Effect of Underfill Mechanical Property on Cu/Low-K Delamination for Lead-Free Flip Chip Packaging

The low dielectric constant (low-K) material and copper (Cu) interconnection in chip enhance the electrical speed for microelectronic devices. The low-K materials generally show low mechanical strength, high coefficient of thermal expansion (CTE) and poor adhesion. Cu/low-K delamination becomes a failure cause during reliability test. Many papers have studied this failure and demonstrated the underfill selection method for preventing the delamination from die corner. This work demonstrates a different low-K failure mode. The delamination was initialed at die inside and propagated to the die corner, which was observed in the lead free flip-chip package after the temperature cycling test (TCT). Simulation was employed to study the possibility of this failure mechanism and provide the underfill selection strategy for this failure. An underfill evaluation experiment confirms the simulation result, eventually.