Thermo-mechanical reliability of the benzocyclobuten (BCB) film in a WLCSP process

A new WLCSP process which enables reliable fabrication of high-performance and low cost package has been developed based on the low dielectric passivation layer. The fabrication process can be reduced by using the newly developed photosensitive-BCB and a conventional photolithography process. However, cracks frequently nucleating on the BCB poses serious reliability issues. In order to prevent the crack generation, the stresses in the BCB layer were reduced by optimizing the relative thickness of the BCB and the underlying stress buffer layer(SBL), and by stabilizing the BCB microstructure with optimal photolithography processes. The microstructure of the BCB was investigated by measuring the molecular weight, the cross link element intensity and the glass transition temperature.