Low Temperature Diffusion Bonding of High Carat Gold Using in Situ Gold-Cadmium Alloys

Abstract

Gold-cadmium alloys are highly suitable for the solid state bonding of gold because their low melting temperatures lead to an enhanced mobility as compared to that in pure gold. Local gold-cadmium alloys are realized in pure gold and 18 carats gold foils. The cadmium diffusion into gold is performed from the thermal decomposition of cadmium sulphide in air at temperatures ranging from 650°C to 850°C. The quantitative analysis (three-dimensional concentration maps) of those alloys is achieved using proton induced X-ray emission (PIXE). From the distributions of Cd in depth we obtain the diffusion coefficients of cadmium into pure gold and 18 carats gold-copper-silver alloys.

Gold substrates locally prepared by this procedure are used as the first component for the diffusion bonding on another pure gold part. Optical and electron microscopies of solders show that the joint region appears as a simple juxtaposition of the two initial gold parts. Electron microscopy of a section through the contact region reveals that the interface almost disappears. The concentration of cadmium in the joint is often below the detection limit for the electron microprobe, but the high sensitivity of the PIXE microprobe is sufficient to determine the composition of the soldered region even with a Cd concentration below 0.1%. The redistribution of cadmium on about 30 microns