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. 

Dimple structure of flexible circuits is a low cost solution for the non-permanent termination. Multiple dimple arrays can be formed on the polyimide base or PET base flexible circuits by simple pressing processes. (Details are shown in Fig. 62.13) A typical example of the dimple termination is the connection of the disposable cartridges of ink jet printers. Over 60 dimples with hard gold plating are formed at the end of flexible circuit for the connection with the ink cartridge. The dimple arrays can make reliable connections over one thousand times. 

Electro plating is capable to provide a reliable metallic surface treatment controlling the thickness and the surface conditions. It is not difficult to manage the plated metal surface shiny or non-shiny. An issue is the all of the metal pad must be connected to the electrodes. It is not a big issue for the simple cable type flexible circuits, which have mostly parallel lines. But it is an serious issue SMT type circuits what have electrically isolated termination pads. They need supplemental lines for only electro plating, and they should be cut after the process. 

All of the termination technologies of flexible circuits have both advantages and disadvantages. 

No technology can satisfy aU of the requirements at the same time. In general, flexible circuits have numerous wiring options, and therefore a thorough review of the technologies and the wiring requirements is required to design the appropriate termination. 

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... 

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... 

PET based thick film flexible circuits have been consumed for the touch panel switches of the electronic terminals such as PDA because of their transparency. 

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. 

Usually, there are several critical points that require exact dimensional accnracy in a flexible circuit. They are guide marks and termination areas, and customers ask exact tolerance for each dimension. The stable dimensions, snch as punched perimeters processed by blanking dies, can be represented with the first article. However, unstable dimensions snch as line width and pad pitch should be checked for each circuit. A statistical method is available. The measurement can be conducted by sampling when Cpk is larger than 1.3, but 100 percent measurement should be done when Cpk is smaller than 1.0. See MIL-STD-414 for help in reducing the number of samples. 

A recent publication describes a pragmatic approach for developing a thermosetting anisotropic conductive adhesive film to electrically connect a flexible circuit to ITO terminal pads [121], The basic components are a solid high molecular weight bisphenol-A diglycidyl ether, a liquid bisphenol-F diglycidyl ether, a phenoxy resin M = Tg = 94°C), (2,3-epoxypropyl)... 

---