Alternative solution for non stick on pad on gold coated palladium bond pad

Au coated Pd bond pad was developed for better reliability has provided advantage which helps to cater for harder copper wire bonding process (as compared to Au wire bonding) that reduces or eliminates silicon cratering. However, this bond pad design introduces another challenge in copper wire bonding: the NSOP. According to Wu-Hu Li et.al, one of the probable root causes is the inter diffusion of Au and Pd layer causing oxidation of the Pd surface. With the formation of PdO on the bond pad surface, good copper wire bonding bond ability was hindered [1]. Similar NSOP was reported in this study. AES analysis on the NSOP pads found foreign element of Sn was detected despite the known Pd and Au. Additionally, O element signal is observed on top of Pd in depth profiling results indicating the influence of O towards NSOP pads. Tracking back to the processes, the source of Sn was from die attach material and this subsequently raised the next concern of flux residue from solder paste that lead to NSOP. XPS was recommended for the pad analysis in this case to characterize Sn, O elements (O from flux or metal oxide) and flux residue. Based on XPS data interpretation, SnO seems to give the responsive behavior to the NSOP pads. In view the effort of metal oxide reduction or elimination is needed, Ar/H2 plasma was chosen as H2 was able to form radical during plasma cleaning process and react chemically with metal oxide. Removal of metal oxide is more effective and at higher rate using Ar/H2 compared to pure ion bombardment effect from argon gas. DOE was conducted using Ar and Ar/H2 plasma cleaning, significant reduction of NSOP rate was observed in the DoE using Ar/H2 compared to Ar and Ar/H2 definitely is needed to achieve high wire bonding yield. Additionally, in order to further understand the surface condition of the pads, TEM/EDX analysis was also carried out. SnPd spikes were observed. Understanding of the correlation between SnPd spikes and NSOP is carried out. FESEM analysis result shows no correlation of NSOP with SnPd spikes as both NSOP and non NSOP pad also having equal amount of SnPd spike.