We report a Finite Element Model to calculate the bending stress of thin and ultra-thin silicon dies embedded in flexible foil substrates (chip-in-foil package) at lower bending radii. The values of fracture strength computed using Finite Element Analysis showed very good agreement with the experimental results. Furthermore, an increase in the fracture or critical stress (bending stress at fracture) of the dies due to embedding in flexible foil substrates was observed. Besides, the impact of foil material and thickness on the bending stress of ultra-thin silicon die is discussed by comparing two foil materials: Stainless Steel and Polyimide.