Reliability of solder joints in flip chip technology: Approaches from fundamental studies of wetting, interfacial reaction, mechanical shear testing, and electromigration

In the first part of this work, the mechanism of the reactive wetting is studied by using the system of molten SnPb solders on Cu. We conclude that the driving force of reactive wetting must take into account, besides the capillary force, the free energy change due to intermetallic compound formation.; In the second part, we tested flip-chip solder bonded Si samples under tensile and shear loading as a function of annealing time. The solder bump was eutectic SnPb and the under-bump 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 SEM observations, the fracture occurs at Cu-Sn/Cr interface. We conclude that it is the metallurgical reaction that has brought the solder into direct contact with 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. To solve the spalling problem, trilayer Cu/Ni(V)/Al thin film metallization is a potential candidate, beside thick Ni(p) UBM. We have used cross-sectional SEM and TEM to study the wetting reaction between molten eutectic SnPb solder and a sputtered trilayer Cu/Ni(V)/Al thin film metallization. No spalling has been observed. This result indicates that the Cu/Ni(V)/Al or Cu₆Sn₅/Ni(V)/Al is a stable thin film metallization for the low temperature eutectic SnPb solder.; In the third part, using thin film strips, we have investigated electromigration of six different compositions of SnPb solders at current density of 10 5 Amp/cm2 near ambient temperature. The six compositions are pure Sn, Sn₈₀Pb₂₀, Sn₇₀Pb₃₀, Sn₆₃Pb₃₇ (eutectic), Sn₄₀Pb₆₀, and Sn₅Pb₉₅. The eutectic alloy, with the lowest melting point and a high density of interfaces, was found to have the fastest hillock growth. As composition moving toward the two terminal phases, the hillock growth rate decreases because the volume fractions of primary phases having more EM resistance increase. But it increases again in the pure Sn. Being the kinetic path of mass transport, the interface between Sn and Pb also serves as the place to initiate hillock and void formation. Besides hillock and void formation, electromigration has induced a substantial microstructural change in the two-phase alloy, e.g., there is a large amount of grain growth of the Pb phase in the eutectic alloy.