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Thick Film Systems for Challenging Applications
Keywords: Thick Film Resistors, Thick Film Conductors, New Generations of Thick Film Materials
There are microelectronic applications employing thick film paste materials that require for success non-standard approach beyond the conventional techniques of problem solving. Some approaches to dealing with certain challenging tasks are described. Particular ruthenium-based composite conductive phases along with set of special glasses provide a solution for such fundamental thick film resistor technology problem as anomalous conduction loss in high ohm value resistors. Another advantage of this technical solution is a possibility of using of ink firing temperatures significantly below or above regular thick film materials firing temperature range. It provides additional opportunities for advanced thick film material systems development. This type of thick film resistive inks possesses unique sintering ability along with unusual dependence of resistivity versus glass content in resistive compositions. In contrast to an exponential dependence of resistivity versus glass content typical for traditional high ohmic resistive compositions, these materials have a parabolic dependence with a minimum value at specific range of ingredients content. While the range of practically constant resistivity is relatively wide, TCR in this stable resistivity range is smoothly altered versus glass content in resistive composition making available another previously unknown precise TCR control tool. Such technical solution allows development of new resistive inks having enhanced stability in resistivity range up to Gigaohms per square. It becomes feasible through formation during ink firing process of microstructures of resistors similar to ceramics. Possible applications for such resistors are the following: ultra-high value resistors; resistors for high temperature down-hole and extreme cold applications; ruthenium-based resistors fired directly in nitrogen atmosphere and compatible with copper conductors; set of resistive inks with resistivity from milliohms per square to Megaohms per square on aluminum nitride substrates; low firing temperature ruthenium-containing resistor series on dielectric coated aluminum substrates from milliohms per square to Megaohms per square, etc. A concept of controlled paste rheology in wide range of shear rates instead of common single-point viscosity test allows predicting of inks flow during screen printing process. It enables development of fine and thick print conductors as well as high resolution serpentine resistors with controlled, uniform and sufficient lines height using conventional screen printing technique. Therefore precise and stable ultra high ohm value resistor networks may be manufactured in a cost-effective way combining high resistivity with high print resolution of advanced resistive inks. In spite of existing opinion that thick film materials have exhausted all other possibilities for further development, they do have great potential for significant progress. Moreover, described new generations of thick film inks may be used in many modern challenging applications for tasks that had been previously impossible due to some limitations inherent to regular thick film inks. Employing of described specific features has allowed development of set of advanced thick film inks fired in the temperature range from 550°C to 1,300°C on various substrates such as alumina, aluminum nitride, stainless steel, aluminum, crystallized glass, and copper. Among these features the following ones have been employed: unique sintering ability of resistors; parabolic dependence of resistivity on glass content; forming of ceramic-type structures during ink firing process; controlled crystallization of ceramic-type high ohmic resistors and dielectric coatings; and controlled paste rheology.
Michail Moroz, Principal Engineer
TT Electronics
Fullerton, CA

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