Contamination and moisture effects on printed circuit board reliability
Printed circuit boards (PCBs) can suffer from a variety of problems if the surface is contaminated with electrically conducting materials. Combined with moisture, this results in a lowering of resistance between tracks and pads and can lead to corrosion of metals. It can also result in the formation of metal filaments, which grow between pads or tracks on rigid or flexible circuits and between oppositely charged metal terminations of components, such as MLCCs, or between the pins of connectors.
The essential conditions required for this are a combination of ionic contamination, moisture and an applied voltage. The process is electrochemical with metal dissolving at one “electrode” – the anode, and being electrodeposited at the opposite electrode – the cathode. The electrodeposited metal normally takes the form of “dendrites” such as those shown below.
Dendrites can be silver, copper, tin, lead or a combination of metals and cause failures in electrical equipment by short circuits. Dendrite growth can be very rapid; failures have been known to occur in less than 30 minutes but can take several months or more. The rate of growth is dependant on the applied voltage, the quantity of contamination and surface moisture. The amount of contamination required for silver dendrites can be extremely small.
Ionic contamination can arise from fluxes used during soldering processes, from handling (fingerprints) and other materials such as dust and dirt. Standard test procedures have been developed to measure the level of contamination which essentially involve washing the whole PCB in a solvent and measurement of the ionic conductivity. The severity of the washing procedures vary from simply removing surface soluble ions to extracting materials that has been adsorbed within the PCB laminate.
The ionic contamination level is calculated from the ionic conductivity of the wash solution and the total board area. The result is an average value across the whole surface. The development of standards of cleanliness and standard test methods date back to the early 1970’s in the US, culminating in the publication of the original American MIL standards. It was from this work that the original pass/fail criteria of 10µg NaCl equivalence per square inch or 1.56µg per cm2 were first proposed. Cleanliness testing is now a routine procedure and manufacturers of both bare and populated boards are commonly asked to ensure their products meet contamination specifications based on this limit.
Equipment used under very dry conditions should not suffer from these problems unless there are large temperature fluctuations that result in condensation occurring on the surface of the circuitry or if the contaminants are hygroscopic and adsorb enough moisture to provide a liquid layer on the surface. However, very dry conditions are not recommended for delicate electrical equipment due to the increased risk from static discharge.
The ideal situation is constant temperature and a relative humidity of 40 – 50 %. At relative humidities above 50%RH, there is an increasing likelihood that water condensation will occur on the surface of circuitry when changes in air temperature occur. Where the PCB surface is at a lower temperature than the ambient air, this cools the air adjacent to the surface resulting in an increase in the local relative humidity. Condensation occurs when the temperature reaches the dewpoint. At the dewpoint temperature, the relative humidity reaches 100%RH and liquid water is formed.
At high relative humidity values but less than 100%RH, a thin moisture layer will be present on the surface which may be enough to decrease surface insulation resistance, cause corrosion or form metallic dendrites. The higher the humidity, the thicker is this moisture layer and the faster corrosion or dendrite growth can occur.
Dendrites cause short circuits when they bridge across tracks or between pads and in some cases this has resulted in arcing and fires. Corrosion leads to open circuits especially with very fine pitch circuitry. Water is essential for both of these processes and the wetter the surface, the worse these can be. However electrical equipment used at high humidity but less than 100%RH will have a thin moisture layer on the surface, which can be sufficient for damage to delicate circuitry. For this reason, particularly sensitive equipment is used in environments where both temperature and humidity are closely controlled. If the equipment is used in locations where this is not possible then an option is to use conformal coatings or potting but these can lead to a wide range of other technical problems if not used correctly.
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