Growth and properties of intermetallics formed during thermal aging of Cu-Al ball bonds

To mimic the growth of intermetallic compounds during and after copper ball bonding, diffusion couples of aluminum and copper were made, annealed at high temperature and chemically analyzed. Two types of couples were investigated: 1) a piece of copper and of aluminum in mechanical contact at a specified temperature 2) a galvanic deposited layer of copper on aluminum. Both types were kept at a specified temperature (range 225-500°C) for a certain time. The layer growth of the five possible intermetallic compounds was determined as a function of time and temperature (up to 500°C) and compared with data published by Funamizu [7]. A large difference in temperature dependence of the layer growth rate constants of the various Cu-Al intermetallic compounds was found. The data of Funamuzi and our measured layer growth rate constants are extrapolated to the temperature regime where standard JEDEC high temperature storage tests (HTS, 150-225°C) are performed. The layer growth of Θ (CuAl₂), η (CuAl) and γ (Cu₉Al₄), as observed in Cu ball bonds during various HTS tests, was compared to the measurements/extrapolations of the layer growth in the aforementioned diffusion couple experiments. The growth of all the five intermetallic compounds CuAl ₂ (Θ), CuAl (η), Cu₄Al₃ (ξ), Cu₃Al₂(δ) and Cu₉Al₄ (γ) layers at high and low temperatures proved to be consistent with Arrhenius equations. Explicit equation for the layer thickness of each intermetallic compound as a function of time and temperature (range from room temperature up to 500°C) are given. In addition the measured hardness's and indentation Young's moduli of the intermetallic compounds grown in the diffusion couples are presented. Consequences of the thermo-mechanical properties of the intermetallic compounds are crucial for the prediction of the long term mechanical behavior of Cu-Al ball bonds.