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Printed circuit boards showing signs of corrosion? Think Dew Point!

We currently have a number of clients trying to establish why their PCBs and other electrical conductors are showing signs of corrosion. The problem boards we are testing are located within enclosures either by the sea or close to poor air quality regions due to industrial stack emissions in high humidity geographical locations.

One can be forgiven for assuming accelerated corrosion was due to the in-service location by the sea (increased airborne salts) or to sulphate and nitrate air contamination from heavy industrial areas causing an increase in air moisture acidity including acid rain.

Although these factors can indeed increase the rate of corrosion and weathering, consideration to the dew point may be the root cause of these corrosion mechanisms due to the inverse relationship between temperature and humidity (Relative Humidity or RH).

The dew point is the temperature when the surrounding air can no longer hold the amount of moisture. As the air cools during the evening and night it condenses initially as tiny droplets onto surrounding surfaces. This is a particular problem if PCBs are housed within an enclosure (perhaps IP rated) with no form of forced air flow or other mechanism to provide adequate ventilation. Examples are cash point machines, parking meters, petrol pump control panels, electrical sub-stations, rail track point change DUTs, etc..

We have reproduced the dew point effect within poor ventilated enclosures many times in our climatic chambers when a pre-programmed test profile simulating a typical tropical climate of 40oC and 80 – 90% RH for 12 hours followed by 20oC and 50% RH for 12 hours – similar to that of day and night at geographical locations such as Florida USA and the South East Asian tropics.

This test profile in our experiment was repeated for 10 cycles representing 10 days and nights.

Once the climatic chamber had reached the 40 / 85, a simple Tupperware container was opened for 10 seconds then closed allowing the air at 85% RH into the container. This container, once closed had poor ventilation.

All is fine for 12 hours while the temperature remains constant, but as soon it drops by 15 – 20oC (as it does at night) the saturated trapped air begins to condense as visible water droplets form on the inside of the container.

No salts or other air contaminants were introduced during these experiments. The situation within the containers was helped when the lid had was 20% open. (Improved ventilation – less water droplets formed).

Perhaps we should repeat this simple experiment with a new PCB placed inside our Tupperware containers perhaps with a small piece of coiled copper wire or tube and watch them turn white and green!!


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