Numerical Studies On The Stress Condition In Silicon Substrate During Copper Thermosonic Wire Bonding

Copper wire has attracted increasing attention in the thermosonic wirebonding of integrated circuits, due to its lower cost and higher strength thanthe traditional gold wire. One of the key problems that limit the large scaleapplication of copper wire is the silicon under pad damage, which is closelyrelated to the stress conditions during wire bonding. The parameters that canaffect stress condition under pad not only contain wire materials, but alsoinclude coating materials and its thickness, bonding parameters and chipstructure etc. Since the wire bonding joint is very small (<0.1mm), it is veryhard to study stress condition using experiment test. In this study, the finiteelement analysis method is used to study the stress condition of the siliconsubstrate in copper wire bonding, which can supply some guidelines forsolving the silicon under pad damage problem.An incrementally coupled “thermal-mechanical-ultrasonic” model isdeveloped based on the characteristics of wire bonding, while ultrasoundsofting, ultrasound vibration and temperature are considered. Results showthat bonding temperature can make big difference for the stress under pad.Experiments results of copper free air ball deformation are employed tovalidate the simulation results.The developed FEA model is employed to calculate the processes of wirebonding using two types of wire materials, i.e., Cu and Au, and the stressconditions under pad are obtained. Results show that using copper wire canlead to higher maximum compressive stress, maximum tensile stress andmaximum shear stress than gold wire, but the shear stress concentration zoneare on the contact edge for both copper wire bonding and gold wire bonding.That’s the reason why using copper wire is easy to lead to defect.The FEA model is used to analyze the stress condition under pad usingthree types of coatings, i.e. Al, Ag, and Ni, and three silver coatingsthicknesses, i.e.4μm,8μm and16μm. Results show that, when using Al, the values included maximum compressive stress, maximum tensile stress andmaximum shear stress under pad are the lowest, and maximum compressivestress and shear stress are the highest when using Ag, and the maximumtensile stress is the highest when using Ni. Considering the shear stresses aremore determinative to under pad damage, the Al coatings are more favorablein copper wire bonding. The thicker the Ag coating, the lower the stresses insilicon under pad. Hence, it is better for reducing pad damage using thickercoating (16μm).These researches can be guidelines to solve the under pad damageproblem in copper wire bonding, and supply some technological assistance forthe application of copper wire bonding in the future.