Simulation and Analysis of Quad Flat No-lead Package (QFN) under Moisture, and Thermal Stress

Abstract

The major problem with quad flat no-lead package (QFN) packages is the delamination between the copper lead-frame and the molding compound (MC) due to relatively weak adhesion. The copper lead frame has excellent electrical and thermal conductivity. However, moisture penetration not only reduces the adhesion between the interfaces of the two materials, but also adversely affects the conductivity of Cu. Interface stratification is attributed to different material properties such as mismatch of coefficient of moisture expansion (CME), surface treatment of the mold pad, thermal strain, vapor pressure at high temperatures, and reduced interface strength due to moisture and temperature effects.The work presented in this paper focused on moisture, thermal and vapor pressure effects on the lead-frame and molding compound in the QFN during the precondition test and reflow process. The causes of delamination were examined both experimentally and via simulations. In an electronic package, the main failure effect stems from molding compound. To accurately simulate the moisture distribution in QFN, in this work, the moisture mechanism pertaining to material properties was investigated. This investigation focused on the MC and epoxy coefficient of moisture expansion and vapor pressure, and the parameters obtained by experimental study were incorporated into a simulation model to verify the fit between the experimental and the simulation findings. The electronic components employed in this work are standard for moisture sensitive testing, according to JEDEC J-STD-020D. To measure the specimen weight gain and geometric size, electronic balance and microscope were used, and these values were obtained under moisture sensitive level 1, level 2 and level 3 in order to establish the moisture desorption rate of the QFN package. And also according to the different degree of oxidation on the leadframe, the surface analysis of the material is carried out by environmental scanning electron microscope (ESEM) to understand the element distribution and interfacial strength. In addition, finite element analysis (FEA) was performed to analyze the stress, warpage and delamination in QFN packages. In this research will discuss the coupling forces of moisture, thermal stress and vapor pressure under MSL-3 and Reflow, and discuss the interface strength under different reliability test stages. To verify the accuracy of simulation modeling, the delamination site was observed and was aligned with the simulation results by applying scan acoustic tomography (SAT). Finally, the reliability of QFN was enhanced by investigating the moisture behavior of the materials co mprising the QFN.