Fluxless solder bumping in flip chip package by plasma reflow

A new fluxless reflow process using Ar+10%H₂ plasma was investigated for application to solder bump flip chip packaging. The 100 μm-diameter Sn-3.5wt%Ag solder balls were bonded to 250 μm pitch Cu/Ni under bump metallurgy (UBM) pattern by laser solder ball bumping method. Then, the Sn-Ag solder balls were reflowed in Ar+H₂ plasma. Without flux, the wetting between solder and UBM occurred in Ar+H₂ plasma. During plasma reflow, the solder bump reshaped and the crater on the top of bump disappeared. The bump shear strength increased as the Ni₃Sn₄ intermetallic compounds formed on initial reflow stage but began to degrease as coarse (Cu,Ni) ₆Sn₅ grew at solder/UBM interface. As the plasma reflow time increased, the fracture within solder bulk decreased and the fracture along solder/UBM interface increased. The off-centered bumps self-aligned to UBM pattern during plasma reflow. The solder ball defect occurred in high power and a long time plasma reflow