Isotropic adhesives

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... 

Reliability of the electrical properties of silicone-based isotropic adhesives has been the major difficulty to overcome and has essentially prevented commercialization. Another problem associated with silicones is that the addition polymerization reaction of silicones must be carefully controlled to prevent cure inhibition from various common chemical contaminants such as amines and sulfides. Other concerns include low-molecular-weight silicone polymer migration onto wirebond pads and very high GTE. There has been some activity in the development of hybrid resins that contain silicone blocks as comonomer with epoxies such that the epoxy processing can be maintained with the added stress reduction property of the silicones [52]. 

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... 

Earlier studies conducted in our laboratory dealing with isotropic adhesive films demonstrated that differences in the compression molding operation could be associated with either the presence or the absence of stress release events (2). Using DSC we sometimes noted unexpected... 

The first two conditions are relatively straight forward. They are applied only to the isotropic adhesive material and are to ensure that neither of these two loading modes exceed the load bearing capacity of the adhesive. In the case of isotropic adherends, typically only these two loading conditions need to be considered. 

Figure 41 Schematic representation of microdevices attached to thick film metallization pads printed on a ceramic carrier in a multichip module carrying both flip chip 1 and surface-mounted 2 components. Enlarged views show the bonding area (a) silver-filled isotropic adhesive with possible irreversible changes due to thermal cycling (b) anisotropic adhesive made of conducting spheres embedded in thermoplastic or...

There are general classifications of conductive adhesive materials based on the method by which conduction is achieved. Isotropic adhesives are the most common, and have been used in flexible connections. They normally contain about 70 wt. % Ag particles, but other metals are used as well [6]. The quantity of metal loading is sufficient to assure that the adhesive is electrically conductive in all directions upon curing, as illustrated in Fig. 4, hence the term isotropic. This is in contrast to adhesives with much lower metal filling, such that there are an insufficient number of particle-to-particle contacts to render the adhesive conductive, as depicted in Fig. 5a. However, if the adhesive is sandwiched and compressed between a pair of terminal pads, the particles between the... 

Gilleo, K. An Isotropic Adhesive for Bonding Electrical Components. European Patent 0,265,077, 1987. 

Masking is required for many micromechanical applications. While Si3N4 is only suitable for a small etching depth because of its significant etch rate in HF, noble metals like gold are sufficient mask materials. In contrast to alkaline etchants, organic materials like certain resists or even some adhesive tapes are well suited to protect the silicon surface in isotropic etchants. 

The transition between cohesive and adhesive failure in a simple bi-material joint has been studied by Kendall (1975). Based on Griffith s energy approach, a criterion is derived for deflection along the interface for a short crack for an isotropic material... 

The nature and type of initiation scheme plays an important role in the performance of the adhesive [194,202-204]. Stresses due to polymerization shrinkage lead to the creation of a gap between the adhesive and tooth material. In the case of bulk chemical initiation, shrinkage stresses tend to create gaps at all interfaces, drawing material inward isotropically. With a photoinitiation scheme, polymerization begins at the free surface and pulls the material away from the dentin towards the free surface [194]. Thus the gap is created at the... 

Three modes are clearly defined for crack propagation from a very thin (radius of the order of 10 gm) notch-machined in the specimen (Fig. 12.3). This notch induces a stress concentration effect, higher than those produced by all the other defects already present in the specimen, which governs the fracture initiation. For isotropic materials, mode I (the most severe) is generally used and gives the lowest value of toughness. In the case of adhesives and laminates, modes II and III are also performed. 

Some researchers have used approximate microscopic descriptions to develop more rigorous macroscopic constitutive laws. A microstructural model of AC [5] linked the directionality of mechanical stiffness of cartilage to the orientation of its microstructure. The biphasic composite model of [6] uses an isotropic fiber network described by a simple linear-elastic equation. A homogenization method based on a unit cell containing a single fiber and a surrounding matrix was used to predict the variations in AC properties with fiber orientation and fiber-matrix adhesion. A recent model of heart valve mechanics [8] accounts for fiber orientation and predicts a wide range of behavior but does not account for fiber-fiber interactions. 

In principle any isotropic material can be reinforced the combination of the materials has to meet the requirement that the reinforcing material has to be stiffer, stronger or tougher than the matrix furthermore there has to be a very good adhesion between the components. In a composite the reinforcement has to carry the stresses to which the composite material is subjected the matrix has to distribute the stresses. By means of a good distribution of the reinforcement the latter blocks the propagation of cracks, which mostly start at the outer surface, and would lead to rupture of the whole object if no blockade were present. By optimum reinforcement the strength of a matrix material can be improved to the tenfold, albeit in one direction. 

Highly Isotropic Dielectric Constant Adhesion High Etch Selectivity High Glass Transition Temperature... 

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

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