Moulding compounds used for encapsulating electronics typically occupy a large portion of package volume and are most exposed to the external environment. Under harsh conditions such as high temperature, humidity, and mechanical vibrations, constituent materials of electronic components degrade, resulting in a change in their thermal, mechanical, electrical, and chemical behaviour. High-temperature ageing of electronic packages causes the oxidation of epoxy moulding compounds (EMC), forming a layer exhibiting significantly different thermomechanical properties. This reflects in the modified mechanical behaviour of the entire package, which accelerates certain failure modes and affects component reliability. Thus, it is crucial to consider gradual degenerative changes in EMC for a more accurate estimation of the component lifetime. This paper proposes a three-step modelling approach to replicate thermo-chemical changes in package encapsulation. A parametric geometry of a test package was incorporated with the ageing stage-dependent changes in thermomechanical properties of the oxidized layer. The mechanical behaviour of oxidized EMC at multiple stages of thermal ageing (at 150°C for up to 3000 hours) was first experimentally characterized and then validated using warpage measurements on thermally aged test packages and Finite Element (FE) simulations. Lastly, a trend-based interpolation of material model parameters for intermediate stages of ageing was followed, and a continuously updated degradation model (physics-based Digital Twin) was achieved. The proposed model is capable of reproducing degraded stages of the test package under thermal ageing along with its modified thermomechanical behaviour. Its limitations and significance in the domain of health monitoring of microelectronics are also discussed.@en