Cu-base leadframes are very susceptible to thermal oxidation when exposed to elevated temperature in the IC packaging process. When the leadframe surface is covered with copper oxide film, poor adhesion of Cu L/F to epoxy molding compound (EMC) were generally reported, resulting in delamination of the Cu/EMC interface. Despite its importance, there have been few systematic studies on the correlationship between the copper oxidation and the Cu/EMC interface adhesion. Therefore the growth kinetics and composition of oxide films which are formed on a commercial leadframe alloy at low temperature has been investigated in this study. Furthermore, pull strength test and failure analysis were conducted so as to understand the role of copper oxide on the adhesion between Cu-base leadframe and EMC.
X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffraction (XRD) studies confirmed that the oxide film consisted of $Cu/Cu_2O(NiO)/CuO(NiO)$/ air in the low temperature oxidaion. $Cu_2O$ was the major oxide and the thickness ratio of $CuO/Cu_2O$ was approximately 0.28 at 150℃ and 0.34 at 200℃.
The kinetic studies on the oxidation showed that oxide growth followed the parabolic rate law in the temperature range of 150℃ to 300℃ and the activation energy for the overall copper oxidation was 17.0 kcal/mol. The activation energies for the cuprous oxide($Cu_2O$) and cupric oxide(CuO)formation were 15.1 kcal/mol and 21.3 kcal/mol respectively. It is estimated from the activation energy of $CuO$ and $Cu_2O$ that the diffusion rate of cuprous ion($Cu^+$) through $Cu_2O$ is faster than that through $CuO$. The difference in diffusion rate of $Cu^+$ occurs possibly because $Cu_2O$ has a higher symmetry and moe open structure than $CuO$ oxide has. The formation of $CuO$ on top of the $Cu_2O$ is supposed to retard the overall oxidation rate.
Computer aided analysis of electron diffraction patterns showed that the epitaxial relationship between copper substrate and $Cu_2O$ oxide is $\{011\}_{Cu}$ // $(011)_{ox}$, $<022>_{Cu}$ // $[022]_{ox}$.
At the early stage of oxidation, pull strength increases rapidly with increasing oxidation time and reaches a maximum point, then decreases with further oxidation. The pull strength is closely related with oxide thickness rather than the oxidation temperature because the kinds of oxides formed and their layer structure are identical within the temperature range used in this work. The optimum copper oxide thickness required for the maximum pull strength ranged from 20 nm to 30 nm. The leadframe with the oxide thickness greater than about 60 nm showed inferior adhesion strength to the fresh leadframe which had received no oxidation treatment.
The adhesion improvement at the initial stage of oxidation was presumably associated with the increase of surface wettability and mechanical interlocking effects resulting from oxidation. Oxidation of Cu-base leadframe gave rise to the changes both in surface chemistry and surface topography which may be beneficial to the interface adhesion. Scanning Electron Microscope (SEM) images taken on the oxidized leadframe showed that the surface was getting rougher as oxidation time increased. Surface roughness could contribute to the adhesion by increasing the bonding area and mechanical interlocking effect. The contact angle showed marked decrease at the early stage of oxidation, which means the surface wettability improvement to epoxy molding compound.
The Auger Electron Spectroscopy(AES) and XPS studies on the fractured surface of Cu L/F and EMC indicated that the adhesion failure took place partly at the $Cu_2O/CuO$ interface and partly by fracture of the EMC itself when the oxide film on the leadframe was thin. On the other hand, the sample with low pull strength due to excessive oxidation showed the adhesive failure at the interface between $Cu$ metal and $Cu_2O$ interface.
