Investigation of Thermal-Mechanical and Moisture Driven Delamination in Lead-free QFN Packages during SMT Reflow Soldering Process

Interface delamination is one of the major failure modes in plastic IC packages. The encapsulating polymers used in electronic packaging are characteristic of high porosity, which makes the material susceptible to the moisture absorption. During reflow soldering process, the entire package is exposed to the soldering temperature as high as 260degC for lead-free solders. Under such temperatures, moisture absorbed by polymer materials evaporates, generating high internal vapor pressures in the voids. Interface delamination may be caused by the combined effect of the thermomechanical stress, hygromechanical stress and vapor pressure. The main objective of this paper is to investigate the effects of temperature, moisture diffusion and vapor pressure on the delamination in lead-free QFN package during SMT reflow soldering process. Finite element simulation on a plastic QFN package is carried out for the entire thermal and moisture history from the JEDEC Level 1 moisture preconditioning (85degC/85%RH for 168 hours) to subsequent exposure to a lead-free soldering process. The energy release rate due to thermal stress, hygro-stress and vapor pressure is computed and studied by using J-integral method. The result shows that the additional mismatch induced by the vapor pressure and moisture in the package is of the same order as the CTE mismatch, which has a quite significant effect on interfacial delamination failure