Termination of Flexible Circuits

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. 

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. 

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

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... 

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. 

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. 

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 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... 

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. 

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. 

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. 

The single-storage heater is heated by an electric element embedded in bricks and rated between 6 and 15kW depending upon its thermal capacity. A radiator of this capacity must be supplied on its own circuit, in cable capable of carrying the maximum current demand and protected by a fuse or miniature circuit-breaker (MCB) of 30, 45 or 60 A as appropriate. At the heater position, a DP switch must be installed to terminate the fixed heater wiring. The flexible cables used for the final connection to the heaters must be of the heat-resistant type. 

When a patch cord is inserted in either of the two normaled jacks, the circuit path is broken and the signal is routed through the cord. A more flexible approach, made possible by newer source and destination equipment (which does not require termination, typically 600 fl), is the half-normaled arrangement shown in Fig. 16.39. The source jack is wired to monitor the source without breaking its path to the destination jack s normahng contacts. 

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