Dielectric strength — the maximum electric field intensity a material can withstand without breakdown — is the electrical property that determines whether a potting compound can provide adequate isolation between conductors in an encapsulated assembly. Every potting compound data sheet reports dielectric strength, but this value is typically measured at ambient temperature. For electronics operating at 125°C, 150°C, or higher, the ambient-temperature dielectric strength is the least relevant data point on the sheet. Dielectric strength degrades with temperature for all polymer materials, and the rate of degradation differs significantly between compound types and formulations. Selecting a high-temperature potting compound based solely on its ambient dielectric strength number is a specification error that produces inadequate isolation at service temperature.
The Physics of Dielectric Breakdown in Polymers
Dielectric breakdown in a polymer potting compound occurs when the applied electric field is sufficient to mobilize charge carriers within the material and initiate a conducting path between electrodes. At ambient temperature, polymer chains are below their Tg and relatively immobile; charge carrier mobility is low and dielectric strength is at its maximum. As temperature increases toward Tg, chain segment mobility increases, the polymer softens slightly, and ion transport becomes more feasible. Above Tg, the polymer is in the rubbery state with significantly increased molecular mobility; dielectric strength decreases sharply.
For a standard epoxy potting compound with Tg of 120°C, dielectric strength at 150°C — which is above the Tg — may be only 50% to 70% of the ambient-temperature value. The compound has softened, moisture absorption has increased, and ion transport is much more active than at ambient temperature. This is why ambient dielectric strength data is inadequate for specifying potting compounds in high-temperature applications.
The second factor affecting dielectric strength at elevated temperature is moisture. Absorbed moisture in a potting compound increases the concentration of charge carriers (hydrogen and hydroxide ions from water dissociation) and directly reduces dielectric strength. At elevated operating temperatures, moisture diffusion into the compound is faster and equilibrium moisture content is higher than at ambient, compounding the temperature effect on dielectric performance.
What Dielectric Data to Request for High-Temperature Applications
For any potting compound being evaluated for service above 100°C, the dielectric strength data required for adequate specification includes:
Dielectric strength at operating temperature. The compound should be conditioned at the intended service temperature and tested at temperature, not after cooling. The test configuration — parallel plate electrodes in oil, short-time test or step test per IEC 60243 or ASTM D149 — should be specified consistently to allow comparison between compounds.
Dielectric strength after moisture conditioning. Conditioning specimens in humid conditions (85°C/85% relative humidity for 96 to 500 hours, depending on the standard) before high-temperature dielectric strength testing characterizes the wet-state performance. This is the worst-case scenario for many application environments and is more representative of actual service than dry-state testing.
Volume resistivity at operating temperature. Volume resistivity measures the leakage current that flows through the compound bulk under DC voltage. At elevated temperature, resistivity drops — the rate of drop with temperature characterizes how well the compound maintains isolation under DC bias. For assemblies with sensitive signal circuits, resistivity at operating temperature is as important as dielectric strength.
Dissipation factor (loss tangent) at operating frequency and temperature. For RF and high-frequency applications, the dissipation factor characterizes the heating generated in the compound by the alternating field and the signal attenuation across the compound. High dissipation factor at operating frequency and temperature can degrade signal quality and cause internal heating.
If you need dielectric property data at elevated temperature and after moisture conditioning for potting compound evaluation, Email Us — Incure provides temperature-dependent dielectric characterization data for our high-temperature compound formulations.
How Different Compound Chemistries Maintain Dielectric Strength at Temperature
Silicone potting compounds maintain dielectric strength exceptionally well at elevated temperature because the silicone polymer does not undergo a conventional glass transition — it remains flexible and its dielectric properties are relatively stable across a wide temperature range. The Si-O bond chemistry provides inherent resistance to charge carrier transport. Silicone compounds rated to 200°C show only modest reduction in dielectric strength across the temperature range from ambient to service temperature.
High-Tg epoxy compounds maintain acceptable dielectric strength at temperatures below their Tg. With dry Tg above 180°C, a high-temperature epoxy compound can provide good dielectric isolation at 150°C service temperature with adequate margin. The key specification is ensuring that the wet Tg — measured after moisture saturation — is still above the maximum service temperature. If moisture saturation reduces Tg below service temperature, dielectric strength drops dramatically.
Polyimide-based compounds maintain dielectric properties to temperatures above 250°C, driven by the thermal stability of the polyimide backbone chemistry. These compounds are the choice for the highest service temperature applications where both epoxy and silicone chemistry approach their performance limits.
Partial Discharge Inception Voltage
For electronics operating at elevated voltage — power electronics, motor drives, isolated converters — partial discharge inception voltage (PDIV) is a more application-relevant specification than bulk dielectric strength. PDIV is the voltage at which partial discharge begins within voids in the compound or at the compound-conductor interface. Partial discharge at voltages well below the dielectric breakdown value progressively damages the insulation and the potting compound over time.
At elevated temperature, PDIV decreases because the dielectric strength of any residual voids decreases and because thermal expansion of the compound may open micro-voids at compound-conductor interfaces that were absent at ambient temperature. Potting to eliminate voids — vacuum-assisted fill, proper surface preparation to ensure adhesion — is the primary control for partial discharge in high-temperature power electronics encapsulation.
Qualification for Specific Voltage and Temperature Requirements
Specifying dielectric performance for a potting compound in a high-temperature application requires defining the full electrical stress condition: working voltage, waveform (AC, DC, or pulsed), maximum repetitive voltage, isolation distance, and maximum service temperature including any transient temperature excursions. From these inputs, the required dielectric strength at service temperature, with moisture conditioning, can be calculated with adequate safety margin. This calculation, rather than ambient datasheet values, should drive compound selection.
Contact Our Team to discuss dielectric strength specification, compound selection for your voltage and temperature requirements, and test protocol development for high-temperature potting compound qualification.
Visit www.incurelab.com for more information.