Isotropic conductive adhesives process

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. 

Most conductive adhesive failures are accelerated by elevated temperature and humidity. In a study of 12 commercially available isotropically conductive adhesives, joint resistance increased between 160 and 35,000% when exposed to 65°C and 85% relative humidity (65/85) [56]. However, some adhesive manufacturers claim resistance change of less than 10% after 1000 h at 60°C and 90% relative humidity [57] and less than 4% after 1000 h at 85/85 [58]. Anisotropically conductive adhesive joints are even more susceptible to early failures under accelerated test conditions due to process variations [16]. Reliability screening tests can be used effectively to iteratively optimize process parameters. 

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)... 

The pressure sensor is a two-shot molding. Production volume is on the high side of 10 million units per year. The printed-circuit board is contacted on the MID by conductive-adhesive gluing (isotropic conductive adhesive). Nonconductive adhesive is also applied for mechanical location. The contact pins of the MID are tiny bumps to which the conductive adhesive is applied in the dispensing process. [143]... 

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. 

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... 

Conductive Adhesives. Isotropically electrically conductive adhesives (ICAs) are widely used in the electronics industry when high-temperature soldering processes are unsuitable. Typical applications include silicon die attachment, component attachment in multi-chip modules (MCMs), surface mounted printed circuit board repair, and elec-tromagnetic/radio frequency (EMI/RFI) shielding. 

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