Section: Interconnects & Packaging

As flexible-circuit manufacturing moves into the next century, polymer technology -the most environmentally friendly approach-will be at the forefront. Developments in the polymer-based components and anisotropic adhesives used in direct die attach ensure a promising future.

Traditional flexible-circuit technology consists of copper conductors, either etched or applied in an additive manner, on a polyimide substrate. Polyimide, though expensive, is generally the substrate of choice for solderability. Tin/lead solder interconnects the circuit components and copper conductors.

The breakthrough application leading to the explosion in polymer-based flex-circuit production was the membrane-switch circuit. Prior to the membrane switch, most keyboards were built using subtractive copper on rigid, glass-epoxy substrates. Since the copper oxidized over time, it was necessary to gold-plate the copper contacts, making these assemblies both costly and heavy.

Most membrane switches consist of two half switches, printed on polyester film, and a dielectric spacer to separates them, made of another piece of polyester film or a selectively printed layer. Depressing the top pad makes contact between the half switches, and current flows through the circuit. An encoder circuit decodes the X, Y signal lines, identifying each key.

In some applications, such as telecommunications, a rubber dome containing a carbon shorting pad is used in place of the top pad. Here, the mating portion of the polymer flex circuit consists of an interdigitated pattern. The carbon pad on the rubber dome provides the short between the two interdigitated half switches.

For many years, technologists experimented with means of reliably attaching active components to the polyester flex. The traditional method, tin/lead solder, couldn't be used due to polyester's low glass-transition temperature. Conductive epoxies were used as a die-attach material for some time and as an interconnect for passive devices. Unfortunately, these materials absorbed moisture, causing the tin/lead metalization on the component lead to oxidize, and the junctions to degrade.

But the arrival of a new material, Poly-Solder, a patented isotropic conductive epoxy, has changed the picture.

Poly-Solder provides a gas-tight junction between the silver-ink conductors and tin/lead-plated component leads by piercing through oxides. Junctions were stable using this material following 1,000 hours of exposure to 60iC and 90 percent relative humidity.

With this innovation, the opportunities for polymer-based circuitry became limitless. Finally, both passive and active components could be attached reliably to a low-cost polyester substrate.

The addition of components to the polyester flex started with the membrane-switch application and has progressed to more complex designs. With the development of more conductive-ink formulations and improved dielectrics, high-density, polymer-based flexible circuits have entered a number of new markets, including computer disk drives, cellular phones and medical electronics.

Those high-density applications demanded higher-performance materials and better circuit processing. Many new applications require 7-mil line and space. Further, the circuits must perform reliably in hostile environments. That demand for higher performance has led to the development of advanced materials.

Today, highly conductive silver inks and hydrophobic dielectrics allow designers to meet the market's demand for finer pitch. Line and space of 5 mil have been achieved using specially formulated inks complemented by anisotropic adhesives engineered for direct die attach.

Processing breaks

Many differences exist between standard and polymer-based flexible-circuit manufacturing. Although much work has been done recently with additive copper on flex, the primary method is still subtractive.

With subtractive processing, the copper conductors are formed by etching, a wet chemical process with significant environmental and waste-disposal issues. With polymer-based technology, inks (conductive and dielectric) are selectively screened onto a flexible substrate.

The polymer-based circuit process begins with the choice of the substrate. Polyester (PET) is suited to less stringent environmental conditions where operating temperatures are below the material's glass-transition temperature (80iC). PET is not only inexpensive-one-tenth the cost of polyimide-but boasts excellent dielectric properties and low moisture absorption, making it the substrate of choice in low-temperature applications. Higher-temperature materials range from polyethylene naphthalate (with a glass-transition temperature of 120iC) to polyetherimide (215iC). Polyimide (Kapton) can be used in the most hostile environments.

After the substrate has been chosen, the circuit is printed using conventional screen-printing techniques. The primary conductive layer, consisting of a highly filled silver ink, is screened first. Recent improvements in screen-making emulsions and printing equipment have driven the standard to 7-mil line and space (high-end manufacturing achieves resolution down to 5-mil line and space), with rates exceeding 2,000 impressions per hour. Each impression may contain many circuit cavities, thereby increasing throughput.

