Section: Interconnects & Packaging

Part 4:Flexible Circuits

Flexible circuitry, which has reinvented itself often in its almost 100-year history, is at it again. This time the technology is being used for new applications in high-density micro-packages. The result is a future that looks bright for flex, currently the fastest-growing segment of the printed-circuit-board business.

The use of thin, dense and flexible substrates for packaging grew out of TAB and tape-carrier-package applications. Its popularity has burgeoned as advanced IC packaging (such as ball-grid arrays and chip-scale packages) and high-density processes (such as flip chip attach) have come on-stream. New materials and processes are being adapted for flex so that it can meet high-density demands and provide other functions and features that rigid pc boards simply cannot furnish.

The need for innovative, cost-effective packaging circuitry that can handle fine-line geometries while offering high density, high reliability and maximum functionality is pushing advances in all aspects of flex circuits. Properties unique to flex make it desirable for applications requiring thinness, light weight, flexure, mechanical strength and the ability to conform to particular shapes and handle harsh environments.

At the same time, its historical stumbling block-cost-is being brought under control by improved processes, advances in less costly base materials and roll-to-roll manufacturing techniques. While board interconnect will likely continue to be the prime application for flex, its use as a substrate for component mounting is likely to be the fastest growth segment.

Innovative processes have opened the way for the construction of ultra-thin, high-density adhesiveless flex circuits for fine-line geometries; featuring widths around 25 microns, they look especially promising for packaging.

Flex circuits have achieved such densities because of developments in parallel processing, colamination and conductive adhesives that eliminate traditional, plated through-holes. Flex is also moving to multilayer implementations for use in medical electronics, smart cards and appliances, thanks to significant advances in the area of base films and adhesiveless and multilayer constructions.

More feed-through holes for flex requires more layers, and multilayer demand is increasing for high-performance use. Initially, such circuits may be costlier than rigid pc boards, but the end products that incorporate them are expected to be cheaper in the long run because of automated manufacturing and the ability to integrate switches and interconnects, and eventually passives, into the flex circuitry.

Multilayer circuits, still a small portion of the flex business, are mainly dedicated to military applications and are still relatively costly. Multilayer fabrication presents manufacturing challenges that come with bonding inherently unstable materials together in large panels. Typical challenges in achieving acceptable yields are ensuring the dimensional stability of layup in lamination and accomplishing precise layer-to-layer registration.

The customization of flex-circuit performance to serve particular requirements, such as electromagnetic-interference (EMI) shielding and impedance control, has appealed to manufacturers of computers and telecommunications gear. Flex's thinness has added to the benefits of its use in portable and consumer electronics, particularly in automotive and power-supply equipment, where heat transfer is a factor. In disk drives, flexure and high density are popular features. Because of its three-dimensional capabilities, flex can achieve the highest wiring density of any printed circuit. Enhanced reliability, smaller size and weight and ruggedness appeal to military and aerospace designers.

Not only how and where flex will be used but how it will be assembled affect its makeup. Base materials selected for flex used as an interconnect will be different from the flex materials used as a substrate with components mounted on it. Desirable characteristics include thinness, low moisture absorption, high density, low cost, processing ease, good adhesion to copper, dimensional stability and, usually, some degree of flexibility.

Two types of film-polyethylene terephthalate (PET) and polyimide (PI)-have dominated as circuit base materials. PI-base film, mainly Kapton, has emerged as the leader where resistance to heat and chemical exposure, dimensional stability and fine-line geometry capability are key. PET films, mainly Mylar, are the least costly and have proved useful in high-volume, cost-sensitive applications for low-function, low-technology flex circuits. Mylar is also used in friendly environments and processes because its glass transition temperature will not withstand mass reflow.

Anisotropic, or Z-axis, adhesives are used as bonding materials when solder cannot be used or when it cannot easily or productively be applied at very fine pitches to polyimide.

Mylar's limitations

Mylar is not used for under-the-hood and disk-drive applications, because of its lack of flexibility, inability to handle extended temperature cycling and inappropriateness for high-reliability, high-density applications, said Tim Patterson, principal R&D engineer at Smartflex Systems (Tustin, Calif.). He noted that low-cost flex, typically polyester, is used in circuits made with 0.1-inch geometries in a pc-board fabrication operation to produce relatively large automotive instrument panels.

Work is being done on polymer materials and polymer surface mount attach processes to enable Mylar films to be used more extensively in high-volume, cost-sensitive applications. Low-function, low-technology flex-circuit applications in high volume are moving to polyester and pressure-sensitive anisotropic adhesive film for bonding, said Pete Herman, engineering director at Pemstar (Rochester, Minn.). The anisotropic film permits high-density interconnection with geometries down to 4-mil pitch and lower electrical conductivity.

