Joon Kwon Moon

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 H2 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 μm/0.4 μm/0.4 μm and 20 nm/4 μm/4 μm/0.4 μm, 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 sol...