Adhesives anisotropic

An important consideration for testing is the quality of the bond between the metal layers and the substrate. The source/drain layer especially is susceptible to scratching (if probes are used) or delamination (if wirebonding, anisotropic adhesives, or heatseal connectors are used). This is an issue for circuits at nodes which will be externally bonded or probed. 

Introduction of anisotropic adhesives for fine-pitch assemhUes... 

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... 

Figure 2.13 Dependence of volume fractions of conductive filler on the electrical conductivity of an anisotropic adhesive. ...

With anisotropic adhesives, pressure is necessary to squeeze the film so that electrical contacts are made between the aligned electrodes of the parts being bonded. While under pressure and while the electrodes make contact with the metal filler particles, the adhesive cures in the area surrounding the connections and fixes the positions of the z-direction connections. 

Several approaches using electrically conductive adhesives instead of solder have been explored and are proving successful. Anisotropic adhesives (See Chapter 1), for example, have been used to connect flex circuits and cables from the separate driver circuits to the panel, avoiding the use of solder connections. More importantly, integrated-circuit chips can be bonded directly to the ITO conductor traces on the panel, a technology called chip-on-glass (COG). IC chips can be flip-chip bonded, then underfilled with a stress-free underfill adhesive if necessary. For protection, the chips may then be encapsulated with epoxy (glob-topped). 

Numerous geometrical factors of the specific interconnection will also influence anisotropic adhesive formulation and processing, including lead planarity, IC pad metallization, and IC test patterns. The planarity of the leads on the substrate and/or device and the compliance of the conductive particles will determine if anisotropically conductive adhesives can be used in a particular application. For systems with large disparities between lead height, no electrical interconnection will be formed, as shown in Fig. 5. Fine-pitch IC packages for surface-mounted applications, such as the plastic quad flat pack (PQFP), often use gullwing leads that olfer much compliance to the joint. Even if the... 

Mimicking the robust dry adhesion displayed by geckos is of interest to materials research for military, consumer, and biomedical applications. Recently, seven key characteristics of gecko adhesion were identified by Autumn (1) directional (or anisotropic) adhesion, (2) robust attachment with minimal mechmical preload, (3) easy detachment, (4) promiscuous adhesion, (5) self-cleaning, (6) non-self-adherent, and (7)... 

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... 

Although dating as far back as the 1960s, anisotropic adhesives have only recently been rediscovered and advanced to a state where they are being used in place of solder to interconnect fine-pitch, high I/O-count flip-chip devices, TAB devices, and high-density interconnect substrates. Anisotropic adhesives are also used to interconnect edge connectors and flex cable to PWBs and flat-panel displays (FPDs). ACAs are commercially available in either paste or film form and as either thermoplastic or thermoset types. 

Alternatively, thin metal coatings were applied either in combination with electron-beam irradiation or applied anisotropically to create tilted structures. The irradiated or coated pillar surfaces shrank more than the opposite surface, resulting in bending of soft-molded pillars, and subsequent anisotropic adhesion behavior [41, 47, 48]. 

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