Effect of the Au bonding pad contamination on the wettability of Au/Sn-3.0Ag-0.5Cu/Au solder joints in flux-free laser jet solder ball bonding process

With the advantages of non-contact, flexible, high energy density and local heating, the laser jet solder ball bonding (SBB) technique is widely used in electronic packaging. The reliability of laser jet SBB solder joints has long been concerned. As is well known, the wettability (i.e., the spreading ability) of the molten solder on the boding pad is a key factor influencing the formation and performance of solder joints. Previous studies have revealed that the oxidation of the solder balls can lead to the poor wetting during the laser jet SBB process. Meanwhile, the surface contamination of the bonding pads is inevitable in mass production, which may be another essential factor causing the soldering quality problem for the flux-free laser jet SBB process and the mechanism is still unclear so far. In this study, the factors leading to the poor wetting of 90 μm diameter Sn-3.0Ag-0.5Cu solder balls on Au bonding pads have been ascertained, in particular the surface contamination of Au bonding pads is investigated by using a surface analytical technique called Time-of-flight Secondary Ion Mass Spectrometry (TOF-SIMS). When the possibility of the oxidation layer of solder balls is excluded, the poor wetting of Au/Sn-3.0Ag-0.5Cu/Au solder joints is caused dominantly by the surface contamination of Au bonding pads. So, the surface elements of the poorly wetted bonding pads and the normal ones were comparatively investigated by employing TOF-SIMS with the intensity of the contamination. The results manifest obviously that the surface contamination of Au bonding pads is closely correlated with the poor wettability. The intensity of Au peak on the normal pad is higher, while being lower on the poorly wetted pads. In addition, the characteristic peaks of few organics are also found, indicating that the Au bonding pads are contaminated and evenly covered by some organic and inorganic contaminants, which directly lead to the poor wettability of solder joints during flux-free laser jet SBB process. Further, the mechanism of the poorly wetted solder joints induced by the contamination is analyzed and the near quantitative results are also discussed.