Effects of thermal, moisture and mechanical loading conditions on interfacial fracture toughness using Brazil-nut specimens

Interfacial delamination is a common failure mode in multi-layered IC packages. In this paper, an experimental technique using Brazil-nut specimens is employed to determine the interfacial fracture toughness of an adhesively sandwiched joint with introduction of edge interfacial crack. The interface studied here is the die attach material/copper leadframe interface. Tensile loads are applied to the specimens at various loading angles until the failure of the samples. Design of experiments (DOE) is applied to study variations in strain rates and thickness of the sandwich structure. In addition, the effects of moisture content and temperature on the interfacial adhesion are investigated. For the DOEs conducted, as the loading angle increases from 20deg to 90deg, the interfacial fracture toughness decreases. It is found that moisture absorption and increase of temperature have detrimental effects on the die attach material and likewise results in a corresponding decrease in fracture toughness. Failure patterns of the delaminated surfaces were observed with the scanning electron microscope (SEM) to determine the cohesive/adhesive modes of failure. The fracture toughness and its corresponding mode mixity are determined from the measured critical load by finite element modeling computation