I live in an area whose local climate encourages corrosion. It is warm, humid, and near the ocean, though not near enough to get any salt-water spray.

Five years after building the house, the AC units are starting to quit, one by one. The outdoor condenser units have PC boards, which the repair guy says are “worn out”. He showed me small areas of slight discoloration on the boards. A yellowish tinge, which doesn’t look like copper corrosion (to me), and I’m surprised that it would affect electrical function. I’m getting a bit suspicious, since replacing the boards costs $250 a pop, and I’m reminded of the razor/blade and printer/ink business models. On the other hand, to be fair, replacing the boards does seem to fix the problem.

So, two questions:

  1. Is this a common problem with all AC units, or did I just get unlucky and buy ones that are especially susceptible to it?
  2. If this is a corrosion problem, is there some goop coating that I could apply that might prevent it?

Edit: To the people requesting photographs: I’m aware that these would be very helpful, but the guy took the boards away with him. Next time (I expect there will be a next time quite soon).

  • 5
    Would say "worn out" is not the correct term. Nothing really on a board to wear out, except maybe relays(parts that move). Connections being corroded is possible, but the boards should be protected from simple humidity(salt would be different). Five years does seem a bit early for failure, but electronics aren't what they used to be.
    – crip659
    Jul 11, 2022 at 0:23
  • 4
    Yeah, I thought “worn out” was just a dumbed-down explanation for the benefit of the bone-head customer (me).
    – bubba
    Jul 11, 2022 at 0:43
  • 5
    Certain components, especially cheap electrolytic capacitors, tend to be weak points in many consumer products. I've had to replace run/starting capacitors and relays (with welded contacts) on outside compressor units.
    – SteveSh
    Jul 11, 2022 at 11:51
  • 2
    Can you post a photo of the boards, coatings may help, hurt or do nothing at all as the electronics compartment is usually a dry location not exposed to the weather. A photo would be helpful at possibly figuring out what the failure actually is from.
    – Ed Beal
    Jul 11, 2022 at 16:27
  • 3
    Please, can you explain for me what you mean by 'corrosion-friendly'? It could mean 'susceptible to corrosion' or 'resistant to corrosion', depending on your point of view!
    – TonyK
    Jul 12, 2022 at 13:28

4 Answers 4


Some electronic components do wear out, for example electrolytic capacitors have a rated lifetime which depends strongly on temperature. The hotter it is, the quicker it'll die. Same for power semiconductors, which age due to electromigration. Most everything is sensitive to heating/cooling cycles which can crack solder joints. Relay contacts wear out. Vibration can break connectors, etc.

That doesn't mean it's not possible to make long-lasting electronics, it justs costs a bit more and requires the will to do it and a competent designer.

For example a capacitor rated for 16V, 1000 hours at 85°C, used on 12V, in 60°C ambient temperature, should last about 10 months. So the cheap chinese wall wart doesn't last long. If the manufacturer invests 50 cents more into a 25V, 10,000 hours at 105°C capacitor, it will last for decades...

enter image description here

^ If it looks like that, it means someone was way overoptimistic about the heat that the component that used to be there could dissipate. So it overheated and barbecued the board.

enter image description here

^ That's liquid damage. Water plus electricity equals electrolysis, so basically all lots of copper is going to be missing from that board. It's dead.

Another example of corrosion caused by sulfur in the air.

Now a board in an exterior AC unit should have conformal coating, which is a sort of varnish that protects it from moisture and contamination. It's very easy to spot, because it looks, well, varnished and shiny:

enter image description here

If the defective board doesn't have conformal coating and doesn't sit in an enclosure that it moisture-proof, then it won't last as long as you'd want. If such a product is intended for exterior use, I'd consider it defective by design.

  • 7
    Great answer. Thank you. My boards don’t look anywhere near as bad as your pictures. But I also didn’t notice the shiny appearance that indicates conformal coating. So, I’m leaning towards “defective by design”. Can I buy a suitable conformal coating in a spray can?
    – bubba
    Jul 11, 2022 at 13:18
  • 7
    Sure, it's available from most places that sell electronic components (mouser, farnell, digikey, reichelt, etc). Make sure to read the manual though, because some components really do not like varnish, especially connectors and switches... a contact with varnish on it isn't gonna work anymore.
    – bobflux
    Jul 11, 2022 at 14:59
  • High resistance contacts - those that aren't making good solid contact between the mating surfaces, are a major source of heat related damage and discoloration of boards. Like bobflux said, you would just exacerbate such a problem with indiscriminate application of an insulating material, like a board coating. When we coat boards, we take special pains to mask off any conducting (electrical and thermal) surfaces.
    – SteveSh
    Jul 11, 2022 at 15:06
  • 4
    If you can post a good resolution sharp picture of the damaged area?
    – bobflux
    Jul 11, 2022 at 15:26
  • 3
    I would add problems with lead-free solder reliability to the list of things that may go wrong. Not worth its own answer as it doesn't exactly fit the description of the OP's problem, but worth to keep in mind none the less.
    – Mołot
    Jul 11, 2022 at 23:21

Keep in mind that even if you're far enough away to avoid getting salt spray, There will still be a lot of salt in the air and with high humidity, that salt will conduct. I see that in South Florida a lot, but not that severe that it takes out a circuit board in five years. Do you use the same AC guy? Gooping up a circuit board would probably make things worse because those electrical parts need to be air cooled and any good goop would prevent that.

  • Maybe “goop” is the wrong word. I was thinking of some sort of thin spray-on protective film.
    – bubba
    Jul 11, 2022 at 0:53
  • 8
    The "goop" is called conformal coating: en.wikipedia.org/wiki/Conformal_coating
    – jnovacho
    Jul 11, 2022 at 9:44
  • 2
    A decent circuit board should have a conformal coating on it, especially one designed for outdoors use.
    – SteveSh
    Jul 11, 2022 at 11:48
  • 1
    @bubba Yes, search for "conformal coating spray". Check the specifications: some are only rated for a very limited temperature range (more than 100°C would be a good idea), and some need to be heat cured (you don't want that). A good brand will have the instructions available before you buy, so make sure you will be able to follow the application instructions. Jul 13, 2022 at 19:52
  • 1
    Will the application of any coating void any warranty? not sure if a warranty is still in effect after 5 years, but I would think you would want to apply the coating essentially as soon as you get the board. Could also ask the company if they have an alternative board that is pre coated.
    – Forward Ed
    Jul 13, 2022 at 21:55

If one is allowed to make the PCB 5% or 10% more expensive, it could be overengineered to outlive the hardware (compressor, tubes, heat exchanger, fan, etc).

It is not that hard - unless one is unlucky with a particularly sneaky part, it is only the power bearing elements that limit the lifespan of the board.

These are the power transistors, the big electrolytic capacitors, the relays and maybe some big diodes and resistors. A power transformer or an inductor may need some attention as well. Everything else (the CPU and the other digital and analog logic ICs, small resistors, inductors and capacitors that populate the board) are much less prone to aging and wear.

Moisture, salt and other chemical factors can be kept away from the board by using a proper enclosure and coating - really not expensive when done in a factory.

What a seasoned EE or a hobbyist does when confronted with a recurring problem (the same part burning over and over) that brings hassle and expenses:

  1. Get a brand new board (or debug the old one if you really have both the time and the knowledge)
  2. Identify the troublemaking part(s) on the broken board.
  3. Get the next bigger size for these parts and replace them for good. E.g. a 100W transistor is replaced with a 150W or 200W one that has the other parameters compatible. 10A relay gets a 25A replacement and so on.
  4. Mount everything back, swearing (the bigger parts may not trivially fit where they belong).

Done. You will get back to the problem when the real hardware breaks.

Applicable to home electronics, cars, private airplanes, weapons and everything having some electronics inside.


No the PCB do not wear out.

However some components might fail, not due to salt.

The darker coloring is normal for the areas with higher current flow.

The component are ceramic, plastic or corrosion resistant devices.

Your AC might rust away way before PCB.

  • 2
    Thanks. So, you think the service guy is either ill-informed or dishonest (or both)?
    – bubba
    Jul 11, 2022 at 9:30
  • 4
    @bubba while service guy might be ill-informed or dishonets, it's not because he's saying that "PCB failed" when in reality "some component on PCB failed"... That's just normal (simplified) language; e.g. as one would say "my car broke down" instead of "wiring on alternator in my car overheated leading to shorting of different electrical pathways needed for proper functioning of a car" Jul 12, 2022 at 19:47

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