PCB corrosion is one of the core issues affecting product reliability in the electronics manufacturing industry. According to industry statistics, approximately 25% of electronic device failures originate from PCB corrosion, especially in harsh environments such as automotive electronics, outdoor communication equipment, and medical instruments. Corrosion can lead to solder joint detachment, wire breakage, and insulation failure, directly shortening product lifespan. As a technical expert with 10 years of experience in the PCB field, the author has observed that most companies' understanding of PCB corrosion remains at the level of "oxidation," lacking systematic knowledge of its types and causes, which results in inadequate targeted prevention and control measures. This article will comprehensively analyze the common types, causes, and identification methods of PCB corrosion based on IPC-TM-650 testing standards and practical experience, providing actionable reference solutions for engineering and technical personnel.

I. Core Types and Technical Definitions of PCB Corrosion

1.1 Classification by Corrosion Mechanism (Based on IPC-9201 Standard)

Chemical Corrosion: Refers to the oxidation-reduction reaction between the PCB surface and environmental chemicals such as moisture, acids, alkalis, salts, and contaminants. Common forms include copper foil oxidation, pad corrosion, and solder mask aging. According to IPC-TM-650 2.6.15 standard, the core of copper foil oxidation is the reaction of Cu with O₂ and H₂O to form CuO (black) or Cu₂O (red). When the oxide layer thickness exceeds 0.05μm, it affects soldering reliability.

Electrochemical Corrosion: This is the most common type of corrosion, triggered by the formation of micro-cells on the PCB surface, requiring the three elements of "electrolyte (moisture), electrodes (different metals or areas with potential difference), and circuit." For example, when an ENIG (Electroless Nickel Immersion Gold) pad comes into contact with a tin-plated pin in a humid environment, a galvanic cell forms, with gold as the cathode and tin as the anode, accelerating the corrosion and dissolution of the tin layer.

Electrolytic Corrosion: Triggered by external electric fields, commonly occurring in power boards and high-frequency boards. When ion contaminants such as flux residue or fingerprint sweat are present on the PCB surface, the electric field promotes ion migration, forming corrosion channels. According to IPC-6012 standard, the characteristic of electrolytic corrosion is "dendritic growth," which can cause short circuits in severe cases.

Physical Corrosion: Caused by physical factors such as mechanical wear, thermal shock, and UV radiation. It manifests as solder mask peeling or copper foil delamination, often occurring in conjunction with chemical corrosion.

1.2 Classification by Corrosion Location

Surface Corrosion: Corrosion on the surface of copper foil, pads, or solder mask, visible to the naked eye, such as blackened pads or bubbling solder mask.

Internal Corrosion: Corrosion in interlayer dielectrics or via walls, which is highly concealed and requires detection through cross-section analysis or X-Ray inspection. Examples include via copper layer detachment leading to poor conductivity.

Solder Joint Corrosion: Corrosion at the IMC (Intermetallic Compound) layer of solder joints, manifesting as graying of solder joints, increased brittleness, and reduced peel strength.

II. Core Causes of PCB Corrosion

2.1 Environmental Factors (60%)

Temperature and Humidity: Corrosion rates increase by 5-10 times when temperature exceeds 60℃ and relative humidity exceeds 85% RH. Particularly in environments with temperature and humidity cycling (e.g., -40℃ to 85℃), moisture condensation inside the PCB accelerates electrochemical corrosion.

Contaminants: Gases such as SO₂, Cl⁻, and NOₓ in industrial environments, and salt spray in marine environments (NaCl concentration > 0.05%) can form acidic or alkaline electrolytes, directly eroding copper foil and pads.

Ultraviolet (UV) Radiation: PCBs in outdoor equipment exposed to sunlight for extended periods experience accelerated solder mask aging and cracking due to UV radiation, leading to moisture penetration and internal corrosion.

2.2 Production Process Factors (30%)

Inadequate Cleaning: Residual flux after SMT placement (especially ROL2/ROL3 activity grade flux) or residual chemical agents after etching processes can act as ionic contaminants, triggering electrolytic corrosion in humid environments.

Improper Surface Treatment: ENIG gold layer thickness < 0.05μm, uneven tin layer in HASL (Hot Air Solder Leveling) processes (thickness < 1.0μm), or damaged OSP (Organic Solderability Preservative) film can fail to effectively isolate moisture and oxygen.

Material Selection Errors: Using ordinary FR-4 materials (Tg < 140℃) in high-temperature environments or failing to select special corrosion-resistant materials such as Rogers RO4350B results in insufficient corrosion resistance.

Design Flaws: Inadequate pad spacing (< 0.15mm), excessively large via holes (> 0.8mm), and lack of conformal coating design increase corrosion risks.

2.3 Storage and Usage Factors (10%)

Storage in humid environments (humidity > 70% RH), lack of sealed packaging, or absence of desiccants in packaging.

Direct finger contact with PCBs during assembly (salt and oil from sweat can cause corrosion).

Lack of waterproofing, dust protection, or damaged protective structures in outdoor equipment.

III. Professional Methods for Identifying PCB Corrosion

3.1 Visual Inspection (Preliminary Screening)

Visual Observation: Use a 10-20x magnifier (e.g., Leica D700M) to inspect for signs such as blackening or greening of copper foil, loss of luster or pitting on pads, and bubbling, cracking, or discoloration of solder mask.

Adhesion Test: Apply 3M tape to the PCB surface and peel it off forcefully. If copper foil or solder mask detaches, with an area exceeding 5%, it indicates adhesion loss due to corrosion (refer to IPC-TM-650 2.4.18 standard).

3.2 Instrumental Testing (Accurate Determination)

Ionic Contamination Test: Use an LD-LZ20 ionic contamination tester to measure ion concentration on the PCB surface. Levels > 1.5μg/cm² indicate corrosion risk (compliant with IPC-TM-650 2.3.25 standard).

Cross-Section Analysis: Examine corrosion products in interlayer or via walls under a microscope after preparing PCB cross-sections, such as copper oxide layer thickness or via copper layer detachment.

X-Ray Inspection: Use X-Ray equipment (e.g., Uni X-Ray) to detect hidden corrosion in internal vias and interlayers, such as voids in via copper layers or layer separation.

Electrochemical Testing: Perform polarization curve tests to determine corrosion rates via corrosion current density. A current density > 10μA/cm² indicates severe corrosion.

3.3 Environmental Simulation Testing (Predicting Corrosion Trends)

Salt Spray Test: According to IPC-TM-650 2.6.11 standard, expose the PCB to a 5% NaCl solution at 35℃ with continuous spray for 48 hours and observe surface corrosion.

Temperature-Humidity Cycling Test: Cycle between -40℃ and 85℃ with humidity ranging from 5% to 95% for 50 cycles, and check for corrosion failure.

Chemical Resistance Test: Immerse the PCB in media such as engine oil or brake fluid (85℃ for 1000 hours) and observe surface changes.

The core of PCB corrosion prevention and control lies in "source avoidance + process control + post-production protection": During the design phase, select corrosion-resistant materials (e.g., high-Tg FR-4, Rogers special boards) and optimize layout (increase pad spacing, incorporate conformal coating design). During production, strictly control surface treatment quality (gold layer ≥ 0.05μm, tin layer ≥ 1.0μm) and ensure thorough cleaning (ion concentration ≤ 1.5μg/cm²). During storage and usage, maintain dry environments, avoid direct contact, and implement protective measures.