The Microstructure Characterization of Ultrasmall Eutectic Bi-Sn Solder Bumps on Au/Cu/Ti and Au/Ni/Ti UBMs

초록

Flip chip technology is increasingly used due to its high packaging densityand good electrical performance. Recently, the pad size and the pitches of chips get smaller and finer because of further miniaturization and integration of electronic components. Bi-Sn solder has raised great attention for low temperature solder alloy. This paper presents the evolution of microstructure in ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Cr and Au/Ni/Ti UBMs. Ultrasmall 58wt%Bi-42wt%Sn solder bumps with the diameter of 50 ㎛were fabricated using lift-off method and reflowed using RTA system. The large Bi phase and the lamellae of Bi-Sn were observed in the Bi-Sn solder bumps formed on Au/Cu/Cr UBM. Increased cooling rate promoted the development of lamellar structure on solder surface. For Bi-Sn solder bumps on Au/Ni/Ti UBM, the Bi phases were mainly segregated on solder surface and two phase-like structures were observed. Anisotropic and faceted intermetallic compounds were found on the solder surface reflowed at slow cooling rate. As the cooling rate increased, the solder surface became smooth and the lamellar structure appeared. The effect on microstructural change and intermetallic formation of cooling rate and UBM structure will be discussed.