Flexible circuit materials Conductors

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. 

HDI applications require finer traces and microvia holes in severe manufacturing and application conditions. For manufacturing convenience, flexible circnit prodncers prefer thinner conductors and substrates. However, physical performances of thinner materials may not be optimal. Thinner materials impact both the performance and manufacturing yield of the final flexible circuit. 

Aluminum conductors have been developed as the low-cost solution. They have been applied for volume production of the keyboards of calculators, antennas of wireless devices, and so on. But they could not be universal conductor materials of standard flexible circuits because of difficulty in soldering and the special chemistries needed for etching. The wireless suspension of disk drives has consumed a large volume of special stainless steel and copper alloy foils because of the special mechanical performance of the circuits. A high-resolution printer head has also utilized thin tungsten foils as the conductor of thermal printer head circuits. Nickel-chromium alloy foils have been developed as the conductor materials of flexible heater circuits. 

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 ... 

Because of the thiu and fragile materials used, flexible circuits have lower mechanical rehabihty than rigid circnit boards. They have a low conductor bond strength and low base snbstrate tear strength. Nevertheless, most of the flexible circuits are subjected to more mechanical stresses due to movement. This means that special care is required in the circuit design to gain higher circuit rehabihty. 

Several combinations of chemical deposition process of the conductor materials and photo-sensitive resists have been tried. Unfortunately, there is no good combination that satisfies the needs for both high-resolutions and physical performances. Further improvement is required to have practical high density flexible circuits. 

Screen-printing has had a remarkable progress to generate fine resolutions. An appropriate combination of the screen printer and screen mask can provide smaller pitch traces than 20 microns as shown in Fig. 63.24. It could be the low cost solution for the volume production for the high density flexible circuits. The process is capable for both conductor materials and insulation materials. A screen printable conductive material could be the key of the process. 

A silver paste conductor with an acryUc resin matrix is the most popular conductor material as the major thick-film conductor material for flexible circuits. It can provide very flexible conductor layers via a simple screen-printing process. Copper-based and carbon-based paste materials have been developed as the low-cost materials, but their conductivity is very low and unstable, and therefore their applicable areas are limited. 

A silver paste thick-film flexible circuit built on polyester film can provide the lowest-cost solution for large circuits. Unfortunately, the conductivity of the traces is much lower than for copper foil conductors, and this material is not available for power circuits or signal layers of high-speed circuits. Standard soldering is not available because of the acryUc-based matrix. 

One of the primary motivations for developing plastic was the burgeoning electronics industry of the late 19th century. Electrical engineers needed a flexible material that was not an electrical conductor, with which they could protect and insulate electrical wires and circuits. Chemists had been experimenting with materials such as phenol and formaldehyde for decades before Baekeland hit upon the right combination of these substances, as well as the right temperature and pressure at which they would combine to form a useable plastic. 

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