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Electromigration Performance of Fine-pitch µPILR™ Interconnects in Flip Chip
Keywords: Electromigration, Flip Chip, Pb-free
High performance devices must sustain large currents at elevated temperatures for extended periods of time. High current density and operating temperatures have a detrimental impact on electromigration performance of a semiconductor package because they accelerate the metal diffusion and the dissolution of the Under Bump Metallization (UBM) into the solder bump causing a failure. In this work, we investigate the electromigration performance of Pb-free µPILR™ interconnects within fine-pitch flip-chip packages. For package level testing, we used a test vehicle with more than 10,000 interconnects on 18x20x0.75mm die packaged on a 40x40x1.19mm substrate. The chip has 0.090mm diameter Sn/2.5Ag solder bumps with a minimum pitch of 0.150mm and maximum pitch of 0.200mm. The packages were tested under different current densities including 37.7kA/cm2 and 45.2kA/cm2 at 125°C and 150°C. The failure analysis reveals 100% failures due to depletion of the Cu UBM along with excessive voiding on the die-side with electrons moving from die to substrate. Voids initiated at the Cu6Sn5-solder interface with large void formation near Cu3Sn-Cu6Sn5 interface. No substrate-side damage was observed in this multi-pair daisy chain structure irrespective of the current direction. A mean time to failure of >4500hrs measured for packages tested at a current density of 45.2kA/cm2 at 125°C. The enhanced electromigration performance is due to the Inter-Metallic Compound formation around the Cu pin on the substrate coupled with the reduction in the solder joint resistance from the unique shape of the µPILR interconnects. In conclusion, this work will discuss a fine-pitch µPILR™ interconnect technology that offers superior electromigration performance and a significant lifetime improvement with delayed electromigration induced failures. 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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