Pb-free solder paste reflow window study for flip chip wafer bumping

A 2x3 full factorial experimental design was used to study the effects of peak reflow temperature (235 degreesC - 265 degreesC) and time-above-liquidus (40-60 sec.) on bump characteristics. Ultra-fine mesh (Type 5, -500/+650) Pb-free solder pastes, including Sn96.5Ag3.5, Sn95.5Ag3.8Cu0.7, and Sn99.3Cu0.7 were used on electroless NiP/Au Under Bump Metallurgy (UBM). Belt speed and zone temperature settings of a reflow furnace were used to obtain the desired profiles. A linear ramp profile, with ramp rate of 1.5 degreesC/sec was selected after initial profile screening experiments. The reflowed solder bumps were characterized in several categories to obtain an optimal reflow window. These categories included wetting characteristics, solder bump shear strength, shear failure mode, flux residue cleanliness, solder void population, and void size. The lowest peak reflow temperature and time-above-liquidus were established for each solder alloy. A low peak temperature reflow (cooler profile) resulted in fewer and smaller voids in the solder bump than a high temperature, long time-above-liquidus reflow condition. Bump shear strength was consistent for a wide range of reflow conditions. All of three Pb-free solders have very wide reflow process window to produce bumps with good solder/UBM integrity and uniform bump geometry.