Flexible circuit materials Polyester

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. 

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. 

Lower cost flexible printed circuits use polyester base materials with more expensive polyimide, including DuPont s Kapton and Toray s Metaloyal, selected for appfications where higher performance is required. Metaloyal is an electrolytically plated two layer flexible substrate film with a 2-18 pm copper layer formed on the surface of the polyimide film by electrolytic plating. Toray claims that superior fine pitch etching capabilities coupled with good flexibility and heat resistance resnlt from the copper plated layer and the high adhesion of base film. 

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. 

All kinds of sheet or board material could be used as stiffener materials for the flexible circuits however, several materials are conunouly used.Typical stiffener materials for the traditional flexible circuits are listed in Table 61.19. Paper phenol boards and glass-epoxy boards are employed for relatively thick requirements. Polyimide films and polyester films are employed for relatively thin requirements. Aluminum plates and stainless steel plates are commonly used as the Stiffener materials of flexible circuits. A specialty of the metal stiffeners is their forming capabilities after they are bonded on flexible circuits. Paper phenol and polyester are not available for thermo setting adhesives because of the low heat resistance. 

The basic idea of pressure contact termination technology is very simple and is not new. Pad patterns of the same pitch plated with gold are generated on a flexible circuit and opposite circuits, and they are attached to each other with uniform pressure using a suitable rubber strip (see Fig. 64.14). There is no high-temperature process to complete the coimectiou polyester base materials are available. The maximum conuection number and density depend on the capability of dimension control and uniformity of the circuits. It is possible to make 200 connections with 150 /tm pitch because of the processes all take place at room temperature. Neither special application equipment nor special constructions on flexible circuits are... 

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. 

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