Flexible circuit applications High density

Metallized PI films are used in electronic applications, e.g. for flexible printed circuit boards. Conventional techniques for the fabrication use adhesive bonding to a copper foil. However, the demand for high density packaging of electronic apparatuses requires a further reduction in the thickness of these substrates. It is possible to sputter metal particles into the surface of the PI film at a thickness of 20 nm, which forms the intermediate layer for subsequent formation of a conductive layer of copper or a copper alloy. 

TABLE 61.4 Major Applications of High-Density Flexible Circuits... 

Application areas New materials for high-density flexible circuits... 

Managing the conditioning of the process is not difficult. Acryhc or epoxy/dry-film types are good for smaU-volume production because of the flexible manufacturing process. They may need some more improvements in electrical performance and chemical resistance if they are to be used in the general application of high-density flexible circuits. The present version... 

It is common for a flexible circuit to be coimected to other circuit boards. However, more traces and a higher density of the connections are then required. The basic design of the end products automatically determines the placement of specialty components such as keyboards, switches, antennas, actuators, printer heads, sensors, microphones, speakers, and buzzers in small electronic applications. Although these components do not require numerous wires, they do need high-density connections due to the extreme miniaturization of portable electronic products. 

Newly developed anisotropic conductive resins have generated several new termination technologies for high-density flexible circuits. Film-type and paste-type anisotropic materials are developed with suitable applicators. The film-type material, called anisotropic conductive film (ACF), has been widely used for the mounting of driver IC chips on the flexible substrate and connections between the flexible substrates and the glass substrates of the LCD devices. The latest material has 30 m pitch connections in one direction for the IC chips on the gold-plated pads of the flexible circuits. Examples of the basic properties are provided in Table 64.5... 

The basic idea of pressure contact termination technology is very simple and is not new. Pad patterns of the same pitch plated with gold are generated on a flexible circuit and opposite circuits, and they are attached to each other with uniform pressure using a suitable rubber strip (see Fig. 64.14). There is no high-temperature process to complete the coimectiou polyester base materials are available. The maximum conuection number and density depend on the capability of dimension control and uniformity of the circuits. It is possible to make 200 connections with 150 /tm pitch because of the processes all take place at room temperature. Neither special application equipment nor special constructions on flexible circuits are... 

Small bump arrays are valuable for the nonpermanent connections of high-density flexible circuits. Various bump constructions on flexible circuits have been developed through different manufacturing processes that are driven by the needs of specific applications. 

Polyimide films are used in a variety of interconnect and packaging applications including passivation layers and stress buffers on integrated circuits and interlayer dielectrics in high density thin film interconnects on multi-chip modules and in flexible printed circuit boards. Performance differences between poly-imides are often discussed solely in terms of differences in chemistry, wiAout reference to the anisotropic nature of these films. Many of the polyimide properties important to the microelectronics industry are influenced not only by the polymer chemistry but also by the orientation and structure. Properties such as the linear coefficient of thermal expansion (CTE), dielectric constant, modulus, strength, elongation, stress and thermal conductivity are affected by molecular orientation. To a lesser extent, these properties as well as properties such as density and volumetric CTE are also influenced by crystdlinity (molecular ordering). 

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