It would be difficult to list the wide range of applications enjoyed by flexible printed circuits. Flex circuits of various construction methods and materials can be found in automotive products, military and aerospace applications, chip carriers (TAB, wire-bond, and flip chip), medical instrumentation and devices (Ref. 1), disk drives, connectors, detonator cables, mobile phones, computers, ink jet printers, and on the list goes. Many of these uses have unique mechanical, electrical, and/or environmental requirements that must be met for the end product to function properly in long term use.

Service conditions can range from one-time use (missile, detonator cables, torpedoes) to long-term service in temperature extremes from liquid nitrogen to constant high temperature operating conditions.

Some of the emerging flex circuit applications include products for high density display/detector array interconnect, opto-electronic packaging, radio frequency devices, specialized chip-on-flex packaging, connectors for high speed devices, and circuits requiring impedance control. Flex circuits provide attractive solutions for the packaging requirements of these devices because they can leverage dielectric material properties, circuit density, heat dissipation, and mechanical characteristics to provide a space-efficient packaging outcome.

Enabling Attributes of Flex Circuits

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While many of the benefits of flex circuits are obvious to those involved in the industry, they may not be to those unfamiliar with flex circuits and their varied applications. Being thin, lightweight, and flexible, these circuits facilitate miniaturization, making them frequent candidates for mobile and/or handheld devices. Some applications require that the circuit be bent or shaped to accomplish the assembly of the device, and are held rigid after the assembly is completed (flex-to-install applications). Others require the circuit to move associated with the use of the product (semi-dynamic or dynamic flex).

Flex circuits also benefit packaging through the many termination/interconnect methods for which they can be used. These interconnect methods can be separated into methods used to connect the flex to higher level packaging, and to integrated circuits/components.

Surface mount and through hole methods (Ref. 2) can be used for mounting passive components or connectors to the flex circuit. Often the flex circuit requires a tin/lead, gold or nickel/gold, or organic solderability preservative (OSP) to facilitate these attach methods. Pressure contacts and crimp connections can also be used for connectors, and often require a nickel/gold finish at the contact locations.

Z-axis conductive adhesives are also used to connect flex circuits to other devices (Ref. 3). A gold surface finish is preferred for this method, and the assembler of the circuit must be concerned with the alignment of the leads on the flex circuit to the matching leads on the desired device. Dimensional stability of the flex circuit, cross-sectional profile of the leads, and the parameters of the attach method combine to influence this alignment, and must be well understood.

Other interconnect methods unique to flexible circuits include tape automated bonding and hot-bar soldering (Ref. 4). Both methods generally require that the insulating material be removed from the area of the leads to be bonded (usually by laser ablation or chemical etching) to form cantilever leads or "flying leads" that span a defined opening. Gold or nickel/gold finishing is required for tape automated bonding, but a tin/lead solder finish is sufficient for hot-bar soldering.

Another unique interconnect method makes use of specially plated palladium dendrites formed on the top surface of copper pads (Ref. 5). This connector method creates a low impedance connection that can be disassembled and reassembled for several thousand make-break cycles with no detrimental affect on the mating surfaces.

Flex circuits can also accommodate ball grid array area interconnect to circuit boards using either high temperature or eutectic solder, wire bonding, and flip chip attach processing for high level interconnect requirements.

From a thermal perspective, flex circuits provide attractive solutions for thermal management requirements because they provide a short thermal path through a thin dielectric layer (which can be augmented with thermal vias), provide a wide range of heatsink options, and benefit from a high surface-to-volume ratio.

And from a electrical standpoint, uniform dielectric and conductor thicknesses, favorable dielectric constant, and the ability to produce fine lines and spaces on the relatively smooth surfaces of the base materials all help to benefit impedance control, signal speed, and routability.

High Density Display/Detector Interconnect

High-density display and detector technology effectively uses flex circuits to interface between the glass-based displays or detectors and the rest of the electronics in the system. In this application, the ability of the flex circuit to be formed around a tight radius during installation is a key attribute. Glass panel detectors and displays that use flex circuit interconnect circuits can be found in leading-edge medical X-ray equipment (Ref. 6) and flat panel displays.

