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Screen Printing in the Dawn of Fuel Cells
December 31, 1969 |Estimated reading time: 7 minutes
The world is ready for fuel cell technologies - more people are concerned with environmental damage from burning fossil fuels. Fuel cell technology is well proven and effective in many applications however, unit cost must be reduced. This article explains how a typical SMT process - screen printing - can aid in the low-cost production of commercial fuel cells.
By Darren Brown, DEK
The world is almost ready for fuel cell technologies. Many people are concerned about environmental damage from burning fossil fuels. Government levies and pressure on exchange prices for oil and gas also are bolstering the economic argument for generators, vehicles, and CHP schemes to leverage the renewable nature and benign emissions of fuel cells.
Through extensive laboratory and prototype testing, fuel cell technology has been well proven and is effective in many applications. Of course, the technology must continue to develop to deliver further power density and efficiency enhancements. However, it is imperative that the unit cost of fuel cells be reduced to consumer commodity levels. Manufacturing capacity must also increase dramatically to meet anticipated demand.
Figure 1.An Inline, automated assembly will enable fuel cells to reach consumer prices.
Lower costs and higher production volumes are closely linked. Therefore, fuel cell production methods must become faster and more automated. For this, the industry needs high-speed, accurate, repeatable, cost-effective equipment capable of supporting high-yield processes to deliver high-quality fuel cells (Figure 1).
Process & Manufacturing Challenges
Dominant fuel cell chemistries include proton exchange membrane (PEM), solid oxide, and direct methanol. Their constituent parts - particularly the membrane, electrodes and backing layers of a PEM fuel cell - require thin deposits of materials such as perfluorocarbon sulfonates (PFSA) and platinum/carbon/ionomer catalysts. The physical and chemical properties of these layers are critical to achieve fuel cell action. Variations in the characteristics, such as changes of thickness or the existence of voids or pinholes in the PEM cell’s membrane, for example, can impair performance and may cause catastrophic failure by allowing the uncontrolled mixing of hydrogen and oxygen. These materials must be deposited in controlled and repeatable quantities onto a substrate that is typically flexible and porous.
Ultimately, the cost per kilowatt of energy produced is the deciding factor to determine the success of the fuel cell age. The major challenges facing this nascent industry are rooted in ensuring sufficient throughput and yield to minimize the cost per unit produced. High-speed techniques for depositing liquid compounds of varying viscosity are required. These should be capable of producing many units simultaneously within a short cycle time. Suitable techniques should also be integrated into a highly optimized production sequence. This will allow fuel cell manufacturers to maximize production efficiency and minimize human intervention.
Another issue that can have an appreciable effect on the overall cost of production is the volume of material wasted after each process. Some electrodes and electrolytes are formed using spray deposition techniques, which tend to waste large volumes of material as overspray. Vapor deposition promises lower levels of waste, but the need to create deposits in specific patterns complicates this approach. Manufacturers must also be able to switch between different combinations of deposit and substrates, create batches of components for a set of fuel cells, and support a number of fuel cell technologies on the same set of equipment.
These challenges bear striking similarities to those faced by the thick-film electronics industry of the early 1970s, and more recently, SMT assembly. The solutions adopted, such as precision screen printing of thick-film polymer inks or simultaneous deposition of large numbers of solder paste deposits, were instrumental in allowing outsourcing of assembly to specialist EMS companies. This model has enabled year-on-year reductions in the cost , while capabilities have increased. If the fuel cell community could replicate that level of success, the world may contemplate a cleaner, more sustainable future.
But there appears to be a fly in the ointment. The resolution required for effective fuel cell component production is much finer than that needed to deposit solder paste onto pads. Deposit thickness also is much thinner, and the characteristics of fuel cell materials are different from those of solder paste - although not so different from those of polymer thick-film materials.
Screen-printing equipment and processes have reached high levels of capability and refinement. Semiconductor manufacturers are using the latest generations of equipment to apply finely metered quantities of diverse materials at silicon-chip level and to the interconnects and packaging surrounding the chip. Screen printing is a proven solution to deposit chip-attach epoxies, placing solder balls of 0.3-mm diameter, and applying low-viscosity materials such as thermal interface material (TIM), lid seal, or transfer molding. It is also being used for backside wafer coating - applying a thin and uniform layer of partial-cure adhesive to an entire silicon wafer of up to 300-mm diameter, containing many individual chips.
