Progress in and Technology of Low-Cost Silver Containing Thick-Film Conductors

Thick-film materials have been proved to possess economic, processing, and functional advantages over other technologies in the high-volume production of miniaturized circuits. Inherent in the adoption of thick-film technology for increasingly diverse applications has been the ability of thick-film materials suppliers to provide progressive performance improvements at lower cost concurrent with circuit manufacturer's needs. Since the first major commercial thick-film adoption in the early sixties when IBM adopted platinum-gold conductors and palladium-silver resistors in their 360 computers, rapid technological advances over the last decade have produced an increasing variety of hybrid circuits and networks. The wide adoption of thick-film technology in all segments of the electronic industry has placed increasing demands on performance and processing latitude. The development of low-cost silverbearing conductors is outlined and the evolution of technology improvements to present day systems is described. Initially the deficiencies of early Pd/Ag conductors are reviewed, particularly solder leach resistance and degradation of soldered adhesion following hightemperature storage, and focus is placed on the first Pd/Ag system which overcame these problems. Extension of this technology and subsequent improvements in both binders and vehicles to fulfill adhesion requirements to AI
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substrates of Varying chemistries and to meet demands for high-speed printing are described also. In the second segment an overview of the present understanding of thickfilm conductor composites from a mechanistic point of view is given. The various types of binder systems commonly employed in conductors are discussed in terms of how they effect a bond between the sintered metal and the substrate, and the advantages and disadvantages of each type. Metallurgical aspects of conductor/solder connections are considered and their effects on bond reliability following exposure to high temperature are discussed. Rbeological considerations of paste design are presented and related to printing performance. In the final segment newer low-cost high-performance materials systems that have evolved over the past two years are considered. The technologies of each system are reviewed in terms of metallurgy, binder, and vehicle. Important functional properties are presented to illustrate cost/performance trade-offs. Special emphasis is given to a recently developed Ag conductor which has outstanding soldered adhesion even after 1000 h of storage at 150°C.