The intermetallic compound (IMC) growth and phase identification of different kinds of copper wire and Al pad thickness

Recently, Cu wire is the most economic and major material on package wire bonding. The key factor of affecting aging test performance is IMC, which Cu wire reacts with Al pad. Because of various and advanced device development, the IMC of different Cu wire type and Al pad thickness are both important topics.Wire types of this paper include the ASE HVM4N copper (Cu_4N) wire, palladium platedcopper (Cu_Pd) wire and goldpalladiumplated copper (Cu_Pd_Au) wires. For comparison with the most popular 0.8 mil, wire diameter is fixed on 0.7 mil. There are three level aluminum pad thickness of 0.8~1.5 (normal) / 1.6~2.8 / 2.9~4.0 micrometer. The aging tests include HTST, TCT and PCT. In order to accelerating the reaction, two HTST aging temperatures, 175°C and 205°C are applied and the aging periods are from 250 h to 2000 h. The microstructures of micro joints are cross-sectioned and examined under scanning electron microprobe (SEM), wavelength dispersive spectroscopy (WDS) and electron probe X-ray microanalyzer (EPMA) to verify the formed intermetallic phases. The Al-rich phase (CuAl2 and CuAl) has high priority at beginning of aging test. After long period aging, the final phase and Cu-rich phase (Cu9Al4) is expected to form. The intermetallic formation sequences of different wire types and pad thickness are shown for a comparison.