Influence of Second Phase Particles on Thermal Conductivity of Bi Alloys

High-Bi alloys are being explored to understand their potential as replacement for high-Pb alloys in high-temperature die-attach applications. Thermal conductivity of these alloys is an important consideration for die-attach applications, where heat dissipation is necessary for reliable operation of the devices. Pure Bi has a thermal conductivity of about 8 W/
$\text{m}\cdot \text{K}$
, which is the lowest among metals. The addition of the alloying elements to Bi had been explored to tailor its thermal, mechanical, and other physical properties. In this study, the role of Cu and Sb on the effective thermal conductivity of the resultant alloys was investigated. The thermal conductivities of these alloys in the bulk form and the three-layer die-attach form were measured using a flash diffusivity technique. Test specimens were developed to replicate a die-attach assembly between the Ni-metallized Si die and Cu substrate using the Bi–
$x$
Sb–10Cu alloy (where
$x =10$
–20 wt%). The three-layered structure was modeled with the unknown diffusivity of an intermediate layer of the Bi alloy. The alloy microstructure comprises a composite of Bi–Sb solid solution filled with Cu
₂
Sb intermetallic particles. The presence of these intermetallic phases is responsible for an effective increase in thermal conductivity of this alloy. With the optimized microstructure developed, the resultant thermal conductivity was obtained at
$\sim 24$
W/
$\text{m}\cdot \text{K}$
, which is a threefold increase compared to pure Bi.