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Recently, the electronic device equipments using a lot of semiconductors are widespread to all industrial fields. Solder joints are used to mount the electronic chips, such as ceramic resistors and capacitors, on the printed-circuit boards in almost all electronic devices. However, since in many cases the thermal expansion coefficients of electronic parts and PCBs have mismatch, cyclic thermal stress and strain causes solder fatigue. Especially in the power electronic module and car electric module, the evaluation of thermal fatigue life for the chip components is important. It is understood that the fatigue lives of some electronic devices show big scatter in the thermal cycle test, even if their design is the same. The dispersion of main design factors of solder joints is thought as one of these reasons. Moreover, the influence of the dispersion grows when the lead-free solder materials are used in the devices. Therefore, it cannot be bypassed as the main issue for the reliability evaluation in the solder joints. In this study, how the dispersion of design factors and the interacting effect between the design factors influences the failure life in lead-free solder joint was investigated by the analytical approach. Sensitivity analyses were carried out to study the main effect of the dispersion of each factor on solder joints. And then, the interacting effect between the factors on the reliability was studied by considering the structural asymmetry due to the unbalanced solder joints. As a result, a practical evaluating technique for the failure life scatter of solder joints was proposed.