Isotropic conductive adhesives electrical conductivity

Electrically conductive adhesives may be isotropic (conduction equally in all directions) or anisotropic (conduction in the z-direction only). Both types are widely used in the assembly and packaging of electronics. 

There are two types of conductive adhesives conventional materials that conduct electricity equally in all directions (isotropic conductors) and those materials that conduct in only one direction (anisotropic conductors). Isotropically conductive materials are typically formulated by adding silver particles to an adhesive matrix such that the percolation threshold is exceeded. Electrical currents are conducted throughout the composite via an extensive network of particle-particle contacts. Anisotropically conductive adhesives are prepared by randomly dispersing electrically conductive particles in an adhesive matrix at a concentration far below the percolation threshold. A schematic illustration of an anisotropically conductive adhesive interconnection is shown in Fig. 1. The concentration of particles is controlled such that enough particles are present to assure reliable electrical contacts between the substrate and the device (Z direction), while too few particles are present to achieve conduction in the X-Y plane. The materials become conductive in one direction only after they have been processed under pressure they do not inherently conduct in a preferred direction. Applications, electrical conduction mechanisms, and formulation of both isotropic and anisotropic conductive adhesives are discussed in detail in this chapter. 

Conductive adhesives are one of the feasible alternatives to lead for electronics assembly. Isotropically conductive adhesives are suitable for standard pitch (50- to 100-mil) surface-mounted components and numerous commercial materials are available (see commercial suppher Ksting, Section VI.E). Anisotropically conductive adhesives are more suited to flex to rigid connections, fine pitch components (15- to 20-mil pitch), and flip-chip assembly (4- to 12-mil pitch) [22]. Adhesives are not ready to replace solder throughout the electronics industry, however, due to questions that remain concerning the reliability of electrical interconnections. Their implementation is currently limited to low-cost applications using polyester substrates and specialty appHcations where solder cannot be used. Additionally, the lack of equipment for large-volume assembly with anisotropically conductive adhesives, which require the simultaneous appUcation of heat and pressure, impedes the acceptance of these promising materials. 

Table 2 lists the requirements for a type I (electrically conductive) adhesive and test results of a typical current generation isotropically conductive adhesive as reported by Estes [44]. These requirements specify test ranges for characteristics that will establish processing, performance, and reliability. 

Materials for use as anisotropically conductive adhesives must satisfy requirements even more stringent than those defined previously for isotropically conductive adhesives. No specifications, however, have been defined specifically for these materials. When used for flip-chip applications, the adhesive not only serves as a physical and electrical interconnection between the device and the substrate, but also serves as the environmental protection and passivation layer. This fact, combined with high adhesive concentrations, makes the ionic contamination levels of these materials more critical than for isotropic conductive adhesives. In addition, the processing of these materials has a greater influence on joint reliability as the anisotropic electrical properties develop only after heat and pressure are applied to the joint. 

IPC-3406 Guidelines for Electrically Conductive Surface Mount Adhesive IPC-3407 General Requirements for Isotropically Conductive Adhesives lPC-3408 General Requirements for Anisotropically Conductive Adhesive Films lPC-4101 Specification for Base Materials for Rigid and Multilayer Boards lPC-4103 Specification for Plastic Substrates, Clad or Unclad, for High Speed/High Frequency Interconnection... 

Conductive adhesive bonding and electrical contact through the isotropic conductive... 

H. Better, R.B. Van Der Plas, and A. Arun-junai. Factors that Influence the Electrical Contact Resistance of Isotropic Conductive Adhesive Joints During Climate Chamber Testing, International Journal of Microelectronic Packaging Materials and Technologies, Vol 1, 1998, p 177-185... 

Z.M. Mo et al.. Electrical Characterization of Isotropic Conductive Adhesive under Mechanical Loading, J. Electron. Mater, Vol 31, 2002, p 916-920... 

J.H. Constable et al.. Continuous Electrical Resistance Monitoring, Pull Strength, and Eatigue Life of Isotropically Conductive Adhesive Joints, IEEE Transactions on Components and Packaging Technologies, Vol 22, 1999, p 191-199... 