Adhesive-bonded flip-chip assemblies on flex are expected to find many applications in the high-volume consumer market. The use of polyester as a substrate material with adhesives is a low-cost direction for smart cards.

Metallized polyimide films are becoming increasingly popular for flex applications for their superior electrical, thermal and chemical properties. Moreover, parallel processing with filled vias and conductive adhesives is being refined for the fabrication of lower cost multilayer flex structures for high-density applications. It is easier to achieve fine pitch with polyimide flex substrates used for component attach than with traditional FR4, and polyimide is less costly than ceramic substrates.

Nevertheless, there is still room for improvement in the properties of flex, especially where high density is needed. Flex materials need better dimensional stability-as good as or better than FR4, according to Shiuh-Kai Chiang, director of technology at Gould Electronics Inc. (Eastlake, Ohio). Reel-to-reel processing, used for flex because it costs less, can stretch material and cause changes in dimensions. But if changes in processes are understood, the process can be engineered to create improvements.

The demand for flex's desirable characteristics has sparked the search for a dimensionally stable material, resulting in the commercialization of a new polyester material: polyethylene naphthalate (PEN). The material is expected to open the door to a wider range of applications previously not considered flex-worthy. But it isn't perfect: although the properties and price structure of PEN bridge the gap between PET and PI, it does not have the flexural capability of either.

The major U.S. supplier of PEN films and polymers, Kaladex PEN, was formerly ICI Polyester (Wilmington, Del.), part of ICI Americas Inc. In July, DuPont agreed to acquire ICI's polyester films resin and intermediates business and all related technologies worldwide. That includes ICI's PET business, which in turn includes its Kaladex PEN films and polymers.

Polyester is said to be a $30 billion industry globally, growing about 7 percent a year. PEN material is also available from Taigen, a Japanese supplier.

DuPont plans to integrate high-performance PEN into its portfolio of flex products and is looking at other film structures that have all the properties of polyimide but are less costly. The real growth for PEN laminates will be in applications that take advantage of the marginal benefits of its solderability and its ability to withstand higher temperatures, according to Brian Swiggett, managing partner at market-research firm Prismark Partners, LLC (Cold Spring Harbor, N.Y.).

Swiggett said that special adhesive systems and surface treatments are needed with PEN for bonding copper foil, that acrylic adhesives must be reformulated and that experiments have been made with epoxies as bonding materials. Typically, Kaladex PEN film is not used for multilayer flex structures.

PEN presents another challenge: According to Tony Cowburn, director of technology at Adflex Solutions (Chandler, Ariz.) and Havant UK, a global circuit-assembly operation, the process window for soldering to PEN is narrow, and component mounting requires great precision because a drop in yields is wipes out the cost advantage.

But PEN is attractive for simple circuits and, long term, will be used where polyimide's performance is not needed, said Jim Fraivillig of Fraivillig Materials (Austin, Texas). He added that PEN absorbs one-tenth the moisture of Kapton, needs no pre-dry process and is cost advantageous-but processes must be geared to the thermals of the material. A higher degree of thinness is possible with PEN than with polyimide because PEN can be extruded and polyimide cannot. Further, it may be possible to use PEN with polymide-circuit constructions.

Advances in the construction of adhesiveless flex have decreased Z-axis expansion rates and heightened the ability to form fine features compared with adhesive-based flex. Adhesiveless structures eliminate the use of traditional acrylic adhesives for bonding and make copper thinner, making it possible to produce fine-line geometries of 50 to 75 microns. Adhesive-based materials often soften during high-temperature processes, while adhesiveless materials are better able to withstand high temperatures and harsh environments. Because the cost of adhesiveless materials is one and a half to two times that of adhesive-based flex, it is used to meet specific needs.

An adhesiveless construction may consist of a polyimide sheet with an embedded seed layer that acts as the adhesion for the copper layered on, or the copper may be in the form of a very thin foil onto which a wet polyimide material is cast and cured in place. Adhesion with that process is particularly good, and copper layers can be made very thin.

Among the suppliers of adhesiveless materials, DuPont's Flex Circuit Materials Group (Research Triangle Park, N.C.) offers 2-mil-thick adhesiveless clad polyimide, Pyralux AP, for flex-circuit construction. It is also shipping samples of adhesiveless clads under 2 mils thick, as well as versions 3 and 4 mils thick, for very high-reliability applications that require impedance control.

