The effects of aged Cu‐Al intermetallics to electrical resistance in microelectronics packaging

A Cu‐Al bonding system exists when copper wire is bonded onto an aluminum bond pad using thermosonic wire bonding technology. Aged Cu‐Al bonding system was analyzed by measuring the intermetallics layer thickness and its correlation to electrical contact resistance. Result shows that the thickness of Cu‐Al intermetallics layer grows almost linearly to aging time. The activation energy needed for Cu atoms to diffuse into Al was calculated using Fick's law; Q=129.66 kJ/mole and D₀=1.628×10⁻⁴ m²/s. The calculation of activation energy and impurity diffusity using Model Kidson also shows linear relationship. Electrical resistance of Cu‐Al intermetallics layer was calculated from contact resistance of Cu‐Al bonding system. The result shows that the electrical resistance of Cu‐Al intermetallics layer increases linearly with intermetallics thickness. Its growth rate that was calculated using Model of Braunovic and Alexandrov is double of Model of Murcko.