Measurement and analysis of interfacial adhesion strength between the silicone encapsulant and the side wall of a SMD LED leadframe cup

Silicone is one of the typical encapsulation materials used in light-emitting diode (LED) packaging. In general, silicone has the merits of good optical properties, high thermal stability, and good electrical insulation. However, it has relatively low mechanical rigidity and poor adhesion strength. Therefore, delamination between the silicone encapsulant and the leadframe cup (or substrate) is one of the major concerns in LED packaging. In the present study, a 3528 SMD LED was investigated. The objective is to characterize the adhesion strength between the side wall of the leadframe cup and the silicone encapsulant inside the cup. This paper presents a customized pin-push test method together with computational stress analysis for the aforementioned purpose. An axisymmetric finite element (FE) model was built since the relevant geometries are conical in nature. The current FE model adopted the same configuration of the corresponding experimental study and the mechanical loading was applied at the bottom of the silicone encapsulant in terms of distributed pressure. Due to the singularity at the junction between the bottom and the side wall, it is obviously not practical to use the maximum stress as the failure criterion. Therefore, an average stress scheme was applied in the present study. Furthermore, the Tsai-Hill failure criterion for bi-axial stress analysis was employed in the stress analysis. The test configuration and the computational model are described and discussed in this paper in detail.