The packaging engineer is concerned with achieving a good connection between the flex circuit and the glass panel. One of the critical metrics to achieve good bonding is the lead-to-lead pitch from one side of the flex circuit to the other. If the lead-to-lead pitch does not adequately align with the pitch on the display, either a short between adjacent leads or a poor connection from the flex to the panel can result. This problem becomes more acute as the circuit density on the display (and the attaching flex circuit) increases, and can be aggravated further if the circuit line profile on the flex circuit is rounded rather than flat along the contacting surface. It is not uncommon for the specified pitch variation to be less than 400 ppm over the width of the circuit

Figure 1: Illustration of lead-lead pitch constraint.

If the end-to-end lead pitch changes by this amount over a 50mm long connector edge, the relative position of the flex circuit lead to the glass panel could be off by as much as 20-microns (Figure 1); 10-microns would be the best case misalignment if the flex circuit can be perfectly centered. This may not be enough misalignment to affect the quality of the interface when the circuit density allows 3-mil lines and spaces, but as the required circuit density pushes to 2-mil lines and spaces and below, this type of lead runout will need to be controlled to 200 ppm or less. With proper controls in place during the manufacturing process (Ref. 7), the dimensional stability of the circuit can be controlled to meet these requirements.

A related technology, microdisplays, provides a similar type of application that takes advantage of thin, lightweight flex circuit connectors (Ref. 8).

Chip-on-Flex Packaging

Chip-on-flex packaging provides an attractive alternative to reduce the packaging interconnect required in some applications. The ability for flex circuits to produce relatively small line widths and spaces provides the opportunity to route wires to chips within one plane, and use a second layer for voltage/ground and shielding requirements.

In some cases, particularly when the package requires two chips with interconnecting wiring, the routing options do not allow for the wiring to be bussed to the edges of the package for the benefit of typical electrolytic finish plating processes.

This problem can be addressed by using a semi-additive pattern plating process that can use a thin base copper material as the commoning medium for the plating process, circumventing the need to use real estate for commoning lines.

This method provides a similar benefit for flip chip packaging, which meets similar constraints in packaging design, since the circuits cannot be connected together in the area beneath the chip, and line/channel limitations do not allow routing of the interior lines to the outside perimeter of the package.

Optoelectronic Packaging

Optoelectronic circuit packaging provides significant challenges to any packaging method used. Mechanical requirements provide significant challenges compared to other types of packages to accomplish the critical alignments needed for proper operation. Similarly, proper electrical function requires impedance controlled high-speed lines in the circuit design. Flex circuits are an ideal solution for optoelectronic packaging requirements. A recent article (Ref. 9) identified potential shortfalls of FR-4 laminate for optoelectronic devices, stating "FR-4 may be too lossy, its dielectric constant too high, and propagation delay unacceptable for very high-frequency systems." The article later re-states the need for material properties that are tailored to improve signal speed for optoelectronic devices.

Flex circuit solutions improve on material characteristics compared to FR-4, and are being implemented in optoelectronic packaging today.

Polyimide dielectric materials commonly used for flex circuits in high-end applications have a dielectric constant in the range of 3.2 to 3.6, compared to 4.3 to 5.0 for FR-4, depending on the specific material and test conditions applied.

The ability to create line/space geometries significantly less than 50 microns increases the attractiveness, and the thin and flexible mechanical aspects of the package give the designer needed latitude in the packaging design.

The thin cross section (approximately 115 microns) allows the circuit to be easily attached to a heatsink or stiffener to provide good thermal management capability and rigidity where needed.

The required mechanical and electrical tolerances to support optoelectronic packaging make it one of the most challenging areas supported by flex circuit designs. It is also likely to be one of the large growth areas for high-density flex interconnect.

Controlled Impedance

Controlled impedance lines are specified for packages using high-speed traces. Optoelectronic devices and connectors for supercomputer applications are among many uses that require controlled impedance nets.

There are several different structure variations for transmission lines, however double-sided flex circuits are capable of using only a few. Two of the most common in practice are the surface microstrip and the buried or embedded microstrip. The characteristic impedance for the surface microstrip is given by:

For packaging efficiency, it is desirable to decrease the designed dielectric thickness, however in order to maintain a desired characteristic impedance, the line width, thickness, or dielectric constant would have to decrease to compensate.

Because flex circuits can be produced with narrow and thin trace cross-sections, impedance controlled nets can be tailored to match requirements somewhat more readily than other packaging methods may allow.

Summary

The characteristics of flexible circuits are ideally suited for many applications that require high density interconnect between other components in a system. Their ability to be formed for installation and attachment, high density circuit definition capability, thermal management, wide variety of circuit termination options, and efficient use of materials and space make flex circuits an attractive solution for a wide variety of current and emerging packaging applications.