-
- News
- Books
Featured Books
- design007 Magazine
Latest Issues
Current IssueOpportunities and Challenges
In this issue, our expert contributors discuss the many opportunities and challenges in the PCB design community, and what can be done to grow the numbers of PCB designers—and design instructors.
Embedded Design Techniques
Our expert contributors provide the knowledge this month that designers need to be aware of to make intelligent, educated decisions about embedded design. Many design and manufacturing hurdles can trip up designers who are new to this technology.
Manufacturing Know-how
For this issue, we asked our expert contributors to share their thoughts on the absolute “must-know” aspects of fab, assembly and test that all designers should understand. In the end, we’re all in this together.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - design007 Magazine
Conductive Ink Technologies for Digital Printing of Flexible Circuitry
November 2, 2006 |Estimated reading time: 7 minutes
<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
In an environment where an increasing proportion of high-volume electronics manufacture is being transferred to low-cost countries, Western nations are looking to new technological developments to provide a firm foothold in low-volume, high-margin production. Digital printing of conductive materials allows a flexible method of production of flex and rigid PCBs without the necessity to provide expensive tooling beforehand. This reduction in non-recurrent engineering provides a significant reduction in cost and turnaround time for low production volumes and gives rise to an alternative to the more labor-intensive processes traditionally involved in PCB production.
PCB manufacture has traditionally been a reasonably labor-intensive process with a large number of steps, each involving some form of human intervention. Traditional PCB manufacture also relies on an initial amount of up-front NRE. This is usually in the form of production tooling such as photo masks or silk screens. This means that the costs of small volume production runs will be severely impacted by the initial tooling costs whereas the cost of large volume runs will be dominated by the costs of the labor and infrastructure involved. The result of this is that high-volume PCB production has inevitably been transferred to low-cost countries as a means of reducing costs.
The convenience usually desired for small production runs provides an incentive to keep production relatively local. Tooling and labor costs however still mean that this is an expensive process with relatively high piece part costs for low volumes. Digital printing of conductive materials provides the potential for a non-labor-intensive production process that also reduces costs by removing the need for initial tooling such as silk screens or photo masks.
Digital Deposition of Conducting Material
Over the last decade a great deal of interest has arisen in the possibility of digitally printing conducting materials. Much of this work has arisen from field of organic electronics where the promise of fully printed electronic devices and displays required printable conductors for contacts and signal bus lines. Much of the initial work focused on conducting polymers such as PEDOT:PSS with later solutions using carbon and metal-based particulate pastes.
Conducting polymers have suffered from comparably low conductivities whereas higher conductivities have been achievable from thicker, screen-printed silver pastes although these materials inevitably require a high temperature sintering stage to achieve optimum conductivities and therefore limit the choice of substrate materials. Inkjet printing of PWBs has, however, not generally been possible using metal particle inks since the particle size and tendency to agglomerate seriously impacts the reliability of the printing process. New developments exploiting metal nano-particles in inks have opened the door to inkjet printing but the thin layers produced and the need for sintering still limit the usefulness of the process.
A low-temperature digital process that provides reasonable conductivity would allow a rapid and flexible route for rapid prototyping and small- to medium-batch production on low cost flexible and rigid substrates.
A New Process
Printable conductive metal pastes have always had to reach a compromise between the rheological and conductive properties of the material. Binders and carriers used to provide flow during printing and adhesion to substrates impact the conductivity of the final composite layer and impede current percolation through the conductive track.
There is a process, however, that provides a means of separating the additive patterning and substrate adhesion from the conductivity requirements of the printed features. Conductive Ink Technologies (CIT) has developed a process in which a catalytic ink is printed on the substrate and UV cured to provide a rapidly processed and adherent base layer. The layer itself is non-conducting but acts as a catalyst for the electroless deposition of metal layers.
