Fluxless plasma bumping of lead‐free solders and the reliability effects of under bump metallization thickness

Purpose To investigate fluxless plasma ball bumping and effect of under bump metallization (UBM) thickness on joint properties using lead‐free solders. Design/methodology/approach A fluxless soldering process was investigated in this study using Ar‐10 percent H 2 plasma reflow. Balls made from two lead‐free solders (Sn‐3.5 weight percent Ag and Sn‐3.5 weight percent Ag‐0.7 weight percent Cu) were reflowed and, also Sn‐37 weight percent Pb as a reference. In particular, the effects of the UBM thickness on the interfacial metallurgical bonding and joint strength were studied. The UBM (Au/Cu/Ni/Al layers) thicknesses were 20 nm/0.3 μ m/0.4 μ m/0.4 μ m and 20 nm/4 μ m/4 μ m/0.4 μ m, respectively. Findings The experimental results showed that in the case of a thin UBM the shear strengths of the soldered joints were relatively low (about 19‐27 MPa) due to cracks observed along the bond interfaces. The thick UBM improved joint strength to 32‐42 MPa as the consumption of the Cu and Ni layers by reaction with the solder was reduced and hence the interfacial cracks were avoided. To provide a benchmark, reflow of the solders in air using flux was also carried out. Originality/value This paper provides information about the effect of UBM thickness on joint strength for plasma fluxless soldering to researchers and engineers.