DuPont is involved in programs to investigate alternative technologies for the fabrication of thin metal adhesiveless clads for very-high-density applications and alternative substrate materials for flex. Also, work is under way to upgrade adhesive formulations for better overall electrical performance and improved appearance.

Adding copper

An adhesiveless polyimide film with an additive copper seed layer, called GouldFlex, is being manufactured by Gould Electronics. Very thin dielectric layers-0.5 mil thick-can be made, and the layers are amenable to all three major microvia-formation techniques: plasma, laser and chemical etching. GouldFlex is also used when fine-line features of at least 2 mils and vias of at least 2 mils are required.

The benefit of thin dielectric and thin copper is the resultant long life in dynamic flex applications. Copper is available at 0.2 micron to 35 microns thick, making GouldFlex very flexible. It can be used for hard drives and microelectronic packages, and a potentially sizable application is for suspension flex circuits, to replace copper wires in disk-drive heads.

Sheldahl Inc., a major supplier of flex substrates, has invested more than $50 million to build a plant in Longmont, Colo., devoted to the fabrication of adhesiveless polyimide copper-laminated flex substrates, called Novaclad, aimed at the packaging industry. Production at Longmont is just getting started; the plant is operating at 10 percent of its capacity.

Novaclad, which is intended for static rather than dynamic high-density applications, is available with 5 or 15 microns of copper. Static flex may have to be folded or bent to install or may require flexure only occasionally, as exemplified by high-density substrates for advanced IC packages and high-density circuits in portable communications, computer equipment and engine controls. All flex fabrication at Longmont is done on a 13-inch-wide Web, roll-to-roll, in a step-and-repeat process. While dimensional stability and predictability of materials are a challenge when processed on the Web, those factors are said to be controllable.

Sheldahl's recent participation in a Darpa consortium, which in part involved a commitment to manufacture high-density flex substrates with double metal layers for direct-chip attach, led to the construction of high-density multilayer structures. According to Keith Casson, vice president of the microproducts business at Longmont, Sheldahl itself cannot produce multilayer flex in high volumes but will make and put together high-density double-sided flex layers and partner with others for volume multilayer flex fabrication.

The development of cost-effective, high-performance multilayer interconnect technologies for flex structures is gaining momentum. New applications in packaging are driving the implementation of parallel-processing techniques for flex circuits, along with solid "filled" vias that may be blind, buried or stacked to replace sequential processing techniques. Several companies, including Matsushita and Toshiba already have developed variations on this parallel-processing filled-via technology for the rigid pc board market.

CTS Corp. (Elkhart, Ind.) is among the companies focusing on it for high-density flex for packaging and interconnection. Circuit layers or layer pairs are processed in parallel and then fabricated into a multilayer circuit with blind or buried sintered vias. The sintered via, which establishes a stable metallurgical connection between layers, is one of the primary attributes that differentiates CTS colamination technology from other filled-via ones.

CTS offers three variations of colamination technology for the fabrication of multilayer circuits: ViaPly, double-sided plated-through-hole circuits and via-formed and filled bondply interposers; ViaClad, copper foil and filled-bondply interposers; and CircuitPly, single-sided flex circuits. CircuitPly-in which multilayers are fabricated in a parallel colamination process-permits each layer to be inspected. It is less costly to produce multilayers with this colamination technology than with any sequential buildup technology, according to CTS. After multilayering, flexible material is not very flexible and will not be applicable to dynamic flexure. The main applications for colaminated multilayer flex structures is likely to be packages and other applications that require high density.

A variety of single- and double-sided adhesiveless materials that are compatible with its colamination process has been qualified by CTS. This includes Kapton, Upilex, Novaclad and GouldFlex. The multilayer features that are achievable with CircuitPly are 4-mil vias with 8-mil capture pads, 50-micron lines and 38-micron spaces.

Silver-filled epoxies and proprietary conductive pastes are in demand as cost-effective vias fill materials that substitute for plated holes. Most use is for low-density flex circuits with a limited number of holes because of the high cost of the pastes for a large number of very small holes. Ormet Corp. (Carlsbad, Calif.), a company that worked on Darpa projects developing high-density flex circuits with Sheldahl and Litronics, has developed conductive pastes to fill vias in polyimide flex.

Called Ormet, the conductive copper-based composites use transient liquid-phase sintering to form a stable conductive path as well as a metallurgically alloyed connection among the material, the pads and the flex-circuit layers below and above. Ormet is used to fill the microvias and in some cases also to create circuit layers. The metallurgical contact with the pad is key; Ormet provides an improvement over silver-filled epoxies that have particle-to-particle contact by forming a stable connection that is not sensitive to moisture uptake or thermal cycling. Since the process window is a critical factor, some customization of Ormet may be necessary, depending on the surfaces on which it is used.

