Investigation of diffusion bonding quality by ultrasonic technique

For high-temperature ultrasonic transducers piezoelement-to-metal thermosonic bonding technology was developed based on goldto-gold diffusion. Bond quality strongly depends on bonding parameters – pressure, temperature, ultrasonic power and time. The first step for achieving good bonds was ultrasonic monitoring of the process. In ultrasonic transducers good diffusion bond means good acoustic coupling of piezoelement to the protector and this coupling must withstand high temperatures. Bond quality tests comprise piezoelement electrodes adhesion issues as well. Bismuth titanate and PZT piezoelements were bonded to thick stainless steel protectors and membranes. For bond quality determination normal-incidence immersion scanning method from metal side was chosen. It is based on measurement of reflection/transmission signals in the bond interface and determination of their ratio which defines acoustic coupling. The whole piezoelement bonding interface was scanned automatically in x and y directions with respect to 10 MHz wideband ultrasonic transducer, having lateral beam width 1.4 mm at – 6 dB level. With optimal bonding parameters good bond quality was found. Weak or non-bonded areas are absent. Finally thus bonded transducers were used in successful temperature experiments – pulse response measurements up to 400 °C and tests in a liquid Pb/Bi at 290 °C.