The printed and cured substrate is immersed in a commercially available electroless plating bath and gives rise to the deposition of dense metal layers on top of the base ink. The two-stage process allows the ink to be separately optimized for different substrate materials and different printing vehicles without impacting the conductivity of the final process. Most standard electroless metals can be used, including nickel, cobalt and palladium, but most commonly and widely used is copper. The two stages of the process can be implemented in-line or the electroless plating can be performed later as a batch process.
Typical growth rates for copper range from around 20 nm/min to 90 nm/min (bulk copper equivalent), giving a 30 mO/ sheet resistance in around 10 minutes of plating. Typical resistivities are around 2.5 times that of bulk metal (in the case of
copper) but depend on the plating bath and conditions used.
The optimum conductivity range for the CIT process is >10 mO/? (equivalent of around 1.5 to 2 um of bulk copper). This is suitable for a wide variety of applications including UHF RFID, keyboard membranes, low-current PCBs (signal), low-power heater elements, a wide range of sensor applications as well as many other flex and rigid applications. If higher conductivities or greater current carrying capacity are needed then a post process electroplating is also possible.
Resolution of Inkjet Printing
The CIT process has been developed for piezo-based drop on demand print heads such as those produced by Xaar, Konica Minolta, and Spectra. Typical native resolutions of these print heads would be around 180 to 360 nozzles per inch designed to print at basic resolutions of up to 360 dpi with drop volumes down to around 40 pl. This type of print resolution would typically give features equivalent to around 100 um track width on PET or PI substrates. However, new generation greyscale heads allow variable drop volumes down to around 3 pl which give access to digitally printed features of around 50 um or less.
Digital Manufacturing Systems
A wide range of systems exist for digital production of flex circuits. At the lower end of the range are small scale development systems such as the DMP series from Dimatix. This kind of printer will produce up to A4 sheets at variable resolution using a disposable 16 nozzle print head. Such systems have a low throughput due to the small number of nozzles on the print head but are ideal for development and accurate work.
More suited for production would be systems such as the X4000 range from Xennia Technology or the XY100 from Konica Minolta. Also based on an A4 format, these systems implement larger industrial print heads such as the Xaar Omnidot range or the Konica Minolta KM512 range. These systems provide print swathes of up to 70 mm in width with production rates of up to 1 to 2 square meters per minute. Similar systems are also available in widths of a meter or more with production rates dependant on the number of heads and configuration required.
CIT in collaboration with Preco Industries has also developed the MetalJet 6000: a narrow web digital press for in-line reel-to-reel production of flex circuits and RFID antennas. This system prints and cures on a 140 mm platform and implements our proprietary plating module that dramatically reduces the footprint and complexity of plant needed for in-line electroless plating of web-based material. Current print head technology allows this system to produce flex circuits at up to 0.56 ms-1 (equivalent of 4.7 square meters per minute) and typical run speeds of 0.3 ms-1 (2.5 square meters per minute) for products such as UHF RFID antennas. The system is modular and can be reconfigured to increase production speed and/or deposition thickness.
In the case of most of these solutions, typical turnaround times for 10 square meters of single layer board could be less than 1 hour from submission of a CAD drawing.
Summary
New Digital techniques present an additive and tooling-free method of producing small- to medium-batch sizes of PCB and flex circuits using a rapid process with minimal NRE. The ability to separate jetting properties and ink adhesion from the electrical properties of the material provides independent control of the conductivity of the conducting tracks. Using inkjet printing as a production method provides a high manufacturing throughput with a short turn-around time and without the added cost of up-front tooling.
Dr. Steve Thomas earned a first degree in Materials Science and Metallurgy, Steve performed his PhD and postdoctoral research in organic semiconductors and devices at <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />CambridgeUniversity's Cavendish Laboratory. Following R&D and project management positions in a telecommunications start-up based in Zurich, Switzerland, he returned to the UK to work in various consultancy, intellectual property and technology transfer organizations, including the Technology Transfer office at CambridgeUniversity. In September 2004, Steve joined Conductive Inkjet Technology as an Applications Engineer where he has been intricately involved in the development of their digital deposition of conductive materials.
This paper was originally presented at the European Supply Chain Convention and is revised and published here with permission.