Electronics Guide

Failure Mechanisms and Degradation

Harvesting devices fail through a mix of mechanical, electrical, thermal, and chemical processes that often act together. Repeated mechanical loading drives fatigue in piezoelectric ceramics, springs, and flexures; temperature cycling fatigues solder joints and bond wires through mismatched thermal expansion; and humidity drives corrosion, moisture ingress, and delamination of encapsulants and laminates.

Each transducer class adds mechanisms of its own. Photovoltaic modules lose output to ultraviolet-induced yellowing and browning of the encapsulant, to cell cracking and solder-bond fatigue in the interconnect ribbons, and to potential-induced degradation, in which leakage current between the cells and the grounded frame at high system voltage shifts charge into the cell surface and depresses the fill factor. Thermoelectric modules suffer contact-resistance growth, interdiffusion at the hot junction, and cracked legs from the expansion mismatch across a large temperature gradient. Lead zirconate titanate and similar piezoelectric ceramics depole gradually under high electric field, high mechanical stress, or temperatures approaching the Curie point, so a harvester that survives mechanically may still lose coupling. Electrochemical and capacitive storage elements dry out, vent, or grow internal resistance long before the harvester itself wears out.

Most of these are wear-out mechanisms that progress gradually, which is what makes degradation predictable enough to model and to test against. Random early failures behave differently, and reliability practice separates the two: an infant-mortality population is screened out by burn-in, while a wear-out population is managed by derating, by design margin, and by an honest service-life estimate.

Reliability Modeling and Accelerated Testing

Because field lifetimes are measured in years, reliability is characterized by accelerated testing that applies elevated stress and extrapolates to use conditions through physics-of-failure models. The Arrhenius relationship describes how a higher temperature speeds thermally activated chemical and diffusion processes, scaling with an activation energy characteristic of the mechanism. The Coffin-Manson relationship relates the number of thermal or mechanical cycles to failure to the strain amplitude of each cycle, and it governs solder-joint and flexure fatigue. Peck's model combines temperature with relative humidity for moisture-driven failure such as corrosion and encapsulant delamination.

Standard stress screens turn these models into laboratory practice. Photovoltaic module qualification under IEC 61215, for example, applies 1,000 hours of damp heat at 85 degrees Celsius and 85 percent relative humidity, 200 thermal cycles between minus 40 and plus 85 degrees Celsius, humidity-freeze cycling, ultraviolet preconditioning, and mechanical load and hail impact, with a pass criterion expressed as a bounded power loss after each sequence; IEC 61730 covers the corresponding construction and safety requirements. Highly accelerated life testing pushes samples beyond their specification to find design margins and the mechanism that fails first, while a highly accelerated stress screen applies a shorter version of the same stresses in production to catch manufacturing escapes.

Test results feed life-distribution models, most often the Weibull distribution, whose shape parameter distinguishes infant mortality from a constant hazard rate and from wear-out. From the fitted distribution engineers estimate the metrics that matter to a deployment: the fraction of units surviving to a target age, the warranty return rate, and mean time to failure for a device that is discarded on failure or mean time between failures for one that is repaired. Calculating an acceleration factor honestly requires that the test invoke the same failure mechanism as the field. A screen that cracks a solder joint says nothing useful about an encapsulant that will yellow, and an acceleration factor quoted without naming its mechanism is not a number an engineer should trust.

Environmental Protection and Packaging

Packaging is the first line of defense, and for outdoor and industrial harvesters it frequently determines the service life. Enclosure protection is specified by the IP code of IEC 60529, where the first digit rates the exclusion of solid objects and dust and the second rates water: an IP65 housing is dust-tight and resists water jets, while IP67 adds temporary immersion and IP68 covers continuous immersion under conditions the manufacturer states. Conformal coating, potting compounds, and glass-to-metal or ceramic hermetic seals extend the same idea to the board and the die.

Sealing alone is not sufficient. A closed enclosure that heats and cools each day pumps humid air past its gaskets and condenses water inside, so designers fit vented membranes that equalize pressure while blocking liquid water, or they seal hermetically and add a desiccant. Materials must also survive their environment: ultraviolet-stable polymers outdoors, low-outgassing materials in vacuum, and metals chosen to avoid galvanic couples in salt fog. Because a harvester often shares its enclosure with the sensor and radio it powers, thermal design matters as well, since every degree of steady-state temperature rise shortens the life of the electrolytic and lithium chemistries inside.

Derating and Design Margin

Derating is the cheapest reliability tool available. Operating a capacitor, semiconductor, or connector well below its rated voltage, current, and temperature moves the part away from the stresses that drive wear-out and buys margin against the tolerance and drift that accumulate over a long deployment. The practice is particularly important in harvesting, where the source is intermittent and the load is duty-cycled: a node may sit for hours at open-circuit voltage and then deliver a burst of current far above its average, so parts must be selected for the peak rather than the mean.

