High temperature storage reliability investigation of the Al-Cu wire bond interface

In this study the intermetallic compound (IMC) formation of annealed 50 mu m copper wires, thermosonically nailhead bonded to thick Al-0.5 wt.% Cu pads, was investigated. The annealing experimental matrix covered the full range of Al consumption. The IMC thickness (growth behavior), obtained from FIB cuts, follows an Arrhenius type equation. TEM was used to identify the three formed intermetallic phases - CuAl2, CuAl and Cu9Al4 - which were detected in several interface FIB cuts. The activation energy for the growth of the total IMC stack was found to be 1.26 eV. The impact on bond strength was studied using ball shear test. The test results did not show a significant decrease in interface strength across the full experimental matrix even Al being fully consumed. Special procedures were applied to improve pad surface condition and wire bonding process parameters to get parallel and homogeneous IMC layer growth, and therefore reliable data. (C) 2012 Elsevier Ltd. All rights reserved.