Three dimensional flow analysis for incomplete fill failure during matrix array transfer molding of small QFN packages

Driven by the market demands, high density and low profile packaging options of small QFN are continuously developed. Matrix array molding is becoming more and more popular for encapsulating such small packages due to high productivity. Molding-related quality concerns including incomplete fill, exposed wire and wire sweeping are thereafter increased because of narrowed spaces between die top and mold surface and increased unit density. Several pioneers have studied wire sweep risks, which are regarded as the major failure mode during transfer molding for various thin packaging solutions. But as the leadframe features of small QFN packages are evolving more complex, failure of incomplete fill and air trap induced by uneven flow emerges frequently, which leads to high yield losses during assembly. In this study, three-dimensional mold flow simulation was applied to analyze the potential incomplete fill issue of one typical small QFN package with matrix array molding by using commercial software Autodesk Moldflow Insight (AMI). To analytically comprehend the failure mechanism of incomplete fill, a simplified 3D matrix array model was built to evaluate the flow pattern during transfer molding process. Besides die and mold cavity, leadframe features were incorporated into this model as well. Simulation results reveal that both global and local flow front convergence contributed the most to incomplete fill. Local velocity distribution was affected by global flow front convergence, which in turn determined the failure pattern in each molding matrix. As a result, the central packages in the last several rows suffered the greatest local flow imbalance and the highest incomplete fill risk, which was consistent with repeated short-shot observations. Finally an improved leadframe design was proposed, based on the understandings of the failure mechanism. Both simulation and experiment results were proven that no incomplete fill occurred after implementing new design.