Failure Analysis of Diode Components of Electronic Relay Packages Via Thorough Microstructural Characterization

Abstract

Diode components used in the electronic relay packages of avionics of a weapon system had been found corroded. Two sets of the complete assembly of the damaged diode parts consisting of diode body, plain washer, spring washer and mounting nuts were analysed for finding out the root causes of failures. Detailed metallographic characterization of the failed materials was carried out using optical microscope, scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) analysis within SEM and microhardness study. Inadequate electroless nickel (EN) plating along with significant amount of porosity and presence of MnS type of stringers affected the corrosion resistance of the diode body tremendously for both the sets. Inadequate electroplated Zn layer was observed on spring washer, plain washer and nut, wherein both uniform and pitting corrosion were detected. Important recommendations included: (i) thicker (more than 25 µm) EN coating with as-specified porosity level for diode body, (ii) replacement of electroplated zinc layers with galvanized zinc (of higher thickness) layer on all the fasteners, (iii) precaution in minimizing formation of galvanic couplings observed in the present system, like that between EN-coated diode body and zinc coated fasteners, between mild steel and stainless steel fasteners etc.