Design, Experiment and Analysis of the Solder on Rubber (SOR) structure of WLCSP

A novel solder on rubber (SOR) structure of the advanced wafer level chip scaled packaging (WLCSP), having the capability of releasing the deformation energy which is caused by the CTE mismatch between the silicon chip and the substrate, is proposed herein. In the SOR structure, a metallic trace and solder pad would be formed on the rubber-based polymer, and a solder is attached onto the said pad. Moreover, a delamination layer is designed and fabricated between the metallic trace and the stress buffer layer (SBL), and the metal trace is also designed as curved shape to prevent the over-stretching of the trace. By the failure of the designed delamination layer, the SOR structure could theoretically release more energy than the conventional WLCSP structure. In this paper, the design concept of the SOR structure, experimental measurement of the adhesion strength of the delamination layer and the finite element (FE) analysis of the SOR structure are discussed