Abstract
Wire bonding is the most popular interconnect technology and the workhorse of the semiconductor packaging industry. Wire bonding is widely used for 3D packaging in which multiple dies are often stacked vertically in a ‘stacked die’ configuration. In such packages, one or more dies may be unsupported in an ‘overhang’ (e.g. cantilever beam) configuration. Wire bonding on an overhang die causes die deflection. If not optimized, it may lead to improper ball shape, inconsistent looping, pad crack and die crack issues. Therefore, careful process optimization is needed to have the best outcome in wire bonding performance. This optimization is often tedious and time-consuming. Moreover, recent trends towards minimizing package size (e.g. ultra-thin dies) and increasing number of die stacks add to the challenges of optimizing a wire bonding process for overhang devices.
This paper examines the challenges of wire bonding on overhang devices. Finite element analysis (FEA) of overhang devices is presented. Die deflection data obtained from the FEA correlates well with the experimental results obtained on the ball bonder. The FEA results show that die deflection increases significantly with decreasing die thickness and increasing overhang distance. Other factors such as substrate thickness, and bonding temperature also effect die deflection, although less significantly than die thickness and overhang distance. Various considerations for optimizing a ball bonding process on overhang devices are discussed. Experimental results of ball bonding optimization on 50 μm and 75 μm thick overhang devices with different overhang configurations are presented.
