PCB corrosion is a major cause of failures in electronics. What corrosion does is it increases the resistance of the copper traces on a circuit board and as it accumulates, the board will start to work inefficiently or stop working altogether. Understanding the common causes of PCB corrosion and ensuring they are cared for properly can allow PCBs to work properly throughout the duration of their use.
Corrosion is the process of oxidation that occurs when oxygen bonds with metal, resulting in rust and causing the metal to flake off and or lose its valuable chemical properties. Since PCBs are largely made of metal and are exposed to oxygen, they eventually corrode. However, different kinds of metals will corrode at different rates. Some are more resistant to corrosion, while others corrode rather easily. An example of this would be how copper alloys, silver, gold, and graphite resist corrosion indefinitely while copper, lead, thin layers of tin and nickel are highly susceptible to corrosion. This is one reason metals such as gold and silver are also known as noble metals, while metals that corrode easily such as tin and copper, are known as base metals.
Since circuit boards contain high amounts of copper in the form of copper layers and traces, circuit boards are highly susceptible to corrosion.
Types of Corrosion
One can diagnose corrosion and how they lead to PCB failures by understanding the different types of corrosion.
General Attack Corrosion
The most common type of corrosion, also known as atmospheric corrosion or uniform attack corrosion, is generally caused by a chemical reaction between the oxygen/moisture in the air and copper. That reaction causes the formation of copper oxide. The low electrical conductivity can cause problems in PCBs however the mechanical properties stay intact. This is generally easy to diagnose and prevent.
Localized corrosion as its name implies, is limited to a small area and may be of one of three types:
Filiform Corrosion: Moisture entering under the surface finish can cause a defect in the copper, which then has the potential to spread throughout more of the board.
Crevice Corrosion: Areas under hardware or components on circuit boards can have corrosion in these crevices due to left over matter. Flux, cleaning solution, or other contaminants within such crevices commonly cause this type of corrosion.
Pitting Corrosion: This type of corrosion is seen as cavities or “holes” in the copper. Localized galvanic reactions cause deterioration leading to an increase in the pit diameter and depth, ultimately leading to failure. Corrosion-producing compounds often hide the pits, making it difficult to detect them.
Galvanic corrosion, also known as bimetallic corrosion, occurs when two dissimilar metals such as copper and the metal of a component, gold, or tin are coupled in a corrosive electrolyte. This is similar to pitting corrosion but the key difference is that galvanic corrosion occurs only between electro-chemically dissimilar metals when they are in electrical contact, and the metals are both exposed to the electrolyte.
Electrolytic Dendrite Formation
When there is ionic contamination in moisture, dendrites can form on copper traces. When adjacent copper traces grow dendrites, it can short and cause failure.
When there are chemicals on the grain boundary of a copper trace, inter-granular corrosion can occur. Grain boundaries often contain higher impurities that are more susceptible to such corrosion.
Preventing PCB Corrosion
If you’ve made it this far in the article, it should be apparent that corrosion is primarily due to the presence of moisture or electrolytic contaminants on a circuit board. When PCBs are used in extreme environments such as aerospace or marine, PCBs become highly susceptible to the different types of corrosion. OEMs, manufacturers, and consumers can prevent corrosion by ensuring a few things.
- Adequately cleaning the PCBAs during assembly and ensuring all flux residues have been removed.
2. Ensuring the PCBs stay dry
3. Making sure electrolytes don’t wet the PCB
4. Conformal coating the PCB assemblies to protect it from its environment
The main method to prevent PCB corrosion is by preventing moisture or other liquids from reaching your circuit board. One method is placing the PCBA inside an enclosure with a suitable IP rating.
When your PCB cannot be enclosed in a chassis or another type of protective covering, conformal coating may be utilized. Conformal coatings come in a variety of different forms but some include a simple solder mask, aerosol spray coatings, or epoxy coatings. These all help prevent PCB corrosion. Conformal coatings are not a solution for all boards though. Engineers will have to look at what parts need to be masked off or if the addition of conformal coating will affect the boards use and functionality.
PCB Corrosion Repair
Removing corrosion from a PCB is not a sure fire thing. Once it happens the board is compromised. However, you can clean it off and hope for the best. Make sure to remove all power from the board and isolate as best as possible. To do this you can take a cotton swab with a cleaning solution such as isopropyl alcohol, baking soda and water, or even vinegar to clean it.
Making sure your PCBs are suited for the environment in which they will be performing is essential in preventing corrosion. Making sure you have a trusted assembly partner will also ensure that contaminants don’t affect your board. Vinatronic has experience in PCB Assembly and manufacturing for over 25 years. Feel free to reach out if you have any more questions or need a PCB built up at the highest level of quality!