Reducing the bond frame width in Cu/Sn SLID wafer level packaging

In this paper, we present recent results on mechanical strength and reliability of Cu/Sn bond frames with reduced foot-print, for wafer-level packaging of Micro and Nano-devices. The structures of the test vehicle and intermediate multi-layer of bond frames were designed and fabricated on cap and substrate wafers. Electroplated Cu/Sn frames were bonded at temperature 270 °C in an EVG501 wafer bonder. With a tailored bonding profile (temperature and pressure), a symmetric sandwich structure with Cu3Sn between two Cu layers is formed at the bonding interface, where the amount of over flow of Sn is minimized. To reduce the bond frame foot print, the width of the bond frame is reduced from 200 to 40 μm. Mechanical die-shear and reliability testing show that the minimum bond frame width giving acceptable results is 80 μm. Thinner frames (40 μm width) result in inferior results in the present experiment. For bond frames of 80 μm width and wider, the average shear strength is high (65 MPa). Three types of reliability tests, i.e., annealing test, high temperature storage, and temperature cycling test were performed to evaluate the reliability of the bonded frames after the initial inspection. In the failed dies, we observed propagation of cracks in silicon incomplete bonds. The possible failure mechanisms are discussed and feasible improvement methods are proposed.