Circuitry can be processed in either sheet or roll-to-roll form. Following the printing of the primary layer, dielectrics are screened and UV-cured to provide an insulator between conductive layers. A second conductive layer is then printed over the dielectric, making contact to the primary layer. A cover layer of dielectric follows to protect the second conductive layer.

In the case of a double-sided circuit, holes are punched through the substrate after the first conductive layer is set down, with holes ranging from 0.010 inch in diameter, for high-density designs, to 0.030 inch. Conductive links are printed through the holes to form a conductive path to the backside conductive layer. Constructing circuits in this fashion allows for a total of four conductive layers.

Circuits are normally precut prior to assembly using either soft or hard tooling. Lasers have also been used successfully to cut polyester, in a method based on product volume and tolerances.

Assembly begins with the stenciling of Poly-Solder. Newer versions of this material have been printed down to 15-mil pitch, and attachment of 0.5-mm quad flat packs is now the standard. Conventional pick-and-place equipment is used to place the components. Poly-Solder is then cured at 140iC for 15 minutes in a conveyor oven.

Anisotropic conductive adhesives are being used to more readily handle pitches below 15 mils. These adhesives are employed for flip-chip bonding and LCD attach.

Materials design

The design and construction of a flex circuit are only as good as the materials used. The development of commercial materials for polymer thick-film applications has lagged behind the demands being placed on the technology. To address that, a family of proprietary materials has been developed to meet the quality and reliability requirements of high-density thick-film circuits.

The backbone of a flex-circuit construction is the material used to form the conductive traces. They are printed using a silver-loaded conductive ink specifically formulated for flex-circuit applications. This material has a conductivity in the range 15 /square/mil, with good flexibility. Ink flow has been adjusted for high-resolution screen printing at high production speeds. Using this ink, circuits with 15-mil pitch are produced at rates of 2,000 impressions per hour.

A dielectric coating provides some degree of mechanical protection and, more important, electrical isolation of adjacent conductors. Any polymer coating can provide the former, and to some extent the latter, under dry conditions. But when these materials are subjected to high temperature and high humidity, the deficiencies become apparent.

That is important because in addition to the trend to higher density, which alone places greater demands on dielectric performance, the expansion of markets calls for products to perform in hostile environments. For example, telecommunications devices must perform in tropical climates, where high temperature and humidity are the norm.

Because current commercial dielectric coatings will not meet these demands, a new, UV-cured dielectric coating has been developed with superior humidity performance. Designated PF-114, this material has been formulated with a high degree of hydrophobicity while maintaining excellent flexibility, even after 500 hours at 85iC.

PF-114 also has good adhesion to difficult substrates, such as non-print-treated polyester. Circuits prepared with PF-114, with its low moisture absorption and resistance to electromigration, have shown a minimum five- or sixfold improvement in humidity performance over the best commercial materials.

Insulation resistance

Production circuits were prepared with PF-114 and the best of the commercial materials, then tested by an independent facility at 70iC and 85 percent relative humidity (non-condensing), with a 10-V dc bias. The separation between conductors was 15 mils at the closest point. A leakage current in excess of 20 A was considered a failure.

Circuits with the commercial dielectric displayed an immediate and steady increase in leakage current. All parts failed in less than 400 hours, the circuits exhibiting severe discoloration with evidence of electromigration and dendritic growth.

Parts with PF-114, on the other hand, continued to 1,000 hours with minimal increase in leakage current. When removed from the chamber, the PF-114 parts showed little discoloration and no indication of electromigration.

Multilayer test coupons were prepared with two-print passes of PF-114 for a total of 1.5-mil separation between conductors. The test vehicle had 400 crossover points. When tested at 60iC and 90 percent relative humidity (non-condensing) with a 5-Vdc bias for 1,000 hours, there was no appreciable increase in leakage current between conductors.

