The influence of adhesive materials on chip-on-board packing of MEMS microphone

Adhesive material is commonly used for attaching die onto the printed wiring board (PWB) in the Chip-on-Board (COB) packaging of MEMS devices. However, the polymer-based adhesive usually possesses large difference in the coefficient of thermal expansion (CTE) between silicon chip and PWB. Therefore, the mismatch of CTE could lead to the thermally-induced stress and coupling deformation of multilayer structure in the reflow process as surface-mount technology (SMT) on printed circuit board (PCB). In this study, three different adhesive materials, namely 2025D, 3140RTV and SDA6501, and two different cap materials, namely liquid crystal polymer (LCP) and nickel (Ni), were evaluated the influences on the thermally-induced stress in the ploy-silicon diaphragm of MEMS microphone based on Finite Element Analysis (FEA). According to the results, we obtained the following two findings: (1) The CTE mismatch of LCP cap and the metal (Ni) cap caused different type of thermal deformations of PWB and lower thermally-induced stress and deformation were found in the case of LCP cap, however, different cap materials less affected the thermal stress in the diaphragm. (2) Soft adhesive materials (3140RTV and SDA6501) have better mechanical isolation of PWB thermal deformation due to the buffer layer effects. On the contrary, hard adhesive material (2025D) could be affected by PCB thermal deformation when the thickness of adhesive was less than 30µm, thus, a lower stress in the diaphragm existed due to the stress compensation by PWB thermal deformation. In general, present study provides the basis of selection of adhesive material for COB MEMS packaging.