Flex circuits

A cable or flex circuit is often employed as an interconnection between electronic circuits that are not easily coimected by other means. The interconnection density obtained with a cable or flex circuit is lower than that achieved with a backpanel. 

The worldwide market value for gels and encapsulants, although considerably smaller than even the flex circuit market value, is increasing in importance. As noted earlier (2), encapsulants are expected to grow on a worldwide basis from 412M (1987) to reach 616M in 1990. 

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... 

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

Electrical conductivity in anisotropic adhesives occurs by a mechanism different from that of isotropic adhesives. Although metal fillers are also used, they are used in much lower amounts (0.5-5% by volume) so that the adhesive is essentially an insulator in the x-y directions. On inserting the adhesive between the electrodes (for example, the metal bumps of a flip-chip device with metal pads on a flex circuit) of two parts and applying pressure and heat, the metal particles form a z-direction electrical connection between the electrodes while the surrounding material remains insulating. The... 

In addition, there has been greater use of flexible circuitry and foldable flex circuits requiring a corresponding degree of flexibility in the adhesives used. For these applications, solders or other metallurgical attachment materials have often failed due to cracking, delamination, electrical opens, or changes in electrical parameters. Some... 

Several approaches using electrically conductive adhesives instead of solder have been explored and are proving successful. Anisotropic adhesives (See Chapter 1), for example, have been used to connect flex circuits and cables from the separate driver circuits to the panel, avoiding the use of solder connections. More importantly, integrated-circuit chips can be bonded directly to the ITO conductor traces on the panel, a technology called chip-on-glass (COG). IC chips can be flip-chip bonded, then underfilled with a stress-free underfill adhesive if necessary. For protection, the chips may then be encapsulated with epoxy (glob-topped). 

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... 

Juin 2008 Miasole, Protovoltaic Module Utilizing a Flex Circuit for... 

Woznicki,T., Flex Circuit Manufacturing in a Box Printed Circuit Design and Manufacture , February... 

To achieve the best cost-performance combination for a wiring design with flex circuits, an appropriate design procedure is recommended, as illustrated in Fig. 62.1. The steps in the process are as follows ... 

A flex circuit or a combination with a flex circuit is considered to be the solution for a small wiring space. 

Adhesiveless, copper-clad laminates that have simple and uniform substrate layers and smooth copper surfaces have become popular in high-density flex circuits. Because of thin film materials, there are several choices in small hole generation and copper plating processes. Flex circuits can contain both through-holes and bUnd via holes for double-sided circuits as shown in Fig. 62.7. Micro-size bUnd via holes on double-sided flex circuits confer the ability to make SMT assembly density much higher. 

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