Effect of Long-Term Isothermal Exposure on Chip/Underfill Interfacial Crack Growth Under Mode – I Fatigue Loading

Abstract

Underfills are used between flip-chip die and substrate. Underfilling effectively reduces the shear stress and applied strain on the solder joints. Thermomechanical reliability issues at chip/underfill interface are a major concern since the introduction of underfill (UF) in Flip Chip Ball Grid Array (FCBGA) packages. Delamination at the chip/UF interface is one of the significant failure modes in FCBGA packages. The integrity of the interface is one of the major concerns for reliability. The underhood environments can easily expose the components to extreme temperatures. Most of the components in these applications are designed to be operated from 55°C to -100°C. These interfacial cracks at the chip/UF interface propagate over a period to solder interconnects resulting in failure of the system. Delamination at the interface accelerates with thermal exposure, due to changes in properties of the underfill with long-term exposure to high temperature. Interfacial delamination depends entirely on the fracture toughness of the chip/UF interface. Most of the component interfaces are studied, but chip/UF interfaces in FCBGAs are not widely studied yet. Bi-material beam specimens of chip/UF interfaces are fabricated and tested under fatigue bending loads, to understand the change in fracture resistance and failure modes at the interface. Two chip/UF interfaces are examined in this study. The samples are subjected to long-term isothermal aging at 100°C for 30 days, 60 days, 90 days, and 120 days. The samples are subjected to mode-I, four-point bend fatigue loading. The interfacial crack initiation, propagation, and failure modes are recorded. The rate of crack growth with respect to the number of fatigue cycles has been computed. The fracture toughness for the interfaces is calculated. The change in interface properties is examined with respect to the number of days of aging.