Interface reaction of solder droplet/pad and intermetallic compounds evolution during reflow soldering

The interface structures of solder droplet for SnPb and SnAgCu Al/Ni/Cu pad were investigated by droplet direct laser fabrication method, which is compared with the interface structures for bump and pad under laser reflow bumping. The intermetallics evolution at solder bump/pad interface during subsequent reflow soldering is conducted. The results show that, during the contact reaction between molten solder droplet and pad, Au is dissolved into molten solder droplet and Au-Sn intermetallic compounds are precipitated, while Au doesn't react with solder droplet fully. During the subsequent reflow soldering, Au layer disappears from the interface, Au-Sn compounds are distributed into solder matrix, and Ni layer is exposed to solder and reacts with solder directly. Distinctive difference exists between the intermetallics formed at Pb-free solder/pad interface and SnPb solder/pad interface during reflow soldering process. For the SnPb solder/pad system, the Ni₃Sn₄ layer is formed at the solder/pad interface during reflow. For the SnAgCu solder/pad system, the (Cu_xNi_1-x)₆Sn₅ layer is formed at the solder/pad interface during reflow.