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.
Silver (Ag) foils are bonded to alumina substrates by a low temperature solid state bonding process. The alumina substrate is premetalized with 40 nm titanium tungsten (TiW) and 2.54 μm gold (Au). The bonding temperature is just 260 °C, compatible with the peak reflow temperature of lead-free (Pb-free) solders used in electronic industries. The Ag foil is quite soft and ductile. It can deform to mate with the Au surface on alumina. Thus, only 1000 psi of static pressure is needed to bring Ag atoms and Au atoms within atomic distance on the interface. Ag has superior physical properties. It has the highest electrical and thermal conductivities among the metals. Scanning electron microscope (SEM) images show that the Ag foil is well bonded to the Au layer on alumina. A standard shear test is performed to determine the shear strength of the bonding. The shear strength of five samples tested far exceeds the strength requirement of MIL-STD-883 G standard.
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