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Mechanical Properties of Aluminum Bond Pad Structures with Different Thicknesses
Keywords: Aluminum Bond Pad, Copper Wire Bond, Nanoindentation
Copper has become the most common first- level interconnect type used in microelectronic packages and is projected to make up about 67% of total wirebonds in 2017. Many industries are now showing interest in adopting this technology, not only because of cost savings, but also because copper wirebonds have better electrical and thermal properties, higher stiffness, and less susceptibility to intermetallic related failures than gold wirebonds. However, it is crucial to have a procedure for predicting life time of these wire bonds before they can be integrated in mission critical applications. Currently, the most prevalent evaluation techniques that ensure good reliability are pull testing and bond shear, which are followed by most wire bond manufacturers. However, the correlation of these tests to reliability is still unproven for copper wirebonding. Shear strength, the force required to break the weld between ball bond and bond pad, is dependent on the intermetallic compound (IMC) coverage at the ball bond bond pad interface. Thus, IMC coverage is a key parameter that needs to be optimized to produce high shear strength, while controlling bond size and minimizing aluminum splash. Wire bonding parameters, wire bond properties and bond pad properties all influence IMC coverage, bond size, and aluminum splash. Ample research articles have been published on optimization of wire bond parameters, however, the effects of wire bond and bond pad properties need further study. In previous efforts, it has been shown that the size of bonds is affected by bond pad thickness due to difference in perceived hardness. Bonding over thin aluminum pads can lead to larger ball diameter due to high relative stiffness of the pad structure, while thicker aluminum pads result in smaller ball diameters, owing to the greater dissipation of stress within the aluminum pad. Thus measurement of effective properties of pad structure becomes important in determining bond size and IMC area. This study focuses on measuring the elastic modulus, hardness and yield strength of 7 unbonded aluminum pad structures with varying aluminum thickness: 5000, 5500 , 6750 , 8000 , 10000 , 15000 and 30000 by nanoindentation. Ball bonds on another set of the 7 pad thicknesses are cross sectioned, then elastic modulus, hardness and yield strength is measured by nanoindentation through the thickness of the pads, in (a) region under the ball bond and (b) region away from ball bond. These results along with SEM and other analyses will aid in our understanding of pad aluminum properties that interact with the copper wirebonding process, including ball diameter, IMC, and shear strength. Results also provide crucial material properties to aid in future finite element analyses of copper ball bonds and bond shear testing.
Subramani Manoharan,
University of Maryland
College park, MD
United States

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