One of the reliability limitations of an FBGA package is its sensitivity to moisture, i.e., popcorn failure. This paper studies the moisture absorption behavior of thin FBGA substrate materials in order to improve the package's moisture performance. A set of empirical equation is developed to estimate the amount of moisture absorbed in two different states-the steady state and the transient state-under different environmental conditions. For the steady-or saturation-state, the soldermask volume is used to estimate the saturation point because of its higher sensitivity to moisture absorption as compared to the thin core material. For the transient state, non-Fickian law is employed to estimate the moisture content by computing the constants b and n in the non-Fick equation. Since the empirical equations obtained apply well to different humidity conditions, they can also be applied in a desiccator environment. These equations provide a good and quick calculation on the amount of moisture in the substrates at various steps in the assembly process, improving process control and, hence, the package's overall moisture performance. Published by Elsevier B.V.