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Electromigration Performance of Flip-Chips with Lead-Free Solder Bumps between 30 m and 60 m Diameter
Keywords: Electromigration, Flip-Chip, Long-Term-Reliability
In previous investigations we developed technologies for cost-efficient solder bumping and automated assembly in an industrial environment. Wafer level solder sphere transfer (WLSST) and solder sphere jetting (SB) have been used to provide flip-chips with solder bump diameters down to 30 m for interconnection on organic substrate materials as well as on thin film ceramic. As the solder bump sizes continuously decrease with scaling of the geometries, current densities within individual solder bumps will increase along with higher operation temperatures of the dies. Since electromigration of flip-chip interconnects is highly affected by these factors and therefore an increasing reliability concern, long-term characterization of new interconnect developments needs to be done regarding the electromigration performance using accelerated life tests. Furthermore, a large temperature gradient exists across the solder interconnects, leading to thermomigration. In this study, a comprehensive overlook of the long-term reliability and analysis of the achieved electromigration and thermomigration performance of flip-chip test specimen will be given, supplemented by an in-depth material science analysis. In addition, the challenges to a better understanding of electromigration and thermomigration in ultra fine-pitch flip-chip solder joints are discussed. For all experiments, the specially designed flip-chips with a pitch of 100 m and solder bump sizes of 60 m, 50 m, 40 m, or 30 m, respectively, in diameter have been used. The solder spheres can be made of every lead-free SnAgCu alloy (in our case SAC305) and are placed on a UBM which has been realized for our test chips 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 over a time period up to 10,000 hours with separate daisy chain connections of each test coupon provide exact lifetime data during the electromigration tests. As failure modes have been identified: UBM consumption at the chip side or consumption of the Nickel layer at the substrate side, interfacial void formation at the cathode contact interface, and - to a lesser degree - Kirkendall void formation at the anode side. A comparison between calculated life time data using Weibull distribution and lognormal distribution will be given. Thermomigration in microelectronic solder joints was not a concern until significant miniaturization of electronic devices required to run high current densities with smaller solder joint sizes. A high current density induces electromigration and Joule heating at the same time. The imbalance of Joule heating generated at top and bottom of solder joint causes a temperature gradient which is large enough to induce thermomigration damage. By ensuring the solder operating temperature is well kept below a threshold value by proper thermal management, the life time of the solder joints can be extended.
Rainer Dohle, Dr.-Ing. Elektrotechnik
Micro Systems Engineering GmbH
Berg, Bavaria

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