Cr/Cu/Ni underbump metallization study

In flip chip interconnection using eutectic Pb/Sn solder bumps, a highly reliable underbump metallization (UBM) is required to maintain adhesion and solder wettability. An experimental study investigated the thermal stability of the Cr/Cu/Ni UBM-where Cr act as an adhesive, Cu a solder wettable layer and Ni a barrier. The process window for good thermal stability will reduce silicon cratering failure and intermetallic failure to ensure reliability. The Cu and Ni layers were varied in low, medium and high thickness to study their impact on solder bump strength and failure mechanisms. 5×3 mm full array test chips (with Cr/Cu/Ni UBM) were subjected to thermal stability tests (1) multiple reflow for 1x, 5x, 10x, 20x and (2) high temperature storage at 150°C up to 1000 hrs. Destructive ball shear test and cross-sectional analysis was done. Bump shear results show that the Cr/Cu/Ni UBM, with Ni thickness (low to high) remains stable with respect to the number of reflow cycles. The failures were cohesive (Mode I-within solder). A high Ni thickness inhibited Cu diffusion and suppressed Cu IMC formation at near solder interface. Under high temperature storage, intermetallic growth was accelerated and the excessive intermetallic formed was very brittle. For low Ni thickness, failure mode (Mode I+ Mode II) was observed after aging (>500 hrs). Failure mode remained as cohesive in high Ni thickness UBM. For low to high thickness Cu mini-bumps, shear strength was maintained during multiple reflows and the shearing fracture remains within the solder. Failure mode shifted from Mode I (at t=0) to Mode III interfacial failure (after aging) in Cr/low thickness Cu/medium thickness Ni UBM, when the limited Cu supply led to solder dewetting