Flexible circuit materials

Two issues have arisen regarding RoHS requirements for the flexible circuit materials flame-retardant molecules with bromine in adhesive resins, and heat resistance for high-temperature processing with lead-free soldering. Although the issue of bromine is not actually a part of the RoHS requirement, it has been linked to the general environmental issues of printed circuit materials and processes. 

Substrate material selection may be the most important item to consider for dimensional control of high-density flex circuits. (For a more detailed discussion of flexible circuit materials, see Chap. 61.)... 

Properties desired in cable insulation and flexible circuit substrate materials include mechanical flexibiUty, fatigue endurance, and resistance to chemicals, water absorption, and abrasion. Both thermoplasts and thermosets are used as cable-insulating materials. Thermoplastic materials possess excellent electrical characteristics and are available at relatively low cost. 

Seyam A.M., Formation of textiles structures for giant-area applications in Shur M., Wilson R, Urban D. (eds) Electronics on Unconventional Substrates -Electrotextiles and Giant Area Flexible Circuits 736, Materials Research Society, Warrendale, 2003,25-36. 

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. 

However, up to now, most flexible circuit boards have been based on either polyester or polyimide. While polyester (PET) is cheaper and offers lower thermal resistance (in most cases reflow soldering with standard alloys is not possible), polyimide (PI) is favored where assemblies have to be wave or reflow soldered (with standard alloys). On the other side, the relative costs for polyimide are 10 times higher than for polyester. Therefore, a wide gap between these two dominant materials has existed for a long time, prohibiting broad use of flexible circuits for extremely cost-sensitive, high-reliability applications like automotive electronics. Current developments in the field of flexible-base materials as well as the development of alternative solder alloys seem to offer a potential solution for this dilemma. 

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 circuit boards consist primarily of polyimide-based carriers. The problem of bonding the copper foil on the polyimide carrier has not yet been solved satisfactorily. Due especially to their low bonding strength at elevated temperatures, the production of such materials is very limited. Nevertheless, adhesives for copper-polyimide systems were developed, where one-component epoxy resins (e.g., epoxy-polyester mixtures) and reactive hot melts (e.g., phenolic resin-nitrile rubbers) reached importance. 

K. Desai and C. Sung. Electrospinning nanofibers of PANl/PMMA blends. Materials Research Society Symposium Proceedings, 736 Electronics on Unconventional Substrates— Electrotextiles and Giant-Area Flexible Circuits), Boston, 121 126 (2002). 

The use of high temperature thennoplastics for electronic applications is of considerable and growing interest because of the enhanced thermal and electrical properties of these materials. One such material is GE s Ultem polyetherimide which can be injection molded as well as extruded. This latter property is important for utilizations such as flexible circuits where a pliable film is required. The inherent physical properties of the polymer can be enhanced through the addition of fillers. A potential disadvantage, however, is that the nascent translucence of the polyetherimide is eliminated. Visual clarity may be desirable in certain applications such as automatic climate control systems wherein an LED must be read through a patterned circuit board. In addition, neat polymer material may be desirable because of its improved flow properties relative to filled polyetherimide. 

Several other BTBT and DNTT derivatives were prepared in the year 2014 (14ACR1493). Representative samples are given below. Several of these materials have been used in all-printed transistor arrays, flexible circuits, and in medical applications underscoring their promise as practical semiconductors for electronic device applications. 

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. 

Kevlar, Kapton,Teflon, and RO 2800 are materials developed for specialty applications. The first two, in the form of thin films, are commonly used as substrates for flexible circuits. The latter two... 

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). 

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. 

Table 61.5 shows traditional major materials and typical examples of flexible circuits. A broad variety of the materials are employed to build traditional flexible circuits. Film materials, such as polyimide (PI) films and polyester (PET) films are the specially adapted for use in flexible circuits. For greater flexibility, rolled aimealed (RA) copper foil is used as the major conductor material. 

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. 

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