Bonding of Si chips to low carbon steel boards using electroplated Sn solder

The purpose of this research was to develop a new process to bond silicon (Si) chips to low carbon steel substrates using pure tin (Sn) without any flux.

Iron (Fe) substrates were first electroplated with a Sn layer, followed by a thin silver (Ag) layer that inhibits Sn oxidation thereafter. It is this Ag capping layer that makes the fluxless feature possible. Fluxless processes are more environmentally friendly and more likely to produce joints without voids. The Si chips were deposited with Cr/Au dual layer structure. The bonding process was performed at 240°C in vacuum. The Sn joint thickness was controlled by spacers during the bonding. Scanning electron microscopy images on cross sections exhibited quality joints without visible voids. Energy dispersive X-ray spectroscopy analysis was used to detect joint compositions.

It was revealed that the Sn layer was bonded to a Si chip at the Cr–Sn interface and to the Fe substrate by forming an FeSn₂ intermetallic compound (IMC). The IMC is only 1.1 to 1.5 µm in thickness. Thin IMC is highly preferred because IMC deforms a little in accommodating the coefficient of thermal expansion (CTE) mismatch between Si and Fe. Shear test results showed that the fracture forces of the samples passed the military criteria by a wide margin.

This new fluxless bonding process on Fe should make Fe or low carbon steel a more likely choice of materials in optical modules and electronic packages.