The re-evaluation of mechanical properties of wire bonding

Abstract

Wire bonding is the most popular interconnection technique that has been used in microelectronics packaging due to its maturity and cost effectiveness. The technology advances in the era of miniaturization and multifunction have urges the need for the smaller wire bond size to cope with the decrease of bond pad pitches. Ultimately, this will introduces al lot of technology challenges in the characterization and performance of wire bonding micromechanical properties. The conventional tests such as wire pull and ball shear tests provide inadequate information in respect of bonding and metallurgical response of the interconnection. This is because the evaluations of wire bond performance based on conventional tests are more into qualitative results or failure modes rather than detailed quantitative results. Furthermore, the results obtained through wire pull and ball shear tests will change and introduce a lot of variations as the ball bond diameter become smaller. In the present analysis, nanoindentation test was introduced in order to provide more adequate information about the quality of wire bond. Nanoindentation test provides the micromechanical properties in terms of hardness and reduced modulus value in the small length scale. This will facilitate the micromechanical properties measurement of the wire bond. In addition, the continuous measurement of stiffness provided from nanoindentation test realizes the qualitative results of materials such as deformation, strain hardening effect and creep behaviour. Wire bonding process was prepared using thermosonic wire bonding technology using 25 μm diameter of gold wire on the Aluminium bond pad. The nanoindentation test was conducted at various locations on the ball bond that has been cross-sectioned diagonally prior to the indentation process. To further investigate the micromechanical properties, the location of indentations was divided into two zones namely Zone 1 and Zone 2. Zone 1 is located at the area near to the intermetallic layer of Au and Al, while Zone 2 is located at deformed ball bond created from the inner chamfer of capillary. The results show that the micromechanical properties of ball bond vary throughout the location of indentations. The hardness and the reduced modulus for the indentations that located at the Zone 1 have higher average values compared to that of the indentations that located at the Zone 2. The average value of hardness and reduced modulus for the indentations at the Zone 1 are 1.011 GPa and 88.652 GPa, respectively. While the average value of hardness and reduced modulus for the indentations at the Zone 2 are 0.853 GPa and 70.652 GPa, respectively. In addition, indentation 1 of Zone 1 that located perpendicular to the effect of deformation created from the end of capillary has the highest value of hardness with value of 1.156 GPa. The value of hardness for the indentations 3 and 4 of Zone 1 has the lowest value of hardness with value of 0.928 GPa and 0.834 GPa, respectively. It is known that the hardness and the reduced modulus are related with the yield strength and diffusivity of materials. Thus, the results obtained from nanoindentation test are useful in explaining the strengthening and bondability of ball bond, at least from metallurgical point of view. Therefore, the nanoidentation test is suitable approach to re-evaluate the mechanical properties of wire bonding in addition to the conventional tests.