Interconnect technology is expanding to include printed systems. As detailed in the iNEMI Organic and Printed Electronics Roadmap,  there is a bounty of printed electronics technologies, such as ¡°drop on demand,¡± Xerography, micropen, et al. It is not within the scope of this chapter to detail the principles, advantages, benefits, etc., of each technology; rather, it¡¯s important for electronic packaging and interconnect designers to understand the salient features of printed electronics and digital fabrication technologies as well as the similarities and differences among conventional printed circuit technologies and processes.

Digital fabrication can be viewed as the creation of electro-optical-mechanical-thermal systems by means of either parallel or sequential additive processes, as controlled by a digital device. As with any technology, the designer should consider and take advantage of the technology¡¯s features before applying it. In the case of digital fabrication, those features include adaptive, additive, roll-to-roll processes and the ability to fabricate on curved, conformal, curvilinear surfaces.

Most often, digital fabrication is accomplished in a roll-to-roll process, but panel-based digital fabrication is also possible. The revolutionary aspect of digital fabrication, as compared to conventional printed circuit board, is the ability to simultaneously create electronic and/or optical devices, electrical interconnects, mechanical structures, and thermal management systems. For example, using conventional technology and processes, a television remote control is produced by packaging and assembling tens to hundreds of electronic devices onto PCBs and subsequently assembling the PCBs, interconnect, LED, touchpad, etc., within a plastic housing. Digital fabrication would allow the entire system to be fabricated, layer-by-layer, in a fashion similar to that of stereolithography apparatus (SLA). Applying digital fabrication requires a transformation in thinking by electronic system designers: digital fabrication provides for the 2D and 3D design and fabrication of systems that are not limited to electrical interconnect and passive devices.

In digital fabrication, materials with the necessary electrical, mechanical, thermal, and optical properties to form the desired devices, systems, etc., are deposited onto a substrate. Polymer and plastic are the most common substrate materials, due to their compatibility with low-cost, high-through-put processing. However, rigid substrates can be used as well. The materials used in forming electronic devices and interconnect are, typically, conductive inks and dielectrics. Any material, however, that can be dispensed by the dispensing system (¡°print head¡± plus material control) can be used. The materials used in forming mechanical and thermal structures, likewise, are defined by the application and limited by the capabilities of the ¡°print head¡± and material control system. To date, biological, organic, optical, and many types of functional materials have been printed.

Given that devices and structures are being ¡°built-up,¡± layer-by-layer, there are key differences in the resulting device and feature attributes. For example, vias can be formed as filled or partially filled and can be conductive and/or nonconductive, electrically and/or thermally. Unlike conventional pcb circuit board technology and processes, digital fabrication can be used to produce vias of any geometry. Similarly, individual traces can be of any geometry and can be produced to effect sidewalls with attributes to support application needs. Conductors produced using conventional pcb manufacture technology have sidewalls that are tapered or orthogonal. Digital fabrication could be used to produce conductors with irregular sidewall features, as related to specific performance or mechanical applications. Furthermore, in digital fabrication, mechanical, thermal, and other features could be included in the construction of conductors.

Currently, the primary applications for printed electronics are OLEDs, and RFID. Photovoltaics are also an application for printed electronics, but this field is immature compared to OLEDs and RFID.

Transistors that operate in the tens of megahertz have been printed; recently, researchers at the University of Illinois (Rogers et al.) announced that they had printed silicon circuits on plastic that operate at switching speeds of 500 megahertz. As shown in Figure 27, there are demands for printed, stable, high-speed digital and high-fidelity analog devices; however, materials to produce devices other than low-speed, simple ones are lacking. Accordingly, other than novel devices produced in laboratories, printed electronics have not been used to produce the basic ¡°building-block¡± high-speed digital or analog devices (e.g., FPGAs and operational amplifiers, respectively). These components are fundamental to systems comprised of analog devices.

Similarly, with respect to printing electrical interconnects, digital fabrication is in its infancy. Plated vias, a staple of the layer-to-layer interconnect found in conventional pcb circuit board technology, are nonexistent or crudely implemented in digital fabrication. Moreover, the electrical performance and reliability of vias and conductors are either not known or, with respect to characterization, known incompletely. It will be interesting to observe if and when the creation and maintenance of electronic and interconnect standards are realized.



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