Tape Casting Advanced Materials
May 1, 2007
Tape casting is an important ceramic forming technique widely used in the production of thin sheets of flexible tape. When initially developed more than 50 years ago, the tape casting process enabled production of ceramic capacitors with the improved dielectric properties and smaller footprints required by the emerging microelectronics industry.1 Since then, tape casting has been used to manufacture a variety of multi-layer ceramic package (MLC) products, such as low-temperature co-fired ceramics (LTCCs), solid oxide fuel cells (SOFCs) and piezoelectric smart devices.
Moreover, tape casting has been adopted in non-ceramic, polymer and metal applications, such as polyvinylidene fluoride (PVDF) battery separator films and tape automated bonding (TAB) semiconductor adhesive-metals. Tape casting is often the preferred choice in developing and manufacturing these products because of its suitability, relative simplicity and low implementation cost when compared to other available methods, such as dry pressing, slip casting or injection molding.
The development of new products incorporating nanotechnology, smart materials and biomaterials will provide many new opportunities to exploit the advantages of tape casting as a forming method. For example, developers have successfully tape cast nanocomposites like carbon nanotube (CNT) polymers and layered silica nano-clays, optical materials like photonic crystals and dielectric mirrors, and thin film displays like polymer and organic light-emitting diode (OLED) films.2
While many advanced materials feature novel properties that are useful in a variety of applications, economic methods to produce these materials in large volumes with good quality are necessary for broad commercial acceptance. For example, the complementary metal-oxide-semiconductor (CMOS) manufacturing techniques adopted by the microelectronics industry have helped drive exponential growth with reduced costs and dramatic increases in performance. Conversely, optoelectronic devices are typically assembled by hand with very little automation, and remain bulky and expensive. Today, optoelectronic device manufacturers are attempting to adopt CMOS technology in the hope of realizing the benefits experienced in microelectronics.3
Tape casting is a straightforward method of forming uniformly thin sheets of film that is inexpensive, scaleable, and may be used with any ceramics, metals, or polymers that are readily mixed in a liquid suspension or slip. Developers who successfully produce prototypes on a bench tape caster can be confident that similar results will be reproduced on larger-scale production machines.
Moreover, the established history of successful uses for tape casting provides a strong experience and knowledge base from which new product developers can draw. Developing a new material and application does not automatically mean developing a new forming method, and a proven method like tape casting is always an attractive option.
Process TechnologyThe feed stock for the tape casting process is a slip made from a suspension of ceramic, metal or polymer particles in an organic solvent or water, mixed together with strengthening plasticizers and/or binders.4 The actual tape is formed when the slip is cast onto a flat surface by doctor blade to a carrier film or steel belt (see Figure 1).
Slurries are milled and then mixed in a pressure vessel where viscosity and temperature are controlled, and vacuum de-airing is performed prior to casting. Good slip preparation practices are essential to making high-quality finished products. Inconsistent, non-uniform mixtures are often the cause of tape defects.
After the slip has been doctor bladed to a carrier, the wet tape is dried to remove solvents. Numerous drying systems are used in tape casting, but some common types include heated air that flows from the discharge end to the entrance end above the tape, discretely located radiant heaters mounted directly above the tape, and electric resistance heaters located in the support structure that heat the tape from below.
After drying, tapes may be wound on a reel in preparation for additional manufacturing steps, such as slitting, hole punching, metallizing or firing. While most green ceramic tapes require firing in a furnace, many non-ceramic tapes such as PVDF polymers do not require firing.
Traditional doctor blade casting produces tapes with a thickness range of between 0.1 and 2 mm (0.004 to 0.08 in.), which is suitable for many applications (see Figure 2). More recently, the demand for subminiature electronic chips, such as those used in cellular telephones, has led to the use of systems that cast tapes thinner than what can be achieved by the doctor blade process.
