Moisture induced corrosion in gold and copper ball bonds

High gold prices have led to renewed interest in replacing gold with copper in existing packages and new packages in order to save costs. Although reliability is often cited as a reason for using copper, the main driving force for its use is undoubtedly cost. Perceptions that copper wire is more reliable are based on the notion that the intermetallics grow more slowly and that thinner intermetallics are more reliable and yet old data tend to support the idea that copper is as reliable as gold. More recently however, copper ball bonds on aluminium metallization have been found to fail more than gold during temperature cycling (TMCL) and pressure cooker testing (PCT) [1–3]. The key feature of these tests is the presence of moisture than appears to accelerate corrosion. A proposed solution to this problem is Pd-coated Cu wire, which looks to be a promising but relatively untested solution at present [1–4]. While Pd-coated copper wire may improve reliability, cost-savings will be less compared with bare copper wire. It is important to note however that a recent SEMI survey shows that industry is generally not as confident in copper as wire equipment and materials suppliers [5]. Localized corrosion of aluminium bond pads is well known in microelectronics packaging and in Al-Cu and Al-Cu-Si bond pads, CuAl2 acts as a cathode and aluminium corrodes in the presence of water (electrolyte) [6]. The situation is complicated by the presence of chlorine and other ionic contaminants in addition to moisture. In copper ball bonds, intermetallic coverage in as-bonded balls is very difficult to see but it appears that CuAl2 and Cu9Al4 are the compounds that form initially [7]. Intermetallic growth of Cu-Al compounds is slow compared to Au-Al compounds, which means that in finished packages, a large amount of Al remains whereas in gold ball bonds, with thin 1µm bond pads, aluminium can easily be consumed after encapsulation and moulding and surface mounting. It is plausible that during extended periods of exposure to moisture, stress and ionic contaminants, Cu ball bonds may be more susceptible to localized corrosion than Au ball bonds because slow intermetallic growth permits aluminium corrosion. This paper discusses ball bond corrosion and suggests that for high reliability applications or applications in moist environments, it may be necessary to accelerate the growth of Cu-Al intermetallics to mitigate potential corrosion and failure.