Flexible circuit materials Polyimide

Since the invention of integrated circuits (ICs), polyimides as heat-resistant organic polymers have been applied to insulation materials in electronics devices such as flexible printed circuit boards (FPCs), interlayer dielectrics, buffer coatings, and tape automated bonding (TAB). A polyimide thin layer is easily... 

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]. 

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. 

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. 

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. 

See Table 61.6 for a list of new materials for HDI flexible circuits. There is a significant difference between traditional flexible circuits and HDI flexible circuits. For example, more than 80 percent of traditional flexible circuits were covered by adhesive-based copper-clad laminates that use the traditional polyimide films, Kapton H or Apical AV . Conversely, the majority of high-density flexible circuits used in large-volume apphcations utilize all new materials. 

Polyimide film continnes to be the major substrate material for flexible circuits because only it can snpport high-temperature processes such as soldering and wirebonding. Both Kapton H film by Dn Pont and Apical AV by Kaneka have been used in the flexible circuit industry for... 

There are several barriers to using traditional polyimide films as the major substrate materials in HDI flexible circuits. Dimensional stability is the largest issue. Both Kapton H film and Apical AV by Kaneka have coefficients of thermal efficiency (CTEs) higher than 30 ppm, which is not acceptable for the large-volume production processing of HDI flexible circuits. Relatively high moisture absorption is another major barrier to using these films as the base... 

Several liquid polyimide resins have been developed as the base materials of high-density flexible circuits. Some of these liquid polyimide resins can be photoimaged, and have been used in high volume as the dielectric layer and coverlay in the wireless suspension of hard disk drives. 

These hquid polyimide resins could be the major dielectric materials of special high-density flexible circuits that demand extremely high density, down to 5 micron pitches with 10 micron via holes. The cost of these materials is higher than that of polyimide film. However, they have broader capability for meeting nonstandard requirements such as ultrathin substrates with microvia holes. The properties are very dependent on the manufacturer. 

Fluorized carbon polymer films have been applied as a low-loss substrate material for flexible circuits. However, they do not have good dimensional stability or adhesion characteristics. High cost of the materials is the major reason that they cannot be standard in flexible circuits. In the last 20 years, several heat-resistant films, such as polypalabaUc acid film and polysulfon film, have been developed as alternative substrate materials in flexible circuits instead of polyimide films. Unfortunately, there was no successful material from a business standpoint. LCP films and PEEK have been considered as the new materials of high-speed flexible circuits based on the low dielectric constant and loss tangents. 

Several laminates without adhesive layers have been developed as the advanced materials for the next generation of flexible circuits. Lamination technology using epoxy resin or acrylic resin has been almost eliminated from HDI flexible circuits even though it uses new high-performance polyimide films as the substrates. Three types of adhesive less copper-clad laminates have been developed (see Fig. 61.5) ... 

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. 

Polyimide. Kapton FF and Apical AV are standard substrate materials for traditional flexible circuits. Unfortunately, these films have high moisture absorption rates, which create excessive dimensional instability, making them unsuitable for high-density flexible circuits. 

The use of supplemental plasma etching, called the Etch Back Process, is recommended to achieve high reliability of through-hole multilayer rigid/flexible circuits. The plasma gas provides further etching on both of polyimide films and adhesive layer in the drilled holes. The plasma gas does not etch metal materials and it clears the surface of the copper foils for reliable copper plating as shown in Fig. 65.9. 

Polyimides are high-performance polymers, mainly used as films and as substrates for flexible printed circuits, bar code labels, and transformer and capacitor insulation. The rigid structure is responsible for a Tg greater than 300 °C. Since Kapton is flexible, lightweight and withstands very low and very high temperatures, it is an important aerospace material. Potential uses include as a thin film absorber for solar cells114. 

The major applications of LCP films in electronics are multilayer boards and multichip modules, and flexible printed circuits. Current materials used in the.se applications include polyimide film, high performance thermoplastic film such as polyphenylene sulfide and polyetherether ketone, and fiber reinforced composites such as quartz fiber-polyimide. The price of these materials for electronic packaging is in the range of. 50 to. l(K)/lb, so LCPs can compete very effectively at their current prices. Based on a two to three times increase in price... 

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. 

Aromatic polyimides have been the subject of much attention as a material for use in the electronics and other leading-edge industries. Films are used in flexible printed circuit boards, as wire and cable wrap, motor-slot liners, transformers and capacitors. Molded parts are used in applications requiring resistance to thermally harsh environments such as automotive transmission parts, thermal and electrical insulators, valve seats, rotary seal rings, thrust washers and discs, bushings. 

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