Rigid/flexible circuits

Multilayer rigid/flexible circuits with through-holes... 

There are many different concepts in the construction design of multilayer rigid/flexible circuits. 

FIGURE 653 Folding type rigid/flexible circuit. 

There are many varieties of manufacturing processes for multilayer rigid/flexible circuits because of their complicated structures. Figures 65.5 shows a typical layer construction using... 

FIGURE 65.7 Example of typical multilayer rigid/flexible circuit with via holes. (Source Toshiba.)... 

When the dummy boards are removed after routing, the circuit becomes a rigid/flexible circuit. 

FIGURE Cross section of a through-hole in a multilayer rigid/flexible circuit. 

The use of supplemental plasma etching, called the Etch Back Process, is recommended to achieve high reliability of through-hole multilayer rigid/flexible circuits. The plasma gas provides further etching on both of polyimide films and adhesive layer in the drilled holes. The plasma gas does not etch metal materials and it clears the surface of the copper foils for reliable copper plating as shown in Fig. 65.9. 

FIGURE 65.11 Micro blind via hole on a multilayer rigid/flexible circuit made by a build-up process using resin-coated foil. The hole diameter is 150 micrometer. (Source Photo Machining.)... 

In 1990 the majority of U.S. PCB production resulted from subtractive or print-and-etch processing additive processes were less than 6% of the total multilayer boards accounted for 55.8%. The ratio of rigid to flexible surface areas plated is about 15 1. High performance plastics including polyimide. Teflon, and modified epoxy comprised 6% of the market ( 324 million) flexible circuits were 6.6% ( 360 million) (42). 

Flexible circuits (flex circuits) are analogous to rigid printed-circuit boards except they are fabricated from a thin flexible dielectric film to which a thin copper foil is... 

Chip-on-flex is similar to chip-on-board except that the die, chips, or CSPs are wire bonded, flip chip attached, or epoxy connected to a flexible circuit instead of to a rigid interconnect substrate such as a PWB. 

Data describing the reliability of joints assembled with anisotropically conductive adhesives are incomplete. Several papers have been published, but usually the sample size investigated is small, the accelerated stress tests are not standardized, and the results are highly dependent on device type (e.g., flexible circuit to rigid PWB, surface-mounted components, and flip-chip assembles). Further work is required in this area. 

Furthermore, the overall properties of the base materials for circuit boards are determined essentially by the joint between the copper foil and the laminate. This joint can be realized by both an added adhesive and by the laminating resin itself, which additionally, acts as adhesive. An additional adhesive is needed in the manufacture of flexible circuit boards (e.g., polyimide/copper) or of paper-based rigid circuit boards. During this process the adhesive is deposited on the bottom side of the copper foil after... 

Flexible circuits. Flexible circuits (flex circuits) are analogous to rigid printed-circuit boards except they are fabricated from a thin flexible dielectric film to which is adhesively bonded a thin copper foil. The copper is then photoetched to form a circuit pattern using normal photolithography processes. A plastic film (coverlay) is then adhesive bonded to the etched... 

Although some substrates may be molded plastic (3-D or molded circuits) or flexible film plastic materials (flexible circuits), the largest group of substrates is of rigid laminate construction. The laminate construction consists of many layers of resin-treated fibers, fabrics, or papers, which are called reinforcements. 

That is, the cost of using flexible circuits must be compared to the total cost of rigid circuit boards in terms of the following ... 

The basic difference between flexible circuits and rigid circuit boards is the thin and flexible materials used in the substrates of flexible circuits. Furthermore, because the flexible circuits have complicated constructions, supplemental materials other than copper-clad materials have been required to build a whole circuit. 

As flexible circuits have various mechanical stresses, their conductors also have requirements for more flexibility and toughness than traditional ED copper foils of standard rigid circuit boards. An RA copper foil is the solution for high flexing endurance. It is necessary for dynamic flexing use. RA copper foils are more expensive than ED copper, especially at thicknesses of less than 18 micron. Therefore, high-ductility electrodeposited (HD-ED) copper foils have been developed that have greater flexibility than ED copper foils and a lower cost than RA copper foils. The differences between the copper foils are shown in Table 61.11. 

One of the major differences between flexible circuits and rigid circuit boards is coverlay. In addition to the solder mask used for assembly, coverlay is the mechanical protector for the fragile conductors on flexible circuits. Film-based coverlay and flexible solder mask have been standard materials for traditional flexible circuits. Several types of photoimageable coverlay materials have been developed to satisfy the fine-resolution requirements of FCDI flexible circuits ... 

A screen-printing process similar to that used for rigid circuit boards is available for flexible circuits as the low-cost coverlay process. However, specially conditioned liquid solder mask materials are required to produce appropriate flexibility for the circuits. Standard solder mask materials designed for the rigid boards have small cracks and de-laminations when bending. 

Acrylic or Epoxy/Dry-Film Type. Acrylic or epoxy/dry-fibn types were the first materials developed to act as the flexible photoimageable coverlay for HDI flexible circuits. They have the same product concept as the photoimageable solder mask of rigid circuit boards. The same vacuum laminators and imaging equipment are available for these materials. 

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