Study On Electromigration Properties Of Low-Ag Lead-free Solder Bump

At present, the flip-chip technology plays an important role in electronic packaging, but with the increasing packing density, electromigration (EM) has become a serious problem affecting reliability, and has attracted wide attention. After banning the lead solder, low-Ag lead-free has the most possibility to replace SnPb alloy, we believe it has a better prospect. Because of the importance of electromigration in electronic packaging, so it is necessary to study the electromigration of low-Ag solder.This paper was based on SAC305, Sn0.7Ag0.5Cu and Sn0.7Ag0.5CuBiNi, and the electromigration test was done by different temperature, current density and time. Analyzing the electromigration resistance of these three kinds of solders, also the change of interfacial structures and mechanical properties of solder joints after electromigration failure.The results indicated that by adding elements Bi and Ni in the low-Ag solder, electromigration behavior could be improved obviously. With the time increasing, intermetallic compounds (IMC) disappeared near cathode area where the copper are consumed obviously, a large amount of IMC were observed at the anode. The edge of IMC layers flattened and a layer of Cu₃Sn appeared between IMC layer and Cu pad, Cu₃Sn layer at cathode appeared earlier and thicker than at anode after EM test. The edge of IMC layer flattened under a current density of 0.8×10⁴A/cm² at room temperature for 20 days, on the contrary, the IMC changed little under EM experiment which was after a thermal-aging at 160℃for 100h. Electromigration damages were observed in the bumps under a current density of 5.0×10⁴A/cm² just only one minute, a large amount of Cu₆Sn₅ IMC could be found at anode, IMC disappeared near cathode area where the copper are consumed worse at two edges of the contact interface than at middle obviously.Temperature plays an important role in EM test. With the temperature increasing, the EM failure of solder bumps take place easier. In the condition of T=120℃, J=1.76×10⁴A/cm², t=58h, EM failure could be observed quickly, but not in 25℃and 80℃.The results also indicated that EM failure can lead to the changes of microstructures of solder bumps, the structure at cathode became sparser while voids and cracks assembled here, however it is opposite at the anode. Electromigration also leads to the changes of microhardnesses of solder bumps, and it raised form cathode to anode caused by EM. The elastic modulus at cathode area decreased obviously and the one at anode were as the same as before. With the loading rate increasing, the creep depths (△h) of solder bumps increased before and after current stressing, but creep depth at cathode was bigger than at anode, electromigration failure decreased the creep resistance of solder bumps.