L. Ye et al.. Effect of Ag Particle Size on Electrical Conductivity of Isotropically Conductive Adhesives, IEEE Transactions on Electronics Packaging Manufacturing, Vol 22, 1999, p 299-302... 

There are three types of electrically conductive adhesives typically used in the electrical industry, e.g., isotropic conductive, anisotropic conductive, and nonconductive adhesives. 

Isotropic conductive adhesives have a large amount of conductive fillers (25 30 vol%) with the electrical conductivity provided by the contact between the conductive fillers. 

The electrical conductivity of isotropic conductive adhesives is established in all directions by metal particles, generally silver flakes. These silver flakes are embedded in a matrix consisting of a base polymer, generally epoxy resin, and hardeners such as amines or carboxylic acids. It is the hardeners that ensure the mechanical strength of the joint. The choice of these two components can influence reaction speed and curing temperature. Broadly speaking, any chemical reaction will be fast at a high temperature and slow at a low temperature. 

Constable, J.H. Kache, T. Teichmann, H. Muhle, S. Gaynes, M.A. Continuous electrical resistance monitoring, pull strength, and fatigue life of isotropically conductive adhesive joints. IEEE Trans. Compon. Packag. Technol. June 1999, 22 (2), 191-199. 

Botter, H. Factors that influence the electrical contact resistance of isotropic conductive adhesive joints during climate chamer testing. Proceedings of the 2nd International Conference on Adhesive Joining and Coating Technology in Electronics Manufacturing, Stockholm, Sweden, June 3-5, 1996 30-37. 

Over the past decades, development and use of electrically conductive adhesives have expanded in the microelectronics industry. The major applications are die attachment, liquid crystal display and surface-mounted assembly of packaged components on printed wiring board. Recently, isotropic conductive adhesives (ICA s) have been widely used in electronics applications to replace lead-based soldering due to toxicity of the lead in Sn-Pb solders [1]. 

Anisotropic Material property with behavior differing in dependence on stress direction, for example, wood, electrical properties of special conductive adhesives opposite isotropic. 

Isotropic (Iso (Greek) = same tropos (Greek) = direction) Same properties of a substance in all directions (e.g., electric properties of special conductive adhesives) Opposite anisotropic. 

Electrical conductivity in anisotropic adhesives occurs by a mechanism different from that of isotropic adhesives. Although metal fillers are also used, they are used in much lower amounts (0.5-5% by volume) so that the adhesive is essentially an insulator in the x-y directions. On inserting the adhesive between the electrodes (for example, the metal bumps of a flip-chip device with metal pads on a flex circuit) of two parts and applying pressure and heat, the metal particles form a z-direction electrical connection between the electrodes while the surrounding material remains insulating. The... 

Gaynes MA, Lewis RH, Saraf RE, Roldan JM. Evaluation of contact resistance for isotropic electrically conductive adhesives. IEEE Trans Comp, Packaging, Mfg Tech 1995 18(2) 299-304. Part B. 

Isotropic adhesives conduct current equally in all directions and are the most common and widely used in industry. The anisotropic types, also referred to as z-direction adhesives or anisotropic-conductive adhesives (ACA), although filled with metal particles, are filled at much lower levels (0.5%-5% by volume) than isotropic types (filled 25%-30% by volume). The volume Iraction of filler is well below the percolation threshold at which the adhesive becomes highly conductive in all directions. Because of the low volume Iraction of metal particles, there are no continuous electrically conductive paths in the x-y plane. During the connection process, the anisotropic adhesive, either as a film or paste, is positioned between a flip-chip bumped die or a tape-automated bonded (TAB) die and the corresponding pads of an interconnect substrate. Pressure and heat are... 

Anisotropic conduction. Electrical conductivity in anisotropic adhesives occurs by a different mechanism than for isotropic adhesives. Although metal fillers are also used, they are used in much lower amounts (0.5% to... 

Gaynes, M. A., Lewis, R. H., Saraf, R. F., and Roldan, J. M., Evaluation of Contact Resistance for Isotropic Electrically Conductive Adhesives, IEEE Trans, on Components, Packaging, and Mfg. Tech., Part B, 18(2) 299-304 (1995)... 

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