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 a thin copper foil is 

Retain adhesion while the circuit is flexed Wet and bond well to both the polymer film and the copper foil Have low moisture absorption Have good dielectric properties Have low or no-flow characteristics Be thermally resistant to solder-reflow temperatures 

As with PWBs, flex circuits may be single layer, double sided, or multilayer. 

The reliability of die attach adhesive has been extensively investigated for the past three decades. A complete survey of these studies has been previously published [4]. The following sections underline the critical factors that have an impact on the electrical, mechanical, and thermal properties of the adhesive bonds. For process engineers, the main concern is the evaluation of die attach integrity in relation with the assembly technologies and after a series of accelerated ageing tests. 

In all packages, the heat generated at the junction primarily flows through the silicon die, the solder or adhesive bond, the nickel plating when existing, and the base substrate. In the 40-lead P-DIP, thermal dissipation occurs in any direction through the moulded epoxy in contact with the chip. The experimental data 

The electrical conductivity of gold- and silver-filled adhesives is addressed in Section 6.3.2 and the qualification standards in Section 6.4.2.9. Although this latter directive limits the thermal ageing conditions to 150°C, different studies have been published on the behaviour of organic adhesives at higher temperatures [133,134]. The volume resistivity of commercial adhesives is of the order of 

For solvated polyimide adhesives the lap shear strength test is not appropriate because the large overlap bond area prevents the full release of the solvent [61]. The authors underline that the die shear strength data are also highly scattered and 

The effect of porosity on thermal resistance is addressed in Section 6.6.4.1 but other factors influence the value of jc. The results of tests conducted by bonding Motorola 2N-3055 power devices to TO-3 packages by means of commercial silver-filled epoxies show that jc varies from 0.99 to 4.90°C [133]. The 

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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. 

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). 

The final piece in the MOSFET electronic load puzzle is a method of adjusting the current limit. A simplistic yet flexible circuit to accomplish this is shown in Fig. 5.14. 

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. 

We will continue platform extensions, taking Nafion fluoropolymer membranes from chloralkali cells and catalysts to advanced fuel cells, and Kapton polyimide film from flat electronic computer circuits to complex flexible circuits in cell... 

The plastic film with interconnection layers, denoted (2) in Fig. 16.1, can be made by a process similar to that used to manufacture flexible circuit boards. First, plastic films coated with copper foil are processed by a numerically controlled (NC) drilling machine to make via holes. Plating is then used to make interconnections between top and bottom sides though via holes. Finally, the copper layers are patterned by conventional photolithography and etching. Gold plating is occasionally employed to improve electronic interconnections. 

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]. 

Relatively little attention has been paid to current-density nonuniformity caused by uneven pattern density. This effect has been discussed by Romankiw and coworkers [36-38]. A theoretical investigation of the active-area-density effect was presented by Mehdizadeh et al. [23]. This was followed by an experimental investigation [39, 40]. Some highlights from the above mentioned publications by Mehdizadeh et al. are presented in the next section. Further discussion of the active-area-density effect appears in a recent publication on electroplated thin-film wiring for high-performance packaging [41] and in a paper on the fabrication of flexible circuits [14]. 

The MOSFET is an extremely flexible circuit element because there can be n- and p-channel devices with current that increases or decreases with gate voltage. The capacitance of MOSFET structures can also be used for charge storage. The stored charge can be shifted from one capacitor to an adjacent element by control of the gate voltages. 

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. 

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-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 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). 

T. E. Dueber, M. W. West, B. C. Auman, and R. V. Kasowski. Polyimide based adhesive compositions useful in flexible circuit applications, and compositions and methods relating thereto. US Patent 7 220490, assigned to E. I. du Pont de Nemours and Company (Wilmington, DE), May 22, 2007. 

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. 

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. 

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... 

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. 

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