Have you ever seen corrosion on the solder joints on an old PCB? Or on the aluminium side of a power supply? Why we do have corrosion, and how can we stop it?
Every time we connect different metals with different Fermi levels, the chances are you have created a diode. Zener diodes work just based on this principle. Under proper environmental conditions (e.g. high humidity, water or salt), we may have an electrochemical reaction that will corrode the "Anode". For example, on a joint Aluminium Copper, the Aluminium will rust. The rust will happen in a place where some water may condensate. Without water, this process can't start. In the attached drawing, I have shown how something we do all the time can go wrong. Imagine we are using a PCB connected to ab aluminium base, and we use steel screws (same anodic voltage as copper) to join the PCB (copper) to the aluminium plate. If we have condensation in the proximity of copper/aluminium, then there will be some corrosion around that joint. I'm sure we have all seen this before.
If you google "Galvanic corrosion", you will find tables and a spreadsheet with the anodic voltage of many metals and materials. I suggest choosing the materials with the closest anodic voltages as much as possible to avoid surprises.
For Harsh environments (outdoor, high humidity/salt), I use material with less than 0.15V difference only! And if I can't, I mitigate the corrosion with conformal coatings, insulating, plating, etc.
For normal use (home appliance), I set 0.25V in my designs.
So, in the future, when you bolt your aluminium enclosure to your PCB, I'm sure you will do some research first, and you'll ask the right questions to your customer.
Have you had this issue before? please share your experience with us.
Bye now!
Written by Francesco Poderico
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