Mechanism of electromigration-induced failure in flip-chip solder joints with a 10-μm-thick Cu under-bump metallization Jae-Woong Nah, a) Kai Chen, and K.N. Tu Department of Materials Science and Engineering, University California at Los Angeles, Los Angeles, California 90095-1595 Bor-Rung Su and Chih Chen Department of Materials Science and Engineering, National Chiao Tung University, Taiwan 30050, People’s Republic of China (Received 26 October 2006; accepted 7 December 2006) The electromigration-induced failure in flip-chip eutectic SnPb solder joints with a 10-m-thick Cu under-bump metallization (UBM) was studied without the effect of current crowding in the solder region. The current crowding occurred inside the UBM instead of in the solder joint at the current density of 3.0 × 10 4 A/cm 2 because of the spreading of current in the very thick Cu UBM. In these joints, the failure occurred through a two-stage consumption of the thick Cu UBM in the joint where electrons flowed from the chip to the substrate. In the first stage, the Cu UBM dissolved layer by layer rather uniformly across the entire Cu UBM–solder interface. In the second stage, after half of the Cu UBM was dissolved, an asymmetrical dissolution of Cu UBM took place at the corner where electrons entered from the Al interconnect to the Cu UBM. Experimental observation of dissolution steps of the 10-m-thick Cu UBM is presented. The transition from the first stage to the second stage has been found to depend on the location of current crowding in the flip-chip joints as the UBM thickness changes during the electromigration test. The current distribution in the flip-chip solder joints as a function of UBM thickness was simulated by three-dimensional finite element analysis. The dissolution rate of Cu UBM in the second stage was faster than that in the first stage. The mechanism of electromigration-induced failure in the flip-chip solder joints with a 10-m-thick Cu UBM is discussed. I. INTRODUCTION Under-bump metallization (UBM) is very important in the flip-chip technology because it acts as a solder wet- table layer, diffusion barrier, and adhesion layer. 1 Cur- rently, the most common choices for solder wettable lay- ers are Cu and Au, and the diffusion barrier and adhesion layers are TiW, Ti, Cr, Al, NiV, and Ni. These choices depend not only on a solder bumping process, but also on a balancing of capabilities and costs, as well as the manu- facturer’s skill and experience. When a trilayer thin film of Cr/Cu/Au is applied as UBM, spalling of Cu–Sn com- pounds from the Cu–solder interface occurs and results in a weak mechanical solder joint, which is one of the most serious reliability problems because the Sn-based Pb-free solders react very fast with Cu and the amount of Cu is very limited in the thin-film metallization. 2–4 To overcome the spalling problem, a 5-m-thick electro- plated Cu UBM has been integrated into the UBM so that the chemical reaction will not consume all the Cu and no spalling may occur during aging. 5 However, when the joint is subjected to current stressing, current crowding leads to a rapid dissolution of the 5-m-thick Cu UBM at the corner where electrons entered from Al interconnect to Cu UBM and the joint failed quickly. 6 Due to the demand for high performance and miniaturization in the electronics industry, the problem of electromigration must be overcome; a thicker Cu UBM has been designed to overcome the electromigration-induced failure. 7 Cur- rently, the design rule requires that each flip-chip solder joint of 50 m in diameter carries 0.2 A, which means that the average current density in such a joint is about 10 4 A/cm 2 . 7 The International Technology Roadmap for Semiconductor (ITRS) projections indicated that electro- migration is a near-term issue in high current density packages. 8 a) Address all correspondence to this author. e-mail: jnah@us.ibm.com Present address: IBM T.J. Watson Research Center, Yorktown Heights, NY. DOI: 10.1557/JMR.2007.0084 J. Mater. Res., Vol. 22, No. 3, Mar 2007 © 2007 Materials Research Society 763