The interface behavior of the Cu-Al bond system in high humidity conditions

Abstract

Copper (Cu) wire has been highlighted as a low cost material for use in thermo sonic ball bonding for various package (PKG) groups. This wire has already been applied in high-end PKGs such as BGA, QFP and QFN in mass production, and the volume is feasible. Not only the advantage of a low cost material, but Cu wire has other merits which include higher electrical conductivity and a much slower intermetallic compound (IMC) growth rate than Gold (Au) wire. However, although slow IMC growth between Cu wire and Aluminum (Al) pad helps to inhibit kirkendall void formation which is at the root of conventional reliability failure mechanisms in high temperature conditions, it is not effective in high humidity conditions. In a state of insufficient IMC growth due to Al corrosion, the high humidity reliability of Cu wire on an Al pad is shown to reduce performance below that of Au wire. This issue is more critical in fine wire size or substrate PKG than lead frame one. It is also a barrier that is delaying the universal application of Cu wire in high-end PKGs to the same degree as the use of Au wire. Therefore, the scope of this study is the identification of the mechanism to determine the reliability of the application of Cu wire in high humidity conditions and the creation of a solution guide to overcome this issue. To clear the reason of lower reliability performance of Cu wire from crack generation mechanism, galvanic corrosion and IMC study between Cu wire and Al pad are considered. Reliability evaluation is performed on BGA PKG with Al pad and 4N purity wire is used as the bonding wire. After wire bonding, it is tested at uHAST (unbiased Highly-Accelerated temperature and humidity Stress Test) conditions 130 degrees Celsius (C) / 85% relative humidity. Then, IMC growth behavior and the crack generation mechanism are examined by ion polisher, FE-SEM, TEM and EDS analysis. Furthermore, Palladium coated Cu wire (PCC wire) which is also of interest in the current market, is also tested under the same reliability conditions. The reliability performance of PCC wire is better than that of bare Cu wire due to different metal interactions at the bonding interface. That will be a new alternative material to supplement insufficiency parts of bare Cu wire to the Cu wire market increment.