Tackling IGBT Module Failures in High Humidity: A Deep Dive into Leakage Current and Insulation

Power systems operating in the real world—from offshore wind turbines and coastal solar farms to railway traction systems—are relentlessly exposed to harsh environmental conditions. Among these, high humidity is a particularly insidious threat to the reliability of high-voltage IGBT modules. While thermal stress is a well-understood challenge, the long-term effects of moisture can trigger catastrophic failures that are often harder to diagnose. For engineers designing and maintaining high-reliability power electronics, understanding how humidity leads to failure and how to select the right materials is not just important; it’s critical for ensuring system longevity and safety.

Moisture acts as a catalyst for failure mechanisms that degrade the very core of an IGBT module’s insulation system. It increases leakage currents, accelerates material aging, and ultimately creates pathways for destructive electrical discharges. This article provides a practical, in-depth analysis of the problem, exploring the physics of humidity-induced leakage current and offering actionable guidance on selecting robust insulation materials to design resilient power systems.

Understanding Leakage Current in IGBT Modules

Leakage current in an IGBT module refers to the small current that flows through or across an insulating material when a high voltage is applied. In ideal conditions, this current is negligible. However, in high-humidity environments, it can increase dramatically, posing a significant reliability risk. This phenomenon is not confined to the semiconductor chip itself but involves the entire module package, including the housing, silicone gel, and ceramic substrate.

There are two primary paths for leakage current:

• Surface Leakage: This occurs across the surface of insulating materials. When moisture is present, it can combine with dust, industrial pollutants, or ionic residues to form a thin, conductive film on the surfaces of the module’s internal components. This film provides an unintended path for current to flow between high-voltage terminals.
• Bulk Leakage: This is the current that flows directly through the volume of an insulating material. While high-quality dielectrics have very high bulk resistivity, the absorption of water molecules can alter their chemical structure, lowering this resistance and allowing more current to pass through.

Since IGBT modules are not hermetically sealed, they “breathe” with changes in ambient temperature and pressure, inevitably drawing in moisture-laden air. This moisture diffuses through the housing and encapsulating silicone gel, reaching the high-voltage interfaces within the module. Once inside, it drastically reduces the insulation’s effectiveness, setting the stage for more destructive processes.

The Mechanism: How Humidity Triggers Catastrophic Failures

The journey from a small amount of moisture to a complete module failure is a cascading process of electrochemical degradation. Understanding this sequence is key to preventing it.

1. Moisture Ingress and Contamination: As the module operates, its internal temperature cycles. During cooling phases, moist air is drawn inside. The water vapor then condenses on internal surfaces, mixing with any contaminants present to create a weak electrolyte.
2. Partial Discharge (PD) Initiation: The presence of this conductive moisture film, combined with the high electric fields at the edges of conductors (known as triple junctions, where copper, ceramic, and silicone gel meet), can initiate partial discharges. PDs are tiny, localized electrical sparks that occur within the insulation, much like microscopic lightning. While a single PD event is not destructive, millions of them over time bombard and degrade the insulating materials.
3. Electrochemical Migration (ECM) and Tracking: Under the influence of a DC voltage bias and moisture, metal ions can detach from one conductor and migrate across the insulation surface, forming conductive, tree-like filaments called dendrites. Simultaneously, the energy from partial discharges carbonizes the surface of organic materials like the module housing and the silicone gel. This process forms conductive carbon tracks.
4. Catastrophic Failure: Eventually, these carbon tracks or metallic dendrites can bridge the distance between two high-voltage conductors. This creates a low-resistance path, causing a massive surge of current—a dielectric breakdown or short circuit—that results in the permanent failure of the module.

The First Line of Defense: Selecting the Right Insulation Materials

Since preventing all moisture ingress is impractical, the choice of internal insulation materials becomes the most critical factor in designing humidity-resistant IGBT modules. The primary materials responsible for high-voltage insulation are the silicone gel, the ceramic substrate, and the plastic housing.

