| Applications and Fundamentals of Conductive Adhesives Characteristics of Carbon Nanotubes Properties of Carbon Nanotube Filled Conductive Adhesives Soldering is relied upon heavily for the manufacture of many electrical / electronic products. However, environmental legislation has resulted in a need to replace conventional tin-lead type solders with more environmentally friendly alternatives. Electrically conductive adhesives are rapidly filling this need especially in micro-electronic applications. Electrically conductive adhesives provide several advantages in addition to eliminating lead solder. However, there are several disadvantages and limitations to this technology that have slowed wide-spread conversion. As a result, new polymeric materials, fillers, and processing methods are constantly being sought in the quest for higher conductivity, better performance properties, and easier application. Carbon nanotubes have been considered as appropriate filler for electrically conductive adhesives due to their geometry and high degree of conductivity. These materials are just beginning to see commercial application in certain areas. This article will review the state-of-the-art regarding the formulation and use of conductive adhesives containing carbon nanotubes. The reader may also be interested in an earlier SpecialChem article which describes the fundamentals of achieving high electrical conductivity and the formulation of conductive adhesives with conventional conductive fillers such as sliver, copper, and carbon. 1 Dont hesitate to react about this Article
Electrical conductivity is important in adhesives that must make an electrical interconnection between components and in adhesives that must provide electromagnetic or radio frequency interference (EMI and RFI) functions. Electrically conductive adhesives offer excellent alternatives to lead solder interconnects for microelectronic packaging applications. They are used in manufacturing electronic components for many industrial and consumer applications. Most adhesives systems inherently have very low electrical conductivity due to their organic nature and the very low concentration of free charge carriers in the materials used in their formulation. The electrical conductivity of an insulating polymer can be altered significantly by adding conducting particles like silver, copper, and carbon black. The resulting conductivity is dependent on the conductivity and shape of the fillers and their concentration within the polymer, and conductivity is not determined by the type of polymer resin used in the adhesive formulation. Other important criteria for selecting filler include the surface morphology, oxidation potential, reactivity with other adhesive components, and the fillers aspect ratio, particle size, and particle size distribution. For optimum electrical or thermal conductivity in an adhesive, the conducting particles used as fillers must be so concentrated that they come into contact with each other. This generally requires such a high level of filler loading that other properties such as flexibility and tensile-shear strength are significantly degraded. Typically adhesive formulations can be made conductive by filling with conductive particles. This is true for electrically conducting as well as thermally conducting formulations. The resin provides a mechanical bond between the two substrates and between the conducting particles. However, the conducting filler particles provide the desired electrical interconnection or thermally conductive path as shown in Figure 1 (left). The particles must come into contact with one another; thus, conductivity increases abruptly when a threshold level of well dispersed conductive filler is met as shown in Figure 1 (right). The critical concentration at which the conductivity increases significantly is generally termed the "percolation" level.
|
加载中...
