The wedge microstructure of AlSi1 wire bonds as well as the interface between the bonding wire and the Cu/Ni/Au metallization layer especially for chip on board (COB) assemblies was investigated by focused ion beam (FIB), transmission electron microscopy (TEM), and microhardness measurements. The as-received wires were characterized by fiber texture of ><111> and <100> orientation. With increasing ultrasonic (US) power, the results indicate recrystallization of the grain structure and decreasing microhardness inside the bonded wedge contacts. The interface between the AlSi1 wire and the Cu/Ni/flash-Au metallization layer of the optimized bonds consists of a closed crystalline Au layer. Above this Au layer, a second zone consisting of intermetallic phases was analyzed and identified by electron diffraction as Au8Al3
Ultrasonic wedge-wedge bonding of 25- mum AlSi1 wires is characterized as a dynamic process of hardening and softening. In the first phase of wire bonding, the wedge is predeformed and cold worked by the bondforce. After ultrasonic energy has been switched on, recrystallization starts at the interface. During the bonding time hardening and softening processes alternate and a maximum in hardness is measured. Hardening and softening processes correlate well with the grain structure, the measured grain sizes and a typical plateau in the z-deformation curve of the contact. At the end of the wire bonding process the wedge is recrystallized and softer than the predeformated wedge, but harder than the as-received wire
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