The microstructure characterization of ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallization

Abstract

The microstructure of the ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallizations (UBMs) was investigated as a function of cooling rate. The ultrasmall eutectic Bi-Sn solder bump, about 50 µm in diameter, was fabricated by using the lift-off method and reflowed at various cooling rates using the rapid thermal annealing system. The microstructure of the solder bump was observed using a backscattered electron (BSE) image and the intermetallic compound was identified using energy dispersive spectroscopy (EDS) and an x-ray diffractometer (XRD). The Bi facet was found at the surface of the ultrasmall Bi-Sn solder bumps on the Au/Cu/Ti UBM in almost all specimens, and the interior microstructure of the bumps was changed with the solidification rate. The faceted and polygonal intermetallic compound was found in the case of the Bi-Sn solder bump on the Au (0.1 µm)/Ni/Ti UBM, and it was confirmed to be the (Au 1-x-y Bi x Ni y )Sn 2 phase by XRD. The intermetallic compounds grown form the Au (0.1 µm)/Ni/Ti UBM interface, and they interrupted the growth of Bi and Sn phases throughout the solder bump. The ultrasmall eutectic Bi-Sn solder bumps on the Au (0.025 µm)/Ni/Ti UBM showed similar microstructures to those on the Au/Cu/Ti UBM.

Thermodynamic assessment of phase equilibria in the Sn-Au-Bi system with key experimental verification

“…[12][13][14][15] All the results show that the solubility of Bi in the AuSn 2 and AuSn 4 phases is very limited, which agrees well with the results reported in the previous literature. [12][13][14][15] All the results show that the solubility of Bi in the AuSn 2 and AuSn 4 phases is very limited, which agrees well with the results reported in the previous literature.…”

“…6 Recently, the present authors have developed a thermodynamic database for phase diagrams in micro-soldering alloys including the elements of Pb, Bi, Sn, Sb, Cu, Ag, and Zn. [10][11][12][13][14][15] In the present work, the phase equilibria at 200 C, 250 C, 300 C, and 400 C in the Au-rich portion of the Sn-Au-Bi system and phase transformation were experimentally investigated, and the phase equilibria in the Sn-Au-Bi system were thermodynamically assessed by the CALPHAD method. [10][11][12][13][14][15] In the present work, the phase equilibria at 200 C, 250 C, 300 C, and 400 C in the Au-rich portion of the Sn-Au-Bi system and phase transformation were experimentally investigated, and the phase equilibria in the Sn-Au-Bi system were thermodynamically assessed by the CALPHAD method.…”

Micro-scale metal contacts for capillary force-driven self-assembly

“…Other alloys are used in modern flip-chip bonding or die attach processes, where molten alloys also provide capillary forces for precision alignment, mechanical and electrical connections. The diameter of alloy bumps used in these applications has been shrinking from 100 µm and larger to 46 µm [15], 30 µm [16] and 25 µm [17,18]. In these processes, previously deposited alloy bumps are briefly heated above the alloy melting temperature to 'reflow' the alloy and form intended connections.…”