Causes of Aging in Capacitors and Resistors on PCBA (Printed Circuit Board Assemblies)
2026-01-07
The aging of capacitors and resistors on PCBA refers to the irreversible degradation of their electrical parameters (such as capacitance value, resistance value, dissipation factor, equivalent series resistance, etc.) over time and under operating conditions. This ultimately can lead to diminished circuit performance or functional failure. The root cause lies in the chronic physical and chemical transformation of the internal materials of the components under the combined influence of electrical, thermal, mechanical, and environmental-chemical stresses. The following is a systematic analysis from two dimensions: component types and stress factors.
1. Core Mechanisms of Capacitor Aging
Due to the diversity of dielectric materials, capacitors have varied aging mechanisms and are among the components most prone to aging issues on PCBA.
1."Lifetime Depletion" of Electrolytic Capacitors (Aluminum/Tantalum):
- Electrolyte Drying Out: This is the most typical aging mode for aluminum electrolytic capacitors. The internal liquid or gel electrolyte slowly evaporates or undergoes irreversible chemical reactions and is consumed under high temperatures (especially heat generated in the core). This directly reduces the cathode surface area, manifesting as a decrease in capacitance and a significant increase in Equivalent Series Resistance (ESR). Their lifespan can often be approximated using the "10-degree rule," meaning the lifespan is halved for every 10°C increase in operating temperature.
- Oxide Film Degradation and Reformation: The aluminum oxide dielectric layer on the anode foil may experience localized breakdown and reformation under prolonged voltage application and high temperature, or suffer erosion from impurities like chloride ions, leading to a gradual increase in leakage current and reduced reliability.
- "Crystallization" and Failure of Tantalum Capacitors: Under voltage or current surges, localized thermal runaway may occur at the interface between the manganese dioxide cathode and the tantalum powder anode in tantalum capacitors, increasing the risk of failure.
2."Stress Sensitivity" and "Aging" Characteristics of Multilayer Ceramic Capacitors (MLCCs):
- Dielectric Aging: MLCCs using Class II dielectrics (e.g., X7R, Y5V) contain ferroelectric materials with spontaneous polarization. The orientation of their dipoles relaxes slowly over time, causing the dielectric constant and capacitance value to decrease logarithmically after soldering (recoverable, but requires high-temperature re-firing).
- Mechanical Stress Cracking: There is a significant difference in the Coefficient of Thermal Expansion (CTE) between the ceramic body of an MLCC and the epoxy-glass fiber PCB. Mechanical stress generated during wave soldering, reflow soldering, or drastic environmental temperature changes can lead to micro-cracks inside the ceramic body. Initially, cracks may cause only minor parameter drift, but over time they can propagate, ultimately leading to short circuits or intermittent open circuits, representing a typical "latent" aging failure.
- DC Bias Effect: For some high-dielectric-constant materials, the actual effective capacitance decreases under a DC bias voltage. This voltage dependency may manifest as performance degradation resembling aging in certain circuits.
3.Film Capacitors: Their aging primarily manifests as thermal degradation of the polymer dielectric at high temperatures, and the cumulative effect of the "self-healing" process of the metallized electrodes during overvoltage or surges, leading to a gradual decrease in effective capacitance.
2. Key Factors in Resistor Aging
Resistor aging primarily manifests as irreversible drift in resistance value or open-circuit failure.
1."Interface Corrosion" in Thick Film Chip Resistors (Most Common):
- Termination Sulfidation/Oxidation: In environments containing sulfur (e.g., industrial exhaust, rubber seals) or chlorine (e.g., marine, salt spray), the external silver-based termination electrodes of resistors can react with substances like hydrogen sulfide, forming high-resistance silver sulfide or silver chloride. This product permeates inward from the outside, forming an insulating layer at the critical interface between the resistive element and the termination electrode, causing the resistance value to increase sharply until an open circuit occurs. This is a typical "progressive open" failure.
- Resistive Film Layer Degradation: Long-term power overload or high temperatures can cause microstructural changes in the glass phase and conductive phase within the thick film paste, leading to systematic resistance drift.
- Moisture Ingression: If the protective glaze layer has imperfections, moisture ingress can trigger electrochemical corrosion, accelerating the processes mentioned above.
2.Thin Film/Metal Film Resistors: The precise resistive film in these components may undergo atomic interdiffusion or oxidation with the substrate under prolonged high temperatures, leading to slow resistance changes. Thermal stress from temperature cycling may also cause micro-cracks in the film layer.
3.Wirewound and Alloy Resistors: Their aging is mainly related to the oxidation of the wire or alloy material, with high temperatures accelerating oxide film growth, especially in power applications.
3. Common Accelerating Stresses: Catalysts for Aging
The intrinsic material properties of the components determine their aging "fate," while external stresses control the aging "speed."
- Thermal Stress: High temperature is the primary accelerating factor for aging. The rates of most chemical reactions (such as oxidation, diffusion, electrolyte consumption) follow the Arrhenius model. For every 10-15°C increase in temperature, the reaction rate approximately doubles. Temperature cycling generates alternating mechanical stress due to CTE mismatch, leading to material fatigue and interfacial separation.
- Electrical Stress: Continuous overvoltage accelerates dielectric damage in capacitors and electromigration in resistor films; current overload leads to increased Joule heating in resistors and capacitors, creating a vicious heat-electricity cycle; frequent surges or switching transients cause cumulative damage.
- Environmental Chemical Stress: Humidity is a necessary medium for most chemical corrosion and ion migration reactions. Airborne contaminants (sulfides, chlorides, acid gases) directly attack the external and internal metallic parts of components.
Mechanical Stress: Bending, twisting, or vibration of the PCB directly transfers stress to the component bodies, initiating or propagating existing micro-defects.
The aging of capacitors and resistors on PCBA is a chronic process involving coupled multi-physics fields. It begins with microscopic material defects and gradually evolves into macroscopic faults under the long-term "stewing" of electrical, thermal, mechanical, and chemical stresses. Therefore, reliability design for electronic products essentially involves managing and delaying this inevitable aging process through scientific derating (voltage, current, temperature), careful thermal and mechanical design, appropriate protective processes (e.g., conformal coating), and the selection of high-reliability components. Understanding these causes is fundamental for failure analysis, lifespan prediction, and improving product quality.
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