Fluxless laser reflow bumping of Sn-Pb eutectic solder

A possibility of the fluxless reflow solder bumping via a laser heating was examined using a solder disk on a pre-tinned Cu pad. The laser with a 20W power heated for 1.2 second resulted in a spherical bump with a thin layer of intermetallics at the solder/Cu interface. With the increasing laser power and irradiation time, thicker intermetallic layer and increased dissolution of Cu into the solder matrix resulted. The dissolved Cu precipitated out as an acicular ε-phase aligned perpendicular to the interface. The shear strength of the solder joint was measured comparable to that prepared by the reflow soldering in a furnace.