Flexible circuit materials Adhesive

Two issues have arisen regarding RoHS requirements for the flexible circuit materials flame-retardant molecules with bromine in adhesive resins, and heat resistance for high-temperature processing with lead-free soldering. Although the issue of bromine is not actually a part of the RoHS requirement, it has been linked to the general environmental issues of printed circuit materials and processes. 

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]. 

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... 

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. 

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. 

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. 

Historically, copper-clad laminates with acrylic or epoxy adhesives have been the major materials for flexible circuits. Each manufacturer has developed a special resin grade or special additives to ensure reliable flexibility and bond strength. Other adhesive materials such as phenol resin or silicon resin have been developed however, they have not become standard adhesive materials in flexible circuits. 

The adhesive-based copper-clad laminates stiU represent more than 50 percent of the traditional flexible circuit market. The major properties of the materials are shown in Table 61.12. 

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. 

Several different types of adhesive materials can be used in flexible circuits depending on the application, except for the adhesive layer of copper laminates and fihn coverlays. The major applications are the bonding of stiffeners and multilayer constructions. 

TABLE 61.20 Comparison of Adhesive Materials for Flexible Circuits... 

Thinness and bendability are the major advantages of flexible circuits. However, flexible circuits can lose mechanical strength when it is necessary as part of a composite assembly. The addition of stiffeners on fiexibie circuits is a practical solution to solve the conflicts, but appropriate stiffener board materiais and adhesive materials have to be chosen based on the applications purposes. 

The connection of flexible circuits with anisotropic conductive materials is not new idea. An organic adhesive material that has conductive particles becomes conductive only in the z direction when sandwiched between a flexible circuit and the other device under a pressure... 

Several adhesive resins, especially acryhc-based adhesives developed for flexible circuits are not stable with the permanganate solution that is common for rigid multilayer boards, which means that a very narrow window is allowed for the desmearing of the rigid/flex process. If the materials are dipped too long in the solution, the adhesive layers swell and the rehabihty of the through holed will be damaged. 

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. 

Bismaleimide resins are extensively used in multilayer printed circuit boards. They can also be used in insulation fibres for protective clothing. Due to their ability to adhere and retain adhesive strength at high temperature, bismaleimides are widely used as a base material for flexible printed circuit boards in the electrical and electronics industries. Bismaleimides are often linked with rubber chains for use as flexible linking molecules to reinforce the rubber compound for tyre applications. The double bond of bismaleimides readily reacts with all hydroxy or thiol groups found on the matrix to form a stable carbon-sulfur, carbon-nitrogen or carbon-carbon bond. 

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. 

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