Study Of Electromigration In Lead-free Solder Joints On Electroless Ni-P Substrate

With the continuous miniaturization and integration of electronic products, the size of solder joint is decreasing, resulting in the increasing current density through the solder joint. Therefore, electromigration (EM) becomes one of the most important reliability issues of solder joints. In addition, the lead-free solder replaces the Sn-Pb solder as the major material in electronic industry due to the mandatory requirement of lead-free. The higher Sn content and the higher melting point of lead-free solder induce some new challenges in the reliability of solder joint. Therefore, with the developments of lead-free and miniaturization of solder joint, it was needed to take the EM research of lead-free solder joint urgently. In this work, both Ni-P/Sn/Cu line-type interconnects and Ni-P/Sn3.0Ag0.5Cu/Ni flip chip solder joints were taken to investigate the effects of EM on the interfacial reaction, microstructural evolution, interfacial intermetallic compound (IMC) transformation, EM failure modes and EM failure mechanisms at150℃—200℃under5×103A/cm2～1.5×104A/cm2.The following conclutions are drawn in the present work:(1) The evolution of Ni-P layer (cathode) during EM was the same in both Ni-P/Sn/Cu line-type solder joints and Ni-P/Sn3.0Ag0.5Cu/Ni flip chip solder joints. Ni-P layer was dissolved quickly and then transformed into the newNi3P/Ni2SnP layers. The growths of Ni3P and Ni2SnP followed the t1/2law before the complete consumption of Ni-P layer. When the Ni-P was consumed completely, the formation and propagation of voids and then the formation of crack was observed at the cathode interface, resulting in the fused failure of solder joint at last. During the fused failure, the solder bump of failed joints was molten and two adjacent joints were also molten in a high temperature, so that the pronounced EM-induced interfacial reaction occurred in the molten solder joints. Compared with the Ni-P/Sn/Cu line-type solder joints, the current crowding effect was obvious in the Ni-P/Sn3.0Ag0.5Cu/Ni flip chip solder joints, inducing the most serious consumption of Ni-P at the electron-entry corner.(2) In the Ni-P/Sn/Cu line-type solder joints, the across-solder interaction between Cu and Ni was notably affected by EM. During aging at150℃, the Cu atoms could diffuse across the solder to the Ni-P side quickly, resulting in the transformation of interfacial IMC at the Ni-P/Sn interface from Ni3n4to (Ni,Cu)3Sn4and then to (Cu,Ni)6Sns. When the current stress was applied, EM enhanced the diffusion of Cu atoms to arrive at the Ni-P side if Cu under downwind diffusion, but inhibited it to diffuse to the Ni-P side if Cu under upwind diffusion. For the diffusion of Ni atoms, the atoms were still difficult to arrive at the Cu side even under downwind diffusion.(3) In the Ni-P/Sn3.0Ag0.5Cu/Ni flip chip solder joints, In-situ observation was an appropriate way to investigate the effect of EM on the microstructural evolution of the cross-section. Simulation results showed that the EM-induced damage of Ni under bump metallization (UBM) strongly depended on the crystallographic orientation of Sn in Sn3.OAgO.5Cu solders. When the c axis of Sn grain was along with the direction of current, the serious dissolution of Ni UBM was taken. While when the a axis of Sn grain aligned with the current direction, no obvious consumption of Ni UBM was observed, but the formation and propagation of void at the solder/Ni interface (cathode) were occurred. In addition, four types of stress relaxation behaviors were observed during the microstructural evolution of cross-section:(a) the slight hollow of solder at the cathode;(b) the slight upheaval of solder at the anode, and even the formation of hillock or whisker at the electron-exit corner;(c) the sliding of Sn grain boundary;(d) the localized diffusional creep in a Sn grain.