Novel Aluminium/Copper Fibre-Reinforced Bonding Wires for Power Electronics

For power ICs bonding wires with a high thermo-mechanical fatigue resistance are requested by the mechatronic industry. One possible approach of improvement is the use of novel wires consisting of metal matrix composites. Materials like aluminium-copper-composites exhibit a larger strength than standard aluminium bonding wires. In the presented investigation fibre-reinforced bonding wires based on aluminium and copper have been realised by mechanical alloying, i.e. the use of extrusion, deformation and wire drawing. A soft aluminium matrix of the bonding wire is indispensable in order to bond power ICs without causing excessive stress in the chip, which may result in defects like cratering. The copper-fibres increase the strength of the wire. Furthermore, due to the filaments the electrical conductivity is increased and the thermal coefficient of expansion is decreased. Both phenomena have been confirmed by measurements and are expected to have a positive effect on the thermo-mechanical durability of wire-bonded connections. Bonding experiments and accelerated thermal fatigue stress tests have been performed with the fabricated bonding wires of typically 0.3 mm diameter. With these wires significantly higher bond shear strength could be observed in comparison to common aluminium bonding wires. Good reproducibility of the bonds regarding loop shape and wedge form was also fulfilled. After storage at high temperature the bond pull and shear forces for the Al/Cu wires remained at a significantly higher level in comparison to pure Al wires. Consequently, it has been shown that the strength of the joint between wire and substrate is increased using aluminium and copper-reinforced bonding wires and, therefore, the thermo-mechanical fatigue resistance of bonded interconnections for mechatronic power devices might be improved using these novel wires. A further approach is to add FeNi36 filaments together with copper. This alloy has a very low coefficient of thermal expansion and can further compensate the thermo-mechanical mismatch between substrate and bonding wire