Intermediate Low-Melting-Temperature Solder Thermal Cycling Enhancement Using Bismuth and Indium Microalloying

In general, Sn-Ag-Cu solder is widely used for interconnections in semiconductor device packaging. However, recently, several factors have been considered to implement low-melting-temperature solder (LTS), which has a lower assembly tem-perature than conventional Sn-Ag-Cu solder material. Implementation of LTS solder though has a different driving force per each industry sector. Consumer electronics have a driving force for lower energy consummation towards a carbon net zero strategy compared to the high-performance chip industry sector, which has a different reason based on larger com-ponent size-induced challenges, like dynamic warpage. This is a deformation of printed circuit board (PCB) and package components during the reflow process by elevated temperatures. The behavior of dynamic component changes the package size, material characteristics, and temperature range. Although most of the LTS are based on the low-melting-temperature range of 130-140 degrees C, a separate category of intermediate LTS is formed at around 180-190 degrees C to target an assembly peak temperature of 200-210 degrees C. The study presented here targets a LTS at an intermediate temperature assembly to avoid the most active dynamic warpage temperature region. LTS has significant benefits with less warpage and thermal damage towards the component and assembled board, due to the low reflow peak temperature. To improve the thermal cycling performance by maintaining a low melting temperature, a small amount of indium is used as a microalloy element, with 12 mm x 12 mm ball grid array components on 62-mil-thick boards thermal cycled from - 40 degrees C to 125 degrees C with Sn-based LTS including In and Bi. The microstructure changes during thermal cycling have been observed and electron-backscattered diffraction has been used to find a correlation between crack propagation and localized recrystallization. It was found that the added indium enhanced the thermal cycling performance compared to conventional Sn-Ag-Cu-based solders. To compare the paste-induced composition change which dilutes the indium-containing solder ball, a flux-only assembly has been compared.