Studies on double-layered metal bumps for fine pitch flip chip applications

In this paper, Cu/SnAg double-layered bumps structure was proposed and investigated for the fine pitch flip chip applications. Test chip was designed considering the recent high speed memory device and its pad size and pitch was 60μm and 100μm, respectively. Cu and SnAg bumps were fabricated as a 60μm and 20μm thickness on SiO₂/Ti/TiN/Al/TiW/Cu on Si wafer using the electroplating method. Test chip was flip chip assembled with PCB substrates using thermo-compression bonding method. Because the pitch was very tight, the flip chip bonding of Cu/SnAg double bumps was very difficult and it affected several bonding parameters such as bonding pressure, temperature, time, Cu bump diameter and so on. The bonding results were evaluated through the cross-sectional image of interconnection and the electrical continuity test of daisy chain and bump resistance using 4-point Kelvin structure. The long time reliability tests like thermal cycling test and 85°C/85% test are now in progress after flip chip bonding and underfill dispensing.