Advancement in simulating moisture diffusion in electronic packages under dynamic thermal loading conditions

The normalization approach has been accepted as a routine to overcome the concentration discontinuity at multi-material interfaces in the moisture diffusion simulation by the finite element method. However, applying the normalization approach directly in the commercial finite element software under a dynamic thermal loading condition, such as reflow process, may lead to erroneous results. Special techniques are needed to obtain correct results. In this study, different approaches that can deal with the moisture diffusion under dynamic thermal loading conditions were reviewed and compared with case studies. Advantages and disadvantages of each approach were analyzed and discussed. Theoretical derivation was developed to show the direct concentration approach (DCA) violates the law of mass conservation. The effect of Fourier number on the accuracy of the DCA was also examined. The internal source approach was shown to be a universal method in modeling the moisture diffusion in a multi-material system under dynamic thermal loading conditions. The newest version of ANSYS also showed good performance in solving the diffusion problem under transient thermal loadings if a proper time step size was used.