Fluxless tin bonding of silicon chips to iron

In high power electronic modules for automobiles, several alumina circuit boards are soldered to a copper (Cu) plate. In this project, we look into iron as an alternative basis plate material. The use of iron (Fe) dates back to prehistoric times. Iron was made into various steels by adding other elements. The most popular steel is carbon steel. Steel of low carbon content of 0.1-0.2% is a trade-off among various properties and was chosen for this study. Compared to Cu, iron is less expensive (about 1/25 the price of Cu). It has lower CTE of 11.5 ppm/°C vs. 17 ppm/°C of Cu, higher melting point (1450-1540 °C), and higher tensile strength (635-696 MPa). The only downside is its fair thermal conductivity of 80.4 W·m^-1·K^-1 vs. 401 W·of Cu. However, in a module design, this lower thermal conductivity can be compensated using thinner plate and thinner solder layer. To adapt this alternative iron base plate, iron must be made solderable. In this project, fluxless tin soldering process on low carbon steel is developed. Si chips are bonded to low carbon steel plates. Si chips instead of alumina substrates are used because Si provides the largest CTE mismatch to demonstrate the structure survival at extreme case. The low carbon steel is well cleaned and polished. Nickel (Ni) is electroplated over the steel, followed by Sn solder layer. Si chips are e-beam evaporated with chromium (Cr) as adhesion layer and gold (Au) as capping layer. The two parts are held together and bonded in 100 millitorr vacuum. Quality and microstructure of the joints are examined using scanning electron microscopy (SEM). Composition of the joints is studied with energy dispersive X-ray spectroscopy (EDX). Cross section SEM images exhibit high quality bonding.