Margin also has to cover the end of life rather than the beginning. A storage element that has lost a quarter of its capacity, a photovoltaic panel soiled and degraded after ten summers, and a converter running at its temperature extreme must still close the energy budget together, in the worst season, on the worst day. Sizing a system to its beginning-of-life numbers is among the most common causes of field failure in energy-autonomous deployments.

Protection and Fault Containment

Protection circuits keep a single fault from escalating into damage or a hazard. Overvoltage protection guards downstream electronics against the high open-circuit voltage that piezoelectric, electromagnetic, and photovoltaic sources develop when lightly loaded, and against externally coupled surges; transient-voltage-suppression diodes, shunt and Zener clamps, metal-oxide varistors, and crowbar circuits each suit a different energy and speed. Overcurrent protection with fuses, polymeric resettable devices, electronic fuses, and current limiters bounds the energy a fault can draw from storage, which matters far more than the harvester's own modest output.

Higher-voltage photovoltaic work adds hazards that low-power harvesting does not face, and the electrical codes address them directly. In the United States, Article 690.11 of the National Electrical Code requires that photovoltaic systems with DC circuits operating at 80 volts or more between any two conductors be protected by a listed photovoltaic arc-fault circuit interrupter or equivalent listed components, with narrow exceptions for circuits that are not on or in a building and are run underground or in metallic raceways or enclosures. Such interrupters are evaluated against UL 1699B, the outline of investigation for photovoltaic DC arc-fault circuit protection, which requires the device to detect a series arc on the order of 300 watts and to clear it within a bounded time so that the energy delivered into the fault stays below an ignition threshold; IEC 63027, published in 2023, sets the corresponding international test procedures for DC arc detection and interruption in PV circuits up to 1,500 volts. Ground-fault detection, rapid shutdown of array conductors for the benefit of first responders, galvanic isolation, thermal cutoffs, mechanical stress limits, and fail-safe defaults round out a layered, defense-in-depth approach.

Energy-Limited and Intrinsically Safe Design

In flammable atmospheres the goal shifts from containing a fault to making ignition impossible. Intrinsic safety, defined by IEC 60079-11 and certified under the ATEX framework in Europe and IECEx internationally, limits the electrical and thermal energy available in a circuit so that neither normal operation nor a credible fault can ignite the surrounding gas or dust. Protection levels distinguish equipment safe with two independent faults, with one fault, and in normal operation only, which in turn determines the zones where the equipment may be installed.

Energy harvesting fits this discipline unusually well, because a source that produces microwatts to milliwatts is already close to an energy-limited circuit. The constraint falls instead on what the system stores and how it switches: capacitance and inductance must be held below tabulated limits, storage elements must be isolated or barrier-protected, and surface temperatures must stay below the ignition temperature of the classified atmosphere. Designers who plan for certification from the outset find the requirements far easier to meet than those who add a supercapacitor bank first and seek approval later.

Functional Safety and Risk Assessment

For systems whose failure could cause harm, safety is engineered systematically rather than added after the fact. Hazard analyses such as failure modes and effects analysis and its criticality-ranked extension enumerate how each element can fail and rank the results by severity, likelihood, and detectability; fault-tree analysis works in the opposite direction, from an undesired top event down to the combinations of causes that produce it.

The international functional-safety standard IEC 61508 frames this work for electrical, electronic, and programmable electronic systems. It assigns a safety integrity level, SIL 1 through SIL 4 from lowest to highest, where each step demands roughly an order-of-magnitude further reduction in the probability of dangerous failure and a correspondingly higher rigor of design, verification, and diagnostics. Sector standards adapt the same framework, among them IEC 61511 for the process industries and ISO 26262 for road vehicles. Redundancy, diagnostic coverage, safe-failure fraction, and a clearly defined safe state translate these targets into hardware and firmware. The approach is especially relevant where a harvesting-powered node forms part of a protective function, since a system that is designed never to be serviced must also be designed to fail in a direction that is known and harmless.

Prognostics and Health Management

Prognostics and health management shifts maintenance from fixed schedules to a condition-based strategy. By monitoring indicators of degradation—delivered power, source open-circuit voltage, internal impedance, leakage current, temperature, and vibration signatures—a health-management system detects incipient faults, isolates their source, and estimates the remaining useful life of the device.

Two families of models support the estimate. Physics-based models project a known degradation mechanism forward from measured state, and they extrapolate well when the mechanism is understood. Data-driven models learn the relationship between observed features and time to failure from fleet history, and they cope better with mechanisms no one anticipated. Practical systems fuse the two and report a confidence interval rather than a single date, which is what allows operators to plan service for sensors spread across large or inaccessible installations before an unexpected failure interrupts the application. The economic case is straightforward: for a node deployed on a remote pipeline or inside a machine, the cost of the visit dominates the cost of the hardware, so any forecast that consolidates visits or prevents an emergency callout pays for the instrumentation that produced it.