Hermeticity lifetime of a 63Sn37Pb solder sealed optical fiber feedthrough

A highly accurate prediction of hermeticity lifetime is developed for a eutectic 63Sn37Pb alloy solder sealed optical fiber-Kovar nosetube feedthrough subjected to repetitive thermal cycling. Thermally induced fatigue fracture of the 63Sn37Pb solder/Ni-Au plated Kovar interface is initially generated from creep deformation of the solder and propagates gradually through the junction in the axial direction. A nonlinear axisymmetric finite element analysis of the 63Sn37Pb fiber feedthrough seal is performed using a thermo-elastic creep constitutive equation and the solder joint fatigue life prediction based on accumulated strain energy associated with solder creep imposed by temperature cycling is analyzed. SEM/EDX metallurgical analysis of the solder/Kovar nosetube interface indicates that AuSn₄ intermetallic formed during soldering may also contribute to joint weakening. The optical fiber metallized with Ni₂P-Ni underplate and electrolytic Au overplating exhibits correspondingly similar intermetallic formation at the solder/fiber interface. Combined hermeticity testing and metallurgical analysis carried out on 63Sn37Pb alloy solder sealed optical fiber feedthroughs after repetitive temperature cycling between -40 and +125°C validated the analytical approach taken to identify both the failure site and life expectancy