Printed Films

Materials Science and Applications in Sensors, Electronics and Photonics

Maria Prudenziati, Jacob Hormadaly

• Publisher: Woodhead Publishing
• Year: 2012
• eBook ISBN: 9780857096210
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Whilst printed films are currently used in varied devices across a wide range of fields, research into their development and properties is increasingly uncovering even greater potential. Printed films provides comprehensive coverage of the most significant recent developments in printed films and their applications.Materials and properties of printed films are the focus of part one, beginning with a review of the concepts, technologies and materials involved in their production and use. Printed films as electrical components and silicon metallization for solar cells are discussed, as are conduction mechanisms in printed film resistors, and thick films in packaging and microelectronics. Part two goes on to review the varied applications of printed films in devices. Printed resistive sensors are considered, as is the role of printed films in capacitive, piezoelectric and pyroelectric sensors, mechanical micro-systems and gas sensors. The applications of printed films in biosensors, actuators, heater elements, varistors and polymer solar cells are then explored, followed by a review of screen printing for the fabrication of solid oxide fuel cells and laser printed micro- and meso-scale power generating devices.With its distinguished editors and international team of expert contributors, Printed films is a key text for anyone working in such fields as microelectronics, fuel cell and sensor technology in both industry and academia. - Provides a comprehensive analysis of the most significant recent developments in printed films and their applications - Reviews the concepts, properties, technologies and materials involved in the production and use of printed films - Analyses the varied applications of printed films in devices, including printed restrictive sensors for physical quantities and printed thick film mechanical micro-systems (MEMS), among others

Part I

Materials and properties of printed films

1

Technologies for printed films

M. Prudenziati, University of Modena and Reggio Emilia, Italy

J. Hormadaly, Ben-Gurion University, Israel

Abstract:

The terms ‘printed electronics’ and ‘direct-write deposition’ are increasingly familiar to manufacturers and consumers of electronic products. The trend is to make use of new cost-efficient ways to mass-produce electronic devices and in prospective to open new huge product markets. There are already several applications of functional printed films, but further improvements in performance/cost ratios are envisaged, especially in ‘organic electronics’ and ‘flexible’ products. Development in these fields is progressing rapidly on several parallel paths, each with its own strengths and limitations. In this chapter, a broad overview of technologies capable of printing functional films for the microelectronic and electrical industries is presented. First, the history of the most mature and traditional print process known as ‘screen printing’ is delineated and the steps that have resulted in improved, increasingly integrated systems and diversified materials are retraced. Motivations for ever-increasing resolutions are then approached and technologies on which current research is mainly focused are briefly outlined.

Key words

printed electronics

hybrid circuits

thick-film technology

dispensing systems

ink-jets

reel-to-reel printing systems

evolutionary progress

1.1 Introduction: printed films in microelectronics

1.1.1 An historical perspective

The earliest printed films appeared as screen-printed films in microelectronics for making more efficient and cheaper printed circuit boards (PCB); soon, thick-film technology advanced in a variety of versions, all of them still applied in the manufacture of microelectronic devices, circuits or systems, for either mass or niche markets. The starting point was indeed simple: to screen-print photo-resist layers aimed at defining exposed areas (or vice versa the protected areas) during the process of etching away the copper laminate layer and so creating the pattern of conductive interconnections in the circuit. The applied ink is dried, the whole board treated with the chemical etching solution to dissolve away the unwanted metal and finally the dried ink removed with an organic solvent or alkaline solution. Next, small holes are drilled in the board at predetermined places and components placed on the insulated side of the board with their connecting leads projecting through sufficiently to contact the copper tracks on the other side of the board. Finally, the connections are made secure and permanent by soldering, either manually or by treating the surface with a flux and passing briefly over a wave of molten solder. In this second event again a ‘solder resist’ is printed on the copper side, leaving exposed only those sites where solder is required. Similar strategies and operations are today valid and exercised for the mass production of PCBs and ‘surface mount technology’ (SMT).

The second step for introducing printed films in electronic applications was perhaps the most challenging and inventive: to manufacture the whole set of electric passive components, their interconnections and protective layers on a single face of a dielectric substrate, usually a ceramic substrate. This was accomplished by using the ‘drawing’ technology known as ‘serigraphy’ to lay down defined patterns of pastes or inks containing the required functional materials. The development of the proper ink composition and post-deposition processes needed to accomplish the prospected functionality of the films can be traced to the dawn of the nineteenth century, although remarkably a patent was issued in January 1899 for printed and fired meanders for resistive components prepared ‘after the manner in which lustrous gold, silver and platinum decorations are applied’ (Voigt and Haeffner, 1899), i.e. from a resinate-ink, in the current jargon.

The earliest use of particulate-based films (i.e. films from inks containing metal together with glass powders) reportedly concerned the preparation of capacitors (Deyrup et al., 1945), which pre-dated the birth of the term ‘thick-film’ by at least twenty years. The term ‘thick-film technology’ was in fact coined only in the early 1960s to distinguish it from ‘thin films’, typically applied by a vapor deposition process, vacuum evaporation or sputtering (Bouchard, 1999).

But thick-printed and fired film for passive components and interconnections had already been mounted in 1947 on modules supporting subminiature radio-tubes (Hoffman, 1984); hence we might say that the ‘hybrid microelectronics’, i.e. the use of printed and fired interconnections to add and package active electronic devices, came before the invention of transistors (1949).

Turning back to the definition of ‘thick-films’ we observe that in the early days Kelemen proposed defining them as

inorganic conductive or dielectric films, applied to a ceramic or glass substrate from a composition containing a temporary organic binder, as a coating of the order of 40 μm in thickness, and then fired in air at a temperature not exceeding 1100 °C. In most cases the fired film would be several micrometer thick, but in some instances its thickness may be in the tenth-micron range. (Kelemen, 1970)

This definition evolved substantially over the years.

The ‘boundaries’ implied in Kelemen’s definition have been overcome, leaving only the initial link between ‘thick-films’ and ‘printed films’ unchanged. New classes of substrates have been incorporated including metals, polymers and silicon among others, the thickness of the deposited layers have varied widely, firing in many atmospheres has been used, as well as firing at peak temperatures over 1100 °C.

The very beginning of the thick-film hybrid circuit industry as we know it today occurred in the 1960s with the first ‘fired on’ resistors adopted by the military for the Minuteman missile and by IBM for its System 360 (Bouchard, 1999).

The large credit granted by IBM to thick-films technology (TFT) has contributed greatly to its adoption by the electronic community, promoting recognition that TFT allowed one to lay down passive components with easy and cost-effective techniques on relatively rough and therefore low-cost substrates, without the need for vacuum equipment and with the added advantage that resistors and circuits could be trimmed. These features entailed advantages over both thin films technology and conventional PCB, making thick-film hybrid microelectronics competitive in a wide and diversified spectrum of application areas, including telecommunication, automotive, medical equipment, radio and television, and the full range of consumer electronics.

Within a short period of time, thick-film hybrids invaded a large fraction of the industrial electronic markets, and these successes stimulated a variety of technological efforts in the development of new materials (substrates, paste compositions, circuit structures) and processes, to increase circuit density, operating frequencies, flexibility of design, etc.

Very important innovations in spreading TFT to various application areas soon followed. In the 19...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Woodhead Publishing Series in Electronic and Optical Materials
• Dedication
• Preface
• Part I: Materials and properties of printed films
• Part II: Applications of printed films in devices
• Index