Microstructure and Surface Treatment of 304 Stainless Steel for Electronic Packaging

A fundamental study is carried out to investigate the microstructure and surface treatment possibilities of 304 stainless steel (304SS) substrates. To expose the microstructure, mirror-finished 304SS is applied with a self-mixed etchant containing ammonium sulfite ((NH₄)₂SO₃), iron chloride (FeCl₃), hydrochloric acid (HCl), and nitric acid (HNO₃). Scanning electron microscope (SEM) images exhibit quite clear rocky-wall-like structure. Grain boundaries are clearly observed. Each grain contains many subgrains and the average grain size is 10 μm. Energy dispersive X-ray analysis determines the composition as 69 wt. % iron (Fe), 19 wt. % chromium (Cr), 10 wt. % nickel (Ni), around 2 wt. % manganese (Mn), and less than 0.08 wt. % carbon (C), which agrees well with the data provided by the manufacturer. Diffraction peaks produced by X-ray diffraction (XRD) are able to correspond to profiles found in XRD database, showing that 304SS has face-centered cubic crystal lattice. The Ni strike technique is used as surface treatment to make 304SS bondable to other metals, such as silver (Ag), copper (Cu), and gold (Au), commonly used in electronic packaging. Cross section SEM images show that thick Ag, Cu, up to 50 μm, and Au, up to 70 μm, were successfully plated over the thin Ni layer that was plated on 304SS.