Owing to the very low moisture absorption of PF-114, it has been observed that when circuits exposed to long-term, steady-state humidity are allowed to return to ambient conditions, the insulation resistance returns to near initial values. In the real world, of course, a product would not be subjected to steady-state humidity, but to alternating wet and dry cycles. It is probable that product life would be significantly longer than that predicted by steady-state tests.

The attachment of surface-mount devices is handled with Poly-Solder. This adhesive has been proven through five years of extensive testing, customer evaluation and performance in the field to be a superior material. It exhibits high bond strength and excellent junction stability even under conditions of high temperature and humidity.

To meet the demands of higher-density products, a new version of Poly-Solder has been developed for high-resolution stencil printing. A UV-cured encapsulant, applied over the Poly-Solder junctions, cures to a tough, resilient polymer that provides additional strength to the junction. The typical increase in bond strength of a two-lead device, such as an LED, is approximately 200 percent following the application of this encapsulant.

Cellular phones have been challenging packaging engineers with decreasing sizes and more ergonomic designs. Cell-phone flexible-circuit designs incorporate keypad functions, an LCD with supporting driver, microphone, snap-dome switches and mounting pads for a speaker. Along with the challenges of integrating those functions into the flexible circuit, the cellular-phone operating environment demands a high degree of reliability. Thermal shock, high-humidity conditions and mechanical-shock characteristics must all be considered.

A circuit was designed for a cellular phone using double-sided printed through-hole technology. To maximize space for the circuit traces, through-holes were reduced from the standard .03 inch to .01 inch. The interdigitated switch contacts were of carbon ink, with all silver traces covered with hydrophobic dielectric to minimize long-term effects from moisture penetration.

A 0.5-mm (19.7-mil) pitch, 64-pin QFP and passive components were attached using Poly-Solder, and an LCD was attached using a monosotropic adhesive. The microphone was a radial-leaded device supplied with lead extensions soldered to them. The microphone was laminated to the circuit to form a pressure-fit connection. Snap domes were assembled and secured with tape. The result was a solution that successfully integrated all the required features and eliminated a rigid board assembly.

In another example, a control-grid assembly was built to provide mouse-type positioning control using a pen device. Decoding circuitry was integrated with the control grid to form one flexible-circuit assembly designed using printed multilayer circuitry. Hydrophobic dielectric provided isolation between silver-ink layers.

The challenge of this product was to attach multiple components using a stencil of a single thickness. All 92 components of varying package sizes and types-from 0805 resistors and capacitors to a 64-pin, 32-mil-pitch PLCC-were attached with Poly-Solder.

A third example-the TV remote-was an interesting challenge. Previous versions were a combination of a rigid board with components, and a flexible circuit utilized for the switch pads. The requirement was to integrate both sections into one flexible-circuit assembly.

Since mechanical shock and resistance to moisture absorption were critical to the design, the circuit was required to pass a coffee-spill test. Circuits were placed in a vessel filled with coffee, removed and allowed to operate for 24 hours with a 3-Vdc bias. In order to meet that requirement, the assembly was designed using printed multilayer circuitry.

Minimum effects

The interdigitated switch contacts were made of carbon ink, and all silver traces were covered with hydrophobic dielectric. That minimized the long-term effects of moisture penetration and provided isolation between the silver-ink layers. Components were again attached with Poly-Solder, eliminating lead-based solder-a boon for an environmentally "green" product.

Overall, polymer-based flexible circuits are being more widely used in high-density applications. Improvements in materials technology have made possible circuit densities approaching 5-mil line and space. Specially formulated isotropic and anisotropic conductive adhesives have led the way to 0.5-mm pitch being the standard in component attach and 10-mil pitch the standard in die attach.

PHOTO (BLACK & WHITE): Five-mil-line circuts

PHOTO (COLOR): flex-control grid assembly

PHOTO (COLOR): Flex TV remote was an interesting challenge

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By Stephen J. Arrico, Roger A. Iannetta Jr., and James Whitefield