High-Density Flexible Circuit Termination

New high-density flexible circuits have supplemental structures such as flying leads and micro bump arrays to complete high-density terminations. As volume production is limited, the designers should consider the mamrfacturers process capabilities seriously. 

TABLE 64.1 Termination Technologies for High-Density Flexible Circuits... 

Several types of electronic components can be attached to flexible circuits. Table 64.3 categorizes these from the termination standpoint, assuming the use of high-density flexible circuits. 

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 quality assurance concept for high-density flexible circuits is the same as for other circuit types. However, a difference for high-density flexible circuits is the acceptable defect size, which is one order smaller than that for traditional flexible circuits, necessitating higher inspection capabilities. Furthermore, new high-density flexible circuits contain additional structiues such as flying leads and microbump arrays that require additional inspection capabilities for reliable termination. Exact 3-D accuracy and uniform surface conditions are required. Dimensional allowances are smaller than 0.3 percent, and sometimes 2-ftm accuracy is required. 

Many kinds of equipment have been developed for the high-density permanent terminations of flexible circuits. However, the basic process and equipment look similar. A mounting machine places the devices on the reeled flexible circuit with accurate alignment by CCD cameras. The bonding head adds pressure and heat from the top of the electronic devices. An ultrasonic vibration of the bonding head could help to increase the reliability of the connections. 

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... 

No perfect termination technology can satisfy all requirements, especially for high-density connections with high pin counts. On the other hand, higher performances are required for the new portable electronics to complete reliable terminations in limited spaces. Accordingly, many new termination technologies with flexible circuits have been developed to satisfy many requirements. Nevertheless, it is still difficult to use one termination technology to satisfy... 

Most of the micro bump array constructions of flexible circuits have been developed for high-density termination (e.g., fhp-chip, CSP, test probes, etc.). A suitable design shape and materials should be chosen according to the requirements of the termination. Several examples are shown in Fig. 66.12... 

It seems to be an impossibility to require accurate dimensions on an unstable, thin, flexible material. However, high-density interconnects in particular demand very high accuracy in the termination area of the circuits, even though the dimensional allowance is smaller than the dimensional stability of flexible substrate materials.Therefore, high-accuracy 2-D or 3-D dimension measurement technologies are required. 

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