Process control of high density solder bumps by electroplating technology

In this paper, we describe the process control of high-density solder bumps by electroplating with high uniformity and repeatability. The die size is 10×10 mm and it has 1,520 total I/Os. The electroplated bump is composed of eutectic Sn/Pb with a pitch of 250 μm and height of 100 μm. Ti/Cu is used as the UBM (under bump metallurgy) which is deposited by sole sputtering. The nonuniformity of the bump heights is found to be less than 5% in a 6" wafer. The PR (photoresist) opening is 101±1 μm within wafers and 101±2 μm wafer-to-wafer. The deviation of the PR thickness is within ±2 μm for a typical thickness of 45. The alignment accuracy is better than 1.5 μm. The shear force of the UBM as sputtered is comparable to that incorporated with electroplated Cu reported in the literature. The reliability is investigated for different Cu thicknesses. The reliability test of high temperature storage at 150°C shows that shear force remains constant for more than 650 hours and drops to one half of the original value after 2,000 hours for a 4 μm sputtering Cu UBM. It is also found that the shear force does not decay after ten reflow cycles