Non-continuous IMC in copper wirebonding: Key factor affecting the reliability

In today´s trend in semiconductor wirebonding assembly process, Copper (Cu) wire was presented as viable solution and alternative cost saving options to gold (Au) wire. The use of Cu wire found to be even better than Au, because of its high thermal conductivity, great electrical property and importantly low cost value. However, Cu have some challenges that needs to carefully understand, it natural nature such as its inherent material hardness, easily to get oxide, the growth of Cu/Al intermetallic compound (IMC) and others. In this paper it will discuss the process experimentations on how those challenges are addressed and studied primarily on understanding the formation of intermetallic compound, its relationship to the bondability response. An accelerated stress test prior to the formal qualification stress has been assessed as way to determine how good the bonding process is by evaluating a different wirebonding parameter combination. The findings of the study suggest that IMC formation can be improved by applying an initial USG (ultrasonic generator) Power also known as USG “pre-bleed” parameter as compared to the conventional wirebonding parameter. Accelerated stressing on un-molded units through Autoclave test was use to assess and mitigate the potential reliability risk. The accelerated stressing found out more presence of ball lift failure break mode which corresponds to a low wirepull reading on conditions with a bulk of non-IMC on effective bonding area. These theorized to translate a potential failure mode in the real electrical testing environment.