Specialized versions have been developed for sequential build-up of flex-via layers, with fine lines and blind and buried vias throughout, patterned in photo-imageable dielectric materials. Lines and spaces of less than 50 microm have been demonstrated, with vias as small as 75 micro m.

Another recently developed material for use in multilayer flex structures-a polyimide-based adhesive tape-comes from Sheldahl and is in the prototype stage. The tape can have either lased or punched vias for high-density applications. The vias are filled with conductive pastes that fuse flex layers together. After lamination, these structures will withstand temperatures of -40 Celsius to + 125 Celsius in excess of 1,000 cycles with no connections failing.

A new Z-axis film from 3M Corp. (St. Paul, Minn.), used to create interconnects with flex, permits fine features down to 5.5-mil pitch. Called 7303 Z-axis adhesive, the material is for polyester flex applications such as connecting a membrane switch to a pc board. Newer Z-axis adhesives are able to hold interconnects stable to within a much tighter resistance range than older systems. At l or 2 mils thick, 7303 material makes very thin connections for portable consumer electronics and other types of electronics pressed by space considerations, in comparison to mechanical electrical connections.

In the area of microvia formation, work is proceeding to decide which particular formation technique may be best for a specific application. These decisions are based on a number of factors, including desired via diameter, plated metallization properties and plating adhesion-all of which affect via reliability-as well as cost per hole, the selection of blind or buried vias, production volume and manufacturing costs.

Each of the three major via-formation techniques for flex, laser, plasma and chemical wet etch has distinct advantages and limitations. The choices often depend on what size vias are needed. Higher density requirements may call for employing technologies that make use of conductive adhesives to eliminate traditional plated through holes.

The difference between laser systems is the amount of energy each uses for different tasks and the resulting throughput. Laser processes must be compatible with the particular flex material used. Excimer lasers are able to spot-drill individual holes through the copper/insulation flex substrates as well as perform area scanning of pre-etched openings that have been photo-defined. CO2 lasers ablate polyimide but not copper so a photo-imaging process for copper is necessary, which adds cost.

Excimer lasers are used on the 50-micron-thick base polyimide film used in Sheldahl's Novaclad to create l- and 2-mil vias. Vias are lased at the rate of 100 per second, one by one. Sheldahl also has the potential to mass produce 5,000 1- and 2-mil vias per second by means of a precision glass mask that splits a laser beam, a cost-effective process because of its speed.

Artwork registration necessary to form vias in the correct size and location with the desired results has been reported to be difficult with the use of plasma ablation in volume production of flex circuits. At 3M (Austin, Texas), a high-volume supplier of custom-additive flex, wet chemical etching is used through 2-mil thick polyimide in production for via formation instead of using plasma. The etch process is used only with polyimide. The thickness of the polyimide affects via diameters on the top side of the circuit. The dielectric layer generally consists of 2-mil thick substrates; for high-reliability applications, the via diameter is 3 mils to 4 mils in diameter at the small end and 12 mils at the top surface.

The good news about wet chemical etching is that it is a low-cost, scalable, mass-sheet process. The additive process requires minimal compensation, allowing complex and precise patterns to be placed on flex circuits. At 3M they are scaling this process to suit multilayer flex circuits.The additive flex circuits are adapted to flip-chip reflow processing parameters and applications include array packaging, automotive electronics, computers, medical electronics and disk drives.

The current void in the market for flex-to-flex and flex-to-rigid interconnects is stimulating development work with flex in the connector industry. Packard Hughes Interconnect (Irvine, Calif.), a subsidiary of General Motors, is splitting off Gold Dot high-performance electronic interconnect technology, which has been a success in the military, for use with copper- and polyimide-based flex circuitry in volume commercial applications. The new pressure-actuated connector for high-density interconnects, called EZ-Pac, will be produced in high volume but at first will not be used to mount components. The high-volume commitment to Gold Dot requires a focused factory and Packard Hughes has invested millions in a factory within its present facility. Orders are being taken for the interconnect for delivery in 1999, prototypes are currently being provided from the pilot line and low volumes are being produced for certain customers.

