Creep corrosion failure analysis on ENIG printed circuit boards

The proliferation of the Internet of Things (IOT) and the increasing reliance on cloud computing are critically dependent on the reliable operation of Information Technology (IT) equipment. The associated increase in the power consumption by IT equipment is putting pressure on the data center administrators to expand the temperature-humidity operating envelope and in some cases resorting to airside economizers, thereby, exposing their mission critical IT equipment to gaseous and particulate contamination, especially in regions with high pollution levels. It is therefore incumbent on the IT equipment manufacturers to not only improve the performance of their products but to also make them more robust against the harsher environments in which their products are expected to operate. It is necessary to develop more accurate failure models and as well as more reliability qualification test methods for the known failure modes such as the creep corrosion on printed circuit boards (PCB). Creep corrosion is the corrosion of copper and sometimes silver metallization on PCBs and the creep of the associated corrosion products to the extent that the corrosion products electrically short circuits closely spaced metallized features on PCBs. An iNEMI task force has been successful in developing a flowers of sulfur (FOS) based method for the qualification testing of PCBs for robustness against creep corrosion. The test has proven successful in correctly predicting the high propensity to creep corrosion on PCBs from lots that suffered creep corrosion in the field. The test has also consistently shown very extensive creep corrosion on PCBs from an iNEMI experimental test lot with electroless nickel on immersion gold (ENIG) finish. Since the ENIG finished PCBs are very popular in the consumer electronics industry due to their excellent solderability, there is an urgent need to determine the root cause of their susceptibility to creep corrosion and to provide a solution that will make them more robust against creep corrosion. The morphology of the ENIG finish and the creep corrosion products were analyzed using a range of analytical tools including optical microscopy (OM), focused ion beam (FIB), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Preliminary cross-section analysis has shown that there is a gap formed between two nickel layers at the edge of the solder mask. The gap provides an easy path for copper ion migration and for copper sulfide creep. In an effort to improve the robustness against creep corrosion, ENIG finishes with various phosphorus content and post-treatments were studied. The paper will present the root cause of the severe creep corrosion on the ENIG finished PCBs and ways to improve their robustness against creep corrosion.