International Journal of Fatigue 23 (2001) S325–S335 www.elsevier.com/locate/ijfatigue Mixed mode I/II fracture and fatigue crack growth along 63Sn–37Pb solder/brass interface H. Nayeb-Hashemi * , Pinghu Yang Department of Mechanical, Industrial and Manufacturing Engineering, Northeastern University, Boston, MA 02115, USA Abstract Solder joints are extensively used in electronic packaging. They provide critical electrical and mechanical connections. Single edge notched sandwich specimens, which were made of two blocks of brass joined with a 63Sn–37Pb solder layer, were prepared for a fatigue and fracture study of the joint under mixed mode loading. Mode I and mixed mode I/II fracture toughness, fatigue crack thresholds, and fatigue crack growth rates (FCGR) were measured at room temperature using a four point bending test setup. It was found that the fracture toughness of the joint increased and FCGR decreased upon the introduction of the mode II component. The interface fracture toughness was higher than that reported for pure solder. The data of FCGR correlated well with the power law relation of da/dN=C * (G) m , where G is the alternating energy released rate for a crack under mixed mode loading. It was also observed that both fracture toughness and FCGR were a function of the solder layer thickness. When the solder layer thickness increased from 0.1 to 1.0 mm, the fracture toughness decreased substantially and FCGR increased slightly. For mode I loading, fatigue cracks propagated inside the solder layer. However, for mixed mode loading, once a crack initiated, it changed its direction toward the interface and then propagated along the interface. These observations were related to local mode I and mode II stress fields. Fracture surfaces showed signs of rubbing under mixed mode loading with elongated cavities at the crack tip. However, under mode I loading, fracture surfaces were covered with equi-ax voids.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Fatigue crack growth; Solder/brass interface 1. Introduction Recently there has been interest in using diffusion bonded sandwich-layered structures. Diffusion bonded materials are used in wear resistance dies, cutting and measuring tools, metal–ceramic pressure seals, elastic elements of transducers, radial-turbine wheels, turbine blades, valves, pistons and cylinder liners for internal combustion engines, etc. In the electronic industry, dif- fusion bonding has been used in the design of micro switches, bonding ceramics, etc. To insure that the layered material withstands the ser- vice life of the components, there have been a number of research reports [1–6] towards understanding the microstructure of the interfacial reaction zone, its thick- ness, and their effects on bond strength. However, there * Corresponding author. Tel.: +1-617-373-5515; fax: +1-617-373- 2921. E-mail address: hamid@cow.neu.edu (H. Nayeb-Hashemi). 0142-1123/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII:S0142-1123(01)00144-X has been relatively little research towards understanding fatigue crack growth behavior at the interface of those materials and factors affecting fatigue crack growth rate under mixed mode loading. Willams [7] first addressed the problem of interfacial cracks between dissimilar isotropic materials along a straight interface. He found an oscillating stress singular- ity in a region near the crack tip. This problem was further studied by Erdogan [8,9], England [10], Rice and Shi [11], Dundurs [12], and Comninou [13]. Works done by Park and Earmme [14], Hutchinson et al. [15,16,27,18], Rice [19], Gautesen and Dundurs [20], Sih and Arson [21], Thruston and Zehnder [22], McNaney et al. [23,24], and Shaw [25] further enhanced our understanding of the stress field around an interfa- cial crack. Research reports indicate that interface fracture tough- ness and fatigue crack growth behavior are significantly influenced by the presence of mode II stress field [1,2,6,26,27]. Due to elastic mismatch of the two bonded materials at interface, an interfacial crack tip generally