Stencil printing process of buffer layer for Wafer Level CSP

We have confirmed that a stencil printing using a novel developed printable polyimid paste containing no fillers can be available for polymer film deposition in Wafer Level Chip Size Packaging (WLCSP) process. A thick polyimide buffer layer with openings for solder ball bumping can be deposited on all of the LSIs on a wafer by one time printing process. This stencil printing process does not need an expensive lithography tool and photoresist process, which makes WLCSP more cost effective than ever. In this study, the rheology of polyimide paste was tailored to stencil printing process. The novel printable polyimide paste shows that the viscosity ratio of more than 3.95 to 1 at the shear rate of 1s -1 to 10s -1 respectively, and that viscosity of the paste goes up fast after the shear rate is lowered rapidly. Fine spaces of 40μm between 250μm openings were obtained when the new paste was applied to deposit 10μm thick buffer films on Si wafers. It has been also confirmed that the new paste showed robust stability of the viscosity after more than 100 wafers were continuously printed, leading to narrow variation range of 30μm at the opening size of 385μm within 100 wafers. As for the adhesion and reliability of the printed polyimide films, no peelings were observed on plasma CVD SiN films after 300 hours under the PCT condition of 127°C, 100% humidity and 0.25 MPa. The optimal stencil printing process using a novel developed paste will lead to higher productivity and, thus, to a significant cost reduction of WLCSP.