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FIGURE 62.18 Design of dynamic flexing part of flexible circuits (a) Unacceptable technique, (b) guide board, (c) loose guide, (d) silicone rubber.
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FIGURE 62.19 Circuits configuration of double-sided flexible circhits at flexing parts, (a) Preferred, (b) Acceptable, (c) Unacceptable, (d) Meshed shield layer, (e) Crossing traces, (f) Separated shield layer for dynamic flexing, (g) Fan folded. (h) Coiled.
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DESIGN OF FLEXIBLE CIRCUITS
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FIGURE 62.19 (Continued)
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There is no significant difference between the electrical performance of flexible circuits and that of rigid circuit boards. But there are small differences in conductivity and insulation resistance depending on the materials. Also, there are small differences in dielectric constant and tangent delta. Therefore, similar calculations can be applied to determine electrical performances for a flexible circuit. However, flexible circuits have a relatively long parallel circuit for cabling capability, and impedance should be managed carefully, especially for high-speed circuits with high frequencies.
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CIRCUIT DESIGNS FOR HIGHER RELIABILITY
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Because of the thin and fragile materials used, flexible circuits have lower mechanical reliability than rigid circuit boards. They have a low conductor bond strength and low base substrate tear strength. Nevertheless, most of the flexible circuits are subjected to more mechanical stresses due to movement. This means that special care is required in the circuit design to gain higher circuit reliability. There are several ways to make the reliability higher. Figures 62.20 and 62.21 show several common ideas for increasing the reliability of flexible circuits by modifying conductor patterns. The trace patterns should be smooth slopes between different widths. The pad size should be as large as possible. Figure 62.22 shows an example applied for automobile instrument panels. The conductors are designed to be as wide as possible, and the spaces between conductors are
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FIGURE 62.20 Reliability design for flexible circuits (a) Unacceptable, (b) Preferred.
FIGURE 62.21 Reliable pattern design for flexible circuits, (a) Unacceptable, (b) Acceptable, (c) Preferred.
FIGURE 62.22 Conductor pattern designed for the instrument panel of an automobile.
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FIGURE 62.23 Coverlay opening for reliable solder pads. (a) Unacceptable, (b) Acceptable, (c) Preferred.
FIGURE 62.24 Circuit design at stiffener edge, (a) Unacceptable, (b) Preferred.
kept in the safety range. A suitable coverlay opening could increase the reliability, as shown in Fig. 62.23. The amount of squeeze-out of coverlay adhesives should be minimized, however. The edge areas of stiffener boards are dangerous places in which mechanical stresses are concentrated. Figure 62.24 shows ways of reducing these risks.
62.7 CIRCUIT DESIGNS FOR ROHS COMPLIANCE
As it was described in Chap. 61.11, there are two issues involved in satisfying RoHS compliance and Halogen-free requirements. They are the eliminations of bromine molecules and lead components. A suitable material combination should be selected to survive during the high temperature process of lead-free soldering.The adhesiveless copper laminates could be the suitable materials for this purpose. OSP (Organic Surface Protection) treatment or Tin-Copper plating could be the alternative solution for the surface finishing of the bare conductors instead of the eutectic solder and hot air leveling.
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MANUFACTURING OF FLEXIBLE CIRCUITS
Dominique K. Numakura
DKN Research, Haverhill, Massachusetts
63.1 INTRODUCTION
Basically, it is possible to produce the standard constructions of single- and double-sided flexible circuits and multilayer rigid/flex for small volume if there is a set of manufacturing facilities for multilayer rigid circuit boards. However, it is not easy to achieve high productivity, with high process yield, for volume production of flexible circuits using these facilities, as they are not designed to deal with the unique issues of flexible circuits, such as their thin, flexible material. Therefore, manufacturing facilities for high volume flexible circuits must be designed to take advantage of these differences, including specific equipment and process conditions, as discussed in this chapter. Table 63.1 shows the specialties for processing the flexible circuit materials. Most of the issues are caused by their complicated structures and fragile materials. The coverlay structures and the stiffener constructions require supplemental manufacturing processes, making process yields lower. The varieties of outer shapes make material yields and productivity lower. Thin, fragile materials can easily incur serious mechanical damage due to rough handling.Thin materials can also undergo significant dimensional changes in the manufacturing processes. They cause a pattern shift between the processes and make the process yields lower. High-level automation has basic barriers, discussed later, when applied to the manufacturing process for flexible circuits, because of its unstable materials. Many processes must be conducted by manual methods, which are, of course, labor intensive. Process elements particular to flexible circuits are required to achieve a high manufacturing yield. Also, an appropriate total process design is required to achieve high productivity with high yield. A roll-to-roll (RTR) manufacturing system is a high-productivity solution for volume production of flexible circuits. However, the RTR lines are not flexible for a non-standard construction, and they are available for only the early steps of the long manufacturing processes. Also, RTR processes have many limitations to apply.
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