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

For solder based flip-chip assembly, the deposition of an under bump metallisation (UBM) layer onto the surface of the AI bondpads of the die is the first step in the wafer bumping process. The UBM is necessary, as the fragile AI pad has a tenacious oxide layer that cannot be soldered without the use of strong fluxes and a barrier layer is required to prevent dissolution of the bondpad into the solder during reflow. The requirements of the UBM are therefore to provide a solder wettable surface and to protect the underlying A1 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 the pad and the UBM. This paper reports an investigation of the electroless nickel deposition process for the under bump metallisation of wafers that are subsequently to be bumped using solder paste printing. In particular this work has extended the process from previous trials on 225pm pitch devices to wafers including die with sub lOOpm pitch bondpads. As part of this work, the effect of the various pre-treatment etching processes and zincate activation on the quality of the final electroless nickel bump has been investigated. The use of SEM examination of samples at each stage of the bumping process has been used to aid a detailed understanding of the activation mechanisms and to determine their effects on the electroless nickel bump morphology. In addition shear testing of bumps has been used to determine the best pre-treatment regime to ensure good adhesion of the electroless nickel to the bondpad. Finally, electrical resistance measurements of bumped die have been used to confirm that the pre-treatment procedures are producing a low resistance interface between the AI and electroless nickel.