Isotropically conductive materials

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. 

The classical example of a soUd organic polymer electrolyte and the first one found is the poly(ethylene oxide) (PEO)/salt system [593]. It has been studied extensively as an ionically conducting material and the PEO/hthium salt complexes are considered as reference polymer electrolytes. However, their ambient temperature ionic conductivity is poor, on the order of 10 S cm, due to the presence of crystalUne domains in the polymer which, by restricting polymer chain motions, inhibit the transport of ions. Consequently, they must be heated above about 80 °C to obtain isotropic molten polymers and a significant increase in ionic conductivity. 

Electrically conductive materials can be made to be either isotropically conductive (IC) or anisotropically conductive (AIC). IC materials are the most common and conduct electricity in all directions. AIC materials conduct materials in one axis when compressed between two contact points. 

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. 

Experimental [23] as well as theoretical [24-26] studies of percolation phenomena have been reported. In random and macroscopically homogeneous materials it has been demonstrated [27-29] that at concentrations of metal particles below the percolation threshold (p < Pc) a short-range percolation coherence length, exists. Electrical conductivity is probable for length scales less than Thus even if the metal-filled composite exhibits no bulk electrical conductivity, conduction can occur within domains that are smaller than As the concentration of metal particles approaches oo and the composite becomes isotropically conductive. 

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. 

The following list includes several commercial manufacturers of conductive adhesives commonly used for die attach as well as other interconnection applications. Many manufacturers sell both isotropically as well as anisotropically conductive materials. The list is not intended to be inclusive but merely to provide initial guidance. 

Markley, P. L., Tong, Q. K., Magliacca, D. J., andHahn, T. P., Characterization of Silver Flakes Utilized for Isotropic Conductive Adhesives, Proc. IMAPS, Inti. Symp. On Advanced Packaging, Materials, Processes, Properties, and Interfaces,pp. 16-20(1999)... 

Homogeneous medium. A frequently encountered case is a conducting material that is homogeneous (and isotropic, linear, etc), for which a simplified relation can be deduced from the Formal Graph. The integration along a line of electric field over a length provides in this case the electric tension... 

This anisotropy is less pronounced in molded graphites and carbon materials and the thermal conductivity is more isotropic. Typical values for graphite and other high-conductivity materials are given in Table 5.7.f l... 

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... 

D. Klosterman, L. Li, and J.E. Morris, Materials Characterization, Conduction Development, and Curing Effects on Reliability of Isotropically Conductive Adhesives, IEEE Transactions on Components Packaging and Manufacturing Technology Part A, Vol 21, 1998, p 23-31... 

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... 

As for the thermal expansion coefficient, the isotropy assumption was accomplished simply by using one-third of the volumetric thermal expansion coefficient, as suggested by Lipatov et al. (1991). It should be noted, however, that in special cases of conductive adhesive applications, directional conduction may be required, in which case the use of non-isotropic, directional material properties would be necessary. 

Only very limited work has been conducted to investigate the high-frequency behaviors of IGA joints. Felba et al. [82] investigated a formulation of isotropically conductive adhesives that performed well as a solder replacement in microwave applications. The study involved various different adhesive base materials and several types of main (silver flakes, nickel, and graphite) and additional (soot and silver semiflake powder) filler materials. In order to assess the usefulness of a given adhesive formulation, an additional gap in the gold strip of a standard microstrip bandpass... 

Consider a small control volume V = SxSySz (Fig. 4.27), where the inner heat generation is Q "(T) (heat production/volume) and the heat conductivity is A(T). The material is assumed to be homogeneous and isotropic, and the internal heat generation and thermal conductivity are functions of temperature. 

Such oriented composites should also have electrical anisotropy. Indeed, for the composite material with

sample amounts to 5 x 10 5 Ohm-1 cm-1 which is also below... 

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