Development of conductivity in low conversion temperature silver pastes via addition of nanoparticles
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Silver nanoparticles were incorporated in a dispersion of micron-sized silver spheres for testing as a low-temperature reactive component to form conductive particle networks. The development of conductivity depended on the arrangement of the micron-sized particle network, the amount of material reacted to form necks at the points of contact of micron-sized particles, and sintering of the particle network. Nanoparticles reacted to bond the micron-sized particles, but the stress issues involved in nanoparticle sintering can cause macroscopic cracking. Critical processing variables include the state of particle dispersion, the heating rate, and the fraction of nano-sized material.
I. INTRODUCTION
Thick-film technology has succeeded in manufacturing printed circuits because of its speed and repeatability. A suspension of the desired materials can be directly printed on a desired substrate and examined for quality. The materials that are incorporated into these components depend on the ability of designers to match the components on the basis of the cofiring requirements, material stability during firing and operation, and the achievement of the required electronic performance. Multilayer structures integrating conductor, resistor, and inductor elements are generally restricted by the temperatures needed for sintering the conductive metals and insulating ceramics. These temperatures for silver and glass conductor pastes are between 800 and 950 °C. Formulations also require careful treatment of the thermal expansion of the different tapes and incorporation of materials to reduce the stresses during firing. The manufacture of circuits on polymeric substrates is an emerging materials technology. These applications cannot withstand the high temperatures traditionally employed in multilayer technology and necessitate the use of vacuum deposition or electroplating operations to generate printed circuits. These low-temperature substrates have created a market for polymeric thickfilm technology. Polymer thick film employs a polymeric binder filled with conductive particles such as silver flake, which becomes conductive above a particular volume fraction. The highly anisotropic shape of these flakes impacts the rheology of the system and lowers the total volume loading. The material
properties that can be achieved are limited by the loading of active material (metal flake) in the inactive polymer matrix. Fluid rheology is impacted by particle rotation on the basis of the particle shape in the suspension. For a given size, spherical particles have the most isotropic behavior in rheological flow and form conductive networks without orientation effects. Additionally, spherical particles at high solid loading pass through fine pores more easily and at finer scales than flakes. There are applications such as antenna traces that require high conductivity and low loss at high frequency (GHz). These uses require the higher properties achieved by fully sintered high conductivity metals. Improved material properties depend on the form
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