Under bump metallisation of fine pitch flip-chip using electroless nickel deposition

For solder based flip-chip assembly, under-bump metallisation (UBM) layer deposition on the Al die bondpads is the first step in the wafer bumping process. The UBM is necessary, as the fragile Al pad has a tough oxide layer that cannot be soldered without the use of strong flux and a barrier layer is required to prevent dissolution of the bondpad into the solder during reflow. The UBM requirements are therefore to provide a solder wettable surface and to protect the underlying Al bondpad during and after assembly. In addition, the UBM deposition process itself must remove any oxide layers on the bondpads to ensure a low resistance interface between pad and UBM. This paper reports a study of the electroless nickel deposition process for the UBM of wafers that are subsequently to be bumped using solder paste printing. This work has extended the process from previous trials on 225 μm pitch devices to wafers including die with sub-100 μm pitch bondpads. The effect of the various pre-treatment etch processes and zincate activation on the quality of the final electroless Ni bump has been investigated. SEM examination of samples at each stage of the bumping process has been used to aid detailed understanding of the activation mechanisms and to determine their effects on the electroless Ni bump morphology. In addition, bump shear testing has been used to determine the best pre-treatment regime to ensure good adhesion of the electroless Ni to the bondpad. Finally, bumped die electrical resistance measurements have been used to confirm that the pre-treatment procedures are producing a low resistance interface between the Al and electroless Ni