Gi-Tae Lim

Intermetallic compound and Kirkendall void growth in Cu pillar bump during annealing and current stressing

Cu pillar bump with eutectic SnPb was annealed and the micro structures were observed by scanning electron microscopy. Both of Cu₆Sn₅ and Cu₃Sn grew following parabolic rate law at 120 and 150degC. At 165degC, Cu₆Sn₅ growth was stagnated while Cu₃Sn growth rate was increased after 160 hour when all Sn was consumed. Kirkendall void was formed because of different diffusivities of Cu and Sn. The activation energies of Cu₆Sn₅, Cu₃Sn, and Kirkendall void growth were 1.77, 0.72, and 0.36 eV respectively. Intermetallic compound (IMC) growths during 150degC annealing and in current stressing condition were observed to investigate the effect of current stressing. In current stressing condition, the temperature was 150degC and the current density was 5 times10⁴ A/cm². IMC growth in current stressing condition was faster than that during annealing because of electron wind force.

Size effect on electromigration reliability of pb-free flip chip solder bump

To understand for size effect on electromigration behavior in flip chip Pb-free solder bump, electromigration tests were performed with change of pad open size and solder bump height at 140degC, 4.6times104 A/cm2. Electromigration lifetime increases with pad open size and bump height decreasing. In pad open size change, electromigration lifetime increase with pad open size increasing because applied current decrease with pad open size decreasing. In bump height change, electromigration resistance increase with bump height decreasing due to thermal gradient induced thermomigration effect decreasing. It seems to that electromigration resistance increase with size of solder bump decreasing.

Reliability of Cu pillar bump for flip chip and 3-D SiP

Cu pillar bumps with eutectic SnPb solder were annealed and their microstructures were investigated. Linear relationship was observed between thickness of intermetallic compounds (IMCs: Cu6Sn5, Cu3Sn) and square root of time at 120 and 150degC. Kirkendall voids, formed by the diffusivity differences between Cu and Sn, were observed near the interface between Cu and Cu3Sn. There was a change in slope of the linear relationship between IMCs thickness and square root of time at 165degC when all Sn was consumed. Cu6Sn5 growth rate was retarded, while Cu3Sn growth rate was accelerated. The activation energies for Cu6Sn5, Cu3Sn, and Kirkendall voids growth were estimated to be 1.77, 0.72, and 0.36 eV, respectively. The microstructures of Cu pillar bumps with pure Sn were investigated by in-situ scanning electron microscopy under annealing and high current-stressing conditions. It was found that IMC growth rate under annealing condition obeyed parabolic rate law, while that under high current-stressing condition IMC growth rate did not obeyed linear rate law. IMC growth rate under high-current stressing condition was faster than that under annealing condition which is presumed to be caused by the atomic migration enhancement due to the electron wind force.

Effect of Thermal Aging on Intermetallic Compound Growth Kinetics of Au Stud Bump

Microstructural evolution and the intermetallic compound (IMC) growth kinetics in an Au stud bump were studied via isothermal aging at 120, 150, and 180 C for 300hrs. The AlAu phase was observed in an Al pad/Au stud interface, and its thickness was kept constant during the aging treatment. AuSn, AuSn , and AuSn phases formed at interface between the Au stud and Sn. AuSn , AuSn /AuSn , and AuSn phases dominantly grew as the aging time increased at 120 C, 150 C, and 180 C, respectively, while (Au,Cu) Sn / Cu Sn phases formed at Sn/Cu interface with a negligible growth rate. Kirkendall voids formed at AlAu /Au, Au/Au-Sn IMC, and Cu Sn/Cu interfaces and propagated continuously as the time increased. The apparent activation energy for the overall growth of the Au-Sn IMC was estimated to be 1.04 eV.