Silicone Gel Encapsulant

Silicone gel is the primary encapsulant used to protect the IGBT and diode chips. Its main functions are to provide electrical insulation and to act as a stress-relieving buffer against mechanical and thermal shock. A high-quality gel should exhibit:

• High Dielectric Strength: The ability to withstand a high electric field without breaking down.
• Strong Hydrophobicity: The ability to repel water, preventing the formation of continuous moisture films on critical surfaces.
• Excellent Adhesion: Good adhesion to the chip, substrate, and housing surfaces to eliminate voids or gaps where moisture can accumulate and PD can initiate.

Different gel formulations offer varying levels of performance, and leading manufacturers like Infineon invest heavily in developing advanced gels optimized for harsh environments.

Plastic Housing and the Comparative Tracking Index (CTI)

The plastic housing provides the module’s structure and the primary external insulation barrier. The most important property of this material in humid environments is its **Comparative Tracking Index (CTI)**. The CTI is a standardized measure of an insulating material’s resistance to the formation of conductive tracks on its surface.

The test, defined by the IEC 60112 standard, measures the maximum voltage at which a material can withstand 50 drops of a contaminated solution (0.1% ammonium chloride) without developing a carbon track. CTI values are categorized into Performance Level Categories (PLC), with higher values indicating better tracking resistance.

CTI and Material Performance: A Practical Comparison

The CTI value directly impacts the required creepage distance—the shortest path between two conductors along the surface of an insulator. A material with a higher CTI allows for smaller, more compact designs without sacrificing safety. For power modules used in demanding applications like renewable energy or traction, specifying a high CTI is non-negotiable.

Material Group (IEC 60112)	CTI Value (Volts)	Performance Level Category (PLC)	Typical Materials	Application Suitability
I	≥ 600	0	High-performance polyesters, certain epoxy resins	Excellent for harsh, polluted, and high-humidity environments (e.g., outdoor, traction).
II	400 ≤ CTI < 600	1	Polybutylene Terephthalate (PBT), Polyphenylene Sulfide (PPS)	Good for industrial applications with moderate humidity control.
IIIa	175 ≤ CTI < 400	2-3	Standard Phenolics, some older plastics	Suitable for controlled, clean, and dry environments only. Not recommended for new designs in demanding applications.
IIIb	100 ≤ CTI < 175	4-5	General-purpose plastics	Not suitable for high-voltage power electronics applications.

Practical Strategies for Mitigating Humidity-Related Failures

A multi-faceted approach involving robust design, careful operation, and proactive maintenance is necessary to ensure long-term reliability in humid conditions.

Design and Manufacturing Stage

• Material Specification: Always specify power modules with a housing material that has a CTI rating of 600V (Group I).
• Conformal Coating: Applying a thin, protective conformal coating over the entire power assembly (including the PCB and exposed terminals) adds an extra layer of defense against moisture and contaminants.
• Enclosure Design: Ensure the main system enclosure has an appropriate IP (Ingress Protection) rating to prevent direct water exposure. Implement controlled ventilation or use desiccant packs to manage internal humidity levels.

Operational and Maintenance Best Practices

• Regular Cleaning: Periodically clean the surface of modules and heat sinks to remove dust and salt deposits, which can become conductive when moist.
• Insulation Resistance Testing: Incorporate high-voltage insulation testing (Hi-pot testing) into routine maintenance schedules. An increase in leakage current over time is a clear indicator of insulation degradation and can serve as an early warning of impending failure.
• Environmental Control: Where possible, use dehumidifiers or heaters within converter cabinets to maintain relative humidity below 60%, especially during periods of shutdown.

Conclusion: Designing for Resilience in the Real World

High humidity represents a formidable challenge to the long-term reliability of IGBT modules. The failure mechanism, driven by increased leakage current, partial discharge, and the formation of conductive tracks, can lead to sudden and catastrophic system breakdowns. However, by understanding these underlying processes, engineers can make informed decisions to mitigate the risks effectively.

The foundation of a humidity-resistant design lies in the careful selection of materials. Prioritizing modules from reputable manufacturers like Semikron that utilize high-quality, hydrophobic silicone gels and plastic housings with a CTI rating of 600V is the most critical step. This, combined with robust system-level design practices and a proactive maintenance strategy, provides the best defense against the pervasive threat of moisture, ensuring that high-power systems deliver safe and reliable performance for their entire operational lifespan. If you require assistance in selecting IGBT modules engineered for the most demanding environmental challenges, our team of experienced application engineers is ready to help.