Effect of Al pad surface morphology on the flip-chip solder bump reliability

The effect of aluminum pad surface morphology on flip-chip solder bump reliability is reported in this paper. The influence of the Al surface morphology to the electroless zinc/nickel/gold UBM surface morphology is presented. The reliability of the solder bump as measured by ball shear force is reported. Al pad samples were produced by two different RF sputtering systems: CVC-601 and Varian-3180. The Al targets used in CVC and Varian system were 99%Al-1%Si and 98.95%Al-1%Si-0.05%Ti respectively. The surfaces of the CVC Al samples were smooth while the Varian Al samples were rough. All the samples undergo electroless zinc/nickel/gold plating. The results suggest that after plating, the smooth Al surface resulted in a fine UBM surface while the rough Al surface gave rise to a coarse UBM surface. Ball shear test was conducted after solder balls were bumped on the UBM. Result shows that the fine UBM surface samples have twice the shear force as compared to the coarse UBM surface samples. The analysis of the results indicate that some ball shear occurred at the UBM and the solder interface for samples with rough UBM surface leading to the lower shear force. In order to obtain a high reliability solder joint, it is essential to maintain a good Al pad surface morphology