Ag-Au-Ge Alloys For High Temperature Geothermal and Oil Well Electronics Applications
Authors: P. T. Vianco, R. Grant, J. A. Rejent, T. B. Crenshaw, and A. C. Kilgo Company: Sandia National Laboratories Date Published: 10/14/2012
Abstract: Today, the "new" harsh environments are in energy-related fields where the exploration, production, and management of both fossil and non-fossil fuel resources are forcing sensor and data logging electronics to be exposed to increasingly more extreme conditions of temperature, vibration, and corrosion. A study was undertaken to explore the suitability of the 12.62Ag – 77.32Au – 10.06Ge (wt.%, abbreviated Ag-Au-10.06Ge) for downhole oil and geothermal applications having service temperatures as high as 350°C. The solidus (Ts) and liquidus (Tl) temperatures were approximately 405°C and 431°C, respectively. The substrate materials were the Fe-Ni-Co alloy, Kovar™ (Carpenter Technologies, Reading PA) and alumina having a Ni-over-Mo/Mn metallization. The metrics of mechanical strength (stress-strain, 0.46 mm/min) and hermeticity (He leak test) metrics were used to document the integrity of the ASTM F19 joint configuration. The Ag-Au-10.06Ge solder exhibited excellent wetting of both substrates. Poor performance was observed when there was too small of a solder volume due to a "reverse" transient liquid phase (TLP) effect. The reverse TLP pulled Ge from the filler metal, resulting in constitutional solidification. Hermetic joints were made with (room temperature) pull strengths of 111±7 MPa using a peak reflow temperature (Tp) as low as 455°C. When the joints were tested at 300°C, the pull strength decreased slightly to 74±12 MPa. Data indicated a service temperature ceiling of at least 350°C. for the Ag-Au-10.06Ge solder.