Z-directional adhesives

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... 

Missing from the literature are standardized and comparative physical strength data for adhesives in neat form or as applied to the skin. The most comprehensive examples of the stress-strain relationships of excised human skin are appended (Appendix 2.6.1.) three axes of any material are demonstrated below, where X- and T-axes are lateral (perpendicular to each other), but in the same plane, whereas the Z direction is not in the same plane, but perpendicular to X- and T-axes as demonstrated in Fig. 3.10. 

The distribution of the axial velocity is shown in Fig. 8.6. The color scale ranges from -0.17 m/s (blue) to +0.32 mjs (red). An axial velocity of 0 m/s (turquoise) is present at the walls and also at the rotating screws due to wall adhesion. We observe a flow inside the channels of approx.y 0.1 m/s in the positive z-direction, irrespective of the screw position. In the intermeshing region, also irrespective of the screw position, there is a clear velocity increase of 0.2 m/s compared to the channels. Over the screw tips, a... 

Chemical sensitivity can be conferred to AFM by coating the tip with covalently linked monolayers which affect the tip-surface interaction the method is called chemical force microscopy [77]. Additional modulation of the piezo actuator operating in z-direction and evaluation of the force signal can be used to measure the adhesion force between a surface and a chemically modified AFM tip [78]. Metal coated AFM tips can be used in a scanning electrochemical microscopy (SECM, see p. 264) mode [79] in studies of crystal dissolution or growth where surface processes are associated with considerable fluxes of species. 

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. 

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... 

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. 

A third approach to underfilling involves applying an insulative thermoplastic preform (film) prior to attaching the device. Under pressure and heat, the preform softens and flows around the solder bumps, then solidifies quickly on cooUng. The devices may then be solder reflowed to make the connections or the preform may be formulated to flow and encapsulate the solder during solder reflow. Table 5.6 lists some commercially available preforms used as underfills. Anisotropic film adhesives have also been used as underfill preforms, in which case z-direction conductive paths are formed beneath the solder bumps at the same time that the rest of the insulating film flows and cures around the bumps. Anisotropic paste adhesives can be similarly used (Figure 5.11). 

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. 

Most commercially available anisotropically conductive adhesives are formulated on the bridging concept, as illustrated in Fig. 1. A concentration of conductive particles far below the percolation threshold is dispersed in an adhesive. The composite is applied to the surface either by screen printing a paste or laminating a film. When a device is attached to a PWB, the placement force displaces the adhesive composite such that a layer the thickness of a single particle remains. Individual particles span the gap between device and PWB and form an electrical interconnection. For successful implementation of anisotropically conductive adhesives, the concentration of metal particles must be carefully controlled such that a sufficient number of particles is present to assure reliable electrical conductivity between the PWB and the device (Z direction) while electrical isolation is maintained between adjacent pads (X,Y directions). 

Force curves are used to correlate vertical movement of the scanner with cantilever deflection. A typical force curve is depicted in Figure 7. Force curves are obtained by disabling the scanner movement in the X and y directions and oscillating the scanner in the z-direction. As the tip approaches the sample, no cantilever bending is observed until the gap between the tip and the sample is extremely small. Then, the cantilever will be pulled down due to the van der Waals interactions between the tip and the substrate (or by the presence of a thin hydration layer on the surface). Once in contact, raising the sample produces an upward bend in the cantilever. The applied load is computed by multiplying the reference cantilever deflection value times its spring constant. The force curve can also be used to identify adhesive... 

The connection of flexible circuits with anisotropic conductive materials is not new idea. An organic adhesive material that has conductive particles becomes conductive only in the z direction when sandwiched between a flexible circuit and the other device under a pressure... 

Failure at the interphase is more likely with higher strain gradients, Ay lAz. Of course, in the limit, a strain discontinuity defines failure based on mechanics principles. Based on this premise, a mathematical expression can be obtained to gage the possibility of failure using the shear moduli of the adhesive and the adherend along with the adhesive volume fraction, V a> at the interphase. For this purpose, differentiation of O Eq. 23.95 along the interphase thickness direction (z-direction) results in... 

From the viewpoint of their conduction and mechanical joining, ACAs are similar to ICAs, except that they have lower concentrations of conductive particles. This lower concentration provides unidirectional conductivity in the vertical or z-direction (perpendicular to the plane of the substrate), which is why they are called anisotropic conductive adhesives. In the same way, ACA materials are prepared by dispersing electrically conductive particles in an adhesive matrix at a concentration far below the percolation threshold. The concentration of particles is controlled, so that sufficient particles are present to ensure reliable electrical conductivity between the assembled parts in the z-direction, while insufficient particles are present to achieve percolation conduction in the x-y plane (Kim et al. 2008b). O Figure 50.6 shows a schematic description of an ACA interconnect, showing the electrical conductivity in the... 

Anisotropic conductive adhesives are similar to isotropic conductive adhesives, except that they have a lower conductive filler content (5 20 vol%) and thus provide unidirectional conductivity in the vertical or z-direction (perpendicular to the plane of the substrate). 

The prior solution is based on the assumption that the polymer deforms without restraint. However, the electrode layers are much stiffer than the polymer layer and if they are perfectly bonded to the polymer then they will prevent the polymer from expanding in the lateral x- and y-directions. A biaxial compressive stress develops in the polymer. This in turn causes a positive Poisson tensile strain in the z-direction and reduces the compressive stress strain that will be detected by the gauge. Therefore, to obtain maximum sensitivity, a compliant conductive adhesive should be used between the layers. Also, a prestress can be applied to improve contact. 

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