Resistance Changes on Absorption and Desorption of Hydrogen from Evanohm Resistors

Hydrogen gas, the foaming agent in RTV-5370 silicone foam encapsulating plastic, caused resistance increases in thin-film Evanohm resistors. The hydrogen is a byproduct of the polymerization of hydrosiloxane groups with polymeric hydroxy functional siloxanes. The hydrogen gas is produced over an extended time period and trapped in the sealed circuit box. The gas then diffuses through the silicone resin coating and protective vinyl shrink tubing surrounding the metal film, with a rate that increases with pressure and temperature. The polymerization reaction of the foam can be driven to completion by heating in air at 100° C for 24 hours. Out-diffusion of the hydrogen gas from the affected resistors will occur on heating in air at 100° C for 48 hours or more. By fitting analytic solutions of the diffusion equation to the observed in- and out-diffusion data, the diffusion coefficient for the encapsulant surrounding the resistor film was found to be D = 3 x 10-¿4 exp (¿0.28 eV/kT) cm2/s. The resistance increase is caused by the high solubility of hydrogen in nickel. The hydrogen increases the residual resistivity but not the thermal coefficient of resistance (TCR) of the film. The sensitivity of nickel-containing resistor materials, such as Evanohm or Nichrome, to hydrogen suggests that hydrogen-producing encapsulants should not be used with nonhermetic devices and that no hydrogen should be included in the protective atmosphere of hermetically sealed devices.