Behaviour of platinum as UBM in flip chip solder joints

The use of flip chip technology requires necessarily the implementation of bumps either on the chip pads or on the corresponding pads on the substrate side or both. These bumps provide the electrical as well as the mechanical interconnection. Applying solder bumps for flip chip interconnection it is necessary to have a solderable under bump metallization (UBM) which at the same time provides a diffusion barrier. This investigation is focused on the behaviour of Platinum as UBM or part of it and points out the advantages and disadvantages of this metal in combination with a flip chip solder bump on top of it. Platinum was thereby either evaporated and covered with a goldflash or directly mechanically bumped on the Aluminum pad metallization. The evaporation is based on wafer level processes and can therefore be used for high volume production. Because of the very flexible procedure the mechanical stud bumping is suitable for single chips, substrates, and wafers. The Platinum stud bump was leveled after bumping to have a smooth UBM surface for solder application. For solder bumping in both cases mechanical bumping of solder balls or studs with PbSn and PbSnAg was used. After high temperature storage at 100, 125, 130, and 150°C the phase formation of Pt-Sn intermetallics was investigated. The adhesion of the solder bumps was evaluated by sheartesting with respect to the detected shear mode. To characterize the solder interconnections cross-sectioning was performed and the growing of Pt-Sn phases was analyzed by SEM and EDX. Especially for the evaporated UBM the phase formation is of great interest because of the consumption of the Pt layer, which directly influences the reliability of the flip chip interconnection