Casting Quality

Simple processing procedures can reduce and even eliminate common tape casting defects.

For multi-layer ceramic capacitors (MLCCs), low-temperature co-fired ceramics (LTCCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs) and other products that depend on the thin tapes produced by the tape casting process, high tape quality is crucial. Defects can reduce yields or cause problems such as shorts, gas leakage, differential capacitance or zones of weakness in the finished product. Fortunately, the most commonly occurring defects can often be reduced or completely eliminated by following some simple processing procedures.

Common Defects

In the tape casting process, the tape is typically formed on a continuous web or carrier from fluid in a reservoir, which is metered through a gap between a knife/doctor blade and the carrier substrate to control tape thickness. Common defects include pinholes, agglomerates, streaks and surface contamination.

Pinholes, the most common defect found in tape cast products, are small holes or pits in the dry tape. Caused by entrained air in the slip that shows up as a small bubble in the cast tape, pinholes are usually observed on the air side of the tape (i.e., the top side, which is in the air stream during drying). Some bubbles are caused by solvent evaporation during drying of the cast slurry and are usually found on the bottom side of the tape (i.e., the side that is in contact with the carrier during the casting process). Pinholes that extend through the entire thickness of the tape can cause problems such as shorts in capacitors or the potential for gas leakage in fuel cell electrolyte layers. Pinholes that do not extend through the tape can create problems such as differential capacitance and zones of weakness after sintering.

Two different types of agglomerates can cause defects in thin tapes. The first type is an agglomerate of undissolved binder, which is caused by clumps of powdered binder that do not go into solution during the mixing and homogenization phase of slip preparation. These agglomerates cause holes in the sintered layer after binder burnout has been completed.

The second type is the so-called "hard agglomerate" of the solid phase in the tape (i.e., the inorganic or metallic phase). Hard agglomerates are clusters of particles that are stuck together so tightly that they are not broken apart during the dispersion milling procedure. They typically manifest themselves as small peaks or lumps after the tape shrinks during drying, and can cause problems during lamination by piercing the adjacent layer and causing shorts and other quality problems in the structure.

If a single-stage milling process is used during slip preparation (i.e., the solvent(s), dispersant, powder, plasticizer and binder are all added to the mill simultaneously), the binder might attach to multiple particles of an agglomerate and bind the agglomerate together to form a potential defect. Typical defects include lumps and bumps on the cast surface, which show up after drying and remain after sintering. These defects can also create porous areas in the tape, which cause the potential for shorts and gas leakage through the sintered layer.

Streaks, which are thin lines that are parallel to the casting direction, are regions of thickness variation in the tape that are usually caused by bubbles that become trapped under the doctor blade during casting. Another potential source of streaks is caused by the feeding mechanism for slip into the doctor blade reservoir. Even though the slip experiences considerable mixing and turbulence as it moves from the reservoir and under the doctor blade, some "memory" of the pouring or other slip feeding technique often exists. In other words, if a manifold is used for feeding slip to the reservoir, the slip streams might join together in some regions and form streaks, which are then carried through onto the surface of the tape downstream from the doctor blade. Streaks can cause variations in tape properties, especially if the tape is an active material such as a capacitor or varistor, and they can also be potential sources for cracking during subsequent sintering operations.

Surface contamination is caused by dirt and dust in the drying air train that settles onto the wet tape surface. Dust particles are drawn into the machine by the exhaust fans that pull room air into the drying chamber, usually through a heater of some sort. The dust particles settle onto the surface of the wet tape cast product and cause surface defects such as pits or holes when they burn out during sintering.

Processing for Quality

Most common tape casting defects can be avoided or at least minimized by using simple processing procedures. For example, the use of two-stage milling for slip preparation virtually eliminates the potential for forming binder-induced agglomerates. In a two-stage process, the powder, a dispersant and the solvent(s) are added to the mill first. A dispersion milling procedure is then instituted by rolling for up to 24 hours on a set of rollers. After the powder is broken down into individual particles and the particles are coated with the dispersant, the second-stage chemicals-the plasticizer(s) and binder-are added. The mix is then homogenized by rolling for an additional 12-24 hours.

The most common technique for removing air from the slip before casting is vacuum de-airing. A partial vacuum, accompanied by gentle stirring or agitation, tends to lower the viscosity in a pseudoplastic slip, thereby making air removal easier. In a laboratory situation, a vacuum desiccator can be used in the range of 635 to 700 mm of mercury. This low vacuum can be generated using a vacuum "rough" pump or an air aspirator (venturi pump), which works on the Bernoulli Principle. Too high a vacuum will tend to remove a large volume of solvent along with the air bubbles. For small volumes of slip (4 liters or less), the time for complete air removal is generally five to eight minutes. Production de-airing is usually done in large (up to 55 gallon) containers with air-driven stirrers and can take hours instead of minutes.

Vacuum de-airing does not work well for all applications. For example, water-based slurries sometimes foam during vacuum de-airing. These types of slips are de-aired best by using slow rolling or stirring for 24 hours or longer in the presence of a defoaming agent. Figure 1 shows a laboratory de-airing station, and Figure 2 shows a small-scale production de-airing station.

After the slip is de-aired, the next step is to filter out agglomerates of binder, particulate agglomerates, and/or any residual bubbles that remain. Bubbles might not be entirely removed by the filter; however, they will be broken down into smaller bubbles that have less of an effect on the dried tape. Filtration is accomplished by pumping the fluid slip through a medium such as a nylon mesh screen cloth. Many different types of filtration media and filtration stations are available, but the objective is the same-to remove potential defect-creating sources larger than a specific size, which is set by the opening size in the filter.

In a simple filter assembly, as shown in Figure 3, where a filter cloth is clamped between the two metal plates and the gasket, the openings in the filter cloth can be any size ranging from <10 microns to >200 microns. The size selected is determined by the size of the defects that need to be removed, the thickness of the tape to be produced and the time allocated to the filtration process. If the tape being manufactured is thick (0.025 in. or greater), the filtration cloth selected can have larger openings than for a thin tape, since the defects will have less of an impact on the final properties of the tape. Filter cloths with very small openings also usually have a very small percentage of open area. The filtration process will be very slow in this case, since the slip flows through the cloth with difficulty.

In large-scale manufacturing, filters very similar to those used in water filtration can be used. With replaceable filter cartridges of a specific micron size, these filters usually have a much larger area available for the filtration process. Most in-line filtration operations have a parallel path setup, so plugged filters can be changed during the casting operation without stopping the cast. Filters are "throw-away" items that are discarded after one use. The filter material selected must be insoluble and non-reactive with the solvent system being used in the tape formulation.

Most surface contamination defects can be prevented by passing the air through a HEPA filter before it goes into the drying chamber. The HEPA filter used should remove at least 95% of all airborne particles 3 microns and larger. Figure 4 shows a HEPA filter in place on the air intake for a typical tape casting machine.

Preventing Tape Defects

Common defects such as pinholes, streaks, agglomerates and surface contamination can cause major problems in the very thin tapes that are used in the manufacture of MLCCs, LTCCs and fuel cells. By using routine processing techniques, manufacturers can prevent and eliminate these defects and consistently produce high-quality tapes.

For more information on tape casting processes, contact Richard Mistler at 1038 Lafayette Dr., Yardley, PA 19067; (800) 641-1034 or (215) 493-2008 (phone/fax); e-mail tapecast@juno.com; or visit http://www.drblade.com.