Piezoelectric Ceramics and Flexible Printed Circuits (FPCs) Interconnection Using Anisotropic Conductive Films (ACFs) for Ultrasound Transducers Assembly

Abstract

For several decades, non-conductive pastes (NCPs) have been widely used for the mass production of ultrasound transducers assembly using piezoelectric ceramics and FPCs. The NCPs interconnection is established through direct metal to metal contact. In more details, the surface of piezoelectric ceramics is roughly grinded to make point contacts before metallization. And the NCPs are filled in non-contacted area between metallized piezoelectric ceramics and metal electrodes of FPCs. However, the point contacts result in higher electrical resistance. Also, the electrical conduction and the reliability of NCP interconnection can be deteriorated especially in moisture and at high temperature environment due to the polymer expansion. In addition, the piezoelectric ceramics such as PZT cannot maintain the polarization above its Curie temperature. Therefore, the curing temperature should be below 150°C. Therefore, longer curing times at 150°C are required for NCPs to avoid the depolarization of piezoelectric ceramics. Furthermore, sometimes surface grinding of piezoelectric ceramics may produce cracks which lead to the reduction of production yield. Therefore, it is desirable for lowering the electrical resistances, increasing the reliability and shorter curing time without grinding process. As a result, low temperature anisotropic conductive films (ACFs), which consist of adhesive polymer resin and conductive particles, were introduced in order to increase the electrical conduction and reliability without grinding process. In this study, various ACFs, including 3 types of conductive particles (Sn58Bi solder, Au/Ni coated polymer and Ni balls) and 3 types of thermosetting polymer resins (cationic epoxy, imidazole epoxy and acrylic resins), were investigated with respect to mechanical/electrical properties and reliability. In order to lower the ACFs bonding temperature below 150°C, low melting temperature eutectic Sn58Bi solder particles with the melting point of 138°C was used. For the thermo-compression bonding, the piezoelectric ceramics were placed at the bottom and flexible printed circuits boards (FPCBs) were placed on the top of piezoelectric ceramics to guarantee the real temperature of the piezoelectric ceramics below 150°C. As a result, piezoelectric ceramics and metal electrodes interconnection was successfully performed below 150°C with stable contact resistance and solder joint formation even after reliability test and the dicing process. After dicing into FPCBs, no short circuit was found between neighboring electrodes. And ACFs with Sn58Bi solder particles and cationic epoxy resin showed the lowest electrical resistance after bonding, excellent mechanical and electrical performance among various ACFs.