Printed circuit boards (PCBs) can suffer from a variety of problems if the surface is contaminated with electrically conducting materials. When combined with moisture, contamination results in a lowering of resistance between tracks and pads that 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 or between the pins of connectors. The essential conditions required for this are a combination of ionic contamination, moisture and an applied voltage.
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. 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.
Given the spacing between components of the integrated circuits (ICs), when a voltage is applied to a device, voltaic gradients on the order of megavolts/cm can exist across surfaces, accelerating electrochemical corrosion reactions and ionic migration. In ICs, positively biased aluminum metallization is susceptible to corrosion. Combination of the electric fields, the atmospheric moisture, and the contamination by halides leads to corrosion attack on any metals.
Negatively biased aluminum metallization can also corrode in the presence of moisture due to high (basic) pH produced by the cathodic reaction of water reduction. High pH leads to dissolution of the passive surface layer of oxides and aluminum substrate with the corresponding increases in conductor resistance (up to an open circuit).
In the presence of moisture and an electric field, silver ions can migrate to a cathodically (negatively) charged surface and plate out, forming dendrites. The dendrites grow and eventually bridge the gap between the contacts, causing an electric short and possibly arcing and fire. Even a small volume of dissolved metal can result in formation of a relatively large dendrite. Other materials susceptible to the metal migration include gold, tin, lead, palladium, and copper.
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.