Plasma reflow bumping of Sn-3.5 Ag solder for flux-free flip chip package application

A new flux-free 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 bonding 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 in the initial reflow stage but began to decrease as coarse (Cu,Ni)₆Sn₅ grew at the solder/UBM interface. As the plasma reflow time increased, the fracture mode changed from ductile fracture within the solder to brittle fracture at the solder/UBM interface. The off-centered bumps self-aligned to patterned UBM pad during plasma reflow. The micro-solder ball defects occurred at high power prolonged plasma reflow.