Flexible circuit applications

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. 

In poly(imide) (Pl)-based adhesive compositions that are particularly useful in flexible circuit applications, plasticizers are used (35). Organic phosphates in an amount from 15-35% are added, such as triphenyl and tricresyl phosphate. 

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. 

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. 

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. 

PEI resins have been extruded into film and sheet. Since they are thermoplastic, both sheet and film can be formed and drawn in secondary operations. PEI film can be used in many electrical applications, such as the manufacture of capacitors and flexible circuits. These films can also be laminated or coextruded to make various multilayer structures. PEI films are often laminated with metals, such as copper, which are then made into electric circuits. 

PEI resins have been used in a variety of applications such as electrical connectors, internal computer parts, printed-circuit boards, flexible circuits, optical fiber connectors, fire helmets, large appliances, aircraft interiors, trays, microwave cookware, reflectors, motor parts, gears, pumps, lubrication systems, wire coating, industrial applications, bearings, small appliances, films, and fibers. Polyetherimides are used in a wide range of applications. A few of the key markets for PEI resins and some of the benefits they bring to the application are summarized below. 

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... 

Aluminum-clad flexible circuits find use in microwave stripline and radiation-resistance applications. Aluminum and its alloys have good electrical conductivity, are lightweight, and can be plated, soldered, brazed, chem-milled, and anodized with good results. Laminate dielectrics include PPO, polyimide, " epoxy-glass, and polyester. 

TABLE 61.4 Major Applications of High-Density Flexible Circuits... 

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