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. Introduction Flip chip technology has seen increasing demand due to its many advantages including miniaturization and high electrical performance. As a result, a number of methods and various hybrid techniques have been explored in order to achieve high volume production of flip-chip devices at low cost. Presently, solder based interconnection of die to substrates offers a cost effective solution which fits more closely within the current PCB assembly infra-structure. This process involves the deposition of solder material onto the bondpads of the die to form a bump which is subsequently used to form an interconnection with a PCB or other such substrate. A process that has received considerable interest as a low cost wafer bumping technique is electroless nickel plating, followed by solder paste printing. The electroless Ni plating process has been investigated by several groups [l-81 and forms an under bump metallisation layer on the A1 bondpad by means of the deposition of a thin (typically 5-3Opm thick) film of NIP alloy. The electroless nickel deposition technique is a maskless process for which the existing wafer passivation acts as a mask, allowing the deposition of electroless nickel only onto the A1 bondpads. However, the AI surface is not directly active to electroless nickel deposition and must be activated through a number of pre-treatment steps including typically a zincate treatment. This paper reports an investigation of the formation of this type of electroless Ni under bump on the bondpads of wafers including die with sub 100pm pitch bondpads. The effect of the various pre-treatment etching processes and zincate activation on the quality of the final electroless nickel bump were studied by using SEM examination to aid a detailed understanding of the activation mechanisms and to determine their effects on the electroless Ni bump morphology. Bump shear tests were performed to determine the best pre-treatment regime to ensure good adhesion of the electroless nickel to the bondpad and electrical resistance measurements of bumped die were used to ensure that the pre-treatment procedures produce a low resistance interface between the AI and electroless Ni. Finally, stencil printing was used to produce solder bumps on the UBM thereby forming solder balls which can be readily reflowed onto the PCB during the assembly process. Experimental Procedure Wafers used for bumping trials Two types of wafers were used in this work. Wafer type A was manufactured by a commercial organisation and consisted of 6 x 6mm die with daisy chain test structures. The bondpads, which were AI-Cu (1%) alloy, were arranged around the periphery of the die at pitches of 225pm and 300pm. The pads were 3pm in thickness, octagonal in shape with a width across the flats of 90 pm. The opening in the passivation over the pads had a