Failure mode evolution of WLCSP on board by dynamic bend method

A strain-controllable dynamic bending method on WLCSP has been proposed in this paper. In order to identify the principle factor among the effects of stiffness attributed by different board level structures, the 0.4mm pitch WLCSP packages with Sn-4.0Ag-0.5Cu solder ball are used. This combination of WLCSP is considered to have the high stiffness in the structure. It is also shown that there are interactions between the SAC405 solder balls, the Al/Ni/Cu pad plating, the reflow profile and the flux chemistry. The experimental result shows that at the same strain rate range (∼106 µɛ/s), the fracture position occurrence happens in internal die at 11,000µɛ. This data indicates that the brittle fracture position transfers from general IMC layer to higher brittle layer in the component. The variation of the strain energy of materials and the stress concentration position which changes in different package sizes are speculated to be the cause of the fracture position transfer. Therefore, in our research; we will investigate the relationship between the IMC layer and microstructure of under bump metallization (UBM). The influence of different package dimensions will be discussed in this study, too.