The accuracy, resolution, repeatability, and materials-handling capabilities of screen printing can produce fuel cell components. Mature, reliable, highly automated platforms with conveyorized input and output that enable fast cycle times and easy integration into high-volume automated assembly lines are proven and readily available. These machines use high-speed linear motors and accurate, repeatable position encoders to achieve positional accuracy and repeatability at high throughput rates with few stoppages and little human intervention.
One of the most critical elements of a screen-printing process is the screen or stencil itself. Its properties define the size, thickness, and shape of the deposit. In a metal stencil, aperture characteristics are critical. In an emulsion screen, the pitch and thickness of the mesh and the properties of the emulsion have the most influence over results. Individual requirements of the application determine whether a metal stencil or emulsion screen will produce the best results. When depositing small volumes of materials at close spacing, such as precision solder-paste deposits for fine-pitch ICs, a metal stencil is optimal. For coating the backside of a semiconductor wafer, on the other hand, an emulsion screen may be preferable (Figure 2).
Figure 2. For coating the backside of a semiconductor wafer, an emulsion screen may be preferable.
Both technologies can offer a solution in fuel cell manufacture. For example, each is capable of depositing polymer materials of thickness 50 µm or lower, within ±12.5-µm total thickness variation. This is suitable for producing the ionomeric membrane for a PEM fuel cell, for example, which is typically between 50-175-µm thick. The platinum/carbon/ionomer catalyst layers, 5-50-µm thick, as well as the wet-proofed porous carbon paper backing layers typically 100-300 µm, also are candidates for high-speed, precision manufacturing using emulsion-screen or metal-stencil technologies. One advantage of this approach is that little material is wasted compared to spray or vapor-phase deposition. Doping materials for fuel cell components, particularly the platinum catalyst, are expensive; waste materials will have a significant affect on unit cost when production goes commercial.
Other characteristics of the deposit also are easier to control when using a printing process. When depositing materials using a dispenser or syringe-type arrangement, controlling the shape of the deposit is a complex challenge. Uniform thickness is also difficult to achieve. The printing approach, which forces material onto the substrate through apertures in the screen or stencil using a squeegee or enclosed printing head, creates a deposit of more uniform thickness. Because the size and shape of the deposit is defined by the size and shape of the aperture, complex shapes can be formed quickly and easily, and shapes are extremely repeatable. Deposits can also be located close together, if required.
Enclosed-head Printing
As well as enhancements to resolution and dimensional stability of screen printing, enclosed-head printing has provided an enabling technology for screen-printing processes addressing fine-pitch SMT and semiconductor applications. An enclosed-head printing system* can produce a number of head variations, including low-volume heads for expensive materials such as via-fill materials or low-alpha solder pastes. The enclosed-head system allows other materials such as polymer thick-film conductive inks to be screen-printed. These materials can be difficult to control under production conditions.
Fuel Cell Successes
Several successful processes have been implemented on a screen-printing platform - leveraging precision emulsion screens combined with specially developed tooling for flexible substrates. An enclosed-print head for fuel cell applications has also been adopted, storing the compound for deposition in a pressurized tank, and permitting a finely metered volume to be transferred through the screen onto the substrate at uniform thickness and without voids or pinholes.
Screen printing has further economic advantages, in addition to high throughput and automation. The equipment can be reconfigured to produce a variety of components, simply by changing to a different material, fitting the appropriate screen, and loading new process settings either directly or through a network.
Conclusion
Screen printing presents attractive advantages as a high-throughput, highly-automated, inline solution to create fuel cell components. Suitable platforms for hosting such processes are in production. Suitable screen and stencil technologies are in place, and enclosed-head printing enables control of the diverse constituent materials for manufacturing fuel cell electrodes and membranes in each of the dominant technologies entering use in the near future. *ProFlow enclosed-head printing system, DEK.
Darren Brown, alternative energy development manager, DEK, may be contacted at phone: 44 1305 760760; e-mail: dwbrown@dek.com.