Leadframe Mold Lock Study and Optimization for Flag Size Reduction using Finite Element Analysis and Reliability Testing for 3L SOT223 Package

Current design of 3L SOT223 leadframe would allow for a long bonding wires due to its large die attach flag design as compared to the die size. Long bonding wires have resulted in a number of subsequent process and assembly related problem such as wire sway, wire shorting and lifted wire. One way to reduce the wire length is by reducing the die attach pad (flag) size and at the same time redesigning the leads so they are extended into the package center. By doing this, the second bond positions on the lead are closer to the first bond on the die. This reduction in wire length would ultimately result in improved wire strength and integrity especially during epoxy mold compound flow sweeping on these wires. It would also results in cost saving by reducing the amount of wire used per unit of package. The new design has also resulted in reduced usage of copper material for the leadframe. However, it is suspected that the reduction in flag size will change the way stress acting on the die when mechanical and thermal stresses acting onto the package due to reduced contact area between the flag and mold compound. The need to study the effect of reducing the die attach flag on the die stress has prompted this study, which is to investigate if the proposed changes as mentioned above will result in a better stress performance of the die as compared to the current leadframe design. The approach used in this study is by using finite element analysis (FEA) modeling tool and doing reliability data to support the finding of the FEA analysis. The existing leadframe was designed with several locking features added to reduce thermal and mechanical stress on the die especially at trim and form process. These locking features are in the form of two round-shaped through hole and few v-grooves on all leads and the tab. Initial findings suggest that the new leadframe design would result in higher stress level especially due to mechanical loading, compared to the existing leadframe. This i- nitial finding is based on the new leadframe with similar locking features (size and position) as the existing leadframe. Further analysis of the locking features has resulted in optimized locking features in terms of size and dimension, which resulted in leadframe with better stress performance compared to the existing leadframe. Reliability data fully supports this finding with improved delamination and other reliability indexes were achieved. This paper will discuss in great detail the approach used in solving the issue, the various design option considered as well as the optimization work and related engineering theory. It concludes that with optimized locking features, the new leadframe can perform equally better compared to existing leadframe, and more importantly, at the same time the requirement of shorter wire is met.