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SPECIAL CONSTRUCTIONS OF FLEXIBLE CIRCUITS
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FIGURE 66.13
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Membrane switch made by thick film process.
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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.
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Manufacturing Process It is a very simple process to build a thick-film flexible circuit. There are two steps: 1. Screen printing on a flexible film followed by baking. 2. Blanking. This still requires that the process conditions be controlled carefully.The basic properties of these circuits change significantly depending on these conditions. A pre-baking step is recommended for polyester film substrate to eliminate thermal shrinkage during baking for printed conductor paste. It is not difficult to apply the roll-to-roll manufacturing system for large volume because of the simplicity of the processes. The latest technologies of the printers and screens are able to produce 10 to 15 micron line space with low cost. The electrical properties and trace densities are very depend on the conductor paste. A combination with screen printable nano conductive paste will provide fine conductive traces by a simple manufacturing process.
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SHIELDING OF THE FLEXIBLE CABLES
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When the flexible circuits are used as the cables for the high-speed telecommunications, appropriate shielding systems are required. Generally, the cable will be bent frequently, therefore the shielding layers should also have high flexibilities.
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(c) FIGURE 66.14 Shielding of the flexible cables (a): Screen-printing of silver paste (b): Screen-printing of carbon paste (c): Lamination of thin aluminum foil.
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Several materials and constructions are available for the shielding of the flexible cables as shown in Fig. 66.14. Screen-printing of silver paste provides both high shielding effect and high flexibility. Screen-printing of carbon paste provides low cost solution. But its shielding effects are limited. Thermal lamination of a thin aluminum foil with thin flexible adhesive provides a reliable solution for both shielding efficiencies and flexibilities.
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FUNCTIONAL FLEXIBLE CIRCUITS
Various new processes have been developed to build functional elements and devices on the flexible substrates. They have been generating more functions than wiring or assembling board of the discrete components. They are the similar ideas as embedded passive technologies of multilayer boards, but they generate more values with flexible substrates. Table 66.2 shows several material examples and applications.
SPECIAL CONSTRUCTIONS OF FLEXIBLE CIRCUITS
TABLE 66.2 Functional materials for flexible circuits Materials NiCr, W Functions Embedded resistors Heater Mechanical sensors Embedded capacitance Transparent electrode Embedded active devices Transistors and diodes Sensor devices Flexible displays
BaTiO Tin oxide Poly silicone
Organic EL
Figure 66.15 shows examples of sputtered nickel/chromium alloy. The first one is embedded resistors and the second one is a small strain gage built on the flexible substates.
FIGURE 66.15 Examples of functional flexible circuits (a): Embedded resistance (b): Small strain gage built on flexible substrate. (Source: MicroConnex).
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QUALITY ASSURANCE OF FLEXIBLE CIRCUITS
Dominique K. Numakura
DKN Research, Haverhill, Massachusetts
67.1 INTRODUCTION
Because of the different design concepts and materials, there are several significant differences in quality assurance for flex circuits vs. that for typical rigid circuit boards. In addition, special test methods are required to guarantee the finished quality of high-density flex circuits, and new inspection technologies have been developed for this purpose.
67.2 BASIC CONCEPTS IN FLEXIBLE CIRCUIT QUALITY ASSURANCE
The basic concept of quality assurance for flexible circuits is the same as that for rigid boards, in the sense that any critical defect must be eliminated before shipping.This includes failures due to opens and shorts; dimensional degradation of pad arrays, serious defects on conductors, substrates, coverlay, and so on. These defects need to be inspected for each circuit, whether it is flexible or rigid. Technically, however, there are several differences in quality assurance for flexible circuits because of their additional functions, such as dynamic flexing capability. A common issue is damage incurred during the manufacturing process because of the fragile materials. Even though the fine traces are inspected by a high-resolution automatic optical inspection (AOI) system during processing, it is possible that subsequent processes may cause new damage to the inspected circuits. This is a special issue for flying leads, which have no mechanical support under the traces. Therefore, a final inspection is necessary at the end of the manufacturing process. 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 structures 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-mm accuracy is required.
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