EZ-Pac is a reusable interconnect that can, without changing impedance, accommodate systems that process signals at speeds as high as 1 GHz. Because of the new speed capabilities, the interconnection of EZ-Pac to backplanes is expected to be an important application and a distinct advantage over traditional connectors. EZ-Pac's gold dots are small precisely shaped bumps integrated with the copper traces on the flexible circuits and coated with gold. An electrical connection is made with a pressure interface without using solder. Other attributes that the flex interconnect configuration offers over traditional connectors are very high density, controlled impedance and improved thermals that can withstand temperatures ranging from -40 degrees C to +125 degrees C. Interconnect geometries can be as small as 2-mil lines and 2-mil spaces, and via-hole diameter capability is 8 mils at present. It can be used in high-speed computers, telecom switching gear, peripherals, disk drives and medical and automotive electronics as well as advanced antennas for global satellite systems.

Diverse products

Flex circuits have been the enabling technology spurring new products inconceivable without it for applications from satellite array sensors for data reception and devices serving medical purposes within the human body. In the medical field, for example, a flex circuit in a so-called smart needle penetrates the vein and sends an audio signal when it senses blood flow. And a placement aid for disposable balloons used in heart catheterization consists of five ICs on a flex circuit. The catheter, inserted in the vein, projects images of the balloon's location for proper placement.

The automotive applications for flex are multiplying. There are companies developing techniques that over the next two years will put multilayer flex in engine-control units to replace rigid printed-wiring-board circuits and take advantage of flex's size, weight, improved thermal transfer and lower system costs.

As flex circuits replace wiring harnesses in the instrument panels of some cars, they will be linked with connectors capable of many mating cycles. Eventually flex circuit used in the fuse and junction box will be combined with the one in the engie-control unit.

GRAPHS: Kaladex PEN films claim breakthrough in dielectric film technology

Flex-to-rigid interconnects are stimulating work with flex in connectors, as in this Packard Hughes assembly.

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By Glenda Derman

The world production of flexible interconnects is growing steadily, driven by a surge of new and improved products that require lighter, smaller, thinner and more space-conscious interconnects. These properties combined with the potential for high-circuit density of flex circuits make them particularly attractive to designers of various portable and high-performance systems. Because its use makes certain functions and features possible, the flexible interconnect can be thought of as an enabling technology for many applications. As a result, the total flex production around the world is projected to be nearly $5 billion by 2000.

Each of the four flex production regions-Japan, the United States, Europe and Asia-has a unique history of flex manufacturing, particular market drivers and outlook for the future.

Japan is the largest producer. However, it is marked by severe price competition because of the emphasis on the highly competitive consumer-electronics market, which accounts for about 32 percent of the total consumption in Japan. For this reason, Japanese manufacturers concentrate on low-cost, high-volume and high-yield roll-to-roll processing of single- and double-sided circuits.

In the United States, the decline in funding for military electronics has seen demand for high-performance, high-cost flexible circuits for military applications begin to wane. Consequently, American flex manufacturers are looking for other markets for their excess capacity. Applications such as computers and related peripherals, communications and automotive electronics became natural targets for them, giving them the freedom to develop new technologies and processes.

Flex manufacturers in Europe focus on circuits where size, weight, thinness and flexibility are most important. This includes smart cards, cellular telephones and specialty applications such as hearing aids and watches.

The major producers in Asia have built market share through high-volume (roll-to-roll in most cases), low-cost production of single- and double-sided circuits to support local industries such as consumer electronics in Korea and computers in Taiwan.

Future uses

A study undertaken to look at future use of flex circuits by major OEMs in markets such as computers, consumer electronics, automotive and telecommunications indicates:

Many OEMs do not plan future developments in flex circuits, but instead rely on flex-circuit fabricators for technology road maps.

Cost is still a major impediment to the use of flex. Many OEMs are reluctant to design in a flex component when a less-expensive one (such as an FR4 board) can be used. For flexible circuits to see widespread implementations, cost reductions are essential.

Increasing flex production will be the result of increased production of existing products that utilize flex circuits, as well as increased use of flex in new designs

Flex-circuit technology will be driven by cutting-edge applications that require the flexibility, form-factor reduction or high-circuit densities afforded by flexible circuits.

One market segment that is driving new flex technologies such as fine-line patterning is semiconductor-chip packaging. The variety and number of packages utilizing flex as the basic interconnect has ballooned in the last few years. Until a few years ago, tape-automated bonding (now called tape-carrier package, or TCP) was the only IC packaging that employed flex. However, with the advent of ball-grid array (BGA) and chip-sized packages (CSPs), the use of fine-line, thin flexible substrates for packaging has surged substantially.

GRAPH: Upward flexibility

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By Thomas Goodman, Senior Analyst, TechSearch International Inc., Austin, Texas