Flexible printed circuit boards

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... 

JP 05125345 (Japanese) 1993 Poly(vinyl butyral) blend adhesive composition for flexible printed circuit boards Ube Industries, Japan H Inoe, S Takabayashi, T Muramatsu, T Funakoshi Formulated resins having good flexibility and heat resistance are useful adhesives for polyimide/Cu foil laminated circuit boards Maleimide-terminated siloxane-imide block copolymers were formulated with poly(vinyl butyral) and melamine resins to give adhesive compositions. 

Matsuwaki [5] prepared polyimide films, (V), having molecular orientation in the machine direction that were used in high-density mounting of flexible printed circuit boards. 

A film made from a poly(ester-imide) containing trimellitic anhydride, an aliphatic diamine and substituted hydroquinone is made by extrusion and is then laminated onto a metal layer. Flexible printed circuit boards were made from this [269]. 

Metallized PI films are used in electronic applications, e.g. for flexible printed circuit boards. Conventional techniques for the fabrication use adhesive bonding to a copper foil. However, the demand for high density packaging of electronic apparatuses requires a further reduction in the thickness of these substrates. It is possible to sputter metal particles into the surface of the PI film at a thickness of 20 nm, which forms the intermediate layer for subsequent formation of a conductive layer of copper or a copper alloy. 

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. 

Blends of PAEK and PEI can offer increased Tg, good chemical resistance at the lower PEI levels and reduced cost relative to pure PAEK. In particular PEI can be used to build the heat distortion temperature (HDT) of PAEK. Such blends find uses where PAEK-like performance is required in combination with an improved HDT. There are also applications in which the PEI is used above its Tg as a melt adhesive. For example, laminates of copper foil and PEEK/ PEI blend films can be produced in which the copper adheres to the hot, amorphous blend which subsequently crystallises to produce a chemical and solder-resistant structure. Mitsubishi has produced flexible printed circuit boards based on this concept [9]. 

PEEK film finds uses as a substrate for flexible printed circuit boards. Mitsubishi has developed a blend of PEEK and polyetherimide (PEI) in which the slow crystallisation characteristics can be utilised to fuse layers of circuit without the need for adhesives. This is a potentially low-cost route to multilayer circuits [14]. Film can also provide a useful moisture barrier in applications such as membrane switches. There are also a number of niche applications such as environmentally resistant RFID tags. 

Fig. 15 An interface based on flexible printed circuit boards (FPCB) for contacting CP (micro-) actuators in the open (a) and closed (b) position. The actuator imit is placed in the open FPCB interface with the actuators aligned to the Cu leads (c). Thereafter, the FPCB interfaee is folded around the actuator unit to provide contact to both sides of the actuators (d). Scale bars 2 mm (Reproduced fi-om Jager et al. (2013b))...

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. 

The sLCP has excellent dielectric characteristics at high frequencies and low moisture absorption as shown in Table 2.1(C). The dielectric properties of the sLCP film were formd to be constant up to a frequency of 25 GHz. Trials of sLCP as a substrate material of flexible printed circuit boards for higher frequency applications have been performed. Interestingly, the sLCP itself has a relatively high thermal conductivity. Its low viscosity allows the incorporation of a larger quantity of fillers to even further improve thermal conductivity. Trials have been continued in order to make use of sLCP as a substrate for metal based copper clad laminates (MCCL). 

Polyimide films are used in a variety of interconnect and packaging applications including passivation layers and stress buffers on integrated circuits and interlayer dielectrics in high density thin film interconnects on multi-chip modules and in flexible printed circuit boards. Performance differences between poly-imides are often discussed solely in terms of differences in chemistry, wiAout reference to the anisotropic nature of these films. Many of the polyimide properties important to the microelectronics industry are influenced not only by the polymer chemistry but also by the orientation and structure. Properties such as the linear coefficient of thermal expansion (CTE), dielectric constant, modulus, strength, elongation, stress and thermal conductivity are affected by molecular orientation. To a lesser extent, these properties as well as properties such as density and volumetric CTE are also influenced by crystdlinity (molecular ordering). 

PCTFE in thin film form is useful for protection against moisture and finds wide application in pharmaceutical blister packaging, electrolmnines-cent lamps and liquid crystal displays. Due to its chemieal stability, PCTFE is also used in chemical industry in the form of tubes, valves, chemical tank liners, 0-rings, seals and gaskets. Because of excellent electrical resistance and water repulsion properties, PCTFE finds applications also in preparation of flexible printed circuit boards, insulation of wires and cables. 

Outside of the drinks industry, PET is often used for photographic film, recording tape, flexible printed circuit boards, transformer insulation and transparent stationery. 

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