For example, some multi-layer ceramic capacitors manufactured today have an overall footprint as small as 0.4 x 0.2 x 0.2 mm.5 A common method used to form the individual ceramic layers in these smaller capacitors is called a slot die. Slot dies, such as the one in Figure 3, cast a layer of tape by pressure pumping the slip through a precision-machined gap onto a precision roll or flat surface. Slot dies have been used to cast a dry tape thickness range of between 0.001 to 0.1 mm (0.000039 to 0.004 in.).6
A variety of doctor blade, slot die and alternative casting heads are currently being used in tape casting systems. Process suitability, ease of use and cost are the major factors that influence the selection of a particular casting head. For example, while slot die systems can produce thinner tapes, they are several orders of magnitude more complex and costly than a doctor blade system.
Forming New MaterialsThe advent of nanotechnology and the improved material properties that have been demonstrated as a result of nanoscale effects provide new opportunities to use tape casting as a forming process. Nanocrystals (particles or molecules <100 nm in size) exhibit vast improvements in strength, hardness and electrical conductivity in comparison to similar bulk material properties. Carbon nanotubes (CNTs), for example, have a theoretical tensile strength nearly 200 times that of carbon steel (200 Gpa vs. 1-2 Gpa).7 These property enhancements are primarily due to interface and quantum effects, and they can be transferred to bulk materials when the nanocrystals are used as a dispersive agent within a larger matrix material.
A complication of using nanomaterials is that small changes in the size and geometry of particles, as well as their distribution within a composite, can have a significant effect on the properties transferred to the bulk material. For example, nanometer variations in the size and shape of silver (Ag) and gold (Au) particles in solution cause a change in the observed color. Moreover, the distribution and polar orientation of the particles can also affect the resulting properties.
Tape casting may be especially useful in controlling the volume percentage and distribution of nanocrystals within a slip preparation. By experimenting with these variables, developers can directly observe property changes in the final product. In addition, the orientation of particles within a cast tape will be affected by the type of casting head used, as well as the kinds and volume percentages of plasticizers or other materials in the slip. These are also variables that can easily be changed in the tape casting process while providing useful feedback to the developer.
Photonic crystals, or photonic band gap materials, are periodic dielectric structures that have a band gap that forbids propagation of a certain frequency range of light.8 In the simplest terms, these materials provide the ability to precisely control the behavior of light waves, producing effects not possible with current art optical devices. In contrast to optical fibers, which rely on total internal reflection to transmit light waves, photonic band gap materials rely on the introduction of defects into a crystal structure to vary refractive indices and guide light.
The presence of point and line defects in specific geometric configurations within a crystal structure may be used as light wave splitters, 90-degree bends or traps. Some fascinating examples of photonic bandgap structures exist in nature, such as brightly colored insect wings (see Figure 4). As light enters the butterfly wing structure, destructive interference results in the scattering of all visible wavelengths other than those that contain the color reflected by the wings, in this case blue.
Tape casting is an ideal platform for forming two-dimensional photonic crystal structures. Multi-layer ceramic band gap materials that manipulate electromagnetic waves in the microwave spectrum have been fabricated in the laboratory using tape casting and screen printing processes.9 Similar structures for use with electromagnetic radiation in the 700 to 1600 nm wavelength region commonly used for optical communications may also be formed by tape casting.
Optical devices, particularly electric-to-optical-to-electric switching devices, are very expensive. Photonic band gap materials formed with tape casting may prove to be a high-performance, economical solution for the many applications that require integration of optical and microelectronic networks. Again, the ease with which variables can be modified within the tape casting process provides a platform to quickly produce these materials and quickly make adjustments as required.
Widespread OpportunitiesDevelopers of advanced materials who hope to have commercial success must consider the economics of their target market. While the performance advantages offered by new materials may appear compelling, low production costs will play a critical role in gaining broad commercial acceptance. For example, a $10,000 nanocomposite racing bicycle may sell to Lance Armstrong or other professional cyclists, but is not likely to sell to the average cyclist without dramatically lower prices.10
New products typically have high manufacturing costs when first introduced, but costs generally fall as economies of scale accelerate the ascent up the production learning curve.11 Accordingly, the low-cost, flexible and scaleable tape casting process is being used more widely by developers of advanced materials when forming uniform sheets of thin films is a requirement.
For additional information regarding tape casting, contact HED International, Inc. at Rte. 31, Box 246, Ringoes, NJ 08551; (609) 466-3600; fax (609) 466-3608; e-mail firstname.lastname@example.org; or visit www.hed.com.