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During the reliability assessment of a flip chip on board assembly (FCOB) by environmental stress tests, the physical characteristics of the field operating conditions are not sufficiently addressed. The failure mechanisms active at the environmental stress test conditions may not be dominant at the service operating conditions. The magnitude and intensity of thermomechanical stresses experienced by a FCOB at service conditions are different from those in the environmental stress tests, which result in discrepancies between dominant failure modes in the field operating conditions and accelerated life test conditions. Driven by the non-field failure modes, design modifications and process improvements during the prototyping stage may be misled. A "service field-oriented reliability assessment" methodology is introduced in this paper to address this issue by using a power cycling technique. The associated experimental power cycling system for FCOBs field reliability assessment is developed in this research. The test vehicles are assembled using fine pitch flip chips and high-density interconnect (HDI) substrates. The in-situ continuities of individual Kelvin solder bumps and daisy chains of the FCOB assemblies are investigated during the power cycling tests. The physical failure modes are revealed by acoustic microscopy, cross sectioning and scanning electron microscopy. "Design for field reliability" is then achievable by the effective design modifications and manufacturing process improvements at the prototyping stage.