Effects of combining hygro-thermal stress on reliability of plastic QFN package

In this paper, finite element analysis (FEA) was used to evaluate the stress distribution on quad flat no-lead (QFN) packages. Different models were established, including moisture diffusion, thermo-mechanical stress, hygro-mechanical stress and hygro-thermo-mechanical stress models. The transient moisture absorption and desorption analysis were performed to estimate the package moisture distribution. A variety of experimental works were designed to study the hygro-thermal stress on the interface of die/mold compound and die attachment delaminaion. After the moisture preconditioning under 85°C/85%RH, it is found that the external mold compound (MC) material of package is almost fully saturated. The differential swelling that occurs between the mold compound and nonpolymeric materials leads to hygroscopic mismatch stresses in the package. The FEA results revealed that there is a significant contribution of hygroscopic swelling induced stress and an obvious effect of hygro-thermo-mechanical stress on package reliability. The experimental results also show that the hygro-thermo-mechanical stress under lead free reflow temperature produce considerable influence to the package integrity after moisture precondition.