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

Joon Kwon Moon, Y. Zhou, Jae Pil Jung

Research output: Contribution to journalArticlepeer-review

5 Scopus citations


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 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.

Original languageEnglish
Pages (from-to)3-9
Number of pages7
JournalSoldering and Surface Mount Technology
Issue number2
StatePublished - 1 Jun 2005


  • Joining processes
  • Organometallic compounds
  • Soldering
  • Strength of materials


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