Wafer Level Chip Scale Package (WLCSP) has been a favorable packaging solution for compact portable consumer electronics. The microelectronics industry introduced Extra Low K (ELK) to enhance electric performances with the cost of diminishing mechanical reliability. The ELK itself and its interfaces are highly fragile and susceptible to fracture. ELK cracking under bumps and ELK inter layer delamination (ILD) from die corners are often observed during and after solder reflow and qualification process such as accelerated thermal cycling (ATC). In this study, the underfill selection and its fillet formation influence to the Chip Packaging Interactions (CPI) of WLCSP was investigated through an experimental technique and numerical analysis. For the experimental assessment, thermo-mechanical interactions between die corner and underfill was investigated. Digital image correlation (DIC) technique with optical microscope was utilized to quantify the deformation behavior and strains of cross-sectioned WLCSP die corner subjected to thermal loading from 25°C to 125°C. The results clearly show captured deformations of die corner area under thermal loading. For the fillet formation influence, it shows that the high underfill fillet configuration gives higher normal strain at the die corner area during thermal cycling. For the underfill selection, it clearly shows that the strain difference at corner solder during thermal cycling caused by two different type of underfill material. Finally, finite element analysis (FEA) was conducted by simulating the thermal loading applied in the experiments and validated with experimental results. Then, using the FEA analysis, parametric study for underfill material properties and fillet height were performed on the ELK reliability of WLCSP. Energy release rate of the die corner crack were obtained and used as damage indicators for die corner ELK delamination.

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