What is creep corrosion? How can we prevent it?
Editorial Team - PCB Directory
Mar 13, 2022
Creep corrosion is defined as a type of corrosion that creeps onto the surface of a PCB board over time. It is caused when copper on the surface of the PCB reacts with a sulfur (S) in a high moisture environment. Copper reacts with sulfur resulting in the formation of Cu2S on the surface of the PCB. This type of corrosion can occur even when the PCB board is not powered on. Creep corrosion on the surface of a PCB leads to the development of dendrites that can result in electrical shorts on the PCB.
A relative humidity level of 45-55% along with some sulfur in the environment can result in creep corrosion. These corrosion attacks can result from exposed copper on the PCBs, such as non-plugged vias, ESD strips, press-fit connectors, guard traces, and test points. PCB areas with increased airflow will see greater attacks since more sulfur and moisture are present on the board in these areas.
Analysis of creep failures shows that the corrosion product (Cu2S or Ag2S) is fairly resistive, so bridging/shorting of two conductors does not cause immediate failure. The PCB can continue operating despite excessive corrosion. But, over time, as the corrosion product increases in thickness, the resistance decreases, leading to functional shorting.
The main part of printed circuit board that is affected by creep corrosion is the PCB surface finish. The surface finish is the coating on the bare copper area of the PCB to provide a solderable surface and to protect the exposed copper circuitry. Various PCB surface finish methods are the Hot Air Solder Level (HASL), Lead-free ASL, Organic solderability preservative (OSP), Immersion Tin (ISn), Immersion silver (ImAg), Electroless Nickel Immersion Gold (ENIG), Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG), and Gold – hard gold surface finish.
Creep corrosion particularly occurs on PCBs with an immersion silver (ImAg) surface finish, but can also occur in NiPd (lead frames) and to a lesser extent in ENIG and OSP surface finishes. Immersion silver (ImAg) would tarnish in the presence of sulfur (i.e., Ag2S forms on the surface).
The primary industries that have high in sulfur environments include water treatment, fertilizer, paper mills, mining (smelting), rubber manufacturing (from vulcanization), petrochemical, clay modeling, and manufacture of sulfuric acid.
When considering all lead-free (LF) products in the market, the failure rate due to creep corrosion are very small. However, even a small number of failures are often not acceptable, especially in Industry. In these cases, it is important to monitor and minimize creep corrosion.
How to prevent creep corrosion?
Click here to learn about other forms of corrosion on PCBs.
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