High Temperature Thin Film Strain Sensor Calibration and Mounting
High temperature thin film strain sensor accuracy depends on rigorous apparent strain polynomial correction, dielectric barrier integrity, and drift mitigation.
Platinum Leadwire Micro Welding
Joining Technique
Localized electrical resistance or parallel-gap welding joins micro-diameter refractory leadwires to fragile sensor bond pads on high-temperature instrumentation assemblies. Performing platinum leadwire micro welding establishes low-resistance, fatigue-resistant electrical connections without melting the underlying insulating ceramic films. Wire diameters ranging from twenty-five to seventy-five micrometres are joined to sputtered or foil terminal pads.
Precise weld schedules govern current pulse duration, electrode force and peak voltage. The process creates solid-state metallurgical bonds capable of withstanding operating temperatures up to one thousand degrees Celsius.
Weld Metallurgy
High thermal conductivity and high melting points of platinum require precise energy delivery to prevent blowouts and wire vaporization. Thermal compression between copper or tungsten electrodes promotes solid-state diffusion across the platinum-substrate interface. Intermetallic compound formation must be avoided when joining platinum to dissimilar alloy bond pads.
Maintaining clean, unoxidized surfaces prior to welding ensures consistent joint resistivity. Weld interfaces exhibit high ductility, which resists thermal fatigue during temperature cycling.
Process Control
Micro-positioning stages and stereo microscopes provide optical alignment of fine wires over miniature gauge bond pads. Capacitive discharge or closed-loop direct-current power supplies deliver sub-millisecond current pulses with programmable energy profiles. Electrode force must remain strictly controlled to avoid punching through thin dielectric isolation layers into conductive base metals.
Post-weld non-destructive pull testing applies proof loads to verify mechanical joint strength. Welder calibration requires periodic verification of electrode tip geometry and electrical contact resistance.
Failure Boundary
Excessive weld energy causes micro-cracking within underlying ceramic insulation layers, resulting in low resistance to ground. Insufficient energy creates weak adhesion joints that fail under aerodynamic shear forces or mechanical vibration. Leadwire routing must provide strain relief loops to accommodate differential thermal expansion between wire and substrate.
Micro-welds exposed to high-velocity hot gas streams require mechanical over-coating with ceramic cement patches. The joining process is unsuitable for applications where base alloys undergo liquid-phase eutectic reactions with pure platinum at elevated temperatures.