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Advances in Wire Bonding Technology for Overhang Applications
Keywords: wire bonding , overhang optimization, overhang characterization
Despite recent advances in 2.5D and 3D packaging, wire bonding is still the most popular interconnect technology and the workhorse of the industry. Technology development and innovation in wire bonding has provided new packaging solutions that improve performance and reduce costs. One application of wire bonding for 2.5D and 3D packaging is in memory devices 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, which leads to improper ball shape, inconsistent looping, pad crack and die crack issues. The magnitude of die deflection is determined by the package configuration (die thickness, overhang distance and package layout) and bonding parameters, such as applied normal force during wire bonding. Within an overhang die, the actual overhang distance for each bond site is different. Corner pads that are farther from the underlying support are more compliant than the center pads, which are closer to the underlying support. Therefore, careful process optimization is needed to have the best outcome in wire bonding performance. This process is often tedious and time-consuming. Moreover, recent trends towards minimizing package size (e.g. ultra-thin dies) and cost (e.g. Ag wire) further exacerbate the challenges of optimizing a wire bonding process for overhang devices. This paper examines the challenges of wire bonding on overhang devices. Finite element (FE) analysis of overhang devices is presented and the data is correlated with the experimental results. It focuses on a new method to automatically characterize the bond sites on an overhang die using the wire bonder. The results of the characterization can then be used to optimize the bonding parameters while keeping within the target maximum die deflection limit. Using this method, wire bonding processes are optimized for both Au and Ag wires on an overhang die. The pros and cons of the two wire types for overhang devices are compared and a wire bonding capability chart as a function of die thickness and overhang distance is developed. A process optimization guideline for overhang wire bonding is presented.
Aashish Shah, Staff Engineer
Kulicke and Soffa Industries Inc
Fort Washington, PA

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