Ute Geissler

Microstructural evolution of ultrasonic-bonded aluminum wires

Abstract The evolution of microstructural gradients, especially crystallographic texture gradients, after ultrasonic wire bonding process and after active power cycling (APC) of high purity, heavy aluminum (Al) wires is studied by electron backscatter diffraction (EBSD) and nanoindentation. The results improve the knowledge about microstructural changes and arrangements after wire bonding and during APC. After ultrasonic deformation by wire bonding, the evolution of a distinct rotated cube (RC) textured area within the wedge was proved by EBSD analysis. The RC texture is discussed as a result of shear deformation and oriented grain growth. Decreased hardness within the RC textured area provides evidence for local softening effects during wire bonding. During APC, besides crack propagation, grain coarsening as well as local low angle boundary migration occurs and the wedge texture changes to an overall random orientation. Effects of microstructure on the crack growth behavior were discussed and suggestions for the improvement of wire bond reliability were derived.

Aluminum-Scandium: A Material for Semiconductor Packaging

A well-known aluminum-scandium (Al-Sc) alloy, already used in lightweight sports equipment, is about to be established for use in electronic packaging. One application for Al-Sc alloy is manufacture of bonding wires. The special feature of the alloy is its ability to harden by precipitation. The new bonding wires with electrical conductivity similar to pure Al wires can be processed on common wire bonders for aluminum wedge/wedge (w/w) bonding. The wires exhibit very fine-grained microstructure. Small Al3Sc particles are the main reason for its high strength and prevent recrystallization and grain growth at higher temperatures (>150°C). After the wire-bonding process, the interface is well closed. Reliability investigations by active power cycling demonstrated considerably improved lifetime compared with pure Al heavy wires. Furthermore, the Al-Sc alloy was sputter-deposited onto silicon wafer to test it as chip metallization in copper (Cu) ball/wedge bonding technology. After deposition, the layers exhibited fine-grained columnar structure and small coherent Al3Sc particles with dimensions of a few nanometers. These particles inhibit softening processes such as Al splashing in fine wire bonding processes and increase the thickness of remnant Al under the copper balls to 85% of the initial thickness.

Correlation between mechanical properties and microstructure of different aluminum wire qualities after ultrasonic bonding

Abstract Three different heavy aluminum wire qualities were investigated regarding their microstructural evolution after ultrasonic bonding by electron backscatter diffraction and nanoindentation. The results complete the findings of our recent research regarding the effect of bonding mechanisms on the wire bond microstructure and its local mechanical properties. Local elastic-plastic material parameters of the bonded wires were approximated on the basis of the elastic anisotropy of crystals and a correlation between hardness and stress.

Thermomechanical description of interface formation in aluminum ultrasound (US)-wedge/wedge-wirebond contacts

Wire-bonding is and will stay one of the essential interconnection methods in electronic packaging. Physics-based modeling of interconnects between the wire material and the substrate metallization has been constantly advanced over the last five years. Due to the variety and complexity of the mechanisms occurring during bonding it is required to clarify some open issues for a thorough interpretation of the metallurgical processes that take place at the interface of the weld and in the wire. One of these issues relates to the thermo-mechanical processes at or in the vicinity in the interface between the wire and the substrate during an ultrasonic wedge/wedge wire-bonding. These processes shall be qualified by a micro-thermomechanical model and also incorporated quantitatively in the bonding process. In this paper a thermomechanical analysis of ultrasonic wire bonding is performed by means of 3D finite element (FE) simulations. On the wire the bonding force is applied as well as ultrasonic vibration, which causes shear stresses in the wire and a frictional movement between the wire and the pad. The change of temperature in the interface between the aluminum wire and the gold metallization of a copper-nickel pad is studied. It is shown that a rise of temperature is obtained due to the plastic deformation of the wire caused by the bonding force, the shear stresses and by the friction with the pad. It is also shown that the maximum temperature is still lower than the temperature required for a proper material transport, proofing that this temperature rise cannot be alone responsible for ultrasonic wire bonding.