(A) study on the solder anisotropic conductive films (ACFs) and acoustic matching layer films (MLFs) for piezoelectric ultrasound transducers (PUTs) packaging

Abstract

Sn52In solder ACFs were fabricated and thermo-compression bonded at 130℃ for 30 seconds to bond PZTs on FPCs. Scanning Electron Microscopy (SEM) / Energy Dispersive X-ray (EDX) analysis of Sn52In solder joint cross-sections showed that the microstructure of Intermetallic Compounds (IMCs) was consisted of Au-In and Sn-Ni phases. Pressure cooker test and dynamic bending test were also carried out to examine the bonding reliability. And the results were compared with conventional metal coated polymer particles ACFs and commercial adhesive. After a pressure cooker test, the final intermetallic compound of Sn52In solder joint was Au-In-Sn ternary phase. Sn52In solder ACFs bonding offer very reliable metallurgical bonding. Acoustic Matching Layer Films were fabricated using B-stage thermosetting polymer resins with various volume fractions of alumina and tungsten powders. After mixing certain thickness MLFs, acoustic matching layers were fabricated using a simple molding process. The thickness of the matching layers can be precisely adjusted from several micron to hundreds of microns, without any grinding process. Experimental values of acoustic impedances of the matching layers were in good agreement with theoretical values calculated by the Devaney model. Piezoelectric Ultrasound Transducers were fabricated with the different bonding temperatures to validate the performance of PUTs regarding the bonding temperatures. The results showed that electro-mechanical coupling factor and ultrasound intensity of PUTs were increased with decreasing bonding temperatures because of PZT thermal degradation effect, which indicates that Sn52In solder ACFs are suitable for PUTs assembly. And a pair of Piezoelectric Ultrasound Transducer was also fabricated to validate the performance of the matching layer by comparing between the non-matched and matched transducers. Using the optimized acoustic matching layer by the MLFs, the fractional band width of the ultrasonic transducer was increased by 37%. The effectiveness of the matching layer using MLFs was successfully verified.