Reflowable Sn-Pb bump formation on Al pad by a solder bumping method

A new solder bump material for reflow soldering on an Al pad was developed by using a rapidly solidified 60 Sn-29 Pb-5 Sb-1 Zn-5 Ag(mass%) alloy wire. The balls made from the wire by arc discharge in Ar+10%H₂ gas were bonded on an Al-1mass%Si film by thermosonic bonding at 398 K using a wire bonding machine. The bonding to the Al-Si film was made by the interdiffusion of Sn, Sb, Ag and Al through microfissures which occurred on the film surface. The as-bonded bump can be spheroidized by heating at 473 K with an inactivated rosin base flux. The shear strength of the reflowed bump is 79 MPa and decreases with increasing reflow temperature. According to the thermal fatigue life test of the solder bump applied to the flip-chip bonding, the striations formed by the fatigue fracture were observed to generate at very fine intervals of 0.08-0.12 μm. These results are good enough to expect practical application of the direct solder bump formation technique on Al pad