High-temperature Reliability of Gold Wire Bonding According to Aluminum Pad Thickness and Palladium Content

The bonding characteristics and high-temperature reliability of gold bonding wires were investigated with respect to the thickness of the aluminum pads and the addition of palladium. Samples of high-purity gold (99.99%, 4N) and gold with 1% palladium added (99% purity, 2N) were processed into 15.3 μm diameter ultra-fine wires through wire drawing and bonded to aluminum pads with thicknesses of 1 μm and 5 μm using ball bonding equipment. The reliability at elevated temperatures was evaluated by annealing at 175oC for up to 1,000 hours. Micro-structural and intermetallic compound (IMC) phase changes at the vertical cross-sections of bonded areas were characterized using SEM-EDS at specific time intervals. For 1 μm aluminum pads, both 4N and 2N gold wires initially exhibited identical bonding ball morphologies. Voids began to form at the lateral sections after 250 hours, accompanied by the formation of Au8Al3 and Au4Al phases. Despite these observations, stable bonding was maintained up to 1,000 hours, with the palladium addition proving advantageous in terms of bonding strength. On the other hand, for 5 μm aluminum pads, IMCs in the 4N wire were stabilized as Au8Al3, maintaining the bond interface for up to 1,000 hours. However, for the 2N wire, significant voids were observed beneath the bond interface as early as 250 hours. These voids were attributed to the incorporation of palladium, which caused the growth of AuAl2 into Au2Al, leading to rapid shrinkage due to phase transformation occurring at a very early stage. Therefore, the addition of palladium is less favorable when using 5 μm thick aluminum pads, as it induces different inter-diffusion phenomena compared to 1 μm pads, resulting in void formation.