Viscosity electrically conductive adhesives

Filler Adhesive component in a solid, finely dispersed form that specifically modifies the processing properties of the adhesive and the properties of the adhesive layer (e.g., metal particles in electrically conductive adhesives, chalkstone, carbon black to increase viscosity). Fillers are not reactive partners in adhesive curing. 

The method and type of equipment used also vary with the type of adhesive. As an example, the viscosity of electrically conductive adhesives is measured according to ASTM D1824, Apparent Viscosity of Plastisols and Organosols at Low-Shear Rates by Brookfield Viscosity The viscosity of typical electrically conductive die-attach adhesives is measured with a Brookfield HBT viscometer with Spindle TB and Speed 5. For higher-viscosity conductive adhesives and for underfill adhesives, a Brookfield RVT or RVF viscometer is used with Spindles 6 or 7 at speeds of 4—10.4 rpm. Another Brookfield viscometer, the Cone-and-plate viscometer with a CP-51 spindle is used for low-to-intermediate viscosity adhesives. Finally, the Brookfield HAT and HBT instruments are used for the high-viscosity (1-2 million cP at 1 rpm) adhesives typically used in SMT applications. ... 

The method and type of equipment used also vary with the type of adhesive. As an example, the viscosity of electrically conductive adhesives is measured aeeording to ASTM D1824, Apparent Viscosity of Plastisols and Organosols at Low Shear Rates by Broolfield ViscosityThe viseosify oftypieal electrically conductive die-attach adhesives is measured... 

Further work in this area is underway employing polyamic acid systems which are known to produce higher viscosity solutions (e.g. polyimides derived from 4,4 -oxydianiline and either BTDA or pyromellitic dianhydride). This is being carried out in the belief that higher viscosity solutions will give rise to higher quality, less brittle films and will, thereby, enable a broader spectrum of metal systems to be studied regarding the adhesive and electrical conductance properties of metal ion filled polyimides. 

Fillers are relatively nonadhesive substances added to the adhesive formulation to improve its working properties, strength, permanence, or other qualities. The improvements resulting from the use of fillers are listed in Table 1.8. Fillers are also used to reduce material cost. By selective use of fillers, the properties of an adhesive can be changed significantly. Thermal expansion, electrical and thermal conduction, shrinkage, viscosity, and thermal resistance are only a few properties that can be modified by the use of fillers. Common fillers are wood flour, silica, alumina, titanium oxide, metal powders, china clay and earth, slate dust, and glass fibers. Some fillers may act as extenders. 

The applications of the SFM include force measurement between surfaces in liquid and vapor, adhesion between similar or dissimilar materials, contact deformation, wetting and capillary condensation, viscosity in thin films, forces between surfactant and polymer-coated surfaces, and surface chemistry. Fluid-electrolyte interactions between conductive surfaces can also be measured [Smith, et. al., 1988]. A typical microforce of 10 nN can be detected over separation distances to a resolution of 0.1 nm with optical interoferometry between reflective surfaces. With electrostatic forces, relatively large separation are measured 1-100 nm, whereas, short range forces such as van der Waals forces take place over distances of less than 3.0 nm. Ultrasmooth and electrically conductive surfaces can be formed by the deposition of a metal film (40 nm thickness) such as Pt on a smooth substrate of mica [Smith, et. al., 1988]. The separation distance between the two surfaces is controlled by a... 

Inkjets are flexible and versatile and can be set up with relatively low effort. However, there are some specifics for the ink. It should have high electrical conductivity and resistance to oxidation. It should be able to dry out without clogging the nozzle during printing and have good adhesion to the substrate. The lower particle aggregation and suitable viscosity and surface tension are important for selection of ink. There are some inconveniences for inkjets. The most conductive inks and pastes are based on silver filler and suffer from brittleness. To achieve a satisfactory conductivity, several passes should be applied. As a result, the thickness of paste will be augmented, which influences the flexibility and elasticity of the textile substrate. 

Partici Shape and Size. The most common morphology of conductive fillers used for ICAs is flake because flakes tend to have a large surface area, and more contact spots and thus more electrical paths than spherical fillers. The particle size of ICA fillers generally ranges from 1 to 20 /rm. Larger particles tend to provide the material with a higher electrical conductivity and lower viscosity (45). A new class of silver particles, porous nano-sized silver particles, has been introduced in ICA formulations (46,47). ICAs made with this type of particles exhibited improved mechanical properties, but the electrical conductivity is less than ICAs filled with silver flakes. In addition, short carbon fibers have been used as conductive fillers in conductive adhesive formulations (36,48). However, carbon-based conductive adhesives show much lower electrical conductivity than silver-filled ones. 

Oxide fillers are electrically nonconductive but are used to provide thermal conductivity. Alumina, the most commonly used filler, is fairly inexpensive. It can be added in high concentrations to epoxies and silicones without significantly increasing the viscosity of the uncured material. Minimum bond line thicknesses are desirable for thermally conductive adhesives because heat flow is proportional to the ratio of thermal conductivity to bond thickness. Alumina-filled epoxies contain up to 75 wt % filler and have thermal conductivities ranging from 1.38 to 1.73 W m K. ... 

Inspection tests before shipments of adhesive products are as follows adhesion strength, tackiness, hardness, viscosity, density, volume shrinkage, thermal expansion, shelf life, pot life, gelation time, water resistance, chemical resistance, weather resistance, flame retardance, electric conductivity, color, optical properties, amount of volatile organic compounds (VOC) emergence, etc. Some of these tests are the same as those tests for acceptance. Before shipment, strength tests of joints are very important for adhesives in terms of performance proof (see Chap. 19). In contrast, for PSAs, tackiness is important (see Chap. 15). Other important properties that should he tested are shown below. 

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