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Electromigration Performance of Pb-Free µPILR Fine Flip-Chip Packages
Keywords: Electromigration, Flip Chip, UBM Pb-fre
The µPILRTM interconnect is a copper pillar structure manufactured as a part of a substrate interconnect pad. In this work, we discuss the details of our test procedure and the electromigration performance of Pb-free µPILR interconnects in a multi-pair daisy chain structure within a 150µm pitch flip-chip package. Four test conditions with two different current densities including 37.7kA/cm2 and 45.2kA/cm2 were investigated at 125°C and 150°C. A specially designed test vehicle with 18mmx20mmx0.75mm die and more than 10000 interconnects packaged on a 40mmx40mmx1.19mm substrate was used. Each test condition utilized 16 packages that were pre-calibrated to obtain the individual Temperature Coefficient of Resistance curves and Joule heating profiles according to the JEDEC-JEP154 standard protocol. The failure analysis reveals bump failure due to depletion of the Cu Under-Bump Metallization (UBM) on the die along with excessive voiding. Voids initiate at the Cu6Sn5-solder interface on the die side, with excessive void formation near Cu3Sn-Cu6Sn5 interface. 100% failures occurred on the die-side with electrons moving from die to substrate. No substrate-side damage was observed in this multi-pair daisy chain structure irrespective of the current direction. Electromigration performance of the µPILR interconnects display a significant lifetime improvement with delayed electromigration induced failures. A mean time to failure > 4500hrs was found for packages tested at a current density of 45.2kA/cm2 at 125°C. The µPILR pin structure forms a large Inter-Metallic Compounds (IMC) around the Cu pin on the substrate. The superior electromigration performance is due to the IMC formation coupled with the reduction in the solder joint resistance from the unique shape of the µPILR interconnects. The enhanced electromigration performance of the µPILR interconnect combined with other reliability benefits make it an excellent alternative to conventional solder joints with thin film stack UBMs, thicker copper UBM or tall copper post on die.
Rajesh Katkar, Sr. Product Engineer
Tessera, Inc.
San Jose, CA

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