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This paper details a modelling approach for assessing the in-service (field) reliability and thermal fatigue life-time of electronic package interconnects for components used in the assembly of an aerospace system. The finite element slice model of a plastic ball grid array (PBGA) package and suitable energy based damage models for crack length predictions are used in this study. Thermal fatigue damage induced in tin-lead solder joints are investigated by simulating the crack growth process under a set of prescribed field temperature profiles that cover the period of operational life. The overall crack length in the solder joint for all different thermal profiles and number of cycles for each profile is predicted using a superposition technique. The effect of using an underfill is also presented. A procedure for verifying the field lifetime predictions for the electronic package by using reliability assessment under accelerated thermal cycle (ATC) testing is also briefly outlined.