Ultrasonic Bonding of AlCu Wire – An Advance to Reliable Interconnects for Power Modules

Aluminum cladded copper core (AlCu) composite wire is investigated as an advanced interconnect material for power modules, which combines aluminium’s bonding compatibility with copper’s superior electrical and thermal properties. Fusing current tests on $500-\mu \mathrm{m}$ diameter wires revealed that AlCu wire achieves a fusing current capacity of 32.5 A, surpassing pure aluminum wire at 29.5 A and approaching copper wire at $\mathbf{4 5. 6 ~ A}$. Incorporating a copper core increases the tensile strength of the AICu wire to approximately three times that of conventional aluminium wire, resulting in increased stiffness and higher bonding forces required during ultrasonic bonding, which risks damaging chip pads. To address this issue, the freshly produced wire was subjected to a controlled heat treatment. This induced recrystallization and grain growth in both the copper core and Al sheath, reducing stiffness and tensile strength. This enabled successful ultrasonic bonding of $500-\mu \mathrm{m}$ AICu wire onto aluminum chip pads of an IGBT chip without inducing damage. Crosssectional analysis confirmed good interface quality with no cracks or delamination. The findings demonstrate the feasibility of wire bonding of that heat-treated AlCu wire. It is known that Cu wire has a significantly higher reliability under thermal-mechanical load. Therefore, AlCu wire is a promising candidate for good bondability combined with enhanced reliability for power electronic.