Measurement of underfill interfacial and bulk fracture toughness in flip-chip packages

Flip-chip package reliability is greatly improved by encapsulating the solder interconnections between a polymeric encapsulant or underfill. However, thermo-mechanical stresses within such packages often lead to failures initiating in the vicinity of chip and underfill interface. In this study, we present experimental results geared towards measuring and understanding such failure mechanisms. We provide the bulk fracture toughness of the underfill material and interfacial fracture toughness between the underfill material and the silicon die. The bulk and interfacial fracture toughness measurements are performed as a function of temperature. We use the single edge notch bending test to calculate the bulk fracture toughness of the underfill and to measure the interfacial fracture toughness, we use a novel technique referred to as the wedge delamination method. The wedge delamination method provides substantial advantage in measuring the interfacial fracture toughness for brittle materials over traditional methods. Using the wedge delamination method we compare the fracture strength between the underfill and silicon at the front-face and side-wall interfaces. Additionally, the influence of dicing technique on fracture toughness is also investigated.