PCB Corrosion: Causes and How to Clean and Prevent.

There are multiple types of corrosion, but general attack corrosion is the most common. With general attack corrosion, oxygen and moisture in the air react with exposed copper surfaces. This reaction forms a copper oxide, which builds up over time and increases the electrical resistance of the copper traces.
As the copper oxide accumulates, it can exceed the tolerance thresholds for circuits and cause electrical failures. However, although the conductivity decreases, the bulk mechanical properties of the copper typically remain intact.

General corrosion affects the entire exposed copper surface more or less uniformly. This makes it easier to detect through visual inspection compared to localized corrosion, which impacts localized, isolated areas. Preventing general corrosion also tends to be simpler as it mainly requires enclosing the circuit board in a protective barrier like a conformal coating to block air and moisture.

Atmospheric corrosion is the most prevalent form of corrosion affecting printed circuit boards (PCBs). It occurs when moisture and oxygen in the atmosphere react with exposed metal traces on the PCB. All metals are susceptible to atmospheric corrosion to some degree. However, copper is particularly prone to it due to its high reactivity. Copper is often used for circuit traces, plated through holes, and other conductive elements in PCBs.

The atmospheric corrosion process starts when moisture accumulates on the exposed metal surface. Water molecules adsorb to the surface, forming a thin film. Oxygen then diffuses through this film and reacts with the copper metal. The corrosion reaction forms copper oxide, which has a lower electrical conductivity than pure copper. Over time, as more copper oxide forms, the electrical resistance of the copper traces increases. Once the resistance exceeds tolerance thresholds for a given circuit, electrical failures can result.
While atmospheric corrosion degrades the electrical performance of copper traces, their mechanical properties – such as hardness and tensile strength – are often still intact. This is because only a thin surface layer is typically affected by atmospheric corrosion. The bulk of the copper remains unchanged.

Localized corrosion refers to corrosion that impacts limited, isolated areas of a copper circuit. There are 3 main types:

1. Filiform corrosion: Filiform corrosion occurs when moisture penetrates beneath the surface finish and contacts exposed defects in the copper cladding. This establishes an electrolytic corrosion cell that allows an electric current to flow laterally. As the cell propagates, it consumes copper in its path, degrading large sections of circuitry over time.
2. Crevice Corrosion: Crevice corrosion develops under components where residual contaminants trapped within crevices provide an electrolyte that supports a corrosion cell. Corrosion primarily attacks the base of the crevice where copper is exposed. As crevice corrosion progresses, it penetrates through copper traces, eventually severing them.
3. Pitting Corrosion: Pitting corrosion forms small cavities or “pits” in the copper cladding due to localized galvanic corrosion cells. Copper is consumed at the pit base while hydroxyl ions accumulate within the pit, creating an acidic environment that accelerates corrosion. The pits grow both deeper and wider over time, eventually penetrating copper traces and causing electrical failures. However, corrosion deposits within the pits often obscure pitting corrosion, making it difficult to visually detect.

Galvanic corrosion occurs when two dissimilar metals are in electrical contact within an electrolyte. In PCBs, this usually involves copper traces coupled with another metal component like gold or tin. When exposed to an electrolyte like moisture, an electrochemical cell is formed. Electrons flow from the more active metal (anode) to the less active metal (cathode), corroding the anode metal at an accelerated rate.
The key difference between galvanic and pitting corrosion is that galvanic corrosion requires two different metals in electrical contact, whereas pitting corrosion involves only one metal. Galvanic corrosion typically affects the entire interface between the two metals.

When ionic contamination is present in moisture that contacts copper traces, dendritic growth can occur on the copper surfaces. Dendrites are branched, tree-like structures that form due to plating of copper ions from the electrolyte onto preferable crystallographic planes.

If adjacent copper traces grow dendrites that intersect, an electrical short circuit can result, potentially damaging circuitry. Dendrites are more likely to form where tracing is more dense and exposure to ionic contaminants is higher.

Intergranular corrosion affects the grain boundaries within copper traces. Grain boundaries tend to have a higher concentration of impurities and defect sites, making them more susceptible to corrosive attack.

When chemicals are present that can penetrate grain boundaries, they can initiate corrosion reactions at the interface between adjacent copper grains. This corrosion then spreads along grain boundaries, potentially separating entire grains from the trace and compromising its integrity.

Flux is essential during soldering to help the solder flow and adhere to metal surfaces. However, residue left from flux can be highly corrosive and cause problems over time.

Traditional fluxes contained chemicals like chlorine and other activated species that actively removed metal oxides during soldering. While effective, residues from these fluxes were highly corrosive and had to be thoroughly cleaned after soldering.
Modern no-clean fluxes use organic acids that decompose at soldering temperatures, leaving behind less corrosive residues. This is usually not an issue for reflow soldering where components experience high temperatures that fully decompose the flux. However, wave soldering may not reach temperatures high enough to fully decompose the flux, leaving acid residues behind. Click to read more detailed wave soldering and reflow soldering process.

Even small amounts of residual flux can cause corrosion problems over time as the acid residues absorb moisture from the environment. This forms an electrolyte that aids electrochemical corrosion reactions. The acids also lower the pH, further accelerating corrosion.

Fretting corrosion occurs due to small reciprocating motions between two contacting metal surfaces. In PCBs, this commonly happens with solder-plated switches as they open and close repeatedly during operation.
Each time the switch closes, the contact points rub against each other in a wiping action. This removes some of the thin oxide film that normally protects the metal surfaces from corrosion. When the oxide layer is breached, the exposed metal can then react with oxygen and moisture in the air to form corrosion products.

Electrolytic corrosion occurs when an electrolyte is present between two conductive traces that have a voltage potential. This allows ionic current to flow and metal ions to migrate, resulting in the plating or deposit of metallic dendrites that can cause short circuits.
For electrolytic corrosion to occur, three things are needed:

• 1) an electrolyte solution containing ions;
• 2) conductive traces that act as electrodes;
• 3) a voltage potential difference between the traces.

When these conditions exist, the conductive traces become the anode and cathode of an electrolytic cell. Ions in the electrolyte migrate toward the cathode trace, gaining electrons and plating out as tiny metal whiskers called dendrites.
The dendrites continue to grow as more metal ions plate out, driven by the electrical potential. Eventually, these dendritic projections from adjacent traces may contact each other and cause an unintended electrical short.