Essential factors influencing the wettability of Sn-3.0Ag-0.5Cu solder balls on Au pad of the right-angle solder interconnect in laser jet solder ball bonding

As an untouched soldering technology, the laser jet solder ball bonding (SBB) provides a flexible packaging method for three-dimensional (3D) packaging and assembly, such as the right-angle interconnect in the hard disk drive (HDD) and temperature-sensitive devices. The microstructure and reliability of SBB interconnects have been widely studied. However, the problem of poor wetting in the SBB process, induced by the surface contamination on bonding pads and the oxidation of solder balls, is a tough issue and the mechanism is still unclear. In this study, the essential factors leading to poor wetting of Sn-3.0Ag-0.5Cu (SAC305) solder balls with a diameter of 70 μm on Au pads have been clarified, in particular the correlation between the depth of the oxidation layer and poor wettability of SAC305 solder balls is investigated by quantitative analysis using scanning electron microscopy (SEM) and auger electron spectroscopy (AES). Examination results of surface morphologies of solder balls by SEM show clearly that the solder ball does not spread completely on both sides of Au pad, indicating a poor wetting caused mainly by the solder balls. Further, the surfaces of the solder balls with poor wettability and the normal ones were comparatively investigated by using AES with Ga ion sputter with different sputtering times of removing a half of the maximum oxidation layer of solder balls. The results manifest that the sputtering time corresponding to the solder balls with poor wettability is 5.6 minutes, much longer than 1.6 min used for the balls with good wettability. In order to confirm the results, the sputtering times of 12 solder balls from the batch with poor wettability as confirmed and 11 solder balls from the batch with good wettability as confirmed were further characterized, and their corresponding yields of 500 solder interconnects in mass production were counted. The statistical results indicate that 2.2 minutes can be determined to be a criterion for the poor wetting. According to the testing result of the standard samples, the sputtering depth is 0.9 nm per minute for the Ga ion sputter. Finally, it has been estimated that the critical depth of the oxidation layer is about 4 nm in 70 μm diameter SAC305 solder balls during laser jet flux-free SBB process. When the oxidation layer is more than 4 nm, the poor wetting occurs.