liquid adhesives used for electronic packaging are epoxies; however, a wide range of other polymers could be used to achieve unidirectional interconnection and a strong structural bond. Common materials available on the market contain gold-coated polymer spheres, solder-type alloys, or solid metal powders such as nickel, gold, or silver.<\/p>\n
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Application: Flip-Chip on Flex for a Smart Card Module<\/p>\n
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A manufacturer of smart cards wanted to reduce the profile size of their cards. The thickest component in the 150 micron smart card was a 130 micron wire bonded chip covered by a glob top encapsulent. By replacing the wire bonded chip and encapsulant with a flip chip that attaches directly to the substrate, the manufacturer could reduce the profile thickness by 70 percent, to 40 microns. Solder could not be used in this case because the polyamide flex circuit used in the smart card was not a solderable material. Also, the anisotropic adhesive could establish an electrical interconnection for the dense pattern of bumps on the flip chip while providing the structural strength required to bond and protect the chip.<\/p>\n
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Cost of Electrically Conductive Adhesives Versus Solder<\/p>\n
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Adhesive assembly requires approximately half as much material as does solder for the same application. When material costs are considered alone, the cost of adhesives is typically several times the cost of solder per gram. However, the processing costs of adhesives can be substantially lower than those of solder as there are fewer steps involved in adhesive bonding.<\/p>\n
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Electronic device assembly using adhesives is basically a three-step process: 1) adhesive is applied to the component or the substrate using automated dispense equipment or a manual syringe dispenser; 2) components are placed on the substrate; and 3) the assemblies are placed in an oven for heat cure, although room-temperature curing is also an option. The reflow soldering is more complicated and typically involves a five-step process: 1) the substrate is treated with flux to prepare the surface; 2) solder is dispensed onto the board via stencil printing or syringe dispense; 3) components are placed on the board; 4) the board is put through a reflow oven and cooled; and 5) the remaining flux is cleaned from the board using solvents.<\/p>\n
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The processing time for the dispense process and heating steps are very similar for both solder and adhesive bonding. However, adhesives do not require the time and expense of flux application and cleaning. Solder flux was once removed quickly and inexpensively using CFCs which evaporated into the environment. In the aftermath of the Montreal Protocol, CFC use has been curtailed, and board manufacturers are often forced to use much less efficient and less cost-effective cleaning methods to remove flux. On the contrary, adhesives are extremely process friendly, reducing hazardous waste chemicals or associated disposal costs.<\/p>\n
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Despite all these factors, solder is often less expensive than adhesives for standard electronic packaging applications. For specialized applications that have heat sensitive components or non-solderable substrates, however, electrically conductive adhesives are frequently the only option.<\/p>\n
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Conclusion<\/p>\n
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Into the future, electrically conductive adhesives will play an increasingly prominent role in the design and production of electronic packages. A number of factors will drive this growing need, including:<\/p>\n
1.the greater demand for fine pitch capabilities;<\/p>\n
2.the need to control greater amounts of heat generated by increasingly powerful integrated circuits,<\/p>\n
3.and the use of non-solderable or thermally-sensitive substrates such as glass andplastics.<\/p>\n
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