Application of finite element analysis to reduce problem of ground ring delamination

Abstract

Exposed pad packages were first introduced for their superior electrical and thermal performance. Some of the exposed pad packages do include a separate ground ring structure that is situated between the die pad and inner end of the leads. The ground ring is directly connected to the exposed pad. Multiple ground wires can then be bonded on the ground rings without the need to increase the pin count on the leadframe. Initial evaluation results indicate that ground ring is very prone to delaminating after undergoing moisture sensitive level (MSL) condition. Delamination occurs when external stress, exceeds the adhesion strength between mold compound and leadframe. Delamination on the ground ring can affect wire bond integrity. The objective of this paper is to reduce ground ring delamination using finite element analysis. Finite element analysis is used to carry out thermal stress simulation. The stress is generated by subjecting the package to a temperature change of −65°C to 150 °C. The thermal stress analysis helps to define area of ground ring with the highest stress concentration. Subsequently, various design parameters such as thickness, dimension, shape and etc are varied and analyzed. The leadframe design combination with the most optimum predicted stress distribution is then identified. Final result indicates that the optimized leadframe design and dimension are the main factors to improve the robustness that able to withstand the thermal stress induced by preconditioning and temperature cycling under automotive requirement.