Thermal fatigue crack in a beam lead in μ BGA, a type of CSP (Chip Scale Package), is identified as a major failure mode under temperature cycling test condition. Thus, accurate understanding of the fatigue behavior of beam lead and evaluating the effect of parameters on the beam lead crack are crucial to ensure the reliability of μ BGA. In this study, the isothermal low cycle fatigue test for three kinds of copper foil used for beam lead, and for the beam lead in an assembled μ BGA were carried out to investigate the fatigue characteristics of the beam lead In order to impose mechanical load on a beam lead, we have developed a micro-mechanical testing machine that enables the low-cycle fatigue test. The experimental results using the testing machine were compared with the fatigue life of beam lead in a fully assembled μ BGA under the temperature cycling test condition.