Osamu Ikeda

Three-Layer Zn/Al/Zn Clad Solder for Die Attachment

Three-layer Zn/Al/Zn clad solders have been developed for high-temperature die attachment. The clad structure is used to improve the wettability and bondability of Zn-Al eutectic solder by preventing oxidation of the Al. The materials were produced by clad-rolling Zn and Al strips. TEM observations revealed that the Zn/Al clad interface was metallurgically bonded and that the Al oxide was almost entirely removed. The melting behavior of Zn/Al/Zn clad solder was examined. Eutectic melting began at the Zn/Al clad interface at 382°C, and all of the material melted within approximately 10xa0s. Unlike conventional Zn-Al solders, Zn/Al/Zn clad solders were successfully bonded without flux. The shear strength of a Zn/Al/Zn clad solder joint was three times that of a Pb-based solder joint. The bondability of Zn/Al/Zn clad solder was superior because the Al oxide films, which prevent bonding between chip and substrate, were fragmented by clad-rolling, and the outer Zn layers prevented Al oxidation during the bonding process.

Heat dissipative Pb-free bonding technology using Al-rich Zn/Al/Zn clad solder

Abstract Al-rich Zn/Al/Zn clad solder were developed as Pb-free solder for a die-attachment. The Zn/Al/Zn clad solder was produced by clad rolling of Zn and Al strips in order to prevent Al from oxidation and improve wettability. The Zn/Al/Zn clad solder was melted at 382°C after solid-state interdiffusion of the Zn and Al layers. Bonding was successfully achieved with bonding pressure of a few kilopascals. Thermal cycle life of Invar-to-Cu substrate joint using the Zn/Al/Zn clad solder was longer than that of Pb-Sn-Ag solder. No Kirkendall voids were observed in the vicinity of the bonded interface after ageing at 250 °C for 1000 h.

Effects of Bonding Conditions on Bondability Using Zn/Al/Zn Clad Solder

Three-layer Zn/Al/Zn clad solders have been developed for high-temperature die attachment. The clad structure is used to improve the wettability and bondability of Zn-Al eutectic solder by preventing Al oxidation. The effects of the bonding conditions on the bondability with Zn/Al/Zn clad solder were investigated. Bonding was achieved in the temperature range from 385°C to 420°C under N2 atmosphere with oxygen concentration below 100xa0ppm. However, the bonding strength of the joint formed under N2xa0+xa04% H2 atmosphere was almost 0xa0MPa, and stripe defects and air gaps remained in the bond layer. To improve the bondability under N2xa0+xa04% H2 and expand the application range, a five-layer Cu/Zn/Al/Zn/Cu clad solder was developed in an attempt to prevent the Zn layers from being oxidized by the outer Cu layers. Cross-sectional observation of the Cu/Zn/Al/Zn/Cu clad solder revealed that the surface was covered by a Cu layer, and that Cu5Zn8 layers grew between the Cu and Zn layers. This clad solder exhibited high shear strength of over 80xa0MPa when formed under N2xa0+xa04% H2 atmosphere, and no stripe defects or air gaps were observed in the bond layer.