Characterization, Prevention and Removal of Particulate Matter on Printed Circuit Boards

Particulate matter contamination is known to become wet and therefore ionically conductive and corrosive if the humidity in the environment rises above the deliquescence relative humidity (DRH) of the particulate matter. In wet condition, particulate matter can electrically bridge closely spaced features on printed circuit boards (PCBs), leading to their electrical failure. Failures attributed to particulate matter have even been observed in data centers where the gaseous contamination levels are low enough to meet the ANSI/ISA-71.04-2013 G1 severity level. The combination of miniaturization of electronic components, the reduction of feature spacing on PCBs and the loosening of the data center temperature and humidity envelope to save energy is making electronic hardware more prone to failure due to particulate matter. The characterization of particulate matter on PCBs is challenging because of the small amount of particulate matter available for analysis. The objective of this paper is to develop and describe a practical, routine means of measuring the DRH of minute quantities of particulate matter (1 mg or less) found on PCBs. Data center particle filtration schemes and means of removing the particulate matter from PCBs will also be presented. Introduction The physical environment surrounding a printed circuit board (PCB) is defined by the temperature, humidity and gaseous and particulate contamination in the air. Environmental factors can cause PCBs to fail in two ways: First, electrical open circuits can result from corrosion, such as the corrosion of silver terminations in surface mount components. Second, electrical short circuits can be caused by (a) copper creep corrosion, (b) electrochemical reactions such as ion migration and cathodic-anodic Filamentation or (c) settled, hygroscopic particulate matter contamination reducing the surface insulation resistance between closely spaced features on PCBs. In 2006, the European Union’s RoHS directive banning the use of lead in solders led to changes in PCB finishes and the elimination of lead from solders [1]. These changes dramatically increased the PCB failure rates due to creep corrosion. Another common failure mode during this period was that of surface mount resistors suffering open circuits due to the corrosion of their silver terminations. The information technology (IT) equipment manufacturers have since learned to make their hardware robust against these two failure modes, which used to occur predominantly in geographies with high levels of sulfur bearing gaseous contamination [2-4]. The failure mode that is much more difficult to deal with and eliminate is that of the electrical short circuiting caused by the accumulated particulate matter in humid environments. The difficulty arises from the intermittent electrical nature of these particles and that the failure leaves no visible evidence besides the presence of deposited particulate matter [5, 6]. The rapid expansion of the IT equipment market in the polluted geographies of Asia that have high levels of fine particulate matter in the ambient air and the increasing use of free cooling is introducing this new, often intermittent, short-circuit failure mode due to particulate matter. The source of particulate matter is both natural and anthropogenic. In terms of size, particulate matter can be divided into two categories: fine and coarse particles. Fine particles (<2.5μm), such as those found in motor vehicle exhaust, diesel particulate matter (DPM), smoke and haze, are of two types: primary and secondary [7, 8]. The primary fine particles are directly emitted from a source, such as a forest fire, volcanoes, construction sites, unpaved roads, fields or smokestacks. The secondary fine particles, which make up most of the fine particulate pollution, are those formed as a result of photochemical reactions in the atmosphere. This is generally due to the presence of oxides of nitrogen and sulfur emitted from power plants, industries and automobiles. Sulfur dioxide and nitrogen dioxide interact with <0.1 μm size carbonaceous material seed particles in a complex, multi-step photochemical process to produce sulfuric and nitric acids. These acids are neutralized by ammonia from fertilizers, decay of biological materials and other sources to produce fine particles dominated by ammonium sulfate, As originally published in the IPC APEX EXPO Conference Proceedings.