Investigation on the effect of copper leadframe oxidation on package delamination

The presence of a copper oxide layer on the leadframe of plastic IC packages was found to cause delamination at the diepad/mold compound interface. The failure mechanism seems to be the presence of voids at the oxide/metal interface, which increased as the degree of oxidation was increased. Elevated temperature processes used in assembly such as die attach curing and wire bonding were found to be the primary causes of oxidation. However, an important observation was made in that the duration of post mold curing was also found to have a great impact on the interfacial integrity of the oxidized interface within an encapsulated package. Depending on the degree of oxidation and the duration of post mold curing, the susceptibility of the package delamination during solder reflow was found to shift from die pad/compound interface to die attach region. This paper also describes a novel method of characterising oxidation using SIMS (Secondary Ion Mass Spectroscopy) depth profiling. SEM (Scanning Electron Microscopy) and XPS (X-ray Photoelectron Spectroscopy) was used to characterise oxide morphology and oxide composition change on the samples. In parallel, the adhesion strength of the oxidized copper leadframe to a mold compound was characterised using an adhesion test. These data were correlated to the degree of package delamination on a QFP package using C-mode Scanning Acoustic Microscopy. This work shows that it is important to control the oxide thickness on the copper leadframe die pad for improved package integrity, especially when larger die pad sizes are considered.