Reliability Study of Lead-Free Flip-Chips with Solder Bumps Down to 30 μm Diameter

The ongoing trend to miniaturized electronics has induced many developments towards size reduction and increasing performance in electronic products. To meet these requirements the involved processes, materials and components in electronics production have to be enhanced for high performance and high reliability. Flip-chip technology is one technology of choice with potential for highest integration. In previous investigations technologies for cost-efficient solder bumping and automated assembly in an industrial environment were evaluated. Wafer level solder sphere transfer and solder sphere jetting were adapted to provide flip-chips with solder bump diameters down to 30 mu m for flip-chip assembly onto printed circuit boards as well as onto thin film ceramic substrates. The reliability tests done so far showed excellent achievable reliability performance of these ultra-fine-pitch assemblies under various test conditions. Since electromigration of flip-chip interconnects is a very important reliability concern, characterization of new interconnect developments needs to be done regarding the electromigration performance in accelerated life tests. For all experiments, specially designed flip-chips with 10 mm by 10 mm by 0.8 mm in size have been used. The silicon die layout provides a pitch of 100 mu m with solder bump sizes between 60 mu m and 30 mu m in diameter. The solder spheres consist of lead-free SnAgCu alloy and are placed on a Ni-P under bump metallization which has been realized in an electroless nickel process. For the electromigration tests within this study, multiple combinations of individual current densities and temperatures were adapted to the respective solder sphere diameters. Online measurements with separate daisy chain connections for each test coupon provide exact lifetime data during the electromigration tests, which are in some cases still in progress. Cross sectioning has been employed for the analysis of thermal diffusion as well as of the impact of electromigration influence on the failure mechanism using optical, SEM and EDX analysis, respectively. Reliability plots will be discussed regarding the electromigration performance for different test conditions applied to the respective test specimen for lifetime estimations.