Screen printing for high-density flexible electronics: new paste materials and advances in screen-printing equipment create a flexible opportunity.

It has always been an industry vision to build electronic circuits on a substrate by means of a simple printing process. It could be one of the reasons why they were named printed circuit boards. This vision has finally been realized by screen printing conductive pastes onto flexible substrates.

In the early era, this technology was applied only to low-technology circuits because screen printing was a low-resolution process not capable of fine-line circuitization, and there was a lower reliability in the material sets used. Instead, the wet etching process with photolithography that uses copper-clad laminate materials has been employed as the manufacturing process of fine-line and complicated, patterned PCBs for the growing electronics industry.

From its inception in the early 1950s, flexible printed circuit manufacturing has been an interactive process changing to encompass new materials, equipment and the demands of customers to package more functionality at a reduced cost. The first applications were military aircraft and missiles where reliability, reduced size and weight and the ability to conform to packaging structures were of prime importance. These early applications were far less dependent on meeting cost goals than the consumer electronics market that predominates today's flex usage. As the market has evolved, so have the manufacturing processes to meet these changing demands.

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Some of the earliest flexible printed circuits were made by screening a conductive paste onto a dielectric base. However, reliability concerns by the military led to the adoption of photolithography on copper-clad substrates as the industry standard. In recent years, a re-emergence of the printing process to mass produce electronic circuits has started to gain favor, as the need to cut production costs continues to drive technical advances in the interconnect industry. Various processes, including printing etch resists on copper substrates to dot matrix deposition and screen printing conductive materials on dielectric base films, have been developed or refined. The circuit density capability of volume production processing has always been the limiting factor.

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Printable Electronics Materials

Recently, researchers and engineers have been reevaluating the printing process as a new electronic manufacturing process because of the capabilities it offers that cannot be achieved by wet chemical and photolithographic methods. A lot of new organic base materials have been developed as the substrate for printable electronics in the last five years. Several material companies have commercialized "nano paste" products that can be used as conductive material to make fine traces. And yet, many researchers have been developing new supplemental materials. They can incorporate more functions than the simple copper foil conductors etched by the photolithography process. They can incorporate the characteristics of high-resistance materials, high-dielectric constant materials, Piezo effect materials, semiconductor materials, electronic luminescence materials and more.

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These new materials can be applied by a simple screen-printing processes to create more electronic functions on the substrates. Currently, only limited quantities of circuits with special constructions are in commercial use at the high-volume production level. The reason is lower resolution and a limited ability to manufacture multilayer circuits compared to traditional photolithography processes. Specifically, no basic printing process has been established for multilayer fine flexible circuits with micro via-hole connections.

Advanced Screen-Printing Technologies

A series of advanced screen-printing processes have been developed as the basic manufacturing technology of "Printable Electronics" by DKN Research and NY Industry. The new processes provide equivalent or more capabilities to build complicated circuit constructions compared to traditional subtractive process of copper foils or laminates with the photolithography.

An optimized combination of the process condition and material can generate 30 micron line/space on the thin flexible substrates. Supplemental processes are capable of generating 80 micron via holes for double-sided and multilayer circuits.

The basic flow of the advanced screen-printing processes is the same as the traditional screen-printing process as shown in FIGURE 1. It is very simple. A paste material is printed on a thin flexible substrate by screen printing and is baked. If necessary, supplemental screen printing is conducted on the conductor traces. The key to this advanced process is the optimized combination of the materials, screen printers and screen masks. An optimized process condition generates 30 micron line/space on a thin but smooth substrate using nano-conductive paste. Printing machines and screen masks are already capable of screen printing 20 micron line/space. Industry is looking for capable nano-conductive paste materials to realize the ultra-fine conductors. FIGURE 2 shows an example of a coiled circuit with a resolution of 50 micron line/space.

Double and Multilayer Processes

Multiple printing processes with appropriate via-hole technologies are able to produce double-sided and multilayer circuits. FIGURE 3 shows one of the via-hole generation processes for double-sided circuits. First, a conductor paste is screen printed on a thin plastic film as the first conductor layer. Second, small holes are drilled on the conductor pads. The piercing machine with a CCD alignment system minimizes the shifts to less than 50 microns. Third, the second conductor layer is screen printed on the other side of the first conductor layer. One more screen printing of the conductor paste is conducted to make via-hole reliability higher.

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FIGURE 4 shows the buildup process of the advanced screen printing to add more conductor layers on the basic constructions of the flexible circuits. An insulation layer is screen printed on the conductor layer with small access openings followed by another screen printing of the conductor paste. The screen printer with a charge couple device (CCD) alignment system minimizes the shifts between the layers.

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Creating Additional Functionality

The advanced printing process creates more functions than copper foil circuits produced by the traditional etching process. The printing process is able to produce not only conductors on substrates but also dielectrics, capacitors, resistors, coils, diodes, transistors, electroluminescence devices and more as the embedded components in the multilayer circuit. Some of the ideas are illustrated in FIGURES 5 and 6. The resistor elements can be formed between two pads of the conductor layer by screen printing a high-resistance material such as carbon paste. High accuracies of the resistances were made by trimming carbon patterns. The capacitance elements need a two-step screen-printing process to generate the conductor layer. The capacitance materials with high dielectric constants are screen printed between the two electrodes that have been previously generated by a separate screen-printing step.

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Material Selections

Materials are the key to good performance of the functional printed circuits made by advanced screen-printing processes. Depending on the intended application, there are a broad range of material choices for each function with printable electronics. Each material has qualities that must be matched to meet the requirements of the environment in which it will be used (TABLE 1).

Following material selection, the printing process is adjusted to meet the design parameters of these materials and the requirements of the final applications. Various types of screen masks are imaged by using the customer's Gerber data to form the circuit patterns, along with the requisite positioning datum points, so that proper alignment can be maintained. Ink selection is critical, matching conductive particulate size and viscosity of the suspension medium. As the line width of the circuit decreases, the particle size needs to be reduced, at times to less than 3 microns, to minimize the chance that an open can occur because of no conductivity between particles within a trace.

FIGURE 7 shows an example of the material selections for the circuit design. The resistance of the conductors is proportional to the length of the traces and inversely proportional to the width and thickness of the traces. Substrate materials make few contributions on the physical performances of the screen-printed traces. Nano pastes shows about three times higher conductivity for the traces compared to the traditional silver paste. Appropriate correction factors should be provided for patterns finer than 100 micron traces. The electrical performances are not proportional in these ranges. The properties will need to be measured for each design.

TABLE 2 indicates the processing capabilities of the advanced screen printing to produce high-density, flexible circuits.

Combinations with Other Devices

The technical capabilities of advanced screen printing are quite broad. Flex fabricators can build many kinds of constructions for the functional printed circuits without the help of other technologies. However, appropriate combinations with other circuit technologies and materials will generate more value.

FIGURE 8 shows an example of the membrane switches. A simple screen-printing process has been used that produces a large volume of membrane switches for the keyboards of electronic products.

FIGURE 9 shows ACP (anisotropic conductive paste) termination of the flexible circuits with the other circuit devices. An ACP material is screen printed on one side of the device. Then, two devices are layered together with bond pile, and appropriate pressure and heat are added to complete the connections. An optimized combination of the ACP material and process condition is capable of making reliable connections of the flexible circuits with 150 micron pitches.

FIGURE 10 shows a unique idea of a lighting system built on a thin, flexible substrate. All of the active and insulation layers are formed on a transparent ITO (indium tin oxide) film by screen printing. It is indicating a great possibility of large-size flexible displays with low cost.

Applications

The combinations of these new technologies will be valuable in producing new ideas for next-generation electronics products such as switching modules, sensor modules, bendable area light sources, thin flat speakers, flexible displays, RF devices, small-size antennas, disposable medical devices and more. New screen printable electronic materials will create more application ideas in the future.

RTR (Roll to Roll) Capabilities

Most of the printing and piercing processes are technically ready to be applied to RTR processes that have high productivity for the volume productions. The use of high accuracy CCD alignment systems will increase the overall process yields of multiple screen-printing processes.

Conclusion

Screen printing is not a new technology in the printed circuit industry. However, the combination of new paste materials and the advanced screen-printing process have created additional opportunities in printable flexible electronics that cannot be made by the traditional photolithography processes beginning with copper foil coated substrates. More ideas will continue to be generated with these new materials for specific applications. Multiple RTR processing will add value to advanced screen printing by achieving low-cost manufacturing for many flexible electronics components.

ROBERT TURUNEN (rturunen@ dknresearch.com) is vice president of new business development of DKN Research, Haverhill, Massachusetts. MASAFUMI NAKAYAMA (nakayama@ c-house.co.jp) is president of NY Industry, Ohtsu, Japan. DOMINIQUE NUMAKURA ([e-mail]) is managing director of DKN Research, Haverhill, Massachusetts.

TABLE 1. Material selection for screen printing.

Substrate         Flexible Film: Polyimide, PET, LCP, PEN, PEEK
                  Others: Paper, cloth, metal foil, hybrid material
Conductor         Silver powder, copper powder, carbon powder
                  Metal coated plastic
                  Nano particulate powder
Insulation        Epoxy resin, acrylic resin, polyester resin,
                  polyimide resin
Functional        Piezo electric materials
Optical           Organic electroluminescence materials,
                  transparent resins
Interconnection   Anisotropic conductive paste, ACF
Active material   Semiconductor paste
Shielding         Silver paste, carbon paste

TABLE 2. Process capabilities of the advanced screen
printing system.

                             CLASS A            CLASS B

Min. trace pitches          60 microns        150 microns
Min. line width             30 microns         70 microns
No. of layers            4 layers or more   4 layers or more
Min. via hole diameter      80 microns        150 microns
Termination pitches        150 microns        300 microns