Development of lead-free ultrasonic wirebonding transducers for microelectronic packaging applications

Abstract

With the use of cerium dioxide (CeO₂) and manganese dioxide (MnO₂) additives, lead-free piezoelectric ceramics (Na₀.₄₇₅K₀.₄₇₅Li₀.₀₅)(Nb₀.₉₂Ta₀.₀₅Sb₀.₀₃)O₃ (abbreviated as NKLNTS) have been successfully fabricated by a conventional mixed-oxide method. Our results reveal that CeO₂ is effective in improving the densification of the NKLNTS ceramics. After the addition of CeO₂, the ceramics can be well-sintered at a higher temperature, and hence have a higher density and better dielectric and piezoelectric properties. For the ceramic added with 0.4 wt% CeO₂ (NKLNTS-0.4), the piezoelectric and dielectric properties become optimum, giving a large piezoelectric coefficient (d₃₃ = 246 pC/N), a large planar-mode electromechanical coupling coefficient (kp = 0.50), a high relative permittivity (εr = 1300) and a low dielectric loss (tan δ = 4%). It has also been shown that MnO₂ is effective in enhancing the mechanical qulity factor Qм and reducing tanδ and kp of the NKLNTS-0.4 ceramics. The NKLNTS-0.4 ceramic added with 0.4 wt% MnO₂ (NKLNTS-0.4-0.4) has the highest Qм (70), the lowest tanδ (1.9%) and the lowest kp (0.43). These make the NKLNTS-0.4-0.4 ceramic become more suitable for the wirebonding applications. A prototype wirebonding transducer has been fabricated using the NKLNTS-0.4-0.4 ceramic, in the form of ring, and titanium alloy front and back plates (abbreviated as NKLNTS-Ti transducer). Finite element analysis has been applied to study the vibration characteristics of the transducer as well as the ceramic rings. On the basis of the simulation results, the optimum dimensions of the rings and the titanium alloy plates were determined. The performances of the NKLNTS-Ti transducer have been evaluated. Because of the better matching of the acoustic impedances, an effective transfer of vibration energy is achieved in the NKLNTS-Ti transducer, thus leading to a large axial vibration (1.73 μm at 0.1 W), which is comparable to that provided by a lead-based transducer currently used in a commercial wirebonding machine. Moreover, the NKLNTS-Ti transducer exhibits a smaller lateral vibration (0.05 μm at 0.1 W), which is essential for producing a small or narrow bond. Aluminum wire has been successfully bonded on the standard die and gold-plated PCB by using the NKLNTS-Ti transducer. The bonds are of good quality and meet all the industrial standards. These clearly show that the NKLNTS-0.4-0.4 lead-free piezoelectric ceramic is a promising candidate for replacing the lead-based ceramics as a driving element in the future generation of wire bonders.