High-Performance Thermoplastics for Rigid-Flex Printed Circuit Boards
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Polyimides. PIs are characterized by their resistance to temperatures in excess of 204'C, high dielectric strength, and excellent mechanical properties. Drawbacks to PI use are poor bonding with adhesives, flammability, and high moisture absorption. Acrylic adhesives. Acrylic adhesives are quasi-thermoset materials characterized by excellent adhesion to PI substrates, excellent resistance to processing chemicals, and excellent adhesion retention under thermal shock conditions such as soldering. However, they melt at 40'C. High-Performance-Engineering Thermoplastics A possible solution to the adhesive failure problem would be to replace the epoxies, PIs, and adhesives currently used in RF-PCBs with high-performance thermoplastic material systems. Using high-performance thermoplastics would enhance the rigid-flex system by increasing reliability and decreasing system cost. High-performance thermoplastics are less hygroscopic and tend to be more dimensionally stable than the materials currently used. Dimensional stability is
ensured by using thermoplastics with high glass transition temperatures (>125°C), below which the polymer molecules are immobile, and by their low moisture absorption, which results in reduced expansion of the material. Using thermoplastics also may simplify the manufacturing process, thus lowering manufacturing costs. Because these materials are remeltable, adhesives are not required, and parts may be joined using simple fusion bonding methods. Fusion bonding of thermoplastics is much simpler, more consistent, and less expensive than adhesive bonding. A fusion-bonded interface is composed of the resin itself; therefore, concerns with adhesive/board material compatibility are eliminated. Joint strength and durability should be the same as board strength and durability. In summary, the benefits of using high-temperature thermoplastics over the standard materials are the following: No adhesive required. Thermoplastics are remeltable; therefore, fusion bonding may be used in place of adhesives. By eliminating the use of adhesives, the fabrication process is simplified, because fusion bonding is less process sensitive than adhesion bonding. Surface preparation is simpler, and there is not as much concern with surface chemistry, adhesive age, humidity, and temperature. Fusion bonding also is faster because there is no cure time required-the material is simply heated to the melt temperature, held for a few minutes (compared to a few hours), and cooled. Decreased moisture absorption. The moisture absorption of high-performance thermoplastics is much lower than that of epoxies and PIs. Reduced moisture absorption will reduce circuit degradation and improve system performance. Expansion coefficient compatibility. Because only one material is used at the bond, there is no differential in the CTE or in the moisture absorption coefficient. Thus, warping and film stresses are reduced. Reprocessability. Unlike thermoset materials, such as epoxies and PIs, where fabrication involves an irreversible chemical proc
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