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BGA Wire Bonding - The Total SolutionNico Onda, Daniel Zanetti, Daniel von Flüe, Zeno Stössel, Michel Garnier1 IntroductionThe demand for more complex IC devices with increased functions per chip and smaller device geometry steadily pushes forward the development of ever newer and more sophisticated BGA package types. However, the BGA technology is not without its problems. This is true in particular for organic substrates, the so-called PBGAS, on which we will focus in this study. Plastic substrate materials, also called laminates, generally consist of polymer resin such as FR4 or BT, and reinforced fiber. These substrates are characterized by a low glass transition temperature and a large thermal expansion coefficient. Heat driven assembly steps, such as curing, wire bonding or molding, must thus be carried out of reduced temperatures where processing can become critical. Wire bond contact formation, for example, decreases with decreasing bond temperature. One reason is the reduced, thermal induced growth of inter-metallic phases. Another reason is the increased negative influence that surface contamination has on any type of attach process. Wire bonding is further altered by other, more package related properties such as warping, softening of the substrate material, large thickness variations of the strips and, finally, by quality variations of the printed layers. From this and in view of the increasing automation in IC assembly, it becomes clear why an overall, that is, a total solution must be envisioned in order to take full advantage of the BGA technology. The different materials and processes required for this technology - from the package layout to the mold compound - cannot be optimized without also considering the requirements of all the other process steps. Wire bonding certainly plays a central role in IC assembly. Special attention must therefore be devoted to this technology if BGA is to become the package of choice. The aim of this work is to give a short update on some technologies that govern today's wire bonding. In Section a) we discuss new findings concerning the effect of plasma cleaning on wirebondability. In Section b) we present new data regarding wire bonding with higher transducer frequencies and discuss the importance of an accurate bond process optimization. Finally, in Section c), we present more recent developments in loop and bond placement control multilevel devices.
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