barcode reading using c#.net SPECIAL CONSTRUCTIONS OF FLEXIBLE CIRCUITS in Software

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SPECIAL CONSTRUCTIONS OF FLEXIBLE CIRCUITS
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66.1 INTRODUCTION
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Because of thin base layers, the flexible circuits can have many special constructions that are impossible to build by traditional manufacturing processes of the rigid circuit boards. These special constructions generate further functional values for the flexible circuits. This chapter introduces the major constructions in use, along with their manufacturing processes.
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66.2 FLYING-LEAD CONSTRUCTION
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Flying-lead construction is a special case of the double access flexible circuit. The base layer under the traces are removed and the conductors are completely free from the organic materials. New HDI flexible circuit applications have arisen involving flying-lead constructions. Applications such as wireless suspension of hard disk drives, interposers for chip-scale packaging, and ultrasound probes depend on the reliability of this high-density interconnect. In competitive markets such as these, it is imperative that flying-lead applications offer low cost and high volume. The TAB (Tape Automated Bonding) is one of the major application of the flying lead costructions. Traditional manufacturing processes used to create flying leads, such as pre-punching or pre-drilling, are not conducive to the quality levels and manufacturing yields required to achieve low cost, and alternative processes are needed, as described later.
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66.2.1 Basic Design A flying-lead structure is composed of a single layer of high-density-pitch copper conductors accessed from both sides of the substrate. Generally, the substrate side of a conductor on a circuit board is not electrically accessible because the conductive foil is typically laminated to the substrate board in sheet form. In the case of a flex circuit, however, the substrate can be
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FIGURE 66.1 Basic constructions for Flying-leads for flexible circuits.
removed by various methods., Dual access is created by removing the coverlayer in the same area on both sides. Figure 66.1 indicates two basic structures of dual-access conductors, in both the single sided and double-sided types of flexible circuits. Figure 66.2 shows actual examples of the fine flying leads built on the single side circuits and double side circuits. Basically, a coverlayer is applied to the side of the conductive layer opposite the substrate.The coverlayer can be processed easily to create the top-side exposure. The flying lead is created when the base substrate is also removed in the same area.
(a) FIGURE 66.2
Typical fine flying leads construction (a) Single side, (b) Double side.
SPECIAL CONSTRUCTIONS OF FLEXIBLE CIRCUITS
FIGURE 66.3 Manufactuirng process for flying-lead flexiblse circuit construction using prepunching method.
66.2.2 Manufacturing Processes for Flying Leads The fragility of flying leads, caused by the lack of dielectric support, is probably the largest disadvantage in the manufacturing process. Proper circuit design and layout is the key to the manufacturing process yield and subsequent cost and physical performance of the circuit. The most conventional manufacturing process for generating a flying-lead construction involves a pre-punching process similar to the device hole process of tape automated bonding (TAB). Figure 66.3 shows this process schematically. There is no proprietary technology or special tooling required for manufacturing the pre-punched window in the base substrate. This process, however, offers the manufacturer very little flexibility and is fairly labor intensive, which makes it difficult to maintain high productivity rates. The handling of unstable punched films, along with thin, fragile copper foils, makes it difficult to achieve the high manufacturing yields required to support the cost model associated with high-density flexible circuits. These unstable film substrates also make it difficult to maintain dimensional accuracy between the fingers and the window, a key element for HDI flexible circuits. Adhesive flow or squeeze-out from the edge of the punched film along the conductor can also be an issue when dealing with small window exposures. Openings of less than 1.0 mm are difficult to create with high dimensional accuracy and yield. Alternative technologies have been developed to generate high-density flying-lead designs with higher accuracy. Laser ablation, plasma etching, and chemical etching are just a few of the possibilities and are illustrated schematically in Fig. 66.4. A comparison of small opening capabilities of each of the processes is illustrated in Fig. 66.5, while a comparison of the technical capabilities of the processes is shown in Table 66.1. With these manufacturing technologies, many kinds of flying-lead constructions became possible.
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