Low-Temperature Bonding of PZT (PbZrTiO3) and Flexible Printed Circuits Using Sn52In Solder Anisotropic Conductive Films for Flexible Ultrasonic Transducers

Solder anisotropic conductive films (ACFs) consist of conductive solder particles, polymer resin, and flux activator. The flux activator removes solder oxide of solder particles by generated acid for the solder wetting during a thermocompression bonding process. In this article, eutectic Sn52In solder (Tm = 118°) particles were used as the conductive solder particle in the solder ACFs to reduce <; 130° ACFs' bonding temperature. Novel Sn52In solder ACFs were fabricated using a comma-roll film coater and used to interconnect Au-coated PZT (PbZrTiO
₃
) and flexible printed circuits (FPCs). PZT and FPCs interconnects using Sn52In solder ACFs were successfully demonstrated with the formation of good solder joints and reliable electrical resistances. According to the energy-dispersive spectrometer (EDS) line scan analysis, excellent solder wetting was confirmed by the presence of Au-In and Ni-Sn intermetallic compounds (IMCs) at the PZT and FPCs interface. Sn52In solder ACFs' joints showed no open-circuit failure due to the excellent metallurgical bonding during a pressure cooker test (100% relative humidity and 2 atm at 121°) and a dynamic bending reliability test (the minimum bending radius of 7 mm). Therefore, Sn52In solder ACFs' bonding can provide excellent metallurgical interconnection for flexible ultrasonic transducer packaging.