Applications of Silver Paste
In nanoscience, silver nanoparticles have been the subjects in the many works due to its specific properties on the optical, electronic, catalytic, and antibacterial materials research. The research on the synthesis of silver nanoparticles was rapidly developed for last decade. It has been known as chemical reduction, electrochemical reduction, irradiation reduction, or micro emulsion methods. Through these methods, it has been possible to prepare the conducting metal nanoparticles which could be applied for metal paste, conducting ink, and conducting adhesive.
Generally, a thick-film silver paste contains a functional phase consisting of particles of different shapes (flake or spherical) and sizes (micrometer, submicron, or nanometer), and these particles are dispersed in an organic vehicle by high-energy ball milling, ultrasonic vibration, and/or mechanical stirring.
A homogeneous thick-film silver paste is a stable system in which all the components are soluble in solvent and no solid or particle phase is excluded. The paste is able to avoid thoroughly the sedimentation and/or agglomeration of the particles applied in conventional pastes and eliminates the constraints of the particle size on fine patterns dispensing.
In recent years, the demand for flexible print circuit (FPC) has been increasing. Ordinary FPC is produced using circuits composed of plastic films onto which copper foil is laminated. In addition, silver printing circuit board (membrane circuit board, MB) that has a structure in which conductive silver paste is screen printed on a PET (polyethylene terephthalate) film to form circuits is available.
Silver conductive paste for screen printing used for formation of circuits in the MB uses silver material in conductive particles. Although silver has a disadvantage that ion migration can occur easily, it is handled with ease since it is more resistant to oxidization compared to copper, which has specific resistance of a similar level, and hence this material is used widely.
Polymer-type conductive paste utilized in the MB is specific in that low-temperature baking at less than 150°C (PET film circuit board is capable of withstanding this temperature) is possible. Silver conductive particles are dispersed in organic binder (polymer) and if printed or baked, conductive silver particles make contact each other thereby ensuring good electrical conductance. However, with this conductive mechanism, there are many contact resistance between conductive particles, specific resistance of the circuit being formed is more than 4.0 × 105 Ω cm which is more than 30 times that of bulk silver.
Conductive paste used in the ceramics substrate is composed by conductive silver particles and glass frit, and its baking temperature is more than approximately 500°C. After baking, conductive particles are sintered and contact resistance between conductive particles is greatly reduced. Therefore, it is possible to form a low-resistance circuit having specific resistance of the order of 106 Ω cm.
As one of applications, current collection wiring of transparent conductive glass substrate (transparent conductive glass) for dye-sensitized solar cells (DSC), which has been attracting attention recently as the next generation solar cells, is cited. With DSC, power is taken out through transparent conductive glass used as the window electrode. Since it has a certain resistance, the power generated is lost in part while the internal resistance of the battery is increased. In order to prevent this, efforts have been made in such a way that current collection wiring is provided to transparent conductive glass so as to reduce the loss to a minimal level.
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