C. Y. Liu

Microstructure-electromigration correlation in a thin stripe of eutectic SnPb solder stressed between Cu electrodes

Room-temperature electromigration occurs in a thin stripe of eutectic SnPb solder stressed by a current density of 105 amp/cm2. Hillocks and voids grow at the anode and the cathode, respectively. While the dominant diffusion species is Sn in this two-phase alloy, the growth of the hillocks, surprisingly, originates from the Pb grains.

Electromigration in Sn–Pb solder strips as a function of alloy composition

Using thin film solder strips, we have investigated the electromigration of six different compositions of Sn–Pb solders at current density of 105 A/cm2 near ambient temperature. The six compositions are pure Sn, Sn80Pb20, Sn70Pb30, Sn62Pb38 (eutectic), Sn40Pb60, and Sn5Pb95. The eutectic alloy, with the lowest melting point and a high density of lamella interfaces, was found to have the fastest hillock growth. As composition moving toward the two terminal phases, the hillock growth rate decreases; but it increases again in pure Sn. The interface between Sn and Pb, being the fastest kinetic path of mass transport, also serves as the place to initiate hillock and void formation.

Electron microscopy study of interfacial reaction between eutectic SnPb and Cu/Ni(V)/Al thin film metallization

The wetting reaction between molten eutectic SnPb solder and a sputtered trilayer Cu/Ni(V)/Al thin film metallization was studied using cross-sectional transmission electron microscopy and scanning electron microscopy. Reaction temperatures were from 200 to 240 °C and reaction times ranged from 1 to 40 min. The initial reaction products were Cu6Sn5 and Cu3Sn. The latter transforms to the former after an annealing greater than 1 min at 220 °C. The Cu6Sn5 grains adhere well to the Ni(V) surface and no spalling of them was observed, even after 40 min at 220 °C. This surprising result indicates that the Cu/Ni(V)/Al or Cu6Sn5/Ni(V)/Al is a stable thin film metallization for low temperature eutectic SnPb solder direct chip attachment to organic substrates. Additionally, Kirkendall voids accompanied Cu3Sn formation, yet the voids disappear when the Cu3Sn transforms to Cu6Sn5.

Effect of current crowding on vacancy diffusion and void formation in electromigration

In multilevel interconnects, current crowding occurs whenever the current changes direction, such as when passing through a via. We propose that in current crowding, the current-density gradient can exert a driving force strong enough to cause excess vacancies (point defects) to migrate from high to low current-density regions. This leads to void formation in the latter. This is a key feature of electromigration-induced damage in very large scale integrated interconnects.

Electromigration in eutectic SnPb solder lines

We have prepared eutectic SnPb solder lines for electromigration study by a process of solder reflow into V-grooves etched on (001) Si wafer surfaces. They are thick lines and are highly reproducible. We report here results of lines of 100 μm in width and 150 to 800 μm in length, stressed by a current density of 2.8×104 A/cm2 at 150 °C in ambient. The accumulation of a large lump of solder, rather than hillocks of Sn and Pb, was observed at the anode, and depletions and voids were observed at the cathode. By measuring the volume of the lump, we have calculated the average effective charge number of electromigration in the eutectic solder to be 33, which is close to the reported value of 47 for self-electromigration in bulk Pb. Using x-ray dispersive analysis, we found that Pb is the dominant diffusing species.

DIRECT CORRELATION BETWEEN MECHANICAL FAILURE AND METALLURGICAL REACTION IN FLIP CHIP SOLDER JOINTS

We tested flip chip solder bonded Si samples under tensile and shear loading as a function of annealing time at 200 °C. The solder bump was eutectic SnPb and the underbump thin film metallization was Cu/Cr deposited on oxidized Si. We found that the failure mode is interfacial fracture and the fracture strength decreases rapidly with annealing time. From scanning electric microscope observations, the fracture occurs at the Cu–Sn/Cr interface. We conclude that it is the metallurgical reaction that has brought the solder into direct contact with the Cr surface. The weak joint is due to the spalling of Cu–Sn compound grains from the Cr surface, especially near the edges and corners of the joint.

Morphology of wetting reactions of SnPb alloys on Cu as a function of alloy composition

We have investigated the wetting angle, side band growth, and intermetallic compound formation of seven SnPb alloys on Cu ranging from pure Sn to pure Pb. The wetting angle has a minimum near the middle composition and increases toward pure Sn and pure Pb, but the side band growth has a maximum near the middle composition. The intermetallic compounds formed are Cu 6 Sn 5 and Cu 3 Sn for the eutectic and high-Sn alloys, yet for the high-Pb alloys, only Cu 3 Sn can be detected. While no intermetallic compound forms between Cu and pure Pb, the latter nevertheless wets the former with an angle of 115°. The driving force of a wetting reaction, which may be affected by the free energy gain in compound formation, is discussed by assuming that rate of compound formation is fast.

Dewetting of molten Sn on Au/Cu/Cr thin‐film metallization

On Au/Cu/Cr thin film surface, a drop of molten Sn first spreads out to wet the surface, but it then pulls back to dewet. The latter is due to the spalling of Cu–Sn compounds and exposing the Cr surface to the molten Sn when all of the Cu film has been consumed by the wetting reaction. Dewetting is clearly undesirable for solder joints in electronic packaging; the phenomenon is presented here.