**PCB (Printed Circuit Board) Corrosion: Core Mechanisms and Common Types**

As the "neural center" of electronic products, the PCB undertakes core functions such as component fixation, electrical connection, and signal transmission. Its reliability directly determines the service life and operational stability of the entire device. PCB corrosion is the primary hidden cause of electronic equipment failures, short circuits, open circuits, and even complete报废. Unlike visible physical damage, corrosion often begins insidiously at the microscopic level, showing no obvious symptoms in the early stages and becoming irreparable once it erupts.

**I. The Essence of PCB Corrosion: Chemical and Electrochemical Decay of Metal Substrates**

The core conductive carrier of a PCB is copper foil, along with plated metals on pads such as tin, lead, nickel, and gold. The essence of corrosion is that these metal substrates undergo chemical or electrochemical reactions with media in the surrounding environment. This leads to the loss, oxidation, and deterioration of the metal layer, thereby destroying conductive paths, altering circuit resistance, and causing abnormal electrical performance. All common metals have a natural tendency to oxidize. Copper, as the core conductive material in PCBs, is relatively chemically active. When triggered by external factors, its oxidation rate can increase geometrically, resulting in irreversible corrosion damage.

From a chemical principle perspective, PCB corrosion is divided into pure chemical corrosion and electrochemical corrosion, with electrochemical corrosion accounting for over 90% of cases, making it the most prevalent and insidious form.

*   **Pure chemical corrosion** involves the direct oxidation-reduction reaction of a metal with corrosive gases or liquids, without the need for an electrolyte. An example is copper reacting directly with acid gases to form copper oxide.

*   **Electrochemical corrosion** requires three essential conditions: two metals with a difference in electrical potential, an electrolyte solution, and a conductive path. These three elements form a micro-galvanic cell. The metal at the anode continuously dissolves and deteriorates, while the metal at the cathode is protected. The copper foil, different plated metal layers, and impurity sites on a PCB easily form such micro-batteries, which is the core reason for the rapid propagation of corrosion.

**II. Common Types of PCB Corrosion and Their Microscopic Manifestations**

**1. Atmospheric Corrosion (Most Common)**

Components like moisture, oxygen, sulfur dioxide, hydrogen sulfide, and salt spray in the atmosphere are the primary media inducing atmospheric corrosion of PCBs. When the relative humidity exceeds 65%, an invisible thin film of water forms on the PCB surface. This water film acts as a natural electrolyte. Combined with oxygen from the air, it rapidly accelerates the oxidation of copper foil, producing cuprous oxide (red) and copper oxide (black). In coastal areas, chloride ions from salt spray penetrate the surface protection layer, destroy the metal passivation film, and form copper chloride, increasing the corrosion rate by more than five times. Sulfur compounds in industrial environments react with copper and silver plated layers, forming highly resistive copper sulfide and silver sulfide, leading to poor contact. In the early stages, this type of corrosion manifests as discoloration and darkening of traces. Later stages show the formation of verdigris, powdery corrosion products, and even induce creeping corrosion where corrosion products spread along gaps in the solder mask, causing short circuits between adjacent traces.

**2. Electrochemical Migration and Dendritic Corrosion (Most Dangerous)**

Electrochemical migration is a form of corrosion unique to PCBs, often occurring under energized operating conditions. Under the influence of an electric field, metal ions migrate along the electrolyte film on the PCB surface and deposit at the cathode, forming tree-like metallic crystals known as dendrites. Common examples are copper dendrites and tin dendrites. Dendrites grow extremely rapidly. Once they bridge adjacent traces, they can cause instantaneous short circuits and destroy components. This type of corrosion is particularly hazardous in high-density PCBs and fine-pitch circuits. It is primarily induced by ionic contaminants remaining on the PCB surface combined with a humid environment and an energized state, making it a common cause of failure in consumer electronics, automotive electronics, and industrial control equipment.

**3. Crevice Corrosion and Galvanic Corrosion (Localized High Incidence)**

*   **Crevice corrosion** typically occurs in confined, narrow spaces such as underneath components, within solder joint gaps, at voids in the solder mask, and inside via sidewalls. These areas have low oxygen content, creating an oxygen concentration cell (differential aeration cell) with the external environment. The localized corrosion rate far exceeds that on the open surface, leading to deep, penetrating corrosion. It is difficult to detect in the early stages but can eventually cause solder joint detachment and via open circuits.

*   **Galvanic corrosion** results from the direct contact of two dissimilar metals with different electrochemical potentials. For example, when copper foil contacts gold fingers, or tin solder contacts an aluminum substrate, the more active metal (like copper or tin) corrodes rapidly. This is commonly seen in interface areas and around connectors.

**4. Chemical Residue Corrosion (Process-Induced)**

This type of corrosion originates from residues of chemical agents used during PCB manufacturing, soldering, and assembly processes. Examples include residues from etchants, plating solutions, fluxes, and cleaning agents. Traditional rosin-based fluxes, in particular, can have high chloride content. Under humid and hot conditions, these residues hydrolyze to form hydrochloric acid, which continuously corrodes the metal traces. If the cleaning process is inadequate and ionic residue levels exceed acceptable limits, it will persistently induce electrochemical corrosion over the long term. This represents a congenital corrosion risk caused by process defects.

**III. Core Hazards of Corrosion on PCBs**

*   **Mild corrosion** can lead to increased trace resistance, signal transmission attenuation, and intermittent equipment faults.

*   **Moderate corrosion** can cause trace thinning, weak solder joints (cold solder), poor contact, and frequent system crashes or reboots.

*   **Severe corrosion** can result in trace open circuits, short circuits, component burnout, complete device failure, and even safety incidents such as electrical fires or electric shock hazards.

It is crucial to note that PCB corrosion is irreversible. Once the metal layer is lost, it cannot be restored by simple cleaning; only targeted repair or replacement is possible. Therefore, understanding the corrosion mechanisms and implementing preventive measures beforehand is of paramount importance.