At present, the electronic industry is actively searching for Pb-free solders due to environmental concerns over Pb-containing solders. Solder joints are widely used to bond chips to their substrates for electrical connection and packaging. Lacking reliability data, many electronic companies will be reluctant to adopt Pb-free solders in the advanced products. Hence, it is timely to review our understanding of structure-property relationship and potential reliability issues of Pb-free solders. A brief history of solder joint processes in electronic manufacturing is presented to serve as a background for the review. It emphasizes the unique phenomenon of spalling of interfacial intermetallic compound in solder reactions. Challenges for Pb-free solders from the point of view of physics and materials are given since the reliability issues of solder joints will remain with us when advanced Cu/low k dielectric interconnect technology is introduced into microelectronic devices.

The evolution of interfacial microstructure of eutectic SnAgCu and SnPb solders on Al/Ni(V)/Cu thin films was investigated after various heat treatments. In the eutectic SnPb system, the Ni(V) layer was well protected after 20 reflow cycles at 220 °C. In the SnAgCu solder system, after 5 reflow cycles at 260 °C, the (Cu,Ni)6Sn5 ternary phase formed and Sn was detected in the Ni(V) layer. After 20 reflow cycles, the Ni(V) layer disappeared and spalling of the (Cu,Ni)6Sn5 was observed, which explains the transition to brittle failure mode after ball shear testing. The different interfacial reactions that occurred in the molten SnAgCu and SnPb systems were explained in terms of different solubilities of Cu in the two systems. The dissolution and formation of the (Cu,Ni)6Sn5phase were discussed on the basis of a Sn–Ni–Cu phase diagram. In the solid-state aging study of the SnAgCu samples annealed at 150 °C for up to 1000 h, the Ni(V) layer was intact and the intermetallic compound formed was Cu6Sn5 and not (Cu,Ni)6Sn5, which is the same as was observed for the eutectic SnPb system.

Intermetallic compounds (IMCs) are formed as a result of interaction between solder and metallization to form joints in electronic packaging. These joints provide mechanical and electrical contacts between components. The knowledge of fracture strength of the IMCs will facilitate predicting the overall joint property, as it is more disposed to failure at the joint compared to the solder because of its brittle characteristics. The salient feature of this paper is the measurement of the fracture toughness and the critical energy-release rate of Cu 3 Sn and Cu 6 Sn 5 intermetallic thin films, which is the result of the interaction between Sn from the solder and Cu from the metallization. To achieve the objective, a controlled buckling test was used. A buckling test in the current work refers to one that displays large transverse displacement caused by axial compressive loading on a slender beam. The stress and strain along the beam can be easily calculated by the applied displacement. Fracture-toughness values of Cu 3 Sn and Cu 6 Sn 5 are 2.85 MPa Ϯ 0.17 MPa and 2.36 MPa Ϯ 0.15 MPa , respectively. Corresponding critical energy-release rate values are 65.5 J/m 2 Ϯ 8.0 J/m 2 and 55.9 J/m 2 Ϯ 7.3 J/m 2 , respectively. The values obtained were much higher than the ones measured in bulk intermetallic samples but correlated well with those values obtained from conventional fracturetoughness specimens when fracture was confined within the intermetallic layers. Hence, the controlled buckling test is a promising fast and effective way to elucidate mechanical properties of thin films.Key words: Fracture toughness, critical energy-release rate, thin intermetallic films, Cu 3 Sn, Cu 6 Sn 5 , controlled buckling test 2m 2m 2m 2m

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