Development of reflowable Sn-Pb alloy bump for Al pad

A direct soldering material with Al thin-film pad has been examined for a bare flip-chip bonding on Printed Wiring Board. A solder ball made from the. 59.6Sn-29Pb-5Sb-1Zn-5Ag-0.2Cu-0.2Ni (mass%) alloy wire was thermosonically bonded on the Al-1%Si pad by using a conventional bump bonding machine. For fine-pitch bonding, the strain of ball deformation can be reduced by finding the optimum value of the ultrasonic power for bond strength and ellipticity of the bonded bump. After reflow treatment up to 300°C, no significant degradation of the bump shear force was observed. The concentration of Ag, Cu and Ni in the Al film interface results in an improvement in heat-resistance at the bond interface against the high reflow temperatures. Furthermore, the consistency of measured resistance values during thermal cycling, high-temperature and high-humidity and pressure cooker tests were sufficient enough to put into practical use in epoxy encapsulated flip-chip assemblies