1184 Kariya, Morihata, Hazawa, and Otsuka Journal of ELECTRONIC MATERIALS, Vol. 30, No. 9, 2001 (Received February 14, 2001; accepted May 16, 2001) Special Issue Paper 1184 Assessment of Low-Cycle Fatigue Life of Sn-3.5mass%Ag-X (X = Bi or Cu) Alloy by Strain Range Partitioning Approach YOSHIHARU KARIYA, 1 TOMOO MORIHATA, 2 EISAKU HAZAWA, 2 and MASAHISA OTSUKA 2 1.—The Open University, Materials Engineering Department, Walton Hall, Milton Keynes, Buckinghamshire, MK7 6AA, U.K. 2.—Shibaura Institute of Technology, Department of Materials Science and Engineering, Shibaura 3-9-14, Minato-ku, Tokyo, 1088548, Japan The fatigue lives and damage mechanisms of Sn-Ag-X (X = Bi and Cu) solder alloys under creep-fatigue interaction mode have been investigated, and the adaptability of the strain partitioning approach to the creep-fatigue of these alloys was examined. Symmetrical and asymmetrical saw-tooth strain profiles components (i.e., fast-fast, fast-slow, slow-fast and slow-slow) were employed. Application of the slow-slow strain mode did not have an effect on fatigue lives of the alloys under investigation. Transgranular fracture observed on the fracture surfaces suggests that creep damage might be cancelled under slow- slow mode. The fatigue lives of all alloys were dramatically reduced under slow- fast mode, which is attributed to intergranular cavitation and fracture during tensile creep flow. On the other hand, the compression creep component gener- ated by fast-slow mode also significantly reduced the life of Sn-3.5Ag and Sn- 3.5Ag-1Cu, while the component did not affect the life of Sn-3.5Ag-xBi (x =2 and 5). The four partitioned strain ranges (i.e., De pp , De pc , De cp , and De cc ) versus life relationships were established in all alloys tested. Thus, it is confirmed that the creep-fatigue life of these alloys can be quantitatively predicted by the strain partitioning approach for any type of inelastic strain cycling. Key words: Strain range partitioning, creep-fatigue interactions, Coffin-Manson plots INTRODUCTION Lead-free soldering for electronic assembly is being driven by environmental and health concerns regard- ing toxicity of lead and, more importantly, by the perceived economic advantage of marketing ‘green’ products. Lead-free solder alloys require various mate- rial characteristics in terms of melting temperature, solderability, material availability, cost, and reliabil- ity. 1 In addition, there are mechanical characteristics, such as fatigue, within the solder joint that will gain in significance when considering mechanical reliability and the implementation of lead-free soldering. When an electronic device is in operation, solder joints become subjected to mechanical and thermal stresses and strains. Of such deformations, it is thermomechanical fatigue that is considered most damaging. Thermomechanical fatigue within a sol- der joint is generated by temperature cycles (both ambient and power switching) reacting with compo- nents, boards, and the solder—all of which have different coefficients of thermal expansion. In addi- tion to thermomechanical fatigue, the solder joint can also be subjected to mechanical fatigue, typically when the board or substrate is bent during mechani- cal handling or from vibration forces. Therefore, an understanding of fatigue properties of solders is re- quired for improved joint design and to facilitate the development of new lead-free solders. The authors have investigated low cycle fatigue properties of Sn- 3.5mass%Ag-X (X = Bi, Cu, In, and Zn) lead-free solder alloys at a relatively high strain rate in a previous study. 2,3 It was found that low cycle fatigue at high strain rate will duplicate mechanical fatigue stresses that will be generated as a result of handling the circuit board. However, this does not address thermally induced fatigue stressing. Since even room temperature represents a homologous temperature that is greater than 0.6 for most solders, creep dam- age developed during any dwell period (i.e., stress