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Warpage is one of the challenging problems for wafer level package (WLP). Especially, the bonding process of multiple wafers will bring additional stress to WLP and warpage of WLP. This paper aims at the stress and warpage characteristics of the WLP (consisting of silicon cover wafer and silicon MEMS wafer) with glass frit bonding. The finite element (FE) method and MOS (Multi-beam Optical Sensor) technology were used. Simulation result indicated that WLP presents almost zero warpage. MOS measurement result indicated that WLP actually presents convex warpage instead of zero warpage. It turned out that the convex warpage results from the temperature difference between WLP wafers (i.e. cover wafer and MEMS wafers) in the course of post-bond cooling. Taken the temperature difference into account, the simulation result was consistent with experiment results. Furthermore, on the basis of the convex warpage value the stress of WLP was also calculated so as to understand the stress distribution and estimate the reliability level of the MEMS device. Keywords-warpage,wafer level package(WLP), finite element method(FEM), micro Electro mechanical systems(MEMS), multibeam optical sensor (MOS)

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