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Thermo-Electromigration Induced Failure in WL-CSP Pb-Free Solder Joints
Keywords: Wafer level chip scale package, thermo-electromigration, pancake void
Owing to the ban of Pb-based solder in consumer electronic products and the trend of miniaturization in wireless and portable devices, the reliability of Pb-free solder interconnection is one of the most challenging problems in electronic manufacturing industry, especially the failure caused by thermo-electromigration. Joule heating due to the on-chip Al interconnects has generated thermomigration to accompany electromigration in solder joints. For a joint of 200 micron in size, a temperature difference of 20C across it will cause a temperature gradient of 1,000 C/cm, which can lead to thermomigration. We define the combined effect as thermo-electromigration. The Weibull failure distribution and mean-time-to-failure (MTTF) of thermo-electromigration in BGA solder ball (Sn1.2%Ag0.5%Cu, or Sn0.65%Cu-500ppmNiGe) with three different UBM structure (Ti/Cu/Cu, Al/Ni(V)/Cu, e-less NiAu) in wafer-level chip scale package is reported in the current study. The failure mode of the samples were characterized by scanning electron microscope and focus ion beam channeling imaging. The size of the silicon die is 3x3mm with 36 solder joints. The diameter of the solder ball is 250 um, the contact opening at the chip side is 200 micron. The thickness of the copper in Ti/Cu/Cu UBM is 7.5 micron and Al/Ni(V)/Cu UBM is 0.85 micron. There are temperature sensors built in each silicon die and the PCB substrate. These sensors are distributed a few micron away from the powered solder bumps, by which the temperature on chip side and board side could be determined. The thermo-electromigration reliability were tested in two ambient temperatures (125 C and 150 C) and two current densities (5000 and 10000 A/cm2). For each condition, 16 chips were tested. For example, for the case of Al/Ni(V)/Cu with SAC1205, Weibull failure distribution analysis shows that at 104 A/cm2 and 150 C, the MTTF is 53.2 hrs, characteristic life of 61.7 hrs. For 104 A/cm2 and 125 C, the MTTF is 288.5 hrs, characteristic life of 364.2 hrs. The failure mode were found to be pancake void formation along the interface at the chip side between solder and the Ni(V) interface. All the interfacial IMCs are dissolved first before the void growth at the interface. The remaining IMC thickness depends on the location as the current re-distribution depends on the location. Temperature gradient at 104A/cm2 is 650 C/cm across the whole solder joint, which is enough to induce thermomigration. In the neighboring solder bumps without current, intermetallic compound were found to migrate to the board side and Sn move against thermal gradient (from cold to hot end). When all the copper at the chip side are consumed and migrate to board side, voids start to form and induce failure.
Luhua Xu, Postdoc Research Fellow
University of California - Los Angeles
Los Angeles, CA

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