Sn based lead-free solder was bonded to Cu substrate at room temperature by means of the surface activated bonding method. Bonding of Sn to Cu was succeeded even in nitrogen atmosphere when the surfaces to be bonded were sputter-cleaned in vacuum.Subsequently the solders were molten on the copper substrate in vacuum or inert gas atmosphere. A good wetting was observed in the both atmosphere without any flux. Dependence of the bond strength of Sn balls on the plastic deformation of the Cu substrate due to the bonding pressure will be discussed in term of the inhomogeneous distribution of local pressure and the contact area at the bonded interface.
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