PCB Trace Corrosion: Why It Happens and How to Prevent It
When most people drop their cell phone in water, they hurriedly dry off the outside of the phone, remove the battery, and toss the phone in a bag of rice. The myth goes that the rice draws water out from the inside of the phone, thus saving your phone’s the precious electronic circuitry, your wallet’s money, and your own sanity.
What people don’t realize is that simply drying out your phone is not a solution. Water ingress can cause the device to short out, killing the phone immediately. But if the device does not break, creep corrosion will accumulate over time. Electronic devices start corroding as soon as they get wet, and they continue corrode as they are operated. This is typically what causes devices to fail.
If you ever took chemistry classes in college, you probably remember that different materials and plating will corrode at different rates when exposed to the environment. Moisture in humid air, air pollutants (atmospheric corrosion), or direct contact with water or an electrolyte solution like gatorade can cause electrical traces in a printed circuit board to start corroding. There are a number of corrosion mechanisms that degrade device performance or cause it to fail outright.
Electrochemical Corrosion Failure
There are two primary electrochemical corrosion mechanisms: galvanic and electrolytic. Galvanic corrosion occurs when two different metals are in electrical contact in the presence of an ionic contamination or an electrolyte solution. There is a natural voltage difference between the two metals (known as the “electrode potential”). As a result, current flows towards the more resistant metal, and the more resistant metal corrodes faster.
This type of corrosion typically occurs at electrical connections between components and traces. PCB corrosion begins once moisture or some other liquid comes into contact with the electrical connection. PCBs in industrial environments, humid environments, or other extreme environments are highly susceptible to this form of corrosion.
Don't let water end your circuit boards.
Electrolytic corrosion occurs when neighboring traces become contaminated by an ionic liquid or electrolytic solution. Small slivers begin to grow between the two traces. If the contaminant is water, metal oxides and hydroxides will start to grow on the exposed metal.
If two neighboring traces in a circuit board are close to each other, there is a chance that the slivers bridge the two traces, causing a short. This short circuit can disable a particular functional block in your PCB. If corrosion occurs between nearby power and ground lines, it could disable the entire device.
Aside from a neighboring copper trace, copper that is adjacent a surface ground plane can corrode when exposed to ionic or environmental contamination. In the presence of halogens, copper converts into copper halide compounds as halogen gases interact with the surface of exposed copper, causing tiny pits to accumulate over time. After soldering, any exposed copper should be cleaned and coated to prevent corrosion if the circuit board will be in an environment where it may be exposed to halogens.
Corrosion From Flux Residue
Flux residues left over from soldering have been a major cause of corrosion since the first time they were used. Solder flux is extremely important during soldering as it combats oxidation during soldering. The flux actually reduces and metal oxides that form at the soldered electrical contact due to heating. The flux also improves wetting of a solder bead to a solder pad.
Older flux materials contain corrosive chemicals like chlorine, and residues had to be removed after soldering. Newer fluxes use organic acids that decompose at high temperatures. This is not usually a problem when components are soldered with reflow soldering, and any leftover residue should not be a problem. But with wave soldering, the solder flux is less likely to reach the required temperature to decompose the acids. Any leftover flux residue is acidic and is highly corrosive.
Soldering iron and other equipment for soldering a circuit board
Preventing Corrosion in Your PCB
Copper and other base metals corrode rather easily, while noble metals and some alloys are highly resistant to corrosion. Some manufacturers can lay PCB traces using gold or gold-nickel alloy. Obviously, the cost of using a precious metal like gold increases the cost of your PCB, and many design rules that are used for copper traces will no longer apply. Gold is still useful as it is a good conductor and has been used in many low-current devices.
Preventing corrosion can be as simple as placing a conformal coating over exposed copper areas. Epoxy coatings, aerosol spray coatings, and solder mask are all effective barriers to oxidation and corrosion. If any coating is used, it should be able to withstand any thermal demands in a circuit board that runs at a high temperature. Identifying where to apply coatings takes experience. Sometimes, you don’t know which components in a circuit board are likely to corrode until they actually corrode.
No matter how you decide to combat corrosion during your PCB assembly, the built-in CAD/CAM tools in Altium Designer® give you the design power you need. You can download a free trial and find out if Altium Designer is right for you. Take time to learn more and talk to an Altium Designer expert today.