Strength Evaluation of Notch Structure for Semiconductor Encapsulant Resin

Plastic encapsulated semiconductor packages may crack at the corner regions of die pads or chips if internal delamination occurs at an elevated temperature during the reflow soldering process. Thus, the structural strength design around the notch structures, which will be formed in the encapsulant resin due to the delamination, is considered one of the most important issues. Especially, it becomes a more critical item of the package development in order to realize the reflow process with lead-free solder materials, whose melting points are higher than that of Sn63-Pb37. In this study, the fracture behavior of notched specimens, which were made of silica particulate-filled epoxy resins and modeled as the corner regions in actual packages, were studied with experimental and numerical analyses. First, the fracture tests of the notch structure of semiconductor encapsulant resin were carried out. A notch tip with several different radii was introduced to the specimen. The specimens were fractured by a three-point bending load. Second, the strength evaluation of the notch structure was carried out. The critical stress distribution σ_C(r)=max.[K_1C/(2 πr)^1/2, σ_B] was used to determine the crack initiation at the notch tip. It is assumed that a fracture occurs when, at any point near the notch tip, the stress distribution exceeds the critical stress distribution determined by fracture toughness and bending strength. Three-dimensional finite element analysis was carried out to obtain the stress distributions around the notch tip in the specimen. The calculated stress distributions around the notch tip were compared with the critical stress distribution to estimate the fracture load of the specimen. Estimated fracture loads at room temperature and at high temperature were compared with the results of the fracture tests. It was confirmed that the predicted results based on the critical stress distribution corresponded very well with the experimental results. The validity of the criterion was confirmed by studying the fracture behavior of the notched specimens of actual silica particulate filled epoxy resins.