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The Growth of Inkjet Technology10th Installment

The Growth of Inkjet Technology10th Installment

In previous columns, we explored advances in inkjet technology using the example of home-use printers. It is said an advantage of inkjet technology is that the fundamental principles of the technology are applicable regardless of whether the object to be printed is very small (such as semiconductor circuitry) or very large (such as a movie poster). In fact, inkjet technology has found wide application in the manufacturing processes of a variety of industrial and commercial products.
In this extra column edition, we would like look at some issues involving industrial applications as well as the development of ink, the aspect of quality control and the implications that these factors have regarding viscosity measurement.

The wide range of applications of inkjet technology

A feature of inkjet technology is the non-contact method of printing which involves precise spraying of fine ink droplets on a medium.
The restrictions imposed on the material to be printed are relatively less stringent in non-contact printing. Thus, although accuracy of reproduction may vary depending on the type of material, printing is possible on a variety of media including plastics, metal, cloth, and wood, in addition to paper.
Additionally, in inkjet printing data from a PC is directly manipulated by the printer to create drawings and images and as such, printing plates which are required in traditional printing processes and equipment are unnecessary. If you print for example, a large size poster by traditional methods, printing plates equivalent in size to the poster are required and thus the largest size of object which can be printed in a single process would be dictated by the largest physical size of printing plate mountable onto the press. When creating a poster of size which exceeds the physical specifications of the press, the image to be printed must be broken down into separate sections and then combined and this entails a painstaking and exacting process of pasting together individual sections.
With the direct printing technology of inkjet printers, it is only necessary to design the equipment so that movement of the head (ink spray nozzle unit) matches the size of the medium to be printed. This is relatively simple compared to conventional methods which may require enlargement of the printing machinery per se with its intricate and complex mechanisms in order to accommodate a larger scale. Of course there are practical boundaries with regard to size with the inkjet method. Using inkjet technology to print something, say the size of a gymnasium, is likely to pose an entirely different set of problems. Thus although inkjet technology makes possible a wide range of applications, it is certainly not the all-purpose, magical solution for all conditions and requirements.
As alluded to at the outset of this column, size considerations in the inverse direction may also be used to emphasize the advantages offered by inkjet technology. Precision printing accuracies achievable by this technology can be illustrated by the example of the printing of the fine wiring patterns of semiconductor circuits.

Printing as a manufacturing method for electronic component wiring circuits

There has been remarkable downsizing in state-of-the-art industrial products, as exemplified by the cell phone. Phenomenally minute, highly detailed integration is involved in the mounting of semiconductor devices on the circuit boards used in such products. Photolithography techniques are normally employed in such applications to etch metallic wiring circuitry of 50É m (1É m = 1/1,000mm) line widths onto boards (to create wiring patterns on circuit boards, similar to printing images onto film with a silver halide camera).
Although the technology of photolithography is stable and despite its use in a wide range of applications, there are drawbacks with the method. The process involved is comparatively complex with major equipment required and there is also the environmental impact related to disposal of substances such as developers and etching agents.
With inkjet technology, ink is sprayed onto a medium to create patterns and images. Therefore if ink containing electrically conductive metallic powders is used in place of large particle-sized pigments, it is possible to create (i.e. print) wiring circuits. This is indeed a simple way to produce electronic circuit boards and as the process does not entail elaborate equipment nor place inordinate burdens on the environment, there are promising advantages which translate into reduced costs and delivery times compared with conventional methods. As the technology allows data from a PC to be directly replicated on a medium, the technology is highly adaptable and compatible with ever changing market needs, enabling fast response to high-mix, low volume production requirements, for example.
Major domestic printer equipment OEMs are developing manufacturing techniques to produce laminated circuit boards which utilize silver nanoparticles, a few nanometers to a few tens of nanometers in diametrical size. Such silver nanoparticles are encased in a covering of organic material which helps to prevent particle agglomeration which is critical as non-uniformity in particle dispersion adversely affects spray accuracy and the rendering of fine wiring patterns.
This aspect is similar to the stable dispersion techniques discussed in our previous column on pigment-based inks. Stable and uniform dispersion of pigment-based inks over a medium is also a goal of home-use printers and approaches such as encapsulation of pigment particles in a layer of resin are being tried. Indeed, it is believed that viscosity measurement will find increasing applications in the development, manufacture, and evaluation of pigments particularly as they relate to agglomeration states and dispersion stability.
In fact, inkjet technology is already being applied in the coating of alignment films on TFT liquid crystal panels (i.e. coating of film formed on the surface of circuit boards). Uniform, high quality thin film coatings can be achieved by precisely monitored spraying of nanogram order droplets. Through such applications we may be able to understand the wide range of possibilities opened up by this technology.
Certainly there can no longer be comparisons of such leading edge technology with the soldered wire radio kits of old.

Viscosity's role in ink development and optimization of equipment operating conditions

Building on our discussion of liquid crystal panels, let us look at the aspect of viscosity. Based on our familiarity with home-use inkjet printers which print characters and photographs, the creation of film coatings (such as alignment film coatings of liquid crystal panels) using inkjet technology may seem odd. If you visualize the application of the color red over the entire surface of a paper, from the micron level, this would be equivalent to smearing the paper with a coating of printer ink.
As a matter of fact regarding coatings, research is actively being conducted in applying inkjet technology to 3-dimensional (relief) modeling which requires control of thickness as well as control of the application area of the ink spray. Relative to our description of printed circuit boards, it may thus be possible to employ inkjet technology to create not only wiring circuitry, but packaged parts by utilizing ink composed of ceramic particles, as an example.
This is still however cutting edge technology, and as mentioned the predominant method currently in use today is photolithography. A variety of traditional techniques including silk screening, dispenser, spray coating, spin coating (where a liquid solution is dripped onto a rotating medium), and dip coating (where a medium is immersed in a liquid solution and removed at a constant speed) are also widely being utilized to form coated surfaces. In practice, many different types of liquid substances are applied as coatings on circuit boards to provide specific functions. Resin-based reinforced impregnation liquids, electrically conductive pastes, electrically conductive parts mounting adhesives, and ink substances for insulation, are some examples. The suitability of such substances to the application is thus very important in insuring control of the quality of printed circuit boards.
These liquid substances may each possess unique properties (viscosity, etc.) that have direct implications relative to manufacturing and coating processes. A rheological approach which enables control of the application of the coating, leveling, thickness of film, and dripping & sagging characteristics, for example, thus becomes indispensable in addressing material deformation and fluidity of such substances.
Leveling refers to the ability of a coating liquid to flow to a smooth, uniform film thickness. Non-uniformity in film thickness (i.e. unevenness of the coating) causes problems, especially in products such as circuit boards which require tight tolerances. Although leveling is generally better the lower the viscosity, dripping and sagging become conversely problematic.
A comparison with the process of spin coating may perhaps enable a better perspective on the aspect of leveling relative to inkjet printing. In spin coating, leveling is semi-imposed by centrifugal force. With inkjet printing however there are no forces at work except gravity in the leveling of the ink. Thus to achieve good leveling characteristics and a uniform film thickness with this technology, the proper control of viscosity as well as an ideal ink structure (e.g. appropriate leveling and volatility composition) are attributes eagerly sought after.
From the rheological standpoint and referencing the publication, Rheology Engineering and Application Technology* <. . . leveling properties of film coatings are determined by the fluidity of the paste and constant yield values are required to prevent dripping and sagging.> Consequently, the measurement of viscosity is essential in controlling the fluidity of a substance and in the establishment of optimum coating conditions.
This said, it is however extremely difficult in practice to determine properties of conductive pastes and other non-Newtonian liquids as has been stated many times in this column. Indeed, the type of conditions under which viscosity measurements should be implemented and the judgment involved in ascertaining whether the values thus obtained are rational and valid indices can only be induced over numerous measurements based on a high degree of expertise. What is ultimately sought is the knowhow which correlates viscosity values to the products assessment on the manufacturing line. Toki Sangyo can provide valuable assistance in this effort with its products, experience and knowhow and ability to offer a wide range of skills encompassing sample measurements to the establishment of equipment operating conditions.

* Excerpt from <Rheology Engineering and Application Technology> (Japanese language);publisher - Fuji Techno Systems (2001).

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