Using PDMS Micro-Transfer Moulding for Polymer Flip Chip Packaging on MEMS

Polymer flip chip process utilizes Ag filled thermoset and/or thermoplastic polymers, in combination with stencil printing processes to form polymer bump interconnects of flip chip IC devices (Estes, 1998). Nevertheless, slumping behavior of conventional polymer interconnect material during bumping process on the wafer chip has constrained the bump height achieved by the most common stencil printing to 50mum (Kulesza et al., 1997). A bumping process to form fine pitch, high aspect ratio polymer flip chip bumps without slumping was developed by producing a through-hole PDMS micro-transfer mould on a metallized wafer (Wong, et al., 2003). In this paper, a MEMS chip (Lee, et al., 2003) was packaged by the PDMS micro-transfer mould technique to evaluate the performance of the interconnection method. Flip chip assembly process was then performed on the printed circuit board (PCB) and some functionality tests was conducted afterwards. The designed dimension of the polymer bump is 150mum in diameter, 90mum in height, and at a 245mum pitch. Bump adhesion force was measured and found to be about 40gram. Electrical conductivity test was also conducted after PCB assembly as preliminary reliability test of the whole assembly process. Results showed that the MEMS chip was functioning well with new polymer bumping method. With this bumping approach, intrinsic slumping problem of the conductive polymer paste is resolved. This low temperature fabrication technique is an important development for MEMS packaging