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In this paper, the reliability of lead-free solder castellations is considered. The newly developed stress-dependent Engelmaier's solder fatigue model is utilized in this task. Based on this model, it is possible to interpret the thermal cycling test results. A very good agreement between the test results and the lifetime predictions is obtained. Using the lifetime prediction model, optimal solder castellation shape is investigated. Based on the findings, the fatigue life can be improved by up to 30% simply by solder pad length optimization. Further increment in lifetime length can be expected if the solder joint shape is optimized with the help of modeling tools presented here. Understanding how the crack propagates in solder material is vital if optimal lifetime behavior is expected.