Flexible circuits Types

Polyimides for microelectronics use are of two basic types. The most commonly used commercial materials (for example, from Dupont and Hitachi) are condensation polyimides, formed from imidization of a spin-cast film of soluble polyamic acid precursor to create an intractable solid film. Fully imidized thermoplastic polyimides are also available for use as adhesives (for example, the LARC-TPI material), and when thermally or photo-crosslink able, also as passivants and interlevel insulators, and as matrix resins for fiber-reinforced-composites, such as in circuit boards. Flexible circuits are made from Kapton polyimide film laminated with copper. The diversity of materials is very large readers seeking additional information are referred to the cited review articles [1-3,6] and to the proceedings of the two International Conferences on Polyimides [4,5]. 

In addition to planar substrates, many new circuit carrier materials are being investigated. Besides MID (see Section 4), flexible circuit technology has proved to be a market driver in the field of PCB production. Today, flexible circuits can be found in nearly every type of electronic product, from simple entertainment electronics right up to the highly sophisticated electronic equipment found in space hardware. With growth expected to continue at 10-15% per year, this is one of the fastest-growing interconnection sectors and is now at close to 2 billion in sales worldwide. 

This CSP type uses a flexible circuit having solder balls or metal bumps as an interconnect interposer between the chip and the next circuit board level. The bare... 

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. 

Flex interposer-based CSPs. This CSP type uses a flexible circuit having solder balls or metal bumps as an interconnect interposer between the chip and the next circuit board level. The bare chip is attached facedown and wire-bonded to the interposer. A thin elastomer, sandwiched between the chip and interposer, cushions the chip and the solder-ball interconnects, relieving stresses (see Fig. 1.13). The interposer generally consists of a metallized, flexible polyimide tape on which are formed electrical connections by photolithographic processes. As a final step, the exposed wire bonds and edges of the chip are molded with epoxy. 

Laser routing can be an effective alternative to mechanical routing especially for flexible circuit board materials. Laser cutting is very accurate, allowing for the profihng of very small parts. There are several types of lasers in nse, and some eqnipment nses more than one type in a single piece of eqnipment. 

Tape-automated bonding (TAB) has not been considered as a type of flexible circuit because of the slightly different manufacturing processes. However, the basic construction and materials of the final products are the same therefore, TAB is categorized as a type of flexible circuit in this book (see Fig. 61.2). 

Coverlays and stiffeners are typical examples of the special components needed to build flexible circuits. Many types of materials have been introduced to accommodate the designs of the end applications. 

Several laminates without adhesive layers have been developed as the advanced materials for the next generation of flexible circuits. Lamination technology using epoxy resin or acrylic resin has been almost eliminated from HDI flexible circuits even though it uses new high-performance polyimide films as the substrates. Three types of adhesive less copper-clad laminates have been developed (see Fig. 61.5) ... 

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 ... 

Dry-film-type and liqnid ink-type photoimageable coverlay materials have been developed with different resin matrixes to satisfy different technical requirements of HDI flexible circuits. A comparison of the materials is given in Table 61.16. Properties of the materials are shown in Tables 61.17 and 61.18. 

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. 

Managing the conditioning of the process is not difficult. Acryhc or epoxy/dry-film types are good for smaU-volume production because of the flexible manufacturing process. They may need some more improvements in electrical performance and chemical resistance if they are to be used in the general application of high-density flexible circuits. The present version... 

Several different types of adhesive materials can be used in flexible circuits depending on the application, except for the adhesive layer of copper laminates and fihn coverlays. The major applications are the bonding of stiffeners and multilayer constructions. 

Electro plating is capable to provide a reliable metallic surface treatment controlling the thickness and the surface conditions. It is not difficult to manage the plated metal surface shiny or non-shiny. An issue is the all of the metal pad must be connected to the electrodes. It is not a big issue for the simple cable type flexible circuits, which have mostly parallel lines. But it is an serious issue SMT type circuits what have electrically isolated termination pads. They need supplemental lines for only electro plating, and they should be cut after the process. 

Several types of electronic components can be attached to flexible circuits. Table 64.3 categorizes these from the termination standpoint, assuming the use of high-density flexible circuits. 

Newly developed anisotropic conductive resins have generated several new termination technologies for high-density flexible circuits. Film-type and paste-type anisotropic materials are developed with suitable applicators. The film-type material, called anisotropic conductive film (ACF), has been widely used for the mounting of driver IC chips on the flexible substrate and connections between the flexible substrates and the glass substrates of the LCD devices. The latest material has 30 m pitch connections in one direction for the IC chips on the gold-plated pads of the flexible circuits. Examples of the basic properties are provided in Table 64.5... 

FIGURE 64.16 Examples of soldering lead-type connection in flexible circuits. 

FIGURE 64.18 Clamp type connector for flexible circuits. 

FIGURE 653 Folding type rigid/flexible circuit. 

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 thick-film circuit is a type of additive manufacturing process. If an organic resin matrix is used, the conductor can be flexible, and this process has been applied in flexible circuits. 

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 structiues 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-ftm accuracy is required. 

This type of coil was prepared from copper cladded printed circuit board material by applying photolithographic techniques. The p.c. board material is available with difierent copper thicknesses and with either a stiff or a flexible carrier. The flexible material offers the opportunity to adapt the planar coil to a curved three dimensional test object. In our turbine blade application this is a major advantage. The thickness of the copper layer was chosen to be 17 pm The period of the coil was 100 pm The coils were patterned by wet etching, A major advantage of this approach is the parallel processing with narrow tolerances, resulting in many identical Eddy current probes. An example of such a probe is shown in fig. 10. 

When there are more than one sources of supply, it is recommended to distribute the loads also in as many sections as the incomers, and provide a tie-circuit between every two sections, to obtain more flexibility. Now fault on one section or source of supply will not result in the loss of power to the entire system. Figures 13.16 and 13.17 illustrate this type of distribution. 

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