Electromigration in Pb-free bumps with different UBM thickness

A study of a dummy flip-chip package was conducted to evaluate electromigration failure of different Pb-free bumps with different UBM (under bump metallization) thickness. The applied current density was 2.3×10⁴ A/cm², and the operating temperatures were 180°C and 160°C. The UBM consisted of Cu and Ni electroplate with thickness of 20 μm and 10 μm (or 5 μm), respectively. Pb-free solder bumps, SnAg and SnAgCu alloys, were utilized to perform the experiment. An SnPb eutectic solder bump was also tested in contrast to the different Pb-free solder bumps. Electromigration failure was observed only at the solder bump/UBM interface with electron current flow from the chip to the substrate. Mass IMC (intermetallic compound) dissolution and void nucleation near the cathode were observed during current stressing. Failure took place in the region of the UBM and UBM/bump interface in the form of solder cracking or delamination. Otherwise, in different Pb-free bumps, Ni followed by Cu migration along the electron wind direction was observed. Pb-free solder bumps had a longer failure time than that of SnPb bumps at high temperatures. Effects of current crowding and IMC polarity are key factors of flip-chip interconnects´ electromigration behavior.