Leadframe Strip Stiffness Simulation for Identification of Issues During Package Singulation Process

Electronic assembly processes of the leadframe strip with high density package designs regularly deal with significant strip warpage after the Post Mold Cure (PMC) process. The Coefficient of Thermal Expansion (CTE) mismatch of the Epoxy Mold Compound to the leadframe and chip causes the strip to warp. This will affect the succeeding assembly process which is package singulation. To ensure that the leadframe strip is flat for singulation process, a certain amount of vacuum pressure is applied to the leadframe strip. One of the root causes that resulted in vacuum process failure is the stiffness of the leadframe strip. This paper deals with Quad-Flat No-lead package (QFN) strip modelling using a Finite Element Analysis (FEA) approach to understand the contributing factors to the stiffness of the strip. To validate the model, the leadframe strip stiffness is measured using a Three-Point Bending (3PB) test. The effect of Silicon (Si) die content to the strip stiffness was analyzed. The accuracy of the model is ensured by validating the simulation results with the experimental data. The deviation of both results is less than 10% for all simulated cases. The simulation trends show that higher die to die pad ratio for equivalent package size leads to higher strip stiffness which result in a harder to flatten the strip during the vacuum mount process. The simulated trend agrees with the actual observation. However, the dependence of the strip stiffness on die-to-die pad ratio is quite weak, as reducing the die-to-die pad ratio from 90% to 60% will decrease the strip stiffness about 3%. The validated model enables the authors to explore more on other factors of the leadframe stiffness for instance, different Bill of Materials (BOM) and other geometrical variations.