barcode reading using c#.net SPECIAL ISSUES WITH HDI FLEXIBLE CIRCUITS in Software

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63.2 SPECIAL ISSUES WITH HDI FLEXIBLE CIRCUITS
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High-density flexible circuits, which have been developed since the mid-1990s, have special constructions and therefore require supplemental processes. In addition, high-grade manufacturing technologies are required to generate finer circuits. Sometimes, they are called HDI
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TABLE 63.1 List of Specialties for Processing Flexible Circuits Areas Special constructions Materials Issues for processing flexible materials Coverlay, stiffeners, and so on Complicated shapes Thin, low stiffness Low dimensional stability Fragile for mechanical forces Difficult for automation RTR available only for beginning
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TABLE 63.2 New Manufacturing Technologies for High-Density Flexible Circuits Processes Raw materials New technologies Stable polyimide film ( 20 ppm CTE) Thin copper foil (9, 12 mm) Adhesiveless copper-clad laminates Laser (excimer, UV:YAG, carbon dioxide) Microhole punching High-accuracy x-y table Plasma etching, chemical etching Direct plating Seed layer sputtering New cleaning reagent Thin dry film ( 15 mm) Liquid resist Collimated light source, auto aligner Laser direct imaging High-resolution etchant Laser drilling Photoimageable coverlay (dry film type and liquid type) Electroless plating for Ni, Au, solder High-speed plating Laser/chemical etching Chemical etching Automatic high-accuracy punching machine Universal AOI Noncontact electrical tests Low-tension EPC system Automatic aligner with CCD camera
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Copper plating Surface cleaning Etching resist Exposure Etching Coverlay Surface finishing Flying leads Guide holes Inspection Roll-to-roll system
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flexible circuits because of new termination technologies. Major items are listed in Table 63.2. Details of the processes are described in each section. Furthermore, ultra high-density flexible circuits have been considered recently as the necessary substrate materials for the next generation electronics packaging. Their circuit densities will be under 20 micron pitches with 20 micron diameter via holes. They need completely new manufacturing technologies, such as an additive process instead of traditional subtractive process. Ultra high-density flexible circuits will be considered separately.
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MANUFACTURING OF FLEXIBLE CIRCUITS
BASIC PROCESS ELEMENTS
Figure 63.1 shows the basic idea of the standard manufacturing process flow for double-sided flexible circuits, with both through-holes and stiffener boards. Basically, most of the manufacturing processes for single-sided flexible circuits are included in these processes. The first half of the manufacturing process (through pattern etching) has few choices, and the automation of the processes are relatively easy. However, the second half has many options, depending on the design of the construction. The second half consumes more labors because of the difficulties of the automation, and its manufacturing cost per unit area is much higher than that of the first half. An appropriate consideration of process design should be conducted to increase the productivity and reduce the total cost. Good process design can reduce the total cost significantly, especially for complicated constructions. Typical ideas are introduced in the following sections.
FIGURE 63.1
Standard manufacturing flow for double-sided flexible circuits.
Preparation of Materials Most of the major materials for flexible circuits, such as copper laminates and coverlay films, are supplied in rolled form. The first process of manufacturing is to slit and cut the rolled materials into workable sizes. It is possible to cut with a manual sharing tool; however, an automatic machine is recommended to get an exact size without handling damage.
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Cleaning processes for the copper foil surfaces of single-sided circuits should be conducted prior to cutting if roll-to-roll processing is available. A chemical etching system and soft brushing systems are recommended to use to reduce the scratches on the copper foils and mechanical stress on the thin substrate films. This could reduce handling damage and increase the manufacturing yield significantly. 63.3.2 Direct Cast Processes For special substrate constructions such as ultra thin substrates or ultra-high-density flexible circuits, a direct casting process of polyimide resin has been developed instead of polyimide films.A photosensitive polyimide varnish is sometimes coated on a carrier stainless foil.Then an electroless and an electro copper layer are plated as the thin conductor layer. The casting process with photo-sensitive polyimide varnish is also available as a reliable coverlay of the HDI flexible circuits. The technology of the direct casting is valuable to build multi-layer constructions by semi-additive process. (See Sec. 63.4.2.) A special surface treatment on the polyimide is required for good bond strength. The thin copper layer is then etched to generate fine traces. Finally, the carrier tape is removed for flexibility. Sometimes the carrier tape is partially etched and kept as a stiffener. 63.3.3 Mechanical Generation of Through-Holes There may be several different processes for each via hole construction in the case of flexible circuits. Suitable processes, including material combination, should be designed according to final requirements (for example, construction, circuit density, reliability, manufacturing volume, available equipment, urgency, and so on) Figure 63.2 shows standard manufacturing processes of the through holes for the doublesided flexible circuits. A mechanical numerically controlled (NC) drilling process on double-sided flexible circuits can be done similarly to that used for rigid double-sided printed circuit boards. More laminates can be stacked up in the setup because of the thin materials.
FIGURE 63.2 Standard manufacturing process for microvia holes in double-sided flexible circuits starting with copper clad laminate (CCL). The hole is created by drilling and then copper plated.
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