Failure analysis of thick film resistors on stainless steel as sensing elements

Thick film technology based sensing elements are becoming a hot research in recent years and the study of their failure modes is badly needed. In this paper, the failure modes of the bonding strength between the 1.4016 stainless steel (SS) substrate and the dielectrics are investigated. Experimental results show that residual stress, surface roughness and cleanness of the diaphragm in the 1.4016SS substrate have a significant effect on the bonding strength of dielectrics on stainless steel. Pre-firing, sand-blasting and etch in a semi-concentrated nitric acid solution of the substrate are effective measures to improve the bonding strength of the substrate and the dielectrics. Thickness of the dielectrics and the amount of dielectrics layers are studied. Results show that insulation between the substrate and the dielectric will be very poor if single layer dielectric is screen printed and fired. Based on experimental results, three-layer dielectrics is suggested to be screen printed and co-fired or fired each layer separately on the substrate to improve insulation and reduce current leakage. The misalignment of screen printing results in the misalignment of the thick film resistors from the position of maximum strains and the accuracy of the resistors. Finite element modeling (FEM) is being conducted to predict maximum radial and tangential strains of the substrate to locate the position of